March 10: Dr. Simon Gustavsson on “Non-exponential energy decay and quasi-particle fluctuations in a superconducting flux qubit”

Title: Non-exponential energy decay and quasi-particle fluctuations in a superconducting flux qubit

Location: 26-214, Thursday March. 10, Noon-1pm. Pizza at 11:45!
(Feel free to bring your own cup and plate)
Abstract: The scalable application of quantum information science begins with reproducible and controllable high coherence qubits. In this work, we present an improved superconducting qubit, leveraging design elements from both charge- and flux-based qubits to achieve a device with large frequency tunability, large anharmonicity, and energy relaxation and dephasing times in excess of 40 μs. By mapping out the noise power spectral density seen by the qubit, we uniquely identify thermal shot noise of residual photons in the readout resonator as the dominant source of dephasing. When implementing the CPMG dynamical-decoupling protocol, we are able mitigate to the adverse influence of the photon shot noise, and improve T2 Echo ~ 40 us to reach T2 CPMG ~ 80 μs ~ 2*T1. In a separate experiment, we measure pronounced non-exponential energy relaxation in a superconducting flux qubit, observing a decay function that exhibits a fast initial decay followed by a much slower decay for long times. When applying a sequence of pi pulses to the qubit and measuring the decay after the last pi pulse, we observe strong modifications to the decay function, including a slow-down of the fast initial decay and a threefold increase of the 1/e-time. If we attribute the non-exponential decay to quasiparticle number fluctuations, the improvements in T1 can be interpreted as qubit-mediated shuffling of quasiparticles between the metallic islands of the device, which will eventually pump them away from the Josephson junctions to a larger ground plane where their contribution to qubit energy relaxation become negligible.

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