Resonance eigenstates of the SQUID-qubit system

TL;DR

This paper analyzes the resonance spectrum of a dc-SQUID coupled to a flux qubit, demonstrating its relevance for understanding qubit readout processes through a combination of analytical and numerical methods.

Contribution

It introduces a well-defined resonance state framework for the SQUID-qubit system, enhancing the understanding of the escape process and qubit measurement.

Findings

Resonance spectrum effectively characterizes the escape process.

Resonance states are suitable for switching-type qubit readout.

Numerical methods are developed for accurate spectrum calculation.

Abstract

We study the complex-valued resonance spectrum of a dc-SQUID coupled to a flux qubit, where the former is treated in the cubic and the latter in the two-level approximation. It is shown that this spectrum is well-defined and contains most of the relevant information on the escape process. Thus, the language of resonance states is precise and well-adapted to switching- (or trigger-) type qubit readout, and a worthwhile complement to the various descriptions of continuous qubit measurement. Initial progress is analytic, but nonperturbative numerical methods have been formulated and should soon yield accurate results for all parameter values.