Mechanism of Polariton Decoherence in the Collective Light-Matter Couplings Regime

Molecular polaritons, the hybridization of electronic states in molecules with photonic excitation inside a cavity, play an important role in fundamental quantum science and technology. Understanding the de-coherence mechanism of molecular polaritons is among the most significant fundamental questions. We theoretically demonstrate that hybridizing many molecular excitons in a cavity protects the overall quantum coherence of phonon-induced decoherence originating from the collective light-matter couplings. The polariton co- coherence time can be prolonged up to 100 fs with a realistic Rabi splitting and quality factor at room temperature, compared to the typical electronic coherence time which is around 15 fs. Our numerically exact simulations and analytic theory suggest that the dominant decoherence mechanism is the population transfer from the upper polariton state to the dark state manifold. Increasing the collective coupling strength will increase the energy gap between these two sets of states, and thus prolong the coherence lifetime. We further derived valuable scaling relations of how polariton coherence depends on the number of molecules, Rabi splittings, and light-matter detunings.