Nanosecond quantum state detection in a current biased dc SQUID

Abstract

This article presents our procedure to measure the quantum state of a dc SQUID within a few nanoseconds, using an adiabatic dc flux pulse. Detection of the ground state is governed by standard macroscopic quantum theory (MQT), with a small correction due to residual noise in the bias current. In the two level limit, where the SQUID constitutes a phase qubit, an observed contrast of 0.54 indicates a significant loss in contrast compared to the MQT prediction. It is attributed to spurious depolarization (loss of excited state occupancy) during the leading edge of the adiabatic flux measurement pulse. We give a simple phenomenological relaxation model which is able to predict the observed contrast of multilevel Rabi oscillations for various microwave amplitudes.

10 pages, 8 figures

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• Multilevel effects in the Rabi oscillations of a Josephson phase qubit
• Intrinsic leakage of the Josephson flux qubit and breakdown of the two-level approximation for strong driving
• Dispersive readout scheme for a Josephson phase qubit
• Transient dynamics of a superconducting nonlinear oscillator
• Broadband noise decoherence in solid-state complex architectures
• Measuring the distribution of current fluctuations through a Josephson junction with very short current pulses