Nanosecond quantum state detection in a current biased dc SQUID

TL;DR

This paper demonstrates a rapid nanosecond-scale measurement technique for the quantum state of a dc SQUID using an adiabatic flux pulse, revealing insights into state detection and decoherence effects.

Contribution

It introduces a fast measurement procedure for dc SQUID quantum states and models the observed contrast loss due to relaxation during measurement.

Findings

Detection contrast of 0.54 for phase qubits.

Residual noise affects quantum state detection.

Relaxation model predicts multilevel Rabi oscillation contrast.

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.