The advent of superconducting quantum circuits as a robust scientific platform and contender for quantum computing applications is the result of decades of research in light-matter interaction, low-temperature physics, and microwave engineering. There is growing interest to use this advancing technology to study domains of light-matter interaction that were previously thought to be beyond experimental reach. Our work is part of an initiative to explore non-equilibrium condensed matter physics using photons instead of atoms. Open questions in this area currently pose significant challenges theoretically due to analytical complexity and system sizes which prohibit complete numerical simulations, thus experiment-based research has the potential to lead to significant advancements in this field. Here we examine phenomena that arise when moving beyond standard single-mode strong coupling towards the realm of many-body physics with light in two distinct directions.

First we study multimode strong coupling, where a single artificial atom or qubit is simultaneously strongly coupled to a large, but discrete number of non-degenerate photonic modes of a cavity with coupling strengths comparable to the free spectral range. This domain, which falls in between small, discrete and continuum Hilbert spaces, is experimentally realized by coupling a qubit to a low fundamental frequency coplanar waveguide cavity. In this system we report on resonance fluorescence and narrow linewidth emission directly resulting from complex qubit mediated mode-mode interactions.

In the second part we explore qubits strongly coupled to photonic crystals, which give rise to exotic physical scenarios, beginning with single and multi-excitation qubit-photon dressed bound states comprising induced, spatially localized photonic modes, centered around the qubits, and the qubits themselves. The localization of these states changes with qubit detuning from the band-edge, offering an avenue of in situ control of bound state interaction. Due to their localization-dependent interaction, these states offer the ability to create one-dimensional chains of bound states with tunable interactions that preserve the qubits' spatial organization, a key criterion for realization of certain quantum many-body models.

The unique domains of light-matter interaction discussed here are a subset of exciting research initiatives growing our general understanding of complex, strongly coupled quantum systems.