Nonreciprocal PT-symmetric phase transition in a non-Hermitian chiral quantum optical system

Phase transitions, non-Hermiticity and nonreciprocity play central roles in fundamental physics. However, the triple interplay of these three fields is of lack in the quantum domain. Here, we show nonreciprocal parity-time-symmetric phase transition in a non-Hermitian chiral quantum electrodynamical system, caused by the directional system dissipation. In remarkable contrast to previously reported nonreciprocal phase transitions, the nonreciprocal parity-time-symmetric phases appear even when the atom-resonator coupling is reciprocal. Nonreciprocal photon blockade is obtained in the nonreciprocal phase region. These results may deepen the fundamental insight of nonreciprocal and non-Hermitian quantum physics, and also open a new door for unconventional quantum manipulation.