Probing excited-state dynamics of transmon ionization

TL;DR

This paper investigates the excited-state dynamics of transmons under strong drives, revealing ionization processes analogous to atomic multiphoton ionization, with implications for quantum readout fidelity.

Contribution

It demonstrates the use of multilevel transmon spectroscopy to quantify ionization thresholds, state transfer, and control of ionization via pulse shaping, extending understanding of driven nonlinear quantum systems.

Findings

Quantified the critical photon number for transmon ionization.

Verified ionization as a Landau-Zener-type transition.

Extended methods to a typical transmon and studied charge dependence.

Abstract

The fidelity and quantum nondemolition character of the dispersive readout in circuit QED are limited by unwanted transitions to highly excited states at specific photon numbers in the readout resonator. This observation can be explained by multiphoton resonances between computational states and highly excited states in strongly driven nonlinear systems, analogous to multiphoton ionization in atoms and molecules. In this work, we utilize the multilevel nature of high-$E_J/E_C$ transmons to probe the excited-state dynamics induced by strong drives during readout. With up to 10 resolvable states, we quantify the critical photon number of ionization, the resulting state after ionization, and the fraction of the population transferred to highly excited states. Moreover, using pulse-shaping to control the photon number in the readout resonator in the high-power regime, we tune the adiabaticity of the transition and verify that transmon ionization is a Landau-Zener-type transition. We further extend these methods to a typical transmon with $E_J/E_C \approx 55$ and probe the offset-charge dependence of ionization dynamics in a timed-resolved manner. Our experimental results agree well with the theoretical prediction from a semiclassical driven transmon model and may guide future exploration of strongly driven nonlinear oscillators.