Nonreciprocal and chiral single-photon scattering for giant atoms

Giant atoms coupled to their environment at multiple locations, give rise to unconventional physics not seen with standard atomic systems. In this work, the authors explore the nonreciprocity of giant atoms and propose a scheme of a three-level giant atom coupled to waveguides providing nonreciprocal and chiral scattering which could be used to build efficient single-photon targeted routers and circulators. Quantum optics with giant atoms has provided a new paradigm to study photon scatterings. In this work, we investigate the nontrivial single-photon scattering properties of giant atoms being an effective platform to realize nonreciprocal and chiral quantum optics. For two-level giant atoms, we identify the condition for nonreciprocal transmission: the external atomic dissipation is further required other than the breaking of time-reversal symmetry by local coupling phases. Especially, in the non-Markovian regime, unconventional revival peaks periodically appear in the reflection spectrum. To explore more interesting scattering behaviors, we extend the two-level giant-atom system to Delta-type and backward difference -type three-level giant atoms coupled to double waveguides with different physical mechanisms to realize nonreciprocal and chiral scatterings. Our proposed giant-atom structures have potential applications of high-efficiency targeted routers that can transport single photons to any desired port deterministically and circulators that can transport single photons between four ports in a cyclic way.