  Improvements both in the photonic confinement and in the emitter design have
led to a steady increase in the strength of the light-matter coupling in cavity
quantum electrodynamics experiments. This has allowed to access
interaction-dominated regimes in which the state of the system can only be
described in terms of mixed light-matter excitations. Here we show that, when
the coupling between light and matter becomes strong enough, this picture
breaks down, and light and matter degrees of freedom totally decouple. A
striking consequence of such a counter-intuitive phenomenon is that the Purcell
effect is reversed and the spontaneous emission rate, usually thought to
increase with the light-matter coupling strength, plummets instead for large
enough couplings.
