Multimode Strong-Coupling Processes in Circuit QED Lattices

TL;DR

This paper explores multimode strong coupling in circuit QED lattices, revealing new phenomena like band gaps, degeneracy lifting, and four-wave mixing involving flat-band modes, advancing understanding of photon-qubit interactions.

Contribution

It introduces a circuit Lagrangian analysis capturing beyond tight-binding effects and experimentally observes strong wave-mixing resonances in multimode circuit QED.

Findings

Emergent band gaps and lifted degeneracies in the photonic lattice.

Observation of strong wave-mixing resonances involving multiple modes.

Dominance of flat-band modes in multiphoton processes.

Abstract

Circuit QED systems provide an ideal platform for exploring the strong-coupling regime of multimode cavity QED. Here we present two new phenomena from multimode strong coupling: a circuit Lagrangian analysis which captures beyond tight-binding effects of strong photon-photon coupling and experimental observation of strong wave-mixing resonances in the qubit response. Our circuit analysis reveals qualitatively new features such as emergent band gaps, lifted degeneracies, broadened flat bands, and frequency-dependent hopping. Within the multimode photon environment, strong qubit-photon coupling in turn gives rise to multiphoton processes involving multiple normal modes. We demonstrate a strong four-wave-mixing process involving excitation of a qubit and simultaneous frequency conversion between modes. Notably, this wave-mixing process is dominated by localized flat-band modes of the photonic lattice, which exhibit the strongest coupling to the transmon qubit.