# Single-photon nonreciprocal transport in one-dimensional coupled-resonator waveguides

## TL;DR

This paper demonstrates nonreciprocal single-photon transport in coupled-resonator waveguides, enabling the design of quantum circulators crucial for quantum networks by breaking time-reversal symmetry.

## Contribution

It introduces a novel scheme for achieving single-photon nonreciprocity and constructs a three-port quantum circulator using coupled-resonator waveguides with broken time-reversal symmetry.

## Key findings

- Single-photon nonreciprocal transfer demonstrated
- Analytical conditions for optimal nonreciprocity derived
- Design of a three-port quantum circulator proposed

## Abstract

We study the transport of a single photon in two coupled one-dimensional semi-infinite coupled-resonator waveguides (CRWs), in which both end sides are coupled to a dissipative cavity. We demonstrate that a single photon can transfer from one semi-infinite CRW to the other nonreciprocally. Based on such nonreciprocity, we further construct a three-port single-photon circulator by a T-shaped waveguide, in which three semi-infinite CRWs are pairwise mutually coupled to each other. The single-photon nonreciprocal transport is induced by the breaking of the time-reversal symmetry and the optimal conditions for these phenomena are obtained analytically. The CRWs with broken time-reversal symmetry will open up a kind of quantum devices with versatile applications in quantum networks.