Beyond Strong Coupling in a Massively Multimode Cavity

TL;DR

This paper introduces the multimode strong coupling (MMSC) regime in cavity QED, demonstrating complex multimode interactions and ultranarrow linewidths, opening new avenues for exploring light-matter interactions.

Contribution

First experimental realization of the MMSC regime in circuit QED, revealing complex multimode phenomena beyond traditional strong coupling.

Findings

Widespread structured resonance fluorescence observed

Capture of complex multimode, multiphoton processes

Emergence of ultranarrow linewidths

Abstract

The study of light-matter interaction has seen a resurgence in recent years, stimulated by highly controllable, precise, and modular experiments in cavity quantum electrodynamics (QED). The achievement of strong coupling, where the coupling between a single atom and fundamental cavity mode exceeds the decay rates, was a major milestone that opened the doors to a multitude of new investigations. Here we introduce multimode strong coupling (MMSC), where the coupling is comparable to the free spectral range (FSR) of the cavity, i.e. the rate at which a qubit can absorb a photon from the cavity is comparable to the round trip transit rate of a photon in the cavity. We realize, via the circuit QED architecture, the first experiment accessing the MMSC regime, and report remarkably widespread and structured resonance fluorescence, whose origin extends beyond cavity enhancement of sidebands. Our results capture complex multimode, multiphoton processes, and the emergence of ultranarrow linewidths. Beyond the novel phenomena presented here, MMSC opens a major new direction in the exploration of light-matter interactions.