Nonreciprocal cavity induced by chiral broadening

Creating nonreciprocal optical components is important for applications such as optical communication, quantum networks, and emerging applications in artificial intelligence and machine learning. In particular, ring cavities with nonreciprocal clockwise (CW) and counterclockwise (CCW) modes play essential roles in quantum nonreciprocity, sensing, and in-memory photonic computing cells. Here, we propose a scheme to realize magnetic-free nonreciprocal cavity modes in a stationary cavity by using the natural chiral broadening hiding in the thermal atomic ensemble. The chiral broadening causes different phase delays for the CW and CCW modes, resulting in a difference in their resonant frequencies, which supports the breaking of their degeneracy. It is found that this difference is proportional to atomic density and interaction length. We experimentally demonstrate such a nonreciprocal cavity and get higher than 98% isolation contrast as an isolator. Since the difference required for these two cavity modes to be completely separated depends on the cavity linewidth, the requirement of atomic density and interaction length in our scheme can be dramatically reduced by increasing the finesse of the cavity, which paves the way for miniaturization and integration of such a nonreciprocal component. Moreover, nonreciprocal cavity modes serve as a chiral reservoir, enabling the exploration of physics driven by this chirality.