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.

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."

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."

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."

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."

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.

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."

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."

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.

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."

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."

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."

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.

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."

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."

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.

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."