Improvement of nonreciprocal unconventional photon blockade by two asymmetrical arranged atoms embedded in a cavity.

We improve the nonreciprocal unconventional photon blockade (UCPB) in an asymmetrical single-mode cavity with two asymmetrical arranged two-level atoms (TLAs) where cavity and atom spatial symmetry breakings are involved in. In order to get direction-dependent UCPB in asymmetrical system, we deduce two restrictions of frequency and intensity through the steady solution of the cavity QED system analytically. The former restriction is exactly the same as that of a single-atom case, and the latter restriction combined with both spatial asymmetries. Controllable UCPB in this model shows an improving nonreciprocal UCPB with wider operating regime which is promoted by two asymmetrical arranged atoms. The most innovation of this work is that the contributions of two spatial symmetry breakings are figured out clearly and they play different roles in nonreciprocal UCPB. The cavity spatial symmetry breaking and weak nonlinearity are essential to quantum nonreciprocity, while the atoms spatial symmetry is not and it only can promote such nonreciprocal UCPB. Our findings show a prospective access to manipulate quantum nonreciprocity by a couple of atoms.