Reversible Optical Isolators and Quasi-Circulators Using a Magneto-Optical Fabry–Pérot Cavity

Nonreciprocal optical devices are essential for laser protection, modern optical communication and quantum information processing by enforcing one-way light propagation. The conventional Faraday magneto-optical nonreciprocal devices rely on a strong magnetic field, which is provided by a permanent magnet. As a result, the isolation direction of such devices is fixed and severely restricts their applications in quantum networks. In this work, we experimentally demonstrate the simultaneous one-way transmission and unidirectional reflection by using a magneto-optical Fabry-P\'erot cavity and a magnetic field strength of 50~. An optical isolator and a three-port quasi-circulator are realized based on this nonreciprocal cavity system. The isolator achieves an isolation ratio of up to 22~ and an averaged insertion loss down to 0. 97~. The quasi-circulator is realized with a fidelity exceeding 99\% and an overall survival probability of 89. 9\%, corresponding to an insertion loss of 0. 46~. The magnetic field is provided by an electromagnetic coil, thereby allowing for reversing the light circulating path. The reversible quasi-circulator paves the way for building reconfigurable quantum networks.