Qubit measurement error from coupling with a detuned neighbor in circuit QED

TL;DR

This paper investigates how a detuned neighboring qubit affects measurement fidelity in circuit QED, revealing conditions under which joint eigenstates are better discriminated and how measurement-induced quantum jumps impact fidelity.

Contribution

It provides a detailed analysis of measurement dynamics in the presence of a detuned neighbor, highlighting the regimes where joint eigenstates improve measurement fidelity and the effects of resonator decay.

Findings

Joint eigenstates are better discriminated than bare states under certain conditions.

Measurement tracks joint eigenstates nearly adiabatically when resonator decay is slow.

Quantum jumps between eigenstates can significantly degrade measurement fidelity.

Abstract

In modern circuit QED architectures, superconducting transmon qubits are measured via the state-dependent phase and amplitude shift of a microwave field leaking from a coupled resonator. Determining this shift requires integrating the field quadratures for a nonzero duration, which can permit unwanted concurrent evolution. Here we investigate such dynamical degradation of the measurement fidelity caused by a detuned neighboring qubit. We find that in realistic parameter regimes, where the qubit ensemble-dephasing rate is slower than the qubit-qubit detuning, the joint qubit-qubit eigenstates are better discriminated by measurement than the bare states. Furthermore, we show that when the resonator leaks much more slowly than the qubit-qubit detuning, the measurement tracks the joint eigenstates nearly adiabatically. However, the measurement process also causes rare quantum jumps between the eigenstates. The rate of these jumps becomes significant if the resonator decay is comparable to or faster than the qubit-qubit detuning, thus significantly degrading the measurement fidelity in a manner reminiscent of energy relaxation processes.