Non-reciprocal few-photon devices based on chiral waveguide-emitter   couplings
  We demonstrate the possibility of designing efficient, non reciprocal few-photon devices by exploiting the chiral coupling between two waveguide modes and a single quantum emitter. We show how this system can induce non-reciprocal photon transport at the single-photon level and act as an optical diode. Afterwards, we also show how the same system shows a transistor-like behaviour for a two-photon input. The efficiency in both cases is shown to be large for feasible experimental implementations. Our results illustrate the potential of chiral waveguide-emitter couplings for applications in quantum circuitry. 
C. Gonzalez-Ballestero, Esteban Moreno, F. J. Garcia-Vidal, and A.   Gonzalez-Tudela
quant-ph physics.optics