strongcoupling

Strong coupling is a term used in physics to describe a regime in which the interaction between two or more systems is sufficiently large that perturbative methods fail and the full dynamics must be treated nonperturbatively. It is typically defined in terms of a dimensionless coupling constant or a comparison of interaction strength with other energy scales in the problem. When the coupling is large, small corrections around a free or weakly interacting solution are no longer adequate, and qualitative changes in behavior may occur.

In quantum field theory, strong coupling occurs when interaction strengths grow large due to the running of coupling constants. A prominent example is quantum chromodynamics (QCD) at low energies, where the strong coupling becomes large, leading to confinement of quarks and gluons and the breakdown of perturbation theory. Nonperturbative methods such as lattice gauge theory, effective field theories, or numerical approaches are then required to study the system.

In quantum electrodynamics and related light-matter systems, strong coupling refers to a regime where the rate of energy exchange between light and matter exceeds their respective loss rates. In cavity quantum electrodynamics, strong coupling is achieved when the coupling g exceeds the cavity and emitter decay rates (g > κ, γ), producing observable vacuum Rabi splitting and distinctive Jaynes–Cummings dynamics. Similar concepts apply to condensed-mary systems, such as exciton–polaritons in semiconductor microcavities, where strong coupling yields polariton quasiparticles.

The term also appears in condensed matter, optomechanics, and lattice theories to denote nonperturbative regimes or expansions. Across fields, strong coupling indicates that interactions dominate over other terms, necessitating nonperturbative analysis or numerical methods.