Abstract
The strong coupling regime is essential for efficient transfer of excitations between states in different quantum systems on timescales shorter than their lifetimes. The coupling of single spins to microwave photons is very weak but can be enhanced by increasing the local density of states by reducing the magnetic mode volume of the cavity. In practice, it is difficult to achieve both small cavity mode volume and low cavity decay rate, so superconducting metals are often employed at cryogenic temperatures. For an ensembles of N spins, the spin–photon coupling can be enhanced by sqrt N through collective spin excitations known as Dicke states. For sufficiently large N the 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 a solid-state system at room-temperature. We generate an inverted spin-ensemble with N ~ 1015 by photo-exciting pentacene molecules into spin-triplet states with spin dephasing time T_2^*sim 3 μs. When coupled to a 1.45 GHz TE01δ mode supported by a high Purcell factor strontium titanate dielectric cavity (V_{mathrm{m}}sim 0.25 cm3, Q ~ 8,500), we observe Rabi oscillations in the microwave emission from collective Dicke states and a 1.8 MHz normal-mode splitting of the resultant collective spin–photon polariton. We also observe a cavity protection effect at the onset of the strong-coupling regime which decreases the polariton decay rate as the collective coupling increases.
Highlights
Collective light-matter interactions are fundamental to cavity quantum electrodynamics[1] and a key feature is the strong coupling regime, where excitations are coherently transferred between different quantum systems over timescales significantly shorter than their lifetimes
In order to satisfy this condition at room-temperature, we utilise a system comprising spin-triplets in pentacene molecules to generate a polarised population of N spins and a cavity with small magnetic mode volume, Vm $ 10À5λ3, where λ is the free-space wavelength
We used the Tavis–Cummings Hamiltonian and a Liouvillian that accounted for decoherence due to cavity decay, spin dephasing and spin-lattice relaxation
Summary
Collective light-matter interactions are fundamental to cavity quantum electrodynamics (cQED)[1] and a key feature is the strong coupling regime, where excitations are coherently transferred between different quantum systems over timescales significantly shorter than their lifetimes. A striking example is the Dicke state,[2] responsible for the enigmatic phenomenon of super-radiance, where the rate of emission from an ensemble of emitters is proportional to the square of their number. We report strongcoupling and long-lived collective Rabi oscillations at roomtemperature between Dicke states, in an optically excited spintriplet ensemble, and a cavity mode supported by a strontium titanate (STO) dielectric resonator at 1.45 GHz. The spin ensemble becomes highly correlated through stimulated emission with suppressed spin decoherence due to a cavity protection effect.[10,11,12]
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