Extending coherence times in superconducting qubits Schoelkopf Lab | via Leo DiCarlo — In arXiv 1105.4652Schoelkopf 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 (Tcav ∼ 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.

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