Full characterization of measurement-induced transitions of a superconducting qubit

TL;DR

This paper thoroughly characterizes how strong measurement drives induce state transitions in superconducting qubits, revealing mechanisms that impact quantum non-demolition measurement fidelity.

Contribution

It provides the first comprehensive experimental analysis of measurement-induced transitions in superconducting qubits, especially in high-power readout regimes.

Findings

Identification of photon absorption and emission mechanisms causing state transitions

Observation of additional transitions due to package modes and material defects

Analysis of high-frequency readout effects on qubit stability

Abstract

Repeated quantum non-demolition measurement is a cornerstone of quantum error correction protocols. In superconducting qubits, the speed of dispersive state readout can be enhanced by increasing the power of the readout tone. However, such an increase has been found to result in additional qubit state transitions that violate the desired quantum non-demolition character of the measurement. Recently, the readout of a transmon superconducting qubit was improved by using a tone with frequency much larger than the qubit frequency. Here, we experimentally identify the mechanisms of readout-induced transitions in this regime. In the dominant mechanism, the energy of an incoming readout photon is partially absorbed by the transmon and partially returned to the transmission line as a photon with lower frequency. Other mechanisms involve the excitation of unwanted package modes, decay via material defects, and, at higher qubit frequencies, the activation of undesired resonances in the transmon spectrum. Our work provides a comprehensive characterization of superconducting qubit state transitions caused by a strong drive.