Quantum theory of loss-induced transparency in coupled waveguides

Several years ago, Guo et al. [Phys. Rev. Lett. 103, 093902 (2009)] demonstrated a counterintuitive phenomenon wherein the transmission of classical light through a coupled pair of waveguides is enhanced as the level of loss in one waveguide surpasses a critical threshold. In this paper, we employ the Heisenberg-Langevin formalism to explore a quantum perspective of this phenomenon. In the case where light is incident in both waveguides, a generally nonzero interference term in the presence of loss appears. Our analysis reveals that loss-induced transparency can manifest at the single-photon level for both separable and entangled photonic states.