Measurement-induced qubit state mixing in circuit QED from up-converted dephasing noise

TL;DR

This paper demonstrates how dephasing noise at specific frequencies causes measurement-induced qubit state mixing in circuit QED, revealing limits to nondemolition readout and linking flux noise spectral density across frequency ranges.

Contribution

It uncovers the mechanism of noise up-conversion causing qubit state mixing and introduces a method to extract flux noise spectral density at GHz frequencies from qubit transition rates.

Findings

Dephasing noise at the detuning frequency causes qubit state mixing.

The qubit transition rate can be used to estimate flux noise spectral density at GHz.

Flux noise spectral density at GHz matches extrapolations from low-frequency data.

Abstract

We observe measurement-induced qubit state mixing in a transmon qubit dispersively coupled to a planar readout cavity. Our results indicate that dephasing noise at the qubit-readout detuning frequency is up-converted by readout photons to cause spurious qubit state transitions, thus limiting the nondemolition character of the readout. Furthermore, we use the qubit transition rate as a tool to extract an equivalent flux noise spectral density at f ~ 1 GHz and find agreement with values extrapolated from a $1/f^\alpha$ fit to the measured flux noise spectral density below 1 Hz.