ultrastrong

Ultrastrong is an adjective used in physics to describe phenomena that occur at interaction strengths that are large compared with the characteristic frequencies of a system. The most common usage is ultrastrong coupling (USC), describing a regime of light–matter interaction in cavity quantum electrodynamics and related platforms where the coupling strength g becomes a substantial fraction of the resonant frequencies ω (the photonic mode) and ω0 (the matter transition). In practice, USC is often defined when g/ω or g/ω0 is on the order of 0.1 or larger, and it is sometimes extended toward regimes where g approaches or exceeds the frequencies, which leads into deep strong coupling.

In this regime the rotating-wave approximation breaks down, and the full quantum Rabi model must be used rather than the Jaynes–Cummings model. Physical consequences include the appearance of virtual photons in the ground state, large-scale hybridization of light and matter (dressed states or polaritons), and observable effects such as the Bloch–Siegert shift. The dynamics become highly non-perturbative and may require non-perturbative methods to predict accurately. Experimental platforms include superconducting circuit quantum electrodynamics, semiconductor microcavities with intersubband transitions, and other engineered light–matter interfaces where strong coupling can be tuned.

Historically, USC was demonstrated in the 2010s in several systems, signaling a shift from perturbative quantum optics toward regimes in which the vacuum and excitation spectra are strongly modified by interaction. The concept informs research in quantum simulation, nonclassical light, and novel polaritonic devices, and it remains an active area of study for understanding fundamental light–matter coupling.