Quantum state detection of a superconducting flux qubit using a DC-SQUID in the inductive mode

TL;DR

This paper introduces a novel method for reading out superconducting flux qubits by measuring the Josephson inductance of a DC-SQUID, enabling single-shot detection with minimal back-action.

Contribution

The paper demonstrates a new inductive readout technique for flux qubits using a DC-SQUID in the inductive mode, differing from traditional switching mode methods.

Findings

Back-action features are qualitatively different from switching mode.

Single-shot readout of flux qubits is feasible with the proposed circuit.

Qubit relaxation and dephasing rates depend on circuit noise spectral density.

Abstract

We present a readout method for superconducting flux qubits. The qubit quantum flux state can be measured by determining the Josephson inductance of an inductively coupled DC superconducting quantum interference device (DC-SQUID). We determine the response function of the DC-SQUID and its back-action on the qubit during measurement. Due to driving, the qubit energy relaxation rate depends on the spectral density of the measurement circuit noise at sum and difference frequencies of the qubit Larmor frequency and SQUID driving frequency. The qubit dephasing rate is proportional to the spectral density of circuit noise at the SQUID driving frequency. These features of the backaction are qualitatively different from the case when the SQUID is used in the usual switching mode. For a particular type of readout circuit with feasible parameters we find that single shot readout of a superconducting flux qubit is possible.