Multilevel effects in a driven generalized Rabi model

TL;DR

This paper investigates how higher-level excitations and resonance shifts occur in a driven multilevel Rabi model, using numerical methods and experimental validation with superconducting circuits, revealing the limits of the qubit approximation.

Contribution

It introduces a numerical approach to analyze multilevel effects in the driven Rabi model and validates the findings with superconducting circuit experiments.

Findings

Higher-level excitations increase with driving power

Resonance frequency shifts are observed under strong driving

Qubit approximation is only valid in the extreme quantum limit

Abstract

We study numerically the onset of higher-level excitations and resonance frequency shifts in the generalized multilevel Rabi model with dispersive coupling under strong driving. The response to a weak probe is calculated using the Floquet method, which allows us to calculate the probe spectrum and extract the resonance frequency. We test our predictions using a superconducting circuit consisting of transmon coupled capacitively to a coplanar waveguide resonator. This system is monitored by a weak probe field, and at the same time driven at various powers by a stronger microwave tone. We show that the transition from the quantum to the classical regime is accompanied by a rapid increase of the transmon occupation and, consequently that the qubit approximation is valid only in the extreme quantum limit.