Room-temperature cavity quantum electrodynamics with strongly coupled Dicke states

The strong coupling regime is essential for efficient transfer of excitations between states in different quantum systems ontimescales shorter than their lifetimes. The coupling of single spins to microwave photons is very weak but can be enhanced byincreasing the local density of states by reducing the magnetic mode volume of the cavity. In practice, it is difficult to achieve bothsmall cavity mode volume and low cavity decay rate, so superconducting metals are often employed at cryogenic temperatures. Foran ensembles ofNspins, the spin–photon coupling can be enhanced byffiffiffiffiNpthrough collective spin excitations known as Dickestates. For sufficiently largeNthe collective spin–photon coupling can exceed both the spin decoherence and cavity decay rates,making the strong-coupling regime accessible. Here we demonstrate strong coupling and cavity quantum electrodynamics in asolid-state system at room-temperature. We generate an inverted spin-ensemble withN~1015by photo-exciting pentacenemolecules into spin-triplet states with spin dephasing timeT23μs. When coupled to a 1.45 GHz TE01δmode supported by a highPurcell factor strontium titanate dielectric cavity (Vm0:25 cm3,Q~ 8,500), we observe Rabi oscillations in the microwave emissionfrom collective Dicke states and a 1.8 MHz normal-mode splitting of the resultant collective spin–photon polariton. We also observea cavity protection effect at the onset of the strong-coupling regime which decreases the polariton decay rate as the collectivecoupling increases.