Cohærence * | Quantum Technology

2012-01-18T19:47:00.002-05:00

Inaugural NASA Quantum Future Technologies Conference NASA Ames Research Center NASA scientists join the best quantum technology experts from academia, government and industry to identify new and exciting opportunities in space exploration, aeronautics, earth and space science where quantum technologies can have the greatest impact. Conference topics include next-generation quantum experiments for measurements of time and distance, navigation, field sensing, and gravity wave detection; scalable quantum computing architectures and algorithms; quantum key distribution for practical secure transmission over long distances, including fiber channels, earth-satellite links, and space-based communications networks.

Conference Website | Live Videoconference Stream

* February 3, 2012 | Videos and presentations are now online at the conference website.

2011-09-12T15:39:00.005-04:00

Quantum to Classical Crossover in Mechanical Systems Leiden
Lorentz Center Workshop on the Quantum to Classical Crossover in Mechanical Systems

In recent years there have been rapid developments in controlling micro- and nanometer-sized mechanical systems—to the point where quantum physics has become essential for understanding the dynamics of these systems. Quantized oscillations of mechanical resonators are now being discussed, and these have potential applications in the field of quantum information science.

New, fundamental tests of quantum mechanics—such as superpositions of states and entanglement between systems—are now within reach for macroscopic objects. These experimental possibilities provide new input to the discussion of how the classical world emerges from underlying quantum physics. A related question, whether quantum physics is needed to understand properties beyond those of the chemical reactions and molecular compositions of biological systems, will also be addressed. This Lorentz Center Workshop will bring together leading experimentalists and theorists in this field of research.

Workshop participants include Dirk Bouwmeester, Yaroslav Blanter, Herre van der Zant, Eva Weig, Markus Aspelmeyer, Hans Briegel, Andrew Cleland, Rosario Fazio, Philip Stamp, Wojciech Zurek, and many more.

2011-07-11T22:30:00.000-04:00

2011-06-23T06:38:00.005-04:00

Extending coherence times in superconducting qubits Schoelkopf Lab | via Leo DiCarlo — In arXiv 1105.4652, Schoelkopf et al report novel implementation of a superconducting transmon qubit strongly coupled to a 5-cm, three-dimensional superconducting cavity, attaining reproducible extension in coherence times of both qubit (T₁ and T₂ > 10 μs) and cavity (T_cav ∼ 50 μs) by more than an order of magnitude compared to the current state-of-the-art superconducting qubits. "This enables the study of the stability and quality of Josephson junctions at precisions exceeding one part per million. Surprisingly, we see no evidence for 1/ f critical current noise. At elevated temperatures, we observe dissipation due to a small density (< 1 − 10 ppm) of thermally excited quasiparticles. These results suggest that the overall quality of Josephson junctions will allow for error rates of 10⁻⁴, approaching the error correction threshold to meet the DiVincenzo criteria for universal quantum computation.

Time domain measurement of qubit coherence (a) Relaxation from |1⟩ of qubit J1. T1 is 60 μs for this measurement. (b) Ramsey fringes measured on resonance with (blue squares) and without (red squares) echo sequence. The pulse width for the π and π/2 pulses used in the experiments is 20 ns. An additional phase is added to the rotation axis of the second π/2 pulse for each delay to give the oscillatory feature to the Ramsey fringes.

2011-06-12T07:07:00.002-04:00

The Quantum Computer is Growing Up: Robust error correction in a quantum processor Rainer Blatt | Innsbruck | Science | KurzweilAI

A more efficient algorithm for error correction in quantum computers has been demonstrated experimentally by physicists at the Institute for Experimental Physics of the University of Innsbruck and the Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences (IQOQI).

The physicists demonstrated the mechanism by storing three calcium ions in an ion trap. All three particles were used as qubits: one ion represented the system qubit while the other two ions represented auxiliary qubits. The system qubit was then entangled with the auxiliary qubits to transfer the quantum information to all three particles.

The physicists applied a quantum algorithm to determine whether an error occurred and, if there was an error, correct it. After making the correction, the auxiliary qubits were reset using a laser beam to enable repetitive error correction.

“For a quantum computer to become reality, we need a quantum processor with many quantum bits. Moreover, we need quantum operations that work nearly error-free; the third crucial element is an efficient error correction.”- Philipp Schindler

2011-06-12T06:44:00.000-04:00

A team of physicists at the University of Innsbruck, led by Philipp Schindler and Rainer Blatt, has demonstrated a crucial element for quantum computers: repetitive error correction. This allows scientists to correct errors occurring in a quantum computer efficiently. The researchers recently published these findings in Science.

2011-03-10T11:56:00.017-05:00

I've recently been selected to train as a scientist-astronaut candidate for commercial suborbital and developing orbital flights with a newly-formed, nonprofit endeavor that counts NASA/ESA astronauts, astronaut trainers and instructors among its astronaut corps and its board of advisors. I'm honored to be selected for the program, and tremendously excited about the opportunity. This is just the start of a long and challenging journey!

The nascent field of commercial spaceflight—and the unique conditions afforded by space and microgravity environments—offer exciting new opportunities to conduct novel experiments in quantum entanglement, fundamental tests of spacetime, and large-scale quantum coherence. In pursuit of these goals, we have the opportunity to inspire our next generation of scientists, researchers and engineers.

Astronaut scientists for hire open new research frontier in space

About the Astronauts: Christopher Altman

Quantum Astronaut: Facebook Page

Virgin Galactic to fly scientists to space

Spaceport America

Bigelow Aerospace Partners with NASA, plans extension to International Space Station, Lunar Outposts

Quantum Experiments in Space and Microgravity

NASA JPL: Innovative quantum technologies for microgravity fundamental physics and biological research

Anton Zeilinger: Quantum Entanglement in Space

2011-01-17T09:52:00.002-05:00

Quantum Entanglement Allows "Teleportation in Time" MIT Technology Review "Conventional entanglement links particles across space. Now physicists say a similar effect links particles through time. Entanglement is so deeply enmeshed in the universe that a measurement in the past has an automatic, time-symmetric, influence on the future—and vice versa." –MIT Technology Review

2010-12-17T10:07:00.008-05:00

Science—Top Breakthrough of the Year Science, UCSB Science Magazine has compiled the top breakthroughs of the year, awarding the most significant scientific advance of 2010 to Andrew Cleland and John Martinis (UCSB). "This year’s Breakthrough of the Year represents the first time that scientists have demonstrated quantum effects in the motion of a human-made object," said Adrian Cho, a news writer for Science. "On a conceptual level it extends quantum mechanics into a whole new realm. On a practical level, it opens up a variety of possibilities ranging from new experiments that meld quantum control over light, electrical currents and motion to—perhaps someday—tests of the bounds of quantum mechanics and our sense of reality. This last grand goal might be achieved by trying to put a macroscopic object in a state in which it’s literally in two slightly different places at the same time."

2010-11-22T17:35:00.004-05:00

TEDxCaltech | Feynman's Vision: The Next 50 Years Caltech In recognition of the 50 year anniversaries of Richard Feynman's visionary talk "There's Plenty of Room at the Bottom" and the inauguration of his revolutionary "Feynman Lectures on Physics," the Institute will host TEDxCaltech on January 14, 2011. TEDxCaltech will be a dynamic celebration of Feynman's spirit, curiosity, and scientific vision, and will take ideas worth sharing from Caltech out into the world by celebrating Nobel Laureate, visionary, and “curious character” Richard Feynman, with the theme “Feynman’s Vision: The Next 50 Years.” Speakers include Scott Aaronson, Immanuel Bloch, Sean Carroll, John Preskill, Lenny Susskind, David Awschalom, Kip Thorne, Charlie Marcus, Don Eigler, Michael Roukes, Craig Venter, and many more.

2010-11-22T17:32:00.008-05:00

Future holds key to quantum physics—Obama awards National Medal of Science to Aharonov National Medal of Science USAToday | The future is affecting the past—all the time, on the quantum level—allowing physicists to effectively select the future they want their particles to have, within limits, and amplifying the results for a desired outcome. "I really believe we are close to a second revolution in physics as big as the one a century ago," Aharonov says. "I feel we are only beginning to free existing quantum theory and to do so, we must think of time in another way."

2010-11-20T13:29:00.004-05:00

I'm recently back from a fellowship in "Quantum Mechanics in Higher Dimensional Hilbert Spaces" at Austrian International Akademie, Traunkirchen, with Anton Zeilinger, Marcus Aspelmeyer, and Caslav Brukner. Photos are now online via the link below.

2010-11-12T10:07:00.004-05:00

Quantum computers may be much easier to build than previously thought Physical Review Letters physorg, arXiv "Quantum computers should be much easier to build than previously thought, because they can still work with a large number of faulty or even missing components, according to a study published today in Physical Review Letters. This surprising discovery brings scientists one step closer to designing and building real-life quantum computing system—devices that could have enormous potential across a wide range of fields, from drug design, electronics, and even code-breaking."

Moving Towards Quantum Computing New York Times "Three major technologies have the potential to move from demonstration computers to practical, highly powerful machines. 'We’re at the stage of trying to develop these qubits in a way that would be like the integrated circuit that would allow you to make many of them at once,' said Rob Schoelkopf, a physicist who is leader of the Yale group. In the next few years you’ll see operations on more qubits, but only a handful. The good news is that while the number of qubits is increasing only slowly, the precision with which the researchers are able to control quantum interactions has increased a thousandfold."

Seth Lloyd—Quantum effects in Biological Systems MIT cbc.ca "Lloyd's biological research, funded by the US Defense Advanced Research Projects Agency, looks at how living things use quantum computation [...] Bird navigation, plant photosynthesis and the sense of smell all represent ways living things appear to exploit the oddities of quantum physics."

2010-10-21T12:44:00.019-04:00

Google Workshop on Quantum Biology "Surprisingly robust quantum effects have been observed in warm biological systems. At the same time, quantum information technology has moved closer to physical realization. This Workshop on Quantum Biology will examine the significance of mesoscopic quantum coherence, tunneling and entanglement in biomolecular membranes, proteins, DNA and cytoskeleton, with particular attention to recently discovered megahertz ballistic conductance in microtubules. Potential utilization of biomolecular quantum information in regulation of cellular activities will be addressed, along with implications for disease and therapy as well as the future development of quantum computation and artificial intelligence." List of Speakers includes Alán Aspuru-Guzik (Harvard), Anirban Bandyopadhyay (Tsukuba), Stuart Hameroff (Tucson), Masoud Mohseni (MIT), Hartmut Neven (Google), Jiří Pokorný (Czech Republic), Elisabeth Rieper (Singapore), Mohan Sarova (Berkeley), Jack Tuszynski (Alberta), and Luca Turin (MIT)

– Quantum Biology · Agenda · Abstracts · Biographies

2010-09-13T16:25:00.018-04:00

"We believe it is timely to set out on a distinct quantum biology agenda. The burgeoning fields of nanotechnology, biotechnology, quantum technology, and quantum information processing are now strongly converging. As quantum engineering and nanotechnology meet, increasing use will be made of biological structures, or hybrids of biological and fabricated systems, for producing novel devices for information storage and processing, to create [novel sensors], and for other tasks. If experiments can shed further light on our understanding of decoherence in biomolecules, at scales where equilibrium thermodynamics no longer applies, this may provide the required foundation for greatly accelerating our progress in manmade quantum computers."

– Anita Goel, Gerard Milburn, Sandu Popescu, Jeff Tollaksen
(Quantum Aspects of Life)

2010-09-01T13:52:00.013-04:00

Are we living in a designer universe?

MIT, Sussex "Creating a new universe would require a machine only slightly more powerful than the LHC—and there is every chance that our own universe may have been manufactured in this way."
– John Gribbin, Telegraph

"A basement universe possesses a fate independent of its parent: harnessing the zero-point energy to trigger inflation becomes a form of applied cosmological engineering. And if basement universes are a naturally occurring phenomenon, as suggested by inflationary cosmological models, the multiverse then takes on the characteristics of an evolutionary algorithm. Though the parent universe in any branching scenario need not have been of intelligent design, once a suitable set of cosmological constants is found through natural inflation, intelligent life could branch out from this point of origin, forming an expanding wavefront of intelligence and altering the evolution of the multiverse itself [...]

Given that the conditions of the Drake equation are met, a potential explanation for the silence in our immediate neighborhood of the cosmos is that inter-universe panspermia supercedes local expansion. Vernor Vinge’s Singularity may not be only technological; it may be physical. The most powerful computer we can imagine would for all intensive purposes resemble a black hole."
– C. Altman, Expansion Scenarios

2010-08-27T18:22:00.006-04:00

Is reality even stranger than quantum mechanics tells us?

"We know that quantum correlations can be stronger than classical—but why aren't quantum correlations even stronger? Either we are missing something very significant to define quantum theory, or these other theories are all around us too."
– Caslav Brukner (New Scientist)

2010-08-12T12:16:00.021-04:00

Any quantum state can be cloned in the presence of closed timelike curves "The possible existence of closed timelike curves (CTCs) draws attention to fundamental questions about what is physically possible and what is not. An example is the "no cloning theorem" in quantum mechanics — which states that no physical means exists by which an unknown arbitrary quantum state can be reproduced, or copied perfectly. We show here that this theorem can be circumvented in the presence of closed timelike curves, allowing for the cloning of an unknown arbitrary quantum state. Since the "no cloning theorem" has played a central role in the development of quantum information science, it is clear that the existence of CTCs would radically change the rules for quantum information technology."
– Tim Ralph, David Ahn, R. B. Mann (arXiv:1008.0221)

2010-08-06T10:29:00.010-04:00

Molecular Simulation with Superconducting Qubits

"Because Nature isn't classical, damnit, and if you want to make a simulation of nature, you'd better make it quantum mechanical. " —Richard Feynman

Georgia, UCSB In arXiv:1008.0701, Pritchett, Martinis et al. introduce a protocol for efficient simulation of molecular dynamics using superconducting qubits. "Recent experimental progress suggests that quantum simulation will be one of the first practical applications of quantum computation. We have shown how quantum computers of only a few qubits can simulate arbitrary quantum systems accurately and quickly, even before they reach the regime of fault tolerant quantum computation."

2010-07-17T08:54:00.021-04:00

Decoherence sources in coupled flux qubits NEC, RIKEN In Phys Rev B and concurrent arXiv preprints, Yoshihara, Nakamura and Tsai study decoherence in coupled superconducting flux qubits. "The microscopic origin of decoherence has been elusive so far. It is crucial to identify and eliminate the source of noise sources in order to improve the performance of these devices—the sensitivity of SQUIDs and coherence of qubits. We have quantified the correlations among flux noise and found that the dominant contribution is by local fluctuations."

2010-06-18T17:42:00.003-04:00

Superconducting qubits as artificial atoms

"The demonstrated resonance wave scattering indicates that superconducting quantum devices can be used as building blocks for controllable, quantum coherent, macroscopic artificial structures — in which a plethora of effects can be realized from quantum optics of atomic systems."

NEC 東京大学 This week's Qulink seminar by Yasunobu Nakamura (NEC) reports on recent developments in exploring the quantum optical properties of a superconducting flux qubit coupled to a 1d microwave transmission line. See also: Electromagnetically induced transparency on a single artificial atom (arXiv); Resonance fluorescence of a single artificial atom (Science); Ultimate on-chip quantum amplifier (Phys Rev Lett).

2010-06-11T16:35:00.011-04:00

Quantum Zeno effect with a superconducting qubit NTT In arXiv 1006.2133, Matsuzaki and Semba provide detailed analysis of the Quantum Zeno effect in superconducting qubits: "Superconducting qubits are a promising system to observe the Quantum Zeno effect. We have studied how a sequence of projective measurements can alter the dephasing process, and suggest experimental requirements to observe the Quantum Zeno effect in existing superconducting qubits. It would be possible to demonstrate our prediction utilizing current technologies."

2010-03-18T19:04:00.008-04:00

First quantum effects seen in visible object UCSB The first ever quantum superposition in an object visible to the naked eye has been observed (New Scientist) "The key was to connect the resonating strip to a superconducting qubit—the qubit acts as a bridge between the microscopic and the macroscopic worlds."

Quantum mechanics harnessed to control macroscopic mechanical system (Wired Science) "The goal of the experiment was to see if we could observe quantum mechanical effects in a large, mechanical object. It’s an exciting piece of work. People are interested in pushing the boundaries of quantum mechanics." The techniques harnessed to measure the effect are based upon research earlier reported in "Quantum Entanglement Visible to the Naked Eye" (Nature, Wired Science, BBC)

Room-temperature quantum coherence in photosynthesis (Wired News)"The Nature findings, made at room temperature in common marine algae, show that macroscopic biological coherence operates under everyday conditions. Moreover, similar results from an experiment on another, simpler light-harvesting structure, announced by Engel’s group last Thursday on the pre-publication online arXiv, suggest that photosynthetic coherence is routine. 'There’s every reason to believe this is a general phenomenon,' said Engel. Scholes’ finding is 'an extraordinary result that shows us a new way to use quantum effects at high temperatures.'"

Nature's hot green quantum computers revealed (New Scientist) "Exactly how these molecules remain coherent for so long, at such high temperatures and with relatively large gaps between them, is a mystery,' says Alexandra Olaya-Castro of University College London, who has been collaborating with Scholes to understand the underlying mechanisms and apply them elsewhere. She believes that the antenna's protein structure plays a crucial role. 'Coherence would not survive without it,' she says. 'The hope is that quantum coherence could be used to make solar cells more efficient. The work is going to change the way we think about photosynthesis and quantum computing, Engel says. 'It's an enormous result.'"

Long-lived quantum coherence in photosynthetic complexes at physiological temperature (arXiv) "We present the first evidence that quantum coherence survives at physiological temperature for at least 300 fs—long enough to perform a rudimentary quantum computational operation. This data proves that the wavelike energy transfer process discovered at 77K is directly relevant to biological function. Microscopically, we attribute this long coherence lifetime to correlated motions within the protein matrix encapsulating the chromophores, and we find that the degree of protection afforded by the protein appears constant between 77K and 277K. The protein shapes the energy landscape and mediates an efficient energy transfer despite thermal fluctuations. The persistence of quantum coherence in a dynamic, disordered system under these conditions suggests a new biomimetic strategy for designing dedicated quantum computational devices that can operate at high temperature."

2010-02-23T16:02:00.005-05:00

Josephson junction neurons via physicsandcake Suzanne Gildert offers insights on a recent Phys. Rev. E preprint that proposes to model biologically realistic neurons using Josephson junction arrays. "These 'Josephson junction neurons' reproduce many characteristic behaviors of biological neurons such as action potentials, refractory periods, and firing thresholds, [and] would be orders of magnitude faster than both traditional computer simulations and biological neural networks." See also Quantum neural networks, backpropagation training, adaptive quantum networks.

Quantum Information Science—DARPA's New Frontier Collins offers this executive-level report on quantum information research at DARPA. "It's been almost a half-century since Intel founder Gordon Moore first observed that ever-shrinking circuitry on silicon chips leads to the doubling of the performance of these chips every 18 months or so. This has been instrumental in bringing rapid progress to the field of information processing. The era of Moore’s Law has been an interesting one, to say the least, but it is nearing its end: Within less than two decades, circuits will have shrunk to the atomic level."

2009-05-27T08:05:00.008-04:00

Kavli Colloquium — From the foundations of quantum mechanics to quantum information Delft Friday, 05 June 2009, "Quantum in Delft" (Leo Kouwenhoven), "Is there quantum in bio?" (Nynke Dekker), "Is ψ real?" (Yuli Nazarov), "From the foundations of quantum mechanics to quantum information" (Anton Zeilinger)

2009-05-05T14:43:00.011-04:00

Multi-Qubit Quantum Coherent Operations College Park "The purpose of this meeting is to identify – on a technology-specific basis – the primary difficulties that are likely to be encountered as qubit systems beyond typical current experiments with one, two, or three qubits are pursued. Topics of interest include, but are not limited to: controllable qubit interactions, miniaturization and integration to increase qubit density, new physics arising from the assembly of many-qubit systems, and process and state validation for larger systems. The workshop aims to address the following questions:

Given a particular qubit technology that has already demonstrated full single qubit control, what are the challenges facing this system as it grows into the multiple qubit regime? In a system with multiple qubits, what will be the mechanism(s) through which these qubits interact? How can these interactions be well enough controlled to achieve reliable, scalable logic gates? Do they contribute to decoherent processes?

What new physics must be taken into account for the benefit of, or hindrance to, many qubit operations? Are there any fundamental results that will inform the community of possible roadblocks that may be encountered when growing systems to the multiple qubit regime?

How can cross-talk from the control/readout channel for one qubit, to a bystander qubit, be reduced to a satisfactory level? What are the specific technologies required to ensure that a robust multi-qubit information processing system can be realized?

As systems grow larger, quantum state and, and even more so, process tomography will become impractical. What other measures will be effective for validating the operations of these systems?"

2009-04-10T08:34:00.009-04:00

National Workshop on Quantum Information Science OSTP|NTSC Via John Preskill, Dave Bacon, Ian Durham and Scott Aaronson — NSTC has announced a workshop in response to the recent Federal Vision for Quantum Information Science: "In January 2009, the United States National Science and Technology Council issued the report A Federal Vision for Quantum Information Science. The report proposes that

“The United States … create a scientific foundation for controlling, manipulating, and exploiting the behavior of quantum matter, and for identifying the physical, mathematical, and computational capabilities and limitations of quantum information processing systems in order to build a knowledge base for this 21st century technology.”

This Workshop on Quantum Information Science (QIS) has been organized in response to the NSTC report. It brings together leading theorists and experimenters drawn from physical science, computer science, mathematics, and engineering who will assess recent progress in QIS and identify major goals and challenges for future research."

2009-04-03T15:01:00.019-04:00

Adaptive quantum networks – We introduce a robust, error-tolerant adaptive training algorithm for generalized learning in high-dimensional, superposed quantum networks, or adaptive quantum networks. The formalized procedure applies standard backpropagation training to a coherent ensemble of discrete topological configurations of individual neural networks, each of which is formally merged under linear superposition. Quantum parallelism facilitates simultaneous training and revision within this coherent state space, resulting in accelerated convergence to optima. The protocol provides quantitative, numerical indicators for optimization of both single-neuron activation functions and reconfiguration of global network topology.

2009-03-06T10:29:00.007-05:00

Density plot of qubit purity ξ after t =20 kicks as a function of chaos parameter K=kT and nonlinear parameter g for T=2, δ=0.2 and ε=0.5.

Chaotic dynamics of a Bose-Einstein condensate coupled to a qubit Shepelyansky, Martin, Georgot arXiv 0812.5067: "We study the coupling between a qubit and a Bose-Einstein condensate moving in a kicked optical lattice. In the regime where BEC size is smaller than the lattice period, the chaotic dynamics of the BEC is effectively controlled by the qubit state. This gives an example of exponentially sensitive control over a macroscopic state by internal qubit states. The realization of such coupled systems is within reach of current experimental techniques."

Interference of interacting matter waves Innsbruck arXiv 0812.4836: "The phenomenon of matter wave interference lies at the heart of quantum physics. It has been observed in various contexts in the limit of non-interacting particles as a single particle effect. Here we observe and control matter wave interference whose evolution is driven by interparticle interactions. In a multi-path matter wave interferometer, the macroscopic many-body wave function of an interacting atomic Bose-Einstein condensate develops a regular interference pattern, allowing us to detect and directly visualize the effect of interaction-induced phase shifts. We demonstrate control over the phase evolution by inhibiting interaction-induced dephasing, and by refocusing a dephased macroscopic matter wave in a spin-echo type experiment. Our results show that interactions in a many-body system lead to a surprisingly coherent evolution – possibly enabling narrow-band and high-brightness matter wave interferometers based on atom lasers."

2009-02-26T05:16:00.007-05:00

Entanglement made Visible Genève In arXiv 0902.2896 and arXivblog summary, Gisin et al. show that macroscopic entanglement – and Bell inequality violations – can be made directly visible to the human retina. "We have shown that quantum experiments with human eyes as detectors appear possible, based on a realistic model of the eye as a photon detector. Entanglement, in principle, can be seen."

The Case for a National Investment: A Policy Essay for the Obama Transition Office In a recent CCC white paper, Aaronson and Bacon present the case for quantum computing as a national mandate. "First, quantum computing must be considered a national security issue. Since quantum computers break the codes used ubiquitously to protect transactions over the Internet, any country obtaining a scalable quantum computer would have the ability to disrupt electronic communication. Second, quantum information science research will help to maintain the US's scientific and technological advantages. Third, quantum computing is the study of the fundamental limits of computing and, as such, offers the potential to revolutionize our understanding of computation itself."

Pentagon explores quantum biology DARPA/DSO has launched a research initiative to investigate quantum mechanical effects in biological systems [initial Wired announcement, Sep. 2008]. "Scientists have recently discovered that quantum energy transfers allow plants and cynobacteria to convert sunlight into chemical energy nearly instantly, and with almost 100 percent efficiency. As energy passes between molecules involved in photosynthesis, a newly-observed 'wavelike characteristic' allows the energy to 'simultaneously sample all the potential energy pathways and choose the most efficient one.' DARPA says there are other biological examples of quantum effects — including an explanation, perhaps, for how birds are able to navigate along the Earth's magnetic fields."

2009-01-15T09:07:00.018-05:00

Quantum effects in biosystems Discover Magazine In Discover (February 2009), Mark Anderson reviews contemporary experiments in biophysics that explore the influence of quantum effects in living systems: "Could quantum mechanisms be driving some of the most elegant and inexplicable processes of life? For years experts doubted it: Quantum phenomena typically reveal themselves only in lab settings, in vacuum chambers chilled to near absolute zero. Biological systems are warm and wet. Most researchers thought the thermal noise of life would drown out any quantum weirdness that might rear its head. Yet new experiments keep finding quantum processes at play in biological systems, says Christopher Altman, alumni researcher from European futurist think tank Starlab. With the advent of powerful new tools like femtosecond (10^-15 second) lasers and nanoscale-precision positioning, life’s quantum dance is finally coming into view."

The Unmanned Air Force USAF In Network World and concurrent Slashdot updates, Lt. Gen. Seip discusses the future of unmanned combat aerial vehicles. "How important have unmanned aircraft become to the US military? Well, how's this: the Air Force says next year it will acquire more unmanned aircraft than manned aircraft. Air Force Lt. Gen. Norman Seip this week said the service is "all in" when it comes to developing unmanned systems and aircraft.' Next year, the Air Force will procure more unmanned aircraft than manned aircraft,' the general said. 'I think that makes a very pointed statement about our commitment to the future of unmanned aircraft, and what it brings to the fight in meeting the requirements of combatant commanders.'"

2008-11-15T19:03:00.017-05:00

Convergence⁰⁸ Mountain View " From 15-16 November 2008 – the world's most dangerous ideas will collide in Mountain View, California. Convergence⁰⁸ examines the world-changing possibilities of nanotechnology and the life-changing promises of biotechnology. It is the premier forum for debate and exploration of cognitive technology ethics – and ground zero of the past and future information technology revolution. Convergence⁰⁸ is an innovative, lively 'unconference' – the first and only unclassified forum dedicated solely to the convergence of NBIC – nano-, bio-, information and cognitive – technology developments."

As my own UNISCA First Committee chair report to the General Assembly – "Converging Technologies: The Future of the Global Information Society" – focused specifically upon these long-term technological and cultural challenges, which we will have to confront both as a society and as a species – I have high hopes that this meeting will provide a stimulating, and unparalleled, open venue for exploration of novel ideas, discussion of alternative paradigms, and fertile grounds for brainstorming original, innovative solutions. My congratulations to the initiative of the conference organizers. I look forward to reporting back upon conclusion of the conference, as there has been an open conference wiki set up at the conference website to contribute novel ideas to the discussion.

2008-10-02T09:47:00.001-04:00

Time in Quantum Mechanics Perimeter Institute The Clock and the Quantum focuses on conceptual and technical issues concerning the role of time in quantum theory – including quantum correlations in time, histories approaches, pre- and post-selected ensembles, time and quantum measurement, and causality under the framework of quantum theory.

Lee Smolin discusses cosmological inflation, the problem of initial conditions, and the interpretation of the "wavefunction of the universe." Lev Vaidman provides a review of the two state vector formalism, which considers backwards-evolving quantum states. Noriyuki Hatakenaka presents a new scheme for testing macrorealism without statistical treatments by combining Leggett-Garg and Greenberger-Horne-Zeilinger (GHZ) inequalities, i.e. a temporal GHZ test using quantum correlations in time. Lucian Ionescu outlines an "upgrade" of the Feynman path integral formalism – where qubits, instead of complex amplitudes – are associated within elementary transitions of a causal network structure.

The conference is first in a series of foundations conferences organized under joint collaboration between the Perimeter Institute and three Australian national universities.

2008-09-09T09:46:00.021-04:00

From Qubits to Black Holes Technion | Macquarie University "Asher Peres (1934-2005), was an Israeli scientist who is widely considered to be one of the pioneering founders of quantum information science. A student of Nathan Rosen, (the “R” of EPR), Asher codiscovered quantum teleportation, a time-reversal test for quantum entanglement, and published numerous works on the foundations of quantum science. His research legacy continues through his many research collaborators, students, textbooks and research papers."

The Technion (Israel), and Macquarie University (Australia), will host the inaugural Asher Peres International Physics School 2008, for senior undergraduates and junior postgraduates in a series of lectures ranging from quantum mechanics, theory and experiments, through to quantum gravity. Held over five days, from November 17-22 (2008), in the historic environs at Chowder Bay on Sydney Harbor, the School will feature lectures from leading scientists from around the world, including Sir Peter Knight, Artur Ekert, Christian Kurtsiefer, Chris Fuchs, Bei Lok Hu, Jason Twamley, Daniel Terno, Gavin Brennen, Alexei Gilchrist, James Rabeau, and James Cresser.

2008-08-17T09:52:00.010-04:00

Progress in Quantum Computing IQSA | LT₂₅ | Lorentz Center – I've recently returned from a series of international conferences and workshops on superconductivity, quantum computation, entanglement and quantum coherence. In Sopot, Poland at the International Conference on Quantum Structures, much of the week was spent in long walks on the shores of the Baltic Sea, holding intense discussions on quantum information theory with Lev Levitin, who will be hosting the IQSA meeting at MIT in two years. We also continued ongoing research with Roman Zapatrin (Starlab) to advance the development of adaptive quantum networks for applications in fault-tolerant quantum computation, associative processing and pattern recognition.

Following IQSA, I moved on to the 25^th triennial International Conference on Low-temperature Condensed Matter Physics, where I met with Keith Schwab following presentation of his group's recent experiments with nanomechanical resonators to probe the boundaries of quantum and classical regimes, as well as discussing present and upcoming experiments in superconducting flux qubit systems with Yasu Nakamura, John Clarke, Robert Schoelkopf, and John Martinis.

Upon conclusion of LT₂₅, a satellite conference on Quantum Decoherence in Quantum Information Systems was held at the Lorentz Center, where I met with Vlatko Vedral to discuss long-term research initiatives in multipartite and macroscopic entanglement in condensed matter systems. Jasper van Wezel presented a review of the limits to quantum behavior related to spontaneous symmetry breaking – summarizing recent results on the quantum to classical transition, and future experiments which may elucidate the process of wavefunction collapse. Dirk Bouwmeester was generous enough to offer a tour of the experimental laboratory setup for the MiniGRAIL gravitational wave detector, which has just undergone several modifications, including improvements to the antenna, cryogenic cool-down systems, improved shielding, redesign of the capacitive transducer and fabrication of a new two-stage SQUID module for more stable operation at low temperatures.

2008-06-19T10:06:00.008-04:00

Space QUEST: Experiments with quantum entanglement in space Vienna | ESA | ISS In a recent submission to the arXiv, Zeilinger's group at University of Vienna, Austria has proposed an experiment—Space-QUEST, Quantum Entanglement Science and Technology—for space-to-ground, entangled-photon Bell Inequality violation measurements to verify quantum nonlocality at distances over thousands of kilometers, in a joint operation between the International Space Station and a ground observatory in the European Union.

Entanglement and nonlocality have been pivotal controversies since the birth of quantum mechanics—Einstein's "spooky action at a distance" implies simultaneous, nonlocal correlations between separate entangled particles. J. S. Bell was the first to confirm the phenomenon experimentally in 1964.

Further refinements and increasing precision in succeeding experiments have consistently shown quantum mechanics to be an explicitly nonlocal theory—the outcome Einstein was most averse to accept. However, long-distance relativistic experiments, such as between orbiting satellites, have been technologically cost-prohibitive to date. The paper will be presented at the 2008 IAC Microgravity Sciences and Processes Symposium, under a proposed joint initiative between the European Space Agency and the International Space Station.

"Testing quantum correlations over distances achievable with systems placed in the Earth orbit, or even beyond, would allow to verify both the validity of quantum physics and the preservation of entanglement over distances impossible to achieve on the ground. Using the large relative velocity of two orbiting satellites, one can perform experiments on entanglement where – due to special relativity – both observers can claim that they have performed the measurement on their system prior to the measurement of the other observer. In such an experiment, it is not possible anymore to think of any local realistic mechanisms that potentially influence one measurement outcome according to the other one."

Zeilinger's group has previously conducted proof-of-principle experiments in the Canary Islands with a 144 km free-space link, using an ESA receiver telescope to receive single entangled photons, cf. Nature Physics, 3:481-486 (2007). A more recent experiment in Italy has demonstrated single-photon downlink communications viability from a near-earth orbit satellite, cf. New Journal of Physics, 10:033038 (2008).

2008-06-19T09:48:00.022-04:00

The Reality Tests Vienna In Seed (June 2008), the Vienna experimental group discusses physical and philosophical implications of new correlations between entangled photons, which violate an inequality proposed by Leggett for nonlocal realistic theories. This new series of experiments invalidates macrorealism in quantum mechanics by more than 80 orders of magnitude. Preliminary coverage of the experimental results was first presented in Nature 446 (871) and PhysicsWorld, 20 April 2007. According to Časlav Brukner: "Quantum mechanics does not always wash itself out – but to observe its effects for larger and larger objects, we would need more and more accurate measurement devices. We just do not have the sensitivity to observe the quantum effects around us. In essence, we do create the classical world we perceive. There could be other classical worlds completely different from ours."

Quantum networks: Entanglement of distant atoms by projective measurement University of Barcelona | ICFO | Spain Quantum cryptography is rapidly developing into a mature and robust technology for secure data transactions in financial, government and military sector applications. In arXiv 0806.1052, Zippilli et al. quantify the role of photon detector efficiency in quantum repeaters, which will be necessary to scale beyond the point-to-point networks currently employed for secure communications.

Presently, state-of-the-art systems employ atom-photon interaction to generate entanglement between distant nodes across a quantum network through projective measurement. "We assess proposals for entangling two distant atoms by measurement of emitted photons, analyzing how their performance depends on the photon detection efficiency – we believe that these concepts are generally applicable to all systems that may be considered for the creation of distant entanglement, including atomic-ensemble, photonic, and solid state implementations."

The groups's objectives are to quantify the importance of detector efficiency as applied to generating remote entanglement across quantum networks. With minor modifications, these results can be extended to the efficiency of quantum teleportation protocols that are also based on projective quantum measurement. "In all such systems, the detection efficiency will have a similar, important role for the use of the entanglement as a resource in quantum technologies."

Space-QUEST: Experiments with quantum entanglement in space Vienna | ESA | ISS In a recent submission to the arXiv, Zeilinger's group at University of Vienna, Austria has proposed an experiment – Space-QUEST, Quantum Entanglement Science and Technology – for space-to-ground, entangled-photon Bell Inequality violation measurements to verify quantum nonlocality at distances over thousands of kilometers, in a joint operation between the International Space Station and a ground observatory in the European Union.

Entanglement and nonlocality have been pivotal controversies since the birth of quantum mechanics – Einstein's "spooky action at a distance" implies simultaneous, nonlocal correlations between separate entangled particles. J. S. Bell was the first to confirm the phenomenon experimentally in 1964.

Further refinements and increasing precision in succeeding experiments have consistently shown quantum mechanics to be an explicitly nonlocal theory – the outcome Einstein was most averse to accept. However, long-distance relativistic experiments, such as between orbiting satellites, have been technologically cost-prohibitive to date. The paper will be presented at the 2008 IAC Microgravity Sciences and Processes Symposium, under a proposed joint initiative between the European Space Agency and the International Space Station.

"Testing quantum correlations over distances achievable with systems placed in the Earth orbit, or even beyond, would allow to verify both the validity of quantum physics and the preservation of entanglement over distances impossible to achieve on the ground. Using the large relative velocity of two orbiting satellites, one can perform experiments on entanglement where – due to special relativity – both observers can claim that they have performed the measurement on their system prior to the measurement of the other observer. In such an experiment, it is not possible anymore to think of any local realistic mechanisms that potentially influence one measurement outcome according to the other one."

Zeilinger's group has previously conducted proof-of-principle experiments in the Canary Islands with a 144 km free-space link, using an ESA receiver telescope to receive single entangled photons [Nature Physics, 3:481-486 (2007)]. A more recent experiment in Italy has demonstrated single-photon downlink communications viability from a near-earth orbit satellite [New Journal of Physics, 10:033038 (2008)].

2008-06-01T08:58:00.005-04:00

Superconducting Qubits RIKEN | UBC | Sherbrooke – In arXiv 0805.0164, Zagoskin and Blais provide a broad and accessible introduction to quantum information processing with superconducting qubits. "From a physicist's standpoint, the most interesting part of quantum computing research may well be the possibility to probe the boundary between the quantum and the classical worlds. The more macroscopic are the structures involved, the better. So far, the most "macroscopic" qubit prototypes that have been studied in the laboratory are certain kinds of superconducting qubits. To get a feeling for how macroscopic these systems can be, the states of flux qubits which are brought in a quantum superposition corresponds to currents composed of as much as 10⁵ - 10⁶ electrons flowing in opposite directions in a superconducting loop."

2008-05-19T10:41:00.012-04:00

Efficient pulsed gates for an oscillator stabilized Josephson qubit IBM Watson In arXiv 0709.1478 and New J. Phys. 10 033027 (2008), Koch, DiVincenzo, Brito and Steffen derive operational specifications for high-fidelity one and two-qubit pulsed gates for a superconducting flux qubit, calculating the Hamiltonian with tunable interaction from initialization to readout.

"The quantitative fact that the values of gate inﬁdelity are at the 1% level – and below – is the major result of this paper."

So, can a "debugged" IBM qubit be used soon for universal quantum computation?

"The short answer is, in our opinion, ultimately yes."

"The answer would certainly be no if the noise threshold for fault-tolerant quantum computation were in the neighborhood of the oft-quoted value of 10⁻⁵. It is not inconceivable for the experiment to get to these values someday, since we ﬁnd that the inﬁdelities decrease much faster than linearly with the assumed noise levels."

"To get to 10⁻⁵, we would need to get to the very daunting levels of 100nΦ₀ at 1Hz for the 1/f noise amplitudes and 100 f s for timing accuracies; there is optimism that both of these numbers are ultimately attainable. Fortunately, while 10⁻⁵ was the threshold as it was understood ten years ago, much recent work shows that with good designs, much higher thresholds are possible. According to Terhal and Burkard – 1% is, in fact, on the high end of the noise levels for which fault tolerance may be possible."

2008-05-14T09:41:00.019-04:00

Photon transmission through sub-wavelength diameter apertures Delft | Optica In Optics Express 16, 10 (abstract, full article) and concurrent TU Delft summary, Photonics review, Adam, Planken et al. report on high time-resolution terahertz mapping of photon transmission through sub-wavelength diameter apertures:

"According to the laws of physics, it is particularly difficult to pass light through a hole smaller than half the wavelength of the light used." The Delft group conducted experiments using extremely high time-resolution measurements in the terahertz (THz) frequency range. The group discovered that even if the hole is up to fifty times smaller than the wavelength used, sufficient light can pass through to allow measurements near the hole – an extremely difficult task using other methods. "Improving the sharpness of THz microscopes, coupled with more sensitive detectors, will improve the viability of creating images of biological cells using this type of measurement."

Prior experiments at Leiden University (Nature 418, 304-306) have also studied photon transmission through sub-wavelength metal films and shown entanglement conservation to be much more robust than expected – surviving the conversion process from surface-plasmon waves, which tunnel through the barrier, before reradiating as photons on the opposite side of the film. "It's a good omen, because it's saying quantum entanglement can survive when you might not expect it to," says Bill Barnes, a photonics expert at the University of Exeter. "If they can survive this, what else can they survive?"

2008-05-08T05:22:00.007-04:00

Time Reversal in Bose-Einstein Condensates Toulouse | CNRS In arXiv 0804.3514, Martin, Georgeot, and Shepelyansky of Quantware MIPS Center investigate time reversibility in Bose-Einstein condensates (BEC). "We show that inside the regime of quantum chaos, time-reversal dynamics can be inverted from explosion to collapse. The accuracy of time reversal decreases with the increase of atom interactions inside BEC, until it is completely lost – though, surprisingly, quantum chaos helps to restore time reversibility. Existing experimental setups similar to Ryu, Behinaein, and Wayper can test the fundamental question of BEC time reversal discussed here."

2008-05-06T15:41:00.015-04:00

DARPA INFOSEC Mandate DARPA | EOP | Congress In Wired briefing 01 May 2008, Danger Room reports on the new DARPA Information Security program mandated by Congress and ratified by the President. The UNISCA First Committee INFOSEC Chair briefing to the UN General Assembly is particularly àpropos to the initiative. "The Defense Advanced Research Projects Agency, or DARPA, was created 50 years ago in response to the Soviets' launch of Sputnik. In less than a year, DARPA put together the infrastructure that guided the American space effort for decades to come. Now, DARPA has been given new marching orders: to help America fight and win battles online.

Under a directive signed by the President – and recently approved by Congress – nearly every arm of the government's security apparatus is starting work on a massive national cybersecurity initiative designed to protect the United States from electronic attack and strike at adversaries online. DARPA's role: to create a cyberwarfare range where all these new forms of electronic combat can be tried out. According to a defense official familiar with the program, "Congress has given DARPA a direct order; that's only happened once before – with the Sputnik program in the '50s."

Danger Room's sister blog, Threat Level, has a good writeup of the cybersecurity initiative, which has been labeled as a Manhattan Project-type effort. In the case of cybersecurity, there is at least talk of big money: about $30 billion dollars. For its part, DARPA's "National Cyber Range" would create a virtual environment where the Defense Department can mock real warfare, both defense and offense.

DARPA today issued an announcement, describing how the range would be a test where the government could conduct unbiased, quantitative and qualitative assessment of information assurance and survivability tools in a representative network environment ; replicate complex, large-scale, heterogeneous networks and users in current and future Department of Defense (DoD) weapon systems and operations ; enable multiple, independent, simultaneous experiments on the same infrastructure ; enable realistic testing of Internet/Global-Information-Grid (GIG) scale research ; develop and deploy revolutionary cyber testing capabilities, and enable the use of the scientific method for rigorous cyber testing.

This is clearly a serious deal for the agency: DARPA Director Tony Tether is a scheduled speaker at the proposers' day workshop scheduled for mid-May, and apparently plans to help handpick the contractors. Tether is known for his close involvement in DARPA contracts. Many of the details surrounding this program will be classified."

2008-04-19T08:06:00.018-04:00

Quantum sensor effect in bird navigation? Institute for Electronic Structure and Lasers, Foundation for Research and Technology, Heraklion | University of Crete In arXiv:0804.2646, arXivblog, and Slashdot reports, Kominis investigates the potential for Zeno effect-based quantum sensing in bird navigation. "How birds use the Earth's magnetic field to navigate has puzzled researchers for decades. In recent years, a growing body of evidence has pointed to the possibility that a weak magnetic field can influence the outcome of a certain type of chemical reaction involving the recombination of pairs of ions in bird retinas. The trouble is that the ion recombination is known to happen too quickly for the Earth's weak magnetic field to have any effect. Now it looks as if the quantum Zeno effect may explain the process (abstract). This is the "watched-pot-never-boils" effect, in which the act of observing a quantum system maintains it for longer than expected. This is extraordinary news, because it means a quantum sensor is determining the macroscopic behavior of living birds. Kominis says we may well see these effects elsewhere, and mentions that a similar mechanism might be at work in photosynthesis."

MagiQ Research Labs Andrew Hammond | MagiQ Technologies MagiQ Technologies (NYC) has founded MagiQ Research Labs in Somerville, MA. The lab will provide a technical, engineering and production facility base for public and private sectors in medical optics, quantum information, fiber sensing, aerospace and defense applications, and has been established under research grants from ARO, DARPA, and NASA.

2008-03-16T06:41:00.009-04:00

Room-temperature quantum oscillations in diamond crystals UCSB|CNSI|Kavli TU Delft|Ames DOE In Science Express, Science and concurrent UCSB press release, Hanson, Awschalom et al. report on striking experimental observations of quantum oscillations in diamond crystals at room temperature. "We were stunned by these unexpected experimental results, and extremely excited by the ability to control and monitor single quantum states, especially at room temperature," said David Awshalom. "To our surprise, when looking at longer times, the oscillations disappeared, then re-appeared. At first it looked like an artifact, but repeated measurements reproduced this behavior," said co-author Ronald Hanson, a postdoctoral student at UCSB during this period who is now a professor at Kavli Institute of Nanoscience Delft, Delft University of Technology, in the Netherlands.

2008-02-17T06:44:00.027-05:00

Workshop on Neuromorphic Computing DSO As profiled in Wired and BAA SN08-16, DSO is hosting a workshop on neuromorphic adaptive plastic scalable electronics to be held on 04 March, 2008. "Briefly, the vision for the anticipated DARPA SyNAPSE program is to enable electronic neuromorphic machine technology that is scalable to biological levels. As compared to biological systems, today’s intelligent machines are less efficient by a factor of one million to one billion in real world, complex environments. The key to achieving the vision of the SyNAPSE program will be an unprecedented multidisciplinary approach that can coordinate aggressive technology development activities in the following SyNAPSE areas: 1) hardware; 2) architecture; 3) simulation; and 4) environment. Hardware includes neuromorphic electronics with novel, high density, plastic, synaptic components; architecture includes neuromorphic design from microcircuit to complete system; simulation includes large-scale digital simulation of neuromorphic circuits and functional neuromorphic systems; and environment includes virtual training, testing and benchmarking for neuromorphic systems realized in hardware or simulation. "

2008-01-18T09:10:00.000-05:00

QuanTalk EU QIST Via the Pontiff, a new web-based initiative supported by the European Union under ERA-Pilot QIST grant, QuanTalk. " The purpose of the project is to provide a central facility for open review and discussion of research in quantum information science worldwide. The heart of the site is the Articles section, wherein we provide three main features, (1) Open scientific discussion of the latest research in quantum information. The system will be familiar to those who use forum systems or blogs, but it differs in its approach to permanence and accountability. (2) A community review process, open peer review, to which authors may submit their work. This feature is not yet available during the current beta test of quantalk.org. (3) An open archive where authors can deposit digital material that they wish to make available to the community. During the beta phase, hosting is restricted to PDF documents."

2007-12-11T08:28:00.000-05:00

Marc Feldman, 1945-2007 Rochester Marc J. Feldman, professor and scientist in the Department of Electrical and Computer Engineering at the University of Rochester, passed away December 4, 2007 at age 62. Feldman was a founder and early pioneer in the field of superconducting quantum computing. As leader of the Superconducting Electronics Laboratory at Rochester, he led a number of major projects to explore advanced computing concepts. "Marc will be missed tremendously by all, not only was he an outstanding scientist in his own right but he was a generous and prolific scientific collaborator. His deep love of science, boundless intellectual energy and gentle sense of humor made it truly a privilege and a pleasure to call Marc our colleague and friend." – Mark Bocko

2007-12-05T08:50:00.000-05:00

Experimental demonstration of Berry's Phase in a solid-state qubit Zürich|Waterloo|Sherbrooke|Yale In Science, arXiv preprint, and concomitant ETH-Zürich report "Geometry for Quantum Computers," researchers in collaboration with the Quantum Device Lab have demonstrated Berry's phase in solid-state circuit quantum electrodynamics, an approach which is inherently robust against certain types of errors. "Geometric phase has been argued to have potential fault tolerance. We demonstrate the controlled accumulation of geometric phase, Berry's phase, in a superconducting qubit, manipulating the qubit geometrically using microwave radiation, and observe the accumulated phase in an interference experiment. "

2007-11-27T06:13:00.000-05:00

Back by popular demand – "Donald Duck" technical thread Biercuk (MTO) As per request, attached are the original posts from the "Donald Duck" technical thread, which were controversially removed from the ∂-wave weblog shortly after they were posted in advance of the widely-reported press demo early this year. Geordie has stated that the comments were not taken down – though his comment (#35) apologizing for their initial removal appears in the original thread and in closing below. For those who were following the discussion in the days leading up to the demo, the abrupt disappearance of a technical thread left a memorable impression as to the status of further critical discussion on the weblog. Still notably absent from the debate is any substantive discussion of standard industry benchmarks: quantitative characterization of fidelity, persistence of entanglement in the presence of decoherence, amenability to 1/f noise, Rabi oscillations, Ramsey fringes, Larmor frequency, T₁, T₂ – as well any third-party referee or peer-reviewed technical publication outlining these hardware requirements.

Donald Duck January 22, 2007 – Look, I am not aware of any theory that says that NP complete problems are amenable to any significant speedup on a quantum computer. (Factoring intergers, i.e. Shor’s algorithm, I remind you is somewhat special—it is not NP complete). In this case, you will not be able to compete with conventional computers. Another thing to keep in mind. The press conference method of announcing scientific results doesn’t have a very good track record. In 1989, chemists Stanley Pons and Martin Fleischmann held a press conference to report they had successfully achieved cold fusion with a simple device. In 2002, a group called Clonaid held a press conference to announce they had successfully achieved human cloning. In both cases, the stories were widely reported in the press but were later debunked. How about some good old-fashioned peer review? And so what if you can find the ground state of a 16 spin Ising model. I’m willing to bet that in this particular physical device that quantum coherence has very little if not nothing at all to do with it.

Geordie January 22, 2007 – Donald: (1) One of the most fundamental results of QC theory is that QCs can quadratically speed up unstructured search. I suggest you visit Eddy Farhi’s website at MIT and download and read some papers on AQC, or visit arxiv.org and search for adiabatic quantum computing. Most of the papers on AQCs are about solving NP-complete problems. (2) We’re not announcing scientific results. We’re announcing a technical capability. When we do announce scientific results they will be via the peer review process. (3) I would take that bet in a second, but unless you really are Donald Duck I would have difficulty collecting.

Donald Duck January 22, 2007– (1) That is precisely my point. Quadratic speedup is not good enough to be competitive with current computing technology. (2) + (3) Well, it’s not completely clear, but it sounds like you are claiming the technical capability to perform adiabatic quantum computation. If this is true you need to prove experimentally that what you have is AQC and not some sophisticated form of thermal annealing. This is what I would really like to see.

Geordie January 22, 2007 – Donald: I suppose if a quadratic speed up isn’t good enough, then a constant pre-factor speed-up must be even less useful…damn thanks for pointing that out…now I can go back to using my trusty ole abacus. You should probably email Intel and AMD and let them know. Damn “computers” and their useless pre-factor speed-ups. I understand that presentation of scientific results in Science or Nature is appealing to the expert community, and we do have plans to do this. But our primary objective isn’t publishing science papers, it’s building quantum computers.

Donald Duck January 23, 2007 – Geordie: True, quadratic speedup for general purpose computing would be nice—if the cost is not too outrageous. But that’s not what we are talking about here. AQC may give quadratic speedup for a few select algorithms, e.g. Grover’s search algorithm. There are also problems known to be exponentially hard using AQC. I think its very much still an open question as to how useful AQC is w.r.t. computing in general. Yet I also think that studying this will perhaps tell us something very fundamental about the nature of computing and possibly physical reality. However, I’m not convinced that there is now, or ever will be, a market for AQC. Back to your device. I read somewhere else that your technology works at -269C, i.e. 4K, so I take that to mean a liquid Helium temperatures. Now from what I hear, individual s.c. flux qubits, including yours, have a energy gap E0 of about 10GHz or 0.5K. My guess is that a modest collection of coupled flux qubits as in your ‘processor’ has a minimum energy gap ~2 orders of magnitude smaller than E0. So the temperature is something like 3 orders of magnitude greater that the minimum energy gap. How is AQC possible here? How can you even initialize the system?

Geordie January 23, 2007 – Donald: There are only two reasons why QCs will ever be built: quantum simulation and solving NP-complete problems. Both of these represent enormous markets. We’ve checked. Re. your questions about temperature: these are excellent questions. As a generalization of your question, think about ANY AQC operating on a “hard” (ie exponentially small gap) problem. Is there any physical system whose temperature is smaller than the gap at an anti-crossing of a hard problem? Of course not. All AQCs have the feature you’re describing, not just our approach. At an anticrossing, the temperature is ALWAYS going to be orders of magnitude larger than the gap. That’s why inclusion of a thermal environment is REQUIRED in order to analyze how to operate an “AQC” (although note that at the anticrossings it’s not really adiabatic anymore). In order to see what happens when T>>\Delta take a look at the TAQC (Thermally assisted adiabatic quantum computation) paper in the sidebar. Qualitatively, the effect of the large temperature is to thermalize the two energy levels involved in the anticrossing, reducing the probability of success by 1/2, which is of course completely acceptable.

Uncle Scrooge January 23, 2007 – The unfortunate reality is that this is really just classical SFQ being used for what is effectively analog computation (i.e. system simulation). The fact that only Z coupling is achieveable attests to this. Further, given that nowhere in any of your discussions does DWave ever mention quantum coherence, T2, phase evolution, or superpositions, one is forced to believe, as I said, that this is effectively a classical machine. Frankly, you really shouldn’t call your SQUIDs qubits, as they are no more qubits than are the SQUIDs in SFQ pulse generators. They are two level systems (clockwise and counterclockwise propagating persistent currents), but the quantum nature of said system is never exploited! Indeed, given that all experimental results to date have shown coherence times of order ~10-100ns for Nb trilayer devices, I’d be shocked to learn that Dwave had somehow overcome this technological hurdle ahead of the entire research community. If I’m incorrect, please publish something demonstrating quantum coherence using your “qubits” and prove me wrong. I’d be thrilled with such a response.

Geordie January 23, 2007 – Scrooge: ::sigh:: OK I understand that for some reason you’re desperate to find some reason why what we’re doing can’t possibly be correct, which is fine. I’m familiar with this approach. It goes something like this: I can’t figure out how to do it, therefore you can’t figure out how to do it. Do you want me to point out the basic flaw in this reasoning or can you figure it out all by yourself. As to your specific comments:
There is NO SFQ in this design. Zero. The qubits are compound junction RF squids. The tunneling matrix elements for each qubit can be controlled by varying the flux applied through the CJJs for each qubit. This approach is well-known and is centrally featured in the superconducting AQC papers I’ve linked to here. As I mentioned earlier the Hamiltonian is of the X+Z+ZZ type. Notice the X? As to your comment that I haven’t talked about T2 etc. As you yourself pointed out scientific results belong in peer-reviewed scientific articles, not in a blog whose objective is to reach a broad audience with a message that isn’t completely incomprehensible because it’s buried under jargon. As I said before, our objective is to build quantum computers, not to publish science papers. If the latter supports the former, we’ll publish. If it doesn’t then it’s just a distraction for us.

Uncle Scrooge January 24, 2007 – Geordie, I did not claim that you are using SFQ, I claim that the behavior of your system is akin to classical SFQ. My apologies if the word choice was confusing. My criticism of your approach has nothing to do with me figuring anything out, or an apparent claim that I have been unsuccessful in doing so. I don’t work in superconducting qubits. However, I know the field, and the MANY MANY players as well as the challeges they face. You are claiming to have surpassed them all by more than an order of magnitude in the number of qubits you can control and manipulate. Such a claim warrants a publication, or a detailed press release, or something to suggest that you have actually just ushered in the computing revolution which you are claiming. You may not be in the business of publishing science papers, but you are in applied science. The validity of technical claims in ANY applied science discipline is upheld by scientific scrutiny, generally facilitated by publishing scientific results. Would you prefer a webinar? Fine, but demonstrate the behavior you are claiming transparently for all to see. Further, you shouldn’t fall back on the fact that this is a blog. I have read DWave’s papers on the arxiv and find the same lack of anything quantum coherent in your published results (e.g. cond-mat/0509557, cond-mat/0501085). Dwave and collaborators certainly know how to make quasi-classical superconducting electronics and SQUIDs, but where are the superposition states? the Rabi or Larmor oscillations? anything suggesting that you are operating and controlling a coherent quantum system? I understand the premise of AQC, but again ask this: Can Dwave demonstrate that their simulator/processor can take an input superposition state and output the appropriate answers in superposition? If so, please provide the data and I will be most impressed and GLADLY give you the credit you are due. In stark contrast to your claim, I am not desperate to find some reason why what you’re doing is incorrect. Nothing could be further from the truth, but I do expect reasonable experimental evidence to support your very significant claims.

Geordie January 24, 2007 – Scrooge: Fair enough! While we obviously can’t release everything we’ve learned from the hardware, what we’re planning to submit for publication should clarify (at least) the issue of the role of QM in the operation of the system.

Uncle Scrooge January 24, 2007 – I’m looking forward to those publications, but have a follow-up question. Your statement that said publications will “clarify the role of QM [quantum mechanics] in the operation of the system,” gives me pause. We understand the role of quantum mechanics in quantum computing; does the DWave system exploit QM in the same way? Or are the effects what one might term semi-classical? For example, QM plays a significant role in the operation of the laser, the FET, and classical SFQ logic, but none of these are coherent quantum devices. By this statement I mean they do not preserve and exploit quantum mechanical phase information. Accordingly, they cannot provide the parallelism which leads to exponential speedup in a quantum computer. How would one describe DWave’s system?

Geordie January 24, 2007 – Scrooge: I am not so sure you’re correct when you say that the role of QM in QC is understood. There are of course lots of things that are known, but there is still alot of unexplored territory. The example you brought up about temperature & the role it plays in AQC is a great example. From the theory perspective, adding environments qualitatively changes the behavior of the system. I don’t believe that even this simple point is widely understood. There are lots of things like this where computation and physics are related in non-trivial ways, and where cross-overs between classical and quantum behavior may affect computational scaling in a way that isn’t just either/or. Also just to be clear I don’t believe that the system we’re building is going to exponentially speed up anything. The objective is the quadratic speed up for unstructured search. Chris (and also Scrooge): The way we operate our AQCs is like this (X_i and Z_i are the pauli X and Z matrices for qubit i):

(1) Turn up the tunneling term in the Hamiltonian to its maximum value (H=\sum_i \Delta_i X_i)
(2) Slowly turn the qubit biases and coupler strengths up to their target values (these define the particular problem instance); after this process the Hamiltonian is H=\sum_i (\Delta_i X_i + h_i Z_i) +\sum_{ij} J_ij Z_i Z_j
(3) Slowly turn the tunneling terms off; after this the Hamiltonian is H=\sum_i h_i Z_i +\sum_{ij} J_ij Z_i Z_j
(4) Read out the (binary digital) values of the qubits

OK so the point of this is that the qubits are only read out when they are in classical bit states by design. The readout devices are sensitive magnetometers called DC-squids which sense the direction of the magnetic field threading the qubit and hence it’s bit state. The computational model is explicitly set up so that superposition states are used only during the “annealing” stage; the readouts never fire during this step. Answers are encoded in bit strings. Each bit string corresponds to a particular solution. If the computation succeeds, the bit string returned ({s_i}) will minimize the energy E=\sum_i h_i s_i +\sum_{ij} J_ij s_i s_j. Hope this helps! Also re. the demo. There will be almost zero technical stuff in the demo. The foxus is on describing how one would use the system as an application developer–what it does and how you interact with it. All of the science-type stuff, including details of operation, won’t be part of the demo.

Geordie January 24, 2007 – Hi everybody: As a favor to our non-technical audience, if you have any technical questions about the system, please email me directly at rose@dwavesys.com and I’ll try to help.

Also Donald and Scrooge: Sorry about cutting your posts, please email me directly & we can continue the discussion. I love the feedback, keep it coming!

2007-11-21T07:50:00.000-05:00

Disruptive Technologies SC07 "The disruptive technologies panel serves as a forum for examining those technologies that may significantly reshape the world of high-performance computing (HPC) in the next five to fifteen years, but which are not common in today's systems. Generally speaking, a disruptive technology is a technological innovation or product that eventually overturns the existing dominant technology or product in the marketplace. Disruptive Technologies showcases these technologies in two panel sessions and in a competitively-selected exhibit showcase." This year's showcase featured quantum computing, optical interconnects, CMOS photonics, carbon nanotube memory, and software for massively-parallel multicore processors. The two panel sessions explored potential for disruptions in each major component of HPC architecture: processors, memory, interconnects, and storage.

Progress in Quantum Computing SC07 Panel discussion and HPCWire summary by DiVincenzo. "Hardware to perform quantum information processing is being developed on many fronts. Representing points of view from academia, government, and industry, this panel will give an indication of how work is progressing on quantum computing devices and systems, and what the theoretical possibilities and limitations are in this quantum arena." Panel members included David DiVincenzo (IBM), Wim Van Dam (UCSB), Mark Heiligman (ODNI), Geordie Rose (∂-wave), and Will Oliver (Lincoln Lab).

Rabi, Ramsey, fidelity, 1/f noise, T₁, T₂ MIT EECS Biercuk (MTO) brings back the "Donald Duck" technical thread calling for further clarification on fidelity, 1/f noise, T₁, T₂ Rabi and Ramsey at the new Vatican. Farhi, Chuang, Shor, and Viola follow-up with the same fundamental questions at Amin and Berkley's MIT talk, covered in further detail by Scott Aaronson at Shtetl-Optimized.

2007-10-28T09:11:00.000-05:00

arXivblog, the physics arXiv weblog.

2007-09-26T07:50:00.000-04:00

Qulink Seminar on Fault-Tolerant Quantum Computation NII|QIS This week's Qulink seminar by Keisuke Fujii (Kyoto) outlines a novel entanglement purification protocol for fault-tolerant quantum computation in the presence of errors. " The protocol works with high noise thresholds for the communication channels and local operations, and achieves high fidelity of purified states. [...] We consider an interesting relationship between the entanglement purification and fault-tolerant computation, which provides a tight upper bound on the noise threshold for fault-tolerant computation. "

Everett @ 50 Oxford Videos, photos and weblog are now online from the Everett@50 conference held in Oxford, 19-21 July. " This year sees the 50th anniversary of the publication of Hugh Everett III’s seminal “Relative State Formulation of Quantum Mechanics.” This is an opportune moment for leading advocates and critics to come together and debate the Everett interpretation. Sponsored by FQXi and hosted in the Philosophy Faculty of Oxford University, forty of the world’s top academics will come together for three days on July 19th, 20th, and 21st to see if Everett’s explanation of quantum mechanics has at last come of age. "

2007-06-18T14:17:00.000-04:00

Superconducting flux qubits: CNOT gate. Horizontal axis represents control qubit rotation; ordinate of colour figures sets number of CNOT gates executed (a) (b) Control qubit input state preparation, (c) (d) target qubit state after CNOT pulse. Plantenberg et al. Nature, 15 June 2007.

CNOT gate demonstrated in superconducting flux qubits Kavli|Delft In Nature 447, 836-839 and concurrent reviews in HPCwire, Scientific American, TU Delft [1] [2] (in Dutch), Plantenberg et al. demonstrate selective execution of controlled-NOT quantum logic gates in a pair of coupled flux qubits. "Complemented with longer coherence times and optimized detector visibility, the presented gates enable experiments on two-qubit quantum algorithms and solid-state qubit entanglement using the four Bell states. This scheme, combined with controllable coupling, forms an attractive and generic approach to implementation of solid-state quantum computing." Detailed analysis of controlled rotations, experimental data and conditional spectroscopy are available at doi:10.1028/nature05896.

Teleportation of massive particles without shared entanglement Queensland|Canberra arXiv quant-ph 0706.0062. " We propose a method for quantum state transfer from one atom laser beam to another via an intermediate optical field, using Raman incoupling and outcoupling techniques. Our proposal utilises existing experimental technologies to teleport macroscopic matter waves over potentially large distances without shared entanglement. "

2007-05-12T09:20:00.001-04:00

Martinis Rescues Schrödinger's Cat UCSB In follow-up to Phys Rev Lett 97, 166805 (2006) , " Undoing a Weak Quantum Measurement of a Solid-State Qubit," New Scientist is reporting on upcoming experimental plans to save Schrödinger's Cat from environmental decoherence.

" We propose an experiment which demonstrates the undoing of a weak continuous measurement of a solid-state qubit, so that any unknown initial state is fully restored. Measurement undoing, or "quantum undemolition," may be interpreted as a kind of quantum eraser, in which the information obtained from the first measurement is erased by the second measurement. The experiment can be realized using charge or superconducting phase qubits. "

Reversible weak measurement holds security implications for the integrity of present-day quantum cryptography protocols. " This could be a very profound discovery. Since the birth of quantum theory we have become used to thinking of quantum measurements as creating reality: until things are measured, they don't have an absolute, independent existence. But if some forms of measurement, such as weak measurement, are reversible, then the fundamentals of quantum mechanics go even deeper than we realised. If you create reality with weak quantum measurements, does undoing them erase the reality you created? "

Asian Conference on Quantum Information Science Kyoto, 03-06 Sep 2007 The AQIS07 Meeting will focus on quantum information science and technology. This is a new interdisciplinary field that bridges quantum physics, computer science, mathematics, and computing technologies. AQIS07, following tradition, will consist of invited talks and selected oral communications and posters. Contributions for short communications and posters will be solicited in research areas that relate to quantum information science and technology, both theory and experiments. This includes, but is not limited to: quantum automata, algorithms and complexity, quantum cryptography, quantum information theory, quantum entanglement, non-locality, quantum error correction, decoherence-free subspaces, quantum optics, NMR and solid-state technologies, quantum processor design, quantum programming languages and semantics. "

Evidence for wavelike energy transfer through quantum coherence in photosynthetic systems Berkeley Lab In Nature 446, 782-786, Fleming et al. report on coherent electron transfer in photosynthetic complexes. " We have obtained the first direct evidence that remarkably long-lived wavelike electronic quantum coherence plays an important part in energy transfer processes during photosynthesis. This wavelike characteristic can explain the extreme efficiency of the energy transfer, because it enables the system to simultaneously sample all the potential energy pathways and choose the most efficient one. " Covered also in Scientific American, Wired, PhysicsWeb, rose.blog.

Tunneling and green tea J Am Chem Soc 129 (18) pp 5846 - 5854 " Tunneling is a ubiquitous phenomenon in nature [...] We had a problem understanding how polyphenols work at such low concentrations. This paper gives theoretical credence to a large amount of experimental evidence of polyphenols as in vitro and in vivo antioxidants. "

Solid-State Qubits with Tunable Coupling NEC|JST|RIKEN In Science 314, 5804, NEC, JST and RIKEN report on tunable coupling between two flux qubits through mutual inductance with a dc SQUID acting as a nonlinear transformer. " ... the research group devised an original mechanism that employs another qubit in between the two qubits for coupling. The coupling qubit is able to turn on and off the magnetic coupling between the two qubits. Control is achieved simply by inputting a microwave. Moreover, coupling operation has been achieved without shortening the lifetime of each qubit." Critical analysis and discussion at Technology Review, rose.blog, nextquant [1] and [2], Scott Aaronson, and Travis Hime on related experiments at Berkeley.

Refuting Nonlocal Realism In Nature 446 (871 - 875) and concurrent arXiv preprint, Zeilinger et al. refute nonlocal quantum realism in experiments with entangled photon pairs. " We measure previously untested correlations between two entangled photons, and show that these correlations violate an inequality proposed by Leggett for nonlocal realistic theories. Our result suggests that giving up the concept of locality is not sufficient to be consistent with quantum experiments – unless certain intuitive features of realism are abandoned. " Via Nature, Scientific American, Quantum Quandaries.

Experimental Realization of Deutsch's Algorithm in a One-Way Quantum Computer Belfast|Vienna In Phys Rev Lett 98, 140501 and concurrent preprint coverage in quant-ph/0611186, PhysOrg, Tame, Zeilinger et al. report on the first experimental demonstration of an all-optical, one-way implementation of Deutsch's quantum algorithm on a four-qubit cluster state. " Experimental results are in excellent agreement with the theoretical model, therefore demonstrating the successful performance of the algorithm." See inset [1].

[1] Experimental implementation of Deutsch's algorithm in a cluster-state quantum computer Tame, Zeilinger et al. Phys Rev Letters 98, 140501 (2007) . (a) Experimental setup. An ultraviolet pump-laser performs two passages through a nonlinear crystal aligned to produce entangled photon pairs. (b) Sketch of the cluster-state configuration. (c) and (d) Real and Imaginary plots of the reconstructed experimental density matrix.

New links in quantum information processing [1] nextquant Recent topics include d-Wave, Top 5 Urban Legends in Quantum Computing, Superconducting Quantum Computing: d-Wave replies on NEC, The Awful Truth about Schrödinger’s Cat. [2] Strange Paths on physics, computation, philosophy – electron flow paths, escaping from the end of the universe, the quantum eraser experiment.

2007-03-14T10:29:00.000-04:00

xQIT MIT|RLE The Keck Foundation Center for Extreme Quantum Information Theory has been established at RLE under directive to investigate the fundamental limitations to quantum computing, communications, sensing and control. Seth Lloyd and Jeff Shapiro will lead the initiative.

2007-02-27T09:42:00.000-05:00

Entangled Quantum Networks ICFO|ICREA|Max-Planck Institute In Nature Physics advance publication 10.1038/nphys549, Acin et al. draw upon the classical percolation methods of statistical mechanics to optimize entanglement distribution through quantum networks. " We argue that there exists an entanglement phase transition in quantum networks which may be exploited to obtain very efficient protocols. This work opens a new set of problems in quantum information theory, which are related to statistical physics, but pose completely new challenges in these fields [...] The work leads to a novel type of critical phenomenon, an entanglement phase transition that we call entanglement percolation. "

Maximizing entanglement in quantum networks. Each node is connected by a state consisting of two copies of the same two-qubit state. The nodes marked in (a) make the optimal measurement for the one-repeater configuration on pairs of qubits belonging to different connections. (b) A triangular lattice is obtained where the maximally entangled state for each connection is the same as for the two-qubit state. Acin et al., Nature Physics, 25 February 2007.

2007-02-21T08:48:00.000-05:00

Entanglement engineering for quantum metrology Innsbruck Entanglement-assisted metrology has previously been demonstrated to enhance measurement sensitivity and improve fidelity in noisy conditions. In a quant-ph update to Nature 443 (316), Roos et al. obtain precision atomic clock measurements in the presence of magnetic field noise by engineering a decoherence-free subspace to enhance coherence times. " We find that entangled states are not only useful for enhancing the signal-to-noise ratio in frequency measurements – a suitably designed pair of atoms also allows clock measurements in the presence of strong technical noise. The applied technique makes explicit use of nonlocality as an entanglement property, and constitutes a new paradigm for designed quantum metrology."

Signatures for generalized macroscopic superpositions Queensland In quant-ph 0701204 and Phys. Rev. Lett. 97, Cavalcanti and Reid develop signature detection criteria for macroscopic quantum coherence in situations which are not limited to only two macrosopically distinct measurement outcomes. " The criteria provide a means to distinguish a single macroscopic quantum state from one based on a mixture of several microscopic superpositions of pointer-measurement eigenstates." Calculations are provided for the case of Gaussian-squeezed and spin-entangled states.

2007-02-09T09:37:00.000-05:00

∂-wave throws down the gauntlet Vancouver Pending third-party referee, peer review or independent verification, D-Wave's press release has been received with expected enthusiasm in the mainstream press and restrained skepticism in the scientific community. "I'll be a bit of a skeptic until I see what they have done. I'm happy these guys are doing it. But the proof of the pudding is in the eating." – Seth Lloyd

2007-01-24T09:48:00.000-05:00

Nondemolition measurement. a) Schematic representation of the qubit excitation pulse (top) and readout pulse (bottom) sequence; b) Probability to detect the oscillator in state h for qubit states; c) Measurement of Rabi oscillations; d) Parameters characterizing measurement errors. Lupascu et al. Nature Physics, 14 January 2007.

Quantum non-demolition measurement of a superconducting two-level system Delft|NTT By minimizing disturbance to the system under investigation, quantum nondemolition measurement (QND) can provide particularly clear signatures of quantum coherence. In Nature Physics and cond-mat 0611505, Lupascu et al. demonstrate nondemolition measurement of superconducting qubits coupled to a nonlinear resonator. "The high correlation between measurement results demonstrates the quantum nondemolition nature of the readout method. The fact that quantum nondemolition measurement is possible for superconducting qubits strengthens the notion that these fabricated mesoscopic systems are to be regarded as fundamental quantum objects. Our results are also relevant for quantum information processing protocols such as state preparation and error correction. " cf. also Kavli Institute announcement in TU Delta (in Dutch).

2007-01-22T07:25:00.000-05:00

High-speed linear optics quantum computing using active feed-forward measurement Vienna In Nature 445, 65-69 and concurrent press summary, Zeilinger's group reports experimental demonstration of feedforward error correction via one-way, highly-entangled cluster states in linear optics. "With present technology, the individual computational step can be operated in less than 150 ns using electro-optical modulators. This is an important result for the future development of one-way quantum computers, whose large-scale implementation will depend on advances in the production and detection of the required highly entangled cluster states."

Retrocausal Signaling UW Via the Pontiff – John Cramer recently conducted a UW seminar outlining recent progress on his plans for an experimental test of retrocausal signaling using entangled photon pairs, notwithstanding Eberhard's theorem. The planned experiment draws upon the thesis of Birgit Dopfer (Zeilinger group, Universität Wien), Wheeler-Feynman theory and Cramer's own transactional interpretation. Concurrent press summary and diagram of the setup are available via SF Gate. cf. also Cramer's online summary of the proposed experiment, Jensen gedanken, double-slit and delayed choice quantum eraser experiments (refs: serafino).

2007-01-21T07:30:00.000-05:00

Quantum Transport in Carbon Nanotubes » links – Condensed matter physics weblog metadatta appended to links. Recent post topics include the Wolf Prize announcement in spintronics and single-quantum dot nanowire light-emitting diodes.

2007-01-19T10:11:00.000-05:00

Proton Tunneling in Molecular Biophysics Rensselaer RPI researchers have employed the SCOREC supercomputing cluster to conduct advanced modeling of protein folding dynamics which incorporates quantum mechanical effects to study the influence of proton tunneling in enzyme catalysis. The group's initial study of intein's role in C-termini protein folding will be used to develop nanoscale switches for applications ranging from drug delivery to novel sensors.

2007-01-18T12:02:00.000-05:00

Measurement-based Quantum Computing with Superconducting Charge Qubits RIKEN Wang, You and Nori report on measurement-based preparation of superconducting cluster states. "The measurement of the current of a few parallel Josephson-junction qubits realizes a novel type of quantum-state selector. Using this selector, one can produce various quantum entangled states and also realize a controlled-NOT gate without requiring an exact control of the interqubit interactions. In particular, cluster states for quantum computation could be produced with only single-qubit measurements."

Measuring the Size of a Schrödinger Cat State München "We propose a measure for the "size" of a Schrödinger cat state, i.e. a quantum superposition of two many-body states with macroscopically distinct properties, by counting how many single-particle operations are needed to map one state onto the other. This definition gives sensible results for simple, analytically tractable cases and is consistent with a previous definition restricted to Greenberger-Horne-Zeilinger-like states. We apply our measure to the experimentally relevant, nontrivial example of a superconducting three-junction flux qubit put into a superposition of left- and right-circulating supercurrent states and find this Schroedinger cat to be surprisingly small."

2006-10-24T12:55:00.000-04:00

Symposium for Hans Mooij Kavli|Delft "We see no fundamental reason why superconducting quantum computers cannot be developed into large-scale systems – though, perhaps before then, other applications will also emerge that are much more interesting ..." On Friday, 20^th October 2006 the Kavli Institute of Nanoscience and Delft University of Technology orchestrated a symposium, "The Best of Nanoscience," for Hans Mooij in honor of his attainment of emeritus professor. A program of special guests including Michael Tinkham, David Auston, David DiVincenzo, Seth Lloyd, Yasunobu Nakamura, Hideaki Takeyanagi, John Clarke, Carlo Beenakker, Paul McEuen, Göran Wendin and many other distinguished visitors convened to celebrate Hans's past accomplishments and to highlight future research milestones, followed by a formal reception in honor of the occasion. cf. links to Quantum Transport party committee, TU Delft announcement, featured interview in ∫ Delft Integraal.

2006-09-08T13:30:00.000-04:00

Entanglement Demonstrated in Superconducting Qubits UCSB|IBM In Science 313, 5792 and concurrent PhysicsWeb, Scientific American reviews, Martinis et al. report experimental verification of entanglement between two superconducting qubits. "By using simultaneous measurement and state tomography, we demonstrated entanglement between two solid-state qubits. Single qubit operations and capacitive coupling between two super-conducting phase qubits were used to generate a Bell-type state. Full two-qubit tomography yielded a density matrix showing an entangled state with fidelity up to 87%. Our results demonstrate a high degree of unitary control of the system, indicating that larger implementations are within reach."

2006-08-28T12:24:00.000-04:00

Symposium on Quantum Technologies Cambridge|MIT "Substantial advances in nanoscale science and engineering have made it possible to engineer a wide range of physical systems whose behaviour is governed by the laws of quantum mechanics. Quantum technologies seek to exploit these quantum effects to develop novel practical applications – from secure communications systems to novel computing devices more powerful than existing computers, new measurement devices more accurate than their classical counterparts, or to facilitate chemical reactions using photonic reagents, which might lead to the discovery new materials – to mention only a few potential applications. This Symposium aims to bring together a range of theoretical and experimental scientists and engineers from academia and industry to discuss the current state of the art of various emerging quantum technologies, and the promises and challenges that lie ahead."

2006-08-14T13:58:00.000-04:00

Macroscopic Entanglement in Quantum Computation Tokyo | 東京大学 "We investigate macroscopic entanglement of quantum states in quantum computers, where we say a quantum state is entangled macroscopically if the state has superposition of macroscopically distinct states. When the solutions are such that the problem becomes hard in the sense that classical algorithms take more than polynomial steps to find a solution, macroscopically entangled states are always used in Grover's algorithm and almost always used in Shor's algorithm. Since they are representative algorithms for unstructured and structured problems, respectively, our results support strongly the conjecture that quantum computers utilize macroscopically entangled states when they solve hard problems much faster than any classical algorithms."

2006-08-10T09:52:00.000-04:00

Time Reversal and Super-resolving Phase Measurements Queensland "We demonstrate phase super-resolution in absence of entangled states. The key insight is to use the inherent time-reversal symmetry of quantum mechanics: our theory shows that it is possible to measure, as opposed to prepare, entangled states. Our approach is robust, requiring only photons that exhibit classical interference: we experimentally demonstrate high-visibility phase super-resolution with three, four, and six photons using a standard laser and photon counters. Our six-photon experiment demonstrates the best phase super-resolution yet reported with high visibility and resolution."

Selective Qubit Coupling via Stripline Cavity Chalmers "We theoretically investigate selective coupling of superconducting charge qubits mediated by a superconducting stripline cavity with a tunable resonance frequency. The frequency control is provided by a flux biased dc-SQUID attached to the cavity. Selective entanglement of the qubit states is achieved by sweeping the cavity frequency through the qubit-cavity resonances. The circuit is scalable, and allows to keep the qubits at their optimal points with respect to decoherence during the whole operation. We derive an effective quantum Hamiltonian for the basic, two-qubit-cavity system, and analyze appropriate circuit parameters. We present a protocol for performing Bell inequality measurements, and discuss a composite pulse sequence generating a universal control-phase gate."

2006-08-04T13:22:00.000-04:00

Entanglement Extraction Leeds Cunha and Vedral review how to obtain spin entangled pairs of fermions from a Fermi gas, outlining the relevant experimental parameters. The experiment can be as "a possible new source of entangled particles; and as a foundational interesting step – testing novel properties of fundamental constituents of matter."

Single-atom Macroscopic Entanglement Resource Texas A&M "We discuss the generation of a macroscopic entangled state in a single atom cavity-QED system. The three-level atom in a cascade configuration interacts dispersively with two classical coherent fields inside a doubly resonant cavity. We show that a macroscopic entangled state between these two cavity modes can be generated under large detuning conditions. The entanglement persists even under the presence of cavity losses."

Nonstatistical Weak Measurements GMU|USC Tollaksen and Aharonov report on nondestructive weak measurement protocols and their application under empirical conditions.

2006-07-27T11:42:00.000-04:00

Local Extraction of EPR Entanglement from Classical Systems Leeds|NUS Kaszlikowski and Vedral outline a novel method of entanglement extraction using independent probes that locally interact with two subsets of a macroscopic system. "Coherent states with large amplitudes are traditionally thought of as the best quantum mechanical approximation of classical behavior. Here we argue that, far from being classical, coherent states are in fact highly entangled. We demonstrate this by showing that a general system of indistinguishable bosons in a coherent state can be used to entangle, by local interactions, two spatially separated and distinguishable non-interacting quantum systems. Entanglement can also be extracted in the same way from number states or any other nontrivial superpositions of them [...] It may well be that nature already uses a phonon-to-electron entanglement transfer scheme similar to this to achieve some sort of coherent macroscopic behavior."

2006-06-26T12:23:00.000-04:00

Backward Evolving Quantum States Tel-Aviv In quant-ph 0606208, Vaidman outlines the theoretical limitations on possible manipulations of a backward-evolving quantum state. "The basic concept of the two-state vector formalism, which is the time symmetric approach to quantum mechanics, is the backward evolving quantum state. However, due to the asymmetry of the memory's arrow of time, the possible ways to manipulate a backward evolving quantum state differ from those for a standard, forward evolving quantum state. The similarities and the differences between forward and backward evolving quantum states regarding the no-cloning theorem, nonlocal measurements, and teleportation are discussed. The results are relevant not only in the framework of the two-state vector formalism, but also in the framework of retrodictive quantum theory."

Retrocausation: Experiment and Theory AAAS Causality – the notion that earlier events can affect later events but not vice-versa – undergirds our experience of reality and physical law. Causality is predicated on the forward unidirectionality of time. However, most physical laws are time symmetric; that is, they formally and equally admit both time-forward and time-reverse solutions. Time-reverse solutions are distressing because they would allow the future to influence the past, i.e., reverse causation. Why time-forward solutions are preferentially observed in nature remains an unresolved problem in physics. While the most convincing explanations invoke the second law of thermodynamics, wavefunction collapse or the expansion of the universe, in the end, purely forward causation is an ad-hoc physical assumption. This symposium will explore recent experiments, theory, and philosophical issues connected with reverse causation. In particular, it is hoped that this meeting will help: 1) generate better theoretical models by which established experimental results can be understood; 2) devise new experiments by which the underlying physics may be more clearly exposed; and 3) establish fruitful research collaborations.

Quantum Cosmology From Future to Past CERN, Cambridge In Phys Rev D and concurrent Physics Web overview, Hawking and Hertog apply Feynman's path integral formalism to quantum cosmology. "In this framework, amplitudes for alternative histories for the universe are calculated with final boundary conditions only. This leads to a top-down approach to cosmology, in which the histories of the universe depend on the precise question asked. We study the observational consequences of no boundary initial conditions on the landscape, and outline a scheme to test the theory."

2006-06-22T12:58:00.000-04:00

Generation and control of Greenberger-Horne-Zeilinger entanglement in superconducting circuits RIKEN In quant-ph 0510169, Wei, Liu, and Nori propose an efficient approach to generate and control quantum entanglement between three macroscopic coupled superconducting qubits. "By conditionally rotating, one by one, selected Josephson charge qubits, we show that their Greenberger-Horne-Zeilinger (GHZ) entangled states can be deterministically generated. The possibility of using the prepared GHZ correlations to test the macroscopic conflict between the noncommutativity of quantum mechanics and the commutativity of classical physics is also discussed."

Coherent State Evolution in a Superconducting Qubit from Partial-Collapse Measurement UCSB Martinis et al, Science 312, 5779. "Measurement is one of the fundamental building blocks of quantum-information processing systems. Partial measurement, where full wavefunction collapse is not the only outcome, provides a detailed test of the measurement process. We introduce quantum-state tomography in a superconducting qubit that exhibits high-fidelity single-shot measurement. For the two probabilistic outcomes of partial measurement, we find either a full collapse or a coherent yet nonunitary evolution of the state. This latter behavior explicitly confirms modern quantum-measurement theory and may prove important for error-correction algorithms in quantum computation."

Flux qubit decoherence sources RIKEN, VTT, NEC In cond-mat 0606481, Yoshihara, Harrabi, Niskanen, Nakamura and Tsai investigate flux qubit decoherence sources, demonstrating an optimal bias condition at which noise sources are well decoupled and coherence time is primarily limited by energy relaxation of the qubit.

2006-05-27T12:04:00.000-04:00

Spheres and connections represent qubits and entanglement bonds in (a) a two-dimensional cluster state. Coloured qubits show a compact CNOT gate, proceeding from the input qubits (yellow), through progression of Y-basis measurements (pink), to output qubits (light blue). (b) Activating additional collision cavities can create higher-dimensional topologies of entanglement such as this helical structure. Blythe and Varcoe, quant-ph 0605190.

Measurement-Based Quantum Computing Sussex In quant-ph 0605190, Blythe and Varcoe present a primer for feedforward quantum computation via crossed atomic beams to create a highly-entangled initial cluster state. Drawing upon previous work by Raussendorf and Briegel, the authors provide specifications for microwave cavity QED based scalable quantum computing architectures. "In a departure from the traditional understanding of a computer as a fixed array of computational elements, we show that cluster state quantum computing is well suited to atomic beam experiments. We show that all of the necessary elements have been individually realised, and that the construction of a truly scalable atomic beam quantum computer may be an experimental reality in the near future."

2006-05-24T11:15:00.000-04:00

Quantum Coherence, Noise and Decoherence in Nanostructures Max-Planck Institute "The modern field of nanoelectronics has brought about novel physical phenomena and created new challenges for their interpretation within quantum theory. Among the most fundamental concepts are quantum coherence and interference effects. An improved understanding of these phenomena is needed both from a fundamental point of view as well as for a variety of potential applications – ranging from highly sensitive detectors to quantum information devices and single-electron logic circuits operating at room temperature. The goal of this meeting is to bring together leading scientists from different subfields of condensed matter physics in order to advance the understanding of decoherence in nanostructures."

Constructive Role of Noise in Complex Systems Max-Planck Institute "Noise is inevitably present in any dissipative systems, and all living organisms operate in the noisy environment. Understanding the role of noise is crucial both in fundamental research in nonlinear physics, and in many applications in engineering, biology and medicine. Recent developments in statistical physics and nonlinear dynamics have shed light on a new, sometimes counterintuitive role which noise plays in nonlinear systems: in a wide range of systems, random forces may bring a system to a more ordered state. This meeting will focus on recent developments in the field of noise and fluctuations in complex nonlinear systems, as well as on applications of new noise-mediated phenomena and theoretical methodologies in experimental physics, biological physics, neuroscience and medicine."

Macroscopic Quantum Coherence and Computing MQC² Workshop "The aim of the workshop is to report on the recent theoretical and experimental results on the macroscopic quantum coherence of mesoscopic systems, as well as on solid state realization of qubits and quantum gates. Particular attention will be given to coherence effects in Josephson devices. Other physical systems, including quantum dots, optical, atomic, and molecular devices, exhibiting macroscopic quantum coherence, will also be discussed."

Quantum Communications in Telecom Networks IEEE "Quantum Cryptography, which carries a promise of fundamentally secure communications, has reached a point of relative maturity and first commercial offerings. Its broad deployment, however, is impeded by many technical challenges. This conference will bring together researchers from universities, industry and government labs, commercial QC system manufacturers, service providers, and funding agencies to discuss the novel physics of single-photon sources, interactions between photonic and material qubits, distant entanglement, single photon detection, fundamental physical constraints on the performance of QC links and networks, and resulting trade-offs among key rate, distance and cryptographic security."

2006-05-19T12:18:00.000-04:00

Entanglement Distribution Revealed by Macroscopic Observations Vienna "Observation of quantum entanglement between increasingly larger macroscopic objects is one of the most promising avenues of experimental quantum physics. Eventually, all these developments will lead to a complete understanding of the simultaneous coexistence of a macroscopic classical world and an underlying quantum realm." In quant-ph 0603208, Kofler and Brukner compute multipartite entanglement measures to reveal quantum correlations in the collective properties of two separated objects – "The present work demonstrates that macroscopic properties can reveal entanglement between two or more macroscopic samples. On the fundamental side, our method demonstrates that there is no principal reason why purely quantum correlations could not have an effect on the global properties of objects."

Macroscopic Einstein-Podolsky-Rosen Pairs in Superconducting Circuits RIKEN In quant-ph 0508027, Wei et al. introduce an efficient method of creating EPR pairs in capacitively-coupled Josephson nanocircuits: "A possible application of the deterministically generated EPR pairs is to test Bell's Inequality at the macroscopic level. The approach proposed can be easily modified to engineer quantum entanglement in other fixed-interaction solid-state systems."

Quantum Computing with Superconducting Qubits NATO ASI Geller, Wilhelm et al. provide a concise overview of research efforts currently underway to develop scalable superconducting quantum circuits in Superconducting Qubits I: Architectures and Superconducting Qubits II: Decoherence – "Josephson junctions have demonstrated enormous potential as qubits for scalable quantum computing architectures. Here we discuss the current approaches for making multi-qubit circuits and for performing quantum information processing with them."

2006-04-27T16:01:00.000-04:00

Defense and Security Applications of Quantum Information SPIE Applied technologies that compute, store, and distribute information based upon quantum mechanical entanglement, superposition, and interference phenomena are currently being pursued and realized in multiple parallel architectures, with high-impact assessment in the fields of cryptography, communications, computation and metrology. The SPIE Defense and Security Symposium, the largest unclassified international meeting of its kind, was held from 17-21 April, 2006.

2006-02-26T12:54:00.000-04:00

Entanglement as a function of effective coupling between light and mirror (k) and effective duration of coupling (x-axis). Maximal entanglement is shown in red.

High-Temperature Macroscopic Entanglement PRL Via Raitio – Aires Ferreira, Ariel Guerreiro, and Vlatko Vedral have published novel results on high-temperature macroscopic entanglement in Phys. Rev. Lett. 96, 060407 [arXiv, physicsweb]. "Can entanglement and the quantum behavior in physical systems survive at arbitrary high temperatures? In this Letter we show that this is the case for a electromagnetic field mode in an optical cavity with a movable mirror in a thermal state [...] Entanglement between a macroscopic mirror and a cavity mode field can arise due to radiation pressure at arbitrarily high temperatures as the system evolves in time. This is very surprising because it is commonly believed that high temperature completely destroys entanglement."

2006-02-24T14:10:00.000-04:00

Tunable flux qubit. (A) Double SQUID with two control coils. (B) Potential of the double SQUID in the symmetric case, relative energy levels. (C) Potential in the asymmetric case. Chiarello, cond-mat 0602464.

Tunable flux qubit manipulated by fast pulses MQC Group Chiarello evaluates the physical parameters for operation of a tunable flux qubit, calculating dissipation and decoherence factors, and discussing the potential for employment of integrated rapid single flux quantum (RSFQ) logic for qubit control.

High fidelity state tomography of capacitively shunted phase qubits UCSB Steffen et al. introduce a novel design concept for superconducting qubits – separating the capacitive element from the Josephson junction for improved qubit performance. Environmental coupling to the qubit is reduced by an order of magnitude; measurement fidelity improves to 90%. "This improved design enables the first demonstration of quantum state tomography with superconducting qubits using single shot measurements."

High-contrast dispersive readout of a superconducting flux qubit Delft Lupascu et al. demonstrate high-contrast state detection of a superconducting flux qubit by probing the microwave transmission of a nonlinear resonator based on a SQUID. "Measured contrast of Rabi oscillations is as high as 87%; of the missing 13%, only 3% is unaccounted for. Experiments involving two consecutive detection pulses are consistent with preparation of the qubit state by the first measurement."

Feedback control for communication with non-orthogonal states LSU Kurt Jacobs examines continuous implementation of optimal measurement for distinguishing between two non-orthogonal states. "Feedback control can be used during measurement to increase the rate at which the information regarding the initial preparation is obtained. Enhancement in the rate of information gain is achieved at the expense of reducing the total information which the measurement can extract in the long-time limit."

2006-01-26T13:22:00.000-04:00

Frontiers in Quantum Nanoscience Queensland/PiTP "Within a few years the lives of most people will be touched by the quantum revolution – a change as profound as cars, flight, antibiotics or the Internet. Most people have heard of nanotechnology as the building of new materials at the molecular or atomic scale. That's the stone-axe age compared to what's coming." Nanoscience and nanotechnology receive much attention in the media today. However almost all current work concentrates on very small scale classical devices. This conference looks ahead to the far more revolutionary developments expected once nanoscience 'goes quantum', and begins to use the full potential of quantum mechanical superposition, phase coherence, and entanglement. Conference resources include public surveys on classical and quantum nanoscience.

2006-01-24T13:33:00.000-04:00

Frequency dependence of multiphoton interference fringes in a superconducting qubit. Qubit switching probability plotted as a function of frequency and flux detuning in the limit of (A) strong driving and (B) weak driving signals. Symmetric patterns in peaks and valleys due to quantum interference are clearly observable. Oliver et al. Science 310.

Superconducting circuits and quantum information RIKEN You and Nori discuss recent advances in quantum information processing with superconducting circuits in the charge, flux and phase regimes. "The device can test Bell inequalities, produce Schrödinger cat states, and simulate the Einstein-Podolsky-Rosen experiment. Quantum engineering of macroscopic entangled states will surely play a central role in several future technologies."

Mach-Zehnder interferometry in a strongly driven superconducting qubit Lincoln Lab In Science 310 and cond-mat 0512691, Oliver et al. demonstrate Mach-Zehnder interferometry in a flux qubit. "The development of artificial atoms with lithographically defined superconducting circuits presents a new paradigm of quantum solid state physics, allowing the realization and exploration of new macroscopic quantum phenomena, and holding promise for applications in quantum computing [...] The generalization of optical Mach-Zehnder interferometry, performed in qubit phase space, provides an alternative means to manipulate and characterize the qubit in the strongly driven regime."

Dephasing of a superconducting qubit induced by photon noise Delft In PRL 95, 257002, Bertet et al. evaluate photon noise-induced dephasing in a superconducting flux qubit coupled to a harmonic oscillator. "Retaining quantum coherence is a central requirement in quantum information processing. Solid-state qubits, including superconducting ones, couple to environmental degrees of freedom that potentially lead to dephasing [...] By careful tuning of flux and current bias, long coherence times can be achieved with flux qubits."

Heisenberg limited measurements with superconducting circuits JPL Guillaume and Dowling describe an assembly of superconducting qubits in a single-mode cavity. Performing collective manipulations of the assembly to generate maximally entangled states, "this method can thus enable Heisenberg limited sensor technology with electric charge or magnetic field superconducting devices."

2005-12-18T13:16:00.000-04:00

Quantum coherent oscillations in a charge qubit Y. Nakamura, Yu.A. Pashkin, and J.S. Tsai. Nature, 398:786, 1999.

Tunable coupling scheme for flux qubits CREST-JST Niskanen, Nakamura and Tsai introduce a design for tunably coupling two flux qubits via a third high-frequency qubit, allowing the qubits to remain optimally-biased and shielded from harmful low-frequency flux noise. "The presented scheme is an experimentally realistic way of carrying out two-qubit gates, and should be easily extended to multiqubit systems."

Quantum phase slip junctions Kavli Institute Delft Quantum phase slip is the exact dual to Cooper pair tunneling in the Josephson junction. In cond-mat 0511535, Mooij and Nazarov propose coherent quantum phase slip junctions. If experimentally verified, these junctions could yield applications as resonators or in fundamental current standards.

Scalable controlled gate operations KU In a recent submission to Physical Review A, Han and Yang present a novel approach to realize scalable, controlled-U gate operations with superconducting qubits coupled to a microwave cavity or in atomic qubits within cavity QED. "The method operates essentially by creating a single photon through one of the control SQUIDs, and then performing an arbitrary unitary transformation on the target SQUID with the assistance of the cavity photon."

Decoherence and quantum measurement of Josephson qubits Stony Brook Doctoral dissertation, Kristian Rabenstein

2005-11-22T12:08:00.000-04:00

Towards Fullerene-Based Quantum Computing Oxford In quant-ph 0511198, Benjamin et al. report on recent investigation of C60 arrays as a potential architecture for coherent quantum information processing. "Molecular structures appear to be natural candidates for a quantum technology: individual atoms can support quantum superpositions for long periods, and such atoms can in principle be embedded in a permanent molecular scaffolding to form an array [...] Here we report our efforts, both experimental and theoretical, to create such a technology based on endohedral fullerenes or ‘buckyballs’. We describe our successes with respect to these criteria, along with the obstacles we are currently facing and the questions that remain to be addressed."

Fullerene Molecules Left: A model of N@C60, illustrating that the nitrogen atom sits at the centre of the fullerene cage. Its electron wavefunction lies almost entirely inside, extending on the cage with only a 2% overlap. Right: The ‘peapod’ nanotube contains fullerenes packed in a pseudo-helical phase.

2005-11-01T13:42:00.000-04:00

Efficient evaluation of decoherence rates in complex Josephson circuits IBM Watson Theoretical analysis of the variables contributing to decoherence in Josephson flux qubits has led to order-of-magnitude extensions of coherence time in these circuits over recent years, assisting in both the design phase and control parameter optimization for increasingly-complex qubit circuitry. In cond-mat 0510843, DiVincenzo, Brito and Koch perform a complete quantitative analysis of the decoherence properties of a Josephson flux qubit, exploring relaxation and dephasing times from two different control circuits along an optimal line in the space of applied fluxes.

2005-10-14T12:58:00.000-04:00

Quantum Time Machines: What, Why and How? Queensland/Tokyo Tim Ralph presents a Qulink seminar on closed timelike curves in context of quantum information processing. "Whether time travel into the past is possible is an undecided physical question. Recently it has been noted that certain models of time travel for quantum particles do not lead to the same difficult paradoxes that arise for classical particles. Furthermore the types of quantum evolutions predicted for these 'quantum time machines' could give rise to a 'super' quantum computer, able to solve problems thought to be intractable by any other means. In this talk I will discuss time machines in general, how quantum mechanics avoids the paradoxes and the unusual evolutions predicted. I will then argue that the requirements for realizing such machines are not as stringent as previously thought and I will propose "horizon technology" experiments which could test these ideas."

Theoretical and Experimental Exploration of Time Reversal Formalism Applied to Entanglement IQC, Waterloo In quant-ph/0510048, Laforest, Laflamme and Baugh investigate time reversal of the Schrodinger equation in the context of teleportation. Experimental results are consistent with the interpretation that information can be seen as flowing backward in time through entanglement. "In this paper, we analyze whether the acausal flow of information in a teleportation protocol can actually be physical, or should only consist of a mathematical model. Using an NMR spectrometer, we have demonstrated experimental results faithful with the interpretation that, conditionally and in principle, entanglement seems like it can break the causality of time."

RSFQ Circuits with Selective Dissipation for Coherent Quantum Information Processing VTT, Finland RSFQ, or rapid single flux quantum logic serves as a central component of HTMT, hybrid technology multi-threaded computing and other prototype high-performance architectures. In cond-mat/0510189, Hassel et al. investigate frequency-dependent damping as a means to reduce dissipation and subsequent decoherence in Josephson junction RSFQ/qubit circuits. "We derive criteria for the stability of such an arrangement, and discuss the effect on decoherence and the optimisation issues. We also design a simple flux generator aimed at manipulating flux qubits."

2005-10-11T18:26:00.000-04:00

Life, the Universe and The Complexity Zoo IQC Waterloo In Shtetl-Optimized, Scott Aaronson waxes poetic on complexity theory: "Why is it so hard to explain that we don't worry about [complexity classes] because we're eccentric anal-retentives, but because we want to know whether a never-ending cavalcade of machines, each richer and more complicated than the last, might possibly succeed at a task on which any one machine must inevitably flounder – namely, the task of outracing time itself, of simulating cosmic history in an eyeblink, of seeing in the unformed clumps of an embryonic universe the swirl of every galaxy and flight of every hummingbird billions of years hence, like Almighty God Himself?"

2005-10-07T11:53:00.000-04:00

Workshop on Quantum and Classical Information Security ARDA/NSA/NSF/Caltech 15-18 December 2005 – "The workshop will bring together researchers from a variety of backgrounds who work on different aspects of classical and quantum information security. Participants will strive to identify issues and problems of common interest that can be effectively addressed by pooling their expertise."

Flux Qubits as Trapped Ions RIKEN In quant-ph 0509236, Liu, Wei, Tsai and Nori propose a scalable superconducting circuit in which the qubits act as 'trapped ions.' The qubits are coupled to a 'vibrating' mode provided by a superconducting inductor-capacitor circuit, and interqubit couplings are selectively controlled by modulating the frequencies of the applied time-dependent magnetic flux.

Parametric Coupling for Flux Qubits Delft Pashkin and McDermott have independently demonstrated entanglement between superconducting qubits using a fixed linear coupling scheme. In cond-mat 0509799, Bertet, Harmans and Mooij propose a scalable architecture for two superconducting charge or flux qubits biased at symmetry points with unequal energy splittings. "The fixed-coupling strategy would be difficult to scale to a large number of qubits, and it is desirable to investigate more sophisticated schemes. Modulating the coupling constant between two qubits at the sum or difference of their two frequencies allows to bring them into resonance in the rotating frame. Switching on and off the modulation amounts to switching on and off the coupling which can be realized at nanosecond speed. We discuss various physical implementations of this idea, and find that our scheme can lead to rapid operation of a two-qubit gate."

2005-09-20T12:30:00.000-04:00

Post Quantum Cryptography PQCrypto2006 Via the Pontiff – The European Network of Excellence for Cryptology (ECRYPT) and its Asymmetric Techniques Virtual Lab (AZTEC) examine the future of cryptography in the quantum computer era: "Will large quantum computers be built? If so, what will they do to the cryptographic landscape? Anyone who can build a large quantum computer can break today's most popular public-key cryptosystems: e.g., RSA, DSA, and ECDSA. But there are several other cryptosystems that are conjectured to resist quantum computers: e.g., the Diffie-Lamport-Merkle signature system, the NTRU encryption system, the McEliece encryption system, and the HFE signature system. Exactly which of these systems are secure? How efficient are they, in theory and in practice? PQCrypto 2006, the International Workshop on Post-Quantum Cryptography, will look ahead to a possible future of quantum computers, and will begin preparing the cryptographic world for that future."

2005-08-26T12:54:00.000-04:00

Quantum Interferometric Sensors JPL|LSU Quantum entanglement garners a number of advantages to metrology and remote sensing applications. Dowling, Kapale et al. have issued a recent summary of progress in quantum interferometric sensors, which harness quantum entanglement for improved measurement sensitivity approaching the Heisenberg limit.

Phase-Slip Flux Qubits TU Delft In a recent paper submitted to the New Journal of Physics special issue on solid-state quantum information processing, Mooij and Harmans introduce phase-slip flux qubits, which harness quantum tunnelling to realize a superconducting qubit without the use of Josephson junctions. Phase-slip qubits potentially hold two distinct advantages over traditional flux qubits: lower sensitivity to charge noise-based sources of decoherence, and well-defined separation of energy levels of more than 500 GHz, allowing for extremely rapid excitation of the qubit.

Energy Level Separation Energy levels as a function of applied flux for different fluxoid numbers. A phase-slip event changes the fluxoid number n. The arrow indicates the operating point at f= ½.

Macroscopic EPR Pairs in Superconducting Circuits RIKEN In quant-ph/0508027, Nori et al. propose an efficient approach for deterministic generation of entangled EPR pairs in coupled Josephson nanocicruits. Realization of the experiment would provide an effective means of testing Bell inequality violations, demonstrating nonlocality of quantum entanglement in macroscopic systems.

2005-08-08T12:30:00.000-04:00

Delft Scientists Split Electron Pairs in Superconductors TU Delft "Scientists at Stichting FOM and Kavli Institute of Nanoscience Delft have demonstrated that electrons that normally travel through superconductors in pairs can be seperated while retaining their quantum mechanical kinship. The formation of electron pairs - so-called Cooper pairs - is such a fundamental property of current flow in superconductors that the Delft experiment is considered a breakthrough. It lays the foundation for the realization of a superconducting entangler capable of injecting pairs of entangled electrons into nanoelectronic circuits, an important building block of the quantum computer scientists have been dreaming of for years."

2005-08-04T11:57:00.000-04:00

Robust Entanglement Innsbruck "It is common belief among physicists that entangled states of quantum systems lose their coherence rather quickly. The reason is that any interaction with the environment which distinguishes between the entangled sub-systems collapses the quantum state. Here we investigate entangled states of two trapped Ca+ ions and observe robust entanglement lasting for more than 20 seconds."

2005-07-20T11:22:00.000-04:00

Restoring Quantum Coherence Pavia, Italy Decoherence remains the foremost limiting factor on practical implementation of quantum information technologies. In quant-ph 0504195, Buscemi et al. show that for qubit and qutrit systems it is always possible to recover quantum coherence by performing controlled measurements upon the environment, and that the minimal information required to invert qubit decoherence is equivalent to the von Neumann entropy exchange of the system.

Measuring Decoherence in a three-level rf SQUID Qubit U Kansas In cond-mat 0507008, Han et al. perform direct and quantitative measurements of dissipation-induced relaxation in a three-level rf SQUID qubit. "Analysis of the system indicates that the dominant sources of qubit dissipation are the flux bias and magnetometer readout circuits. Since this kind of dissipation-induced qubit decoherence can be greatly suppressed with more sophisticated designs we believe it does not impose a fundamental limit to this type of qubit [...] We are developing advanced designs for qubit bias and readout circuits that are predicted to decrease their contributions to the qubit damping by several orders of magnitude."

2005-07-12T10:27:00.000-04:00

Josephson Bifurcation Amplifier for Quantum Measurements Yale In cond-mat 0507248, Devoret et al. construct a new type of amplifier for superconducting qubit readout based on the transition of an rf driven Josephson junction between two distinct oscillation states near a dynamic bifurcation point: "The main advantages of this new amplifier are speed, high-sensitivity, low back-action, and the absence of on-chip dissipation. Using pulsed microwave techniques, we demonstrate bifurcation amplification in nanofabricated Al junctions and verify that the performance predicted by theory is attained."

Flux-Qubit Readout with Frequency Dependent Damping Berkeley, München "Recent experiments on superconducting flux qubits, consisting of a superconducting loop interrupted by Josephson junctions, have demonstrated quantum coherence between two different quantum states. The state of the qubit is measured with a superconducting quantum interference device. Such measurements require the SQUID to have high resolution while exerting minimal backaction on the qubit." In Phys. Rev. B 72, 024513 , Plourde, Wilhelm et al. employ a path-integral approach to analyze the Caldeira-Leggett model, calculating backaction of a shunted symmetric SQUID on a flux qubit. "To test the model, we fabricated a dc SQUID in which each junction is shunted with a thin-film interdigitated capacitor in series with a resistor, and measured the switching distribution as a function of temperature and applied magnetic flux. After accounting for the damping due to the SQUID leads, we found good agreement between the measured escape rates and the predictions of our model. "

2005-06-28T13:14:00.000-04:00

arXiv Notables quant-ph Notable submissions to the arXiv this month include a comprehensive overview of solid-state qubits by Esteve and Vion [0505676], two papers by Nori et al. on macroscopic cat states [0506011] and testing Bell inequalities in Josephson qubits [0408089], Wilhelm and Kack present an efficient readout scheme for flux qubits at the degeneracy point [0505537], Greenberger and Svozil derive a quantum information theoretic analysis of time travel [0506027], Wiesniak, Vedral and Brukner on macroscopic entanglement measures [0503037], Brassard et al. on quantum game theory and pseudo-telepathy [0408052], and a novel quantum storage and information transfer method in superconducting qubits by Wang et al. [0506144].

2005-05-27T12:04:00.000-04:00

Quantum-Classical Interface Sussex, Liverpool In cond-mat/0505390, Mark Everitt et al. explore nonlinear interaction of a quantum mechanical SQUID ring with its environment. Potential applications include large frequency ratio down-conversion between electromagnetic fields, such as in classical THz communications technologies. "With the now very serious interest being taken in the possibilities of creating quantum technologies such as quantum information processing and quantum computing, much attention is being focused on the application of Josephson effect devices, particularly the SQUID ring. The highly non-perturbative nature of the SQUID ring in the quantum regime means that the ring-environment interaction can be very non-linear, and may lead to unexpected results ..."

Entanglement Extraction from a Solid NEST-INFM, Leeds, Vienna quant-ph/0505107 "It has been a common belief that entanglement cannot exist on a macroscopic scale. This is because decoherence effects from many-particle interaction would destroy all quantum correlations. However, it has been predicted that macroscopic entanglement can exist in solids in the thermodynamical limit – even at high temperature – and it is related to critical phenomena. Here we propose an experimental setup to demonstrate entanglement extraction with present-day technology using optical lattices. This demonstrates that entanglement not only exists in solids, but can even be used for quantum information processing or to violate Bell’s inequalities ..."

2005-05-20T12:27:00.000-04:00

An n-qubit controlled phase gate with resonator-coupled SQUIDs U Kansas In quant-ph/0504188, Siyuan Han and Chui-Ping Yang propose a novel method to realize multiqubit controlled phase gates with SQUIDs. The scheme "operates essentially by exchanging a single photon between the controlled SQUIDs and the resonator mode before and after a phase shift performed on the target SQUID."

Transport of atoms in a quantum conveyor belt NIST Gaithersburg cond-mat/0504606 "An atomic-gas Bose-Einstein condensate (BEC) is a coherent source of matter waves – a collection of atoms, all in the same state, with an extremely narrow momentum spread ... We can easily control the velocity and acceleration of the atomic lattice structure as well as its strength, making it a variable 'quantum conveyor belt.' This allows us to explore situations that are difficult or impossible to achieve in solid state systems. The results are often remarkable and counterintuitive."

Signatures of quantum behavior in single-qubit weak measurements Penn State, UC Riverside quant-ph/0505094 "With the recent surge of interest in quantum computation, it has become very important to develop clear experimental tests for 'quantum behavior' in a system. This issue has been addressed in the past in the form of the inequalities due to Bell and those due to Leggett and Garg. These inequalities concern the results of ideal projective measurements, however, which are experimentally difficult to perform in many proposed qubit designs ... Here, we show that weak continuous measurements, which are often practical to implement experimentally, can yield particularly clear signatures of quantum coherence ..."

2005-05-06T12:00:00.000-04:00

Full Protection of Superconducting Qubit Systems from Coupling Errors Munchen, Berkeley quant-ph/0407780 "Solid state qubits realized in superconducting circuits are potentially extremely scalable. However, strong decoherence may be transferred to the qubits by various elements of the circuits that couple individual qubits [...] We propose here an encoding that provides full protection against errors originating from these coupling elements ..."

Asymmetry and Decoherence in a Double-layer Persistent-current Qubit Kavli Institute Delft cond-mat/0405272 "We discuss a superconducting flux qubit design that exploits the symmetries of a circuit to protect the qubit from unwanted coupling to the noisy environment [...] Possibilities for prolonging the relaxation and decoherence times of the studied superconducting qubit are proposed on the basis of the obtained results."

2005-04-24T12:31:00.000-04:00

Detection of Macroscopic Entanglement by Correlation of Local Observables University of Tokyo In quant-ph 0504086, Shimizu and Morimae propose a macroscopic entanglement index for unknown and mixed states. "We propose a correlation of local observables on many sites in macroscopic quantum systems. By measuring the correlation one can detect, if any, 'superposition of macroscopically distinct states,' which we call macroscopic entanglement, in arbitrary quantum states that are (effectively) homogeneous. Using this property, we also propose an index of macroscopic entanglement."

2005-04-15T13:21:00.000-04:00

Spectroscopy on Two Coupled Superconducting Flux Qubits Kavli Institute Delft In PRL 94, 090501 (2005) Mooij et al. report on spectroscopy measurements of two coupled superconducting flux qubits: "The new results support the notion that superconducting flux qubits can be used to study entanglement in macroscopic quantum systems and for the development of nontrivial two-qubit gates [...] We demonstrate that two macroscopic flux qubits can be coupled to form a quantum mechanical four level system."

2005 Agilent Europhysics Prize Agilent Technologies The 2005 Agilent Technologies Europhysics Prize has been awarded to Awschalom, Dietl, and Ohno for their investigation of solid-state magnetic semiconductors and spin coherence. Spintronics is a promising candidate for scalable quantum computation. "We are proud to recognize these scientists for combining advanced materials engineering, insightful theoretical modeling, ingenious experimental techniques, and international collaboration to attain important breakthroughs in spintronics," said Jim Hollenhorst, director of molecular technology at Agilent Laboratories. Last year the award was shared by Mooij, Nakamura, Devoret and Esteve for their demonstration of superconducting circuits as qubits.

2005-04-08T11:20:00.000-04:00

Nonlocal Measurements in Time-Symmetric Quantum Mechanics arXiv Vaidman and Nevo have posted a preprint on nonlocal demolition measurement of backward evolving quantum states which allows for the introduction of novel types of nonlocal variables. The work builds upon upon Aharonov's time-symmetric formalism, which contains the quantum state evolving backward in time from complete measurement performed in the future relative to the time in question. "Demolition measurements of nonlocal backward evolving quantum states require remarkably small resources. This is so because the combined operation of time reversal and teleportation of a local backward evolving quantum state requires only a single quantum channel and no transmission of classical information."

2005-04-01T12:40:00.000-04:00

Quantum Interference Effect Transistors PhysicsWeb Cardamone et al. propose a novel approach to single-molecule transistors, the quantum interference effect transistor, or QuIET. Each transistor consists of two electrodes attached to an organic ring molecule in one of two configurations: the presence or absence of quantum interference in the ring determines the state of the transistor. "One potential advantage of the QuIET approach is that it could work in aqueous environments, such as those inside living organisms, because it is made of organic molecules."

Schematic diagrams of two types of QuIET In each, base voltage modulates the coherent suppression of current between emitter (E) and collector (C) leads. In (a), base voltage controls the distance x between the benzene ring and base lead (B), for example an STM tip. This in turn controls the coupling of the ring to the base lead. In (b), a base complex is introduced between the ring and base lead. The electrostatic effect of the base lead's bias on this molecule alters its coupling to the benzene ring.

Quantum Game Theory arXiv Nash equilibria and game theory profoundly affected the outcome of the 20th Century – preventing escalation of Cold War conflict between the US and USSR, for example. Quantum game theoretic approaches similarly hold the potential to influence strategic developments in the coming century. Quantum communications networks are already operating in research laboratories across the globe. With the recent birth of the DARPA/BBN quantum internet, quantum game theory has left the realm of academia and entered the world of practical applications, showing promise to transform politics, economics, conflict and warfare in the decades to come. In a recent PhD thesis, Iqbal reviews the current state of the field. See also "Quantum Pseudo-Telepathy" by Brassard et al, "Classical Rules in Quantum Games" by van Enk, "Quantum Strategies" by Meyer.