Measurement-Induced Transmon Ionization

TL;DR

This paper investigates transmon ionization during dispersive readout in circuit QED, providing a comprehensive framework that explains the process, identifies multiphoton resonances, and aligns well with experimental data.

Contribution

It introduces a unified theoretical framework with three models that accurately describe transmon ionization, preserving the cosine potential and matching experimental thresholds.

Findings

Identifies multiphoton resonances causing ionization

Provides a simple yet accurate computational method

Achieves agreement with recent experimental photon thresholds

Abstract

Despite the high measurement fidelity that can now be reached, the dispersive qubit readout of circuit quantum electrodynamics is plagued by a loss of its quantum nondemolition character and a decrease in fidelity with increased measurement strength. In this work, we elucidate the nature of this dynamical process, which we refer to as transmon ionization. We develop a comprehensive framework which provides a physical picture of the origin of transmon ionization. This framework consists of three complementary levels of descriptions: a fully quantized transmon-resonator model, a semiclassical model where the resonator is treated as a classical drive on the transmon, and a fully classical model. Crucially, all three approaches preserve the full cosine potential of the transmon and lead to similar predictions. This framework identifies the multiphoton resonances responsible for transmon ionization. It also allows one to efficiently compute numerical estimates of the photon number threshold for ionization, which are in remarkable agreement with recent experimental results. The tools developed within this work are both conceptually and computationally simple, and we expect them to become an integral part of the theoretical underpinning of all circuit QED experiments.