Chirality, Nonreciprocity and Symmetries for a Giant Atom

TL;DR

This paper investigates the origins of chirality and nonreciprocity in giant atoms coupled to waveguides, clarifying the roles of symmetries and proposing methods to achieve nonreciprocal photon scattering without breaking time-reversal symmetry.

Contribution

It provides a detailed analysis of symmetry roles in giant-atom nonreciprocity and introduces a passive scheme for chiral emission without time-reversal symmetry breaking.

Findings

Chiral emission is linked to mirror-symmetry breaking, not time-reversal symmetry.

Nonreciprocity can be achieved through external dissipation that breaks time-reversal symmetry.

Artificial time-reversal symmetry breaking via nonuniform coupling phases does not produce nonreciprocal scattering.

Abstract

Chiral and nonreciprocal quantum devices are crucial for signal routing and processing in a quantum network. In this work, we study the chirality and nonreciprocity of a giant atom coupled to a one-dimensional waveguide. We clarify that the chiral emission of the giant atom is not directly related to the time-reversal symmetry breaking but to the mirror-symmetry breaking. We propose a passive scheme to realize the chiral emission of a giant atom without breaking time-reversal symmetry by extending the legs of the giant atom. We find the time-reversal symmetry breaking via nonuniform coupling phases is artificial and thus cannot result in nonreciprocal single-photon scattering for the giant atom. The nonreciprocity of the giant atom can be obtained by the external dissipation of the giant atom that truly breaks the time-reversal symmetry. Our work clarifies the roles of symmetries in the chirality and nonreciprocity of giant-atom systems and paves the way for the design of on-chip functional devices with superconducting giant atoms.