Quantum-noise-induced nonreciprocity for single-photon transport in parity-time-symmetric systems

Abstract

We show nonreciprocal light propagation for single-photon inputs due to quantum noise in coupled optical systems with gain and loss. We consider two parity-time (𝒫𝒯)-symmetric linear optical systems consisting of either two directly coupled resonators or two finite-length waveguides evanescently coupled in parallel. One resonator or waveguide is filled with an active gain medium and the other with a passive loss medium. The light propagation is reciprocal in such 𝒫𝒯-symmetric linear systems without quantum noise. We show here that light transmission becomes nonreciprocal when we include quantum noises in our modeling, which is essential for a proper physical description. The quantum nonreciprocity is especially pronounced in the 𝒫𝒯 broken phase. Transmitted light intensity in the waveguide of incidence is asymmetric for two waveguides, even without noise. Quantum noise significantly enhances such asymmetry in the broken phase.