Bell-inequality violation in light transmitted through disordered emitter ensembles

Abstract

We show that light transmitted through a disordered ensemble of weakly coupled two-level emitters can violate a Bell inequality in a continuous-wave Franson-type measurement. The Bell signal is determined by two steady-state output-field correlations, the equal-time intensity correlation $g^{(2)}(0)$ and the phase-sensitive two-photon coherence $R$. We compute these quantities for bidirectional propagation through disordered emitter ensembles using a fourth-order cumulant expansion of the many-body master equation. The resulting Bell inequality violation appears in two distinct regimes, an antibunched regime where equal-time coincidences are suppressed and a bunched regime where photon pairs remain phase coherent. Extrapolating the numerics to a representative weak single-emitter coupling $β=0.01$ predicts violation in the antibunched regime for atom numbers $N\simeq75$-$220$, and in the bunched regime for $N\gtrsim250$.