Dynamics of Transmon Ionization

TL;DR

This paper investigates how strong microwave drives can cause transmon qubits to ionize, leading to degraded measurement fidelity, by analyzing the dynamics of the system and identifying parameter-dependent resonances.

Contribution

It introduces a numerical and semiclassical analysis of transmon ionization under strong drives, revealing parameter-dependent resonances causing qubit escape.

Findings

Transmon ionization occurs at specific photon populations.

Ionization can happen at low photon numbers, affecting fidelity.

Resonances depend strongly on system parameters.

Abstract

Qubit measurement and control in circuit QED rely on microwave drives, with higher drive amplitudes ideally leading to faster processes. However, degradation in qubit coherence time and readout fidelity has been observed even under moderate drive amplitudes corresponding to few photons populating the measurement resonator. Here, we numerically explore the dynamics of a driven transmon-resonator system under strong and nearly resonant measurement drives, and find clear signatures of transmon ionization where the qubit escapes out of its cosine potential. Using a semiclassical model, we interpret this ionization as resulting from resonances occurring at specific resonator photon populations. We find that the photon populations at which these spurious transitions occur are strongly parameter dependent and that they can occur at low resonator photon population, something which may explain the experimentally observed degradation in measurement fidelity.