Inversion of Qubit Energy Levels in Qubit-Oscillator Circuits in the Deep-Strong-Coupling Regime

TL;DR

This paper experimentally investigates superconducting flux qubits in the deep-strong-coupling regime, revealing significant energy shifts and state inversions due to intense qubit-oscillator interactions, aligning with quantum Rabi model predictions.

Contribution

First experimental observation of large Lamb shifts and qubit state inversion in deep-strong-coupling superconducting circuits, confirming theoretical models.

Findings

Observed Lamb shifts exceeding 90% of qubit frequencies

Detected inversion of qubit ground and excited states

Results agree with quantum Rabi model predictions

Abstract

We report on experimentally measured light shifts of superconducting flux qubits deep-strongly coupled to LC oscillators, where the coupling constants are comparable to the qubit and oscillator resonance frequencies. By using two-tone spectroscopy, the energies of the six lowest levels of each circuit are determined. We find huge Lamb shifts that exceed 90% of the bare qubit frequencies and inversions of the qubits' ground and excited states when there are a finite number of photons in the oscillator. Our experimental results agree with theoretical predictions based on the quantum Rabi model.