Unidirectional Dark-to-Bright Rescue in Cavity-Coupled Quantum Transport

Strong light-matter coupling in optical microcavities can transport energy ballistically across an emitter array, but the same coupling buries most of the excitation in a manifold of dark states that grows with system size and traps energy outside the transport channel. We show that the off-diagonal (non-Condon) part of the exciton-phonon coupling opens a one-way escape route from this trap, driving population irreversibly from dark states into the radiative channel. This rate is fixed by a photonic-weight conservation law rather than by dark-bright overlap which evacuates the dark manifold at a rate independent of system size. The mechanism contributes to transport with near-complete efficiency with four signatures being single exponential dark state decay, a size scaling efficiency gap, distinct temperature behavior, and a resonance in the escape rate at vibrational bath modes. Beyond polariton transport, it recasts dark states from a parasitic loss channel into an engineered dissipative resource, with implications for light harvesting and dissipation based quantum control.