| Abstract: There is a recent realization that the properties of a material can be modified just by placing it in an optical cavity. The quantum vacuum fields surrounding the material inside the cavity can cause nonintuitive modifications of electronic states, producing a vacuum-dressed material with novel properties. Existing theoretical predictions include cavity-enhanced, cavity-induced, and cavity-mediated enhancement of electron–phonon coupling and superconductivity, electron pairing, anomalous Hall effect, ferroelectric phase transitions, quantum spin liquids, and photon condensation. The so-called ultrastrong coupling (USC) regime is a prerequisite for observing these effects, which arises when the interaction energy becomes a significant fraction of the bare photonic mode and matter excitation frequencies [1]. Most intriguingly, when a material is ultrastrongly coupled with cavity-enhanced vacuum electromagnetic fields, its ground state will contain virtual photons. This nonperturbative virtual driving without external fields can lead to phase transitions in thermal equilibrium. This talk will describe our recent studies of USC phenomena in various solid-state cavity quantum electrodynamics systems in search of such vacuum-induced phases of matter [2–11]. We utilize the phenomenon of Dicke cooperativity [5,12], i.e., many-body enhancement of light–matter interaction, to explore quantum-optical strategies for creating, controlling, and utilizing novel phases in condensed matter enabled by the quantum vacuum. |