First-principles study of dispersive readout in circuit QED

TL;DR

This study uses first-principles simulations to analyze dispersive readout in circuit QED, revealing how bath spectrum details influence qubit relaxation and fidelity at different drive amplitudes.

Contribution

It introduces a comprehensive first-principles approach to model dispersive readout, surpassing simplified master equations by including microscopic bath spectra.

Findings

Qubit T1 decreases with drive amplitude when a Purcell filter is used.

Lindblad master equations show qualitative discrepancies compared to first-principles results.

Bath spectrum details critically affect qubit relaxation and measurement fidelity.

Abstract

The speed and fidelity of dispersive readout of superconducting qubits should improve by increasing the amplitude of the measurement drive. Experiments show, however, that beyond some drive amplitude there is always a saturation or drop in fidelity, often associated with a decrease in qubit energy relaxation time $T_1$. A simple Lindblad master equation does not capture the latter effect. More involved approaches based on effective master equations rely on strong assumptions about the spectra of the system and the bath and only partially agree with observations. Here, we perform a first-principles simulation of the full unitary dynamics of dispersive readout by considering the circuit QED Hamiltonian coupled to a microscopic model for the measurement transmission line, allowing for its arbitrary spectrum, including filters. Our access to the dynamics of the bath degrees of freedom allows us to investigate the emission spectrum of the system as a function of drive power. We show how the dependence of qubit $T_1$ on readout drive amplitude is sensitive to the details of the bath spectrum. In particular, we find that $T_1$ drops with increasing drive amplitude when a Purcell notch filter is placed at the qubit frequency, and that the Lindblad master equation shows general qualitative defects compared to the first-principles model.