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

TL;DR

This paper demonstrates that quantum noise induces nonreciprocal single-photon transport in parity-time symmetric optical systems, especially in the broken phase, revealing a fundamental quantum effect on light propagation.

Contribution

It introduces quantum noise into $ ext{PT}$ symmetric systems showing nonreciprocal photon transport, a novel insight into quantum effects in such optical setups.

Findings

Quantum noise causes nonreciprocity in $ ext{PT}$ symmetric systems.

Nonreciprocity is enhanced in the $ ext{PT}$ broken phase.

Asymmetry persists even without noise, but is significantly increased by quantum noise.

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 ($\mathcal{PT}$) 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 $\mathcal{PT}$ 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 $\mathcal{PT}$ 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.