Broken selection rule in the quantum Rabi model

TL;DR

This paper reports the spectroscopic observation of a sign-changing transition in the quantum Rabi model, enabled by ultrastrong coupling, revealing a fundamental selection rule violation in light-matter interaction.

Contribution

It demonstrates the first experimental detection of a sign-changing transition in the quantum Rabi model within the ultrastrong coupling regime.

Findings

Sign-changing transitions are observable in ultrastrong coupling.

Sign-preserving selection rules are violated in the ultrastrong coupling regime.

This work confirms theoretical predictions about selection rule breaking.

Abstract

Understanding the interaction between light and matter is very relevant for fundamental studies of quantum electrodynamics and for the development of quantum technologies. The quantum Rabi model captures the physics of a single atom interacting with a single photon at all regimes of coupling strength. We report the spectroscopic observation of a resonant transition that breaks a selection rule in the quantum Rabi model, implemented using an $LC$ resonator and an artificial atom, a superconducting qubit. The eigenstates of the system consist of a superposition of bare qubit-resonator states with a relative sign. When the qubit-resonator coupling strength is negligible compared to their own frequencies, the matrix element between excited eigenstates of different sign is very small in presence of a resonator drive, establishing a sign-preserving selection rule. Here, our qubit-resonator system operates in the ultrastrong coupling regime, where the coupling strength is 10\% of the resonator frequency, allowing sign-changing transitions to be activated and, therefore, detected. This work shows that sign-changing transitions are an unambiguous, distinctive signature of systems operating in the ultrastrong coupling regime of the quantum Rabi model. These results pave the way to further studies of sign-preserving selection rules in multiqubit and multiphoton models.