Mirror symmetric on-chip frequency circulation of light

Integrated circulators and isolators are important for developing on-chip optical technologies such as laser cavities, communication systems and quantum information processors. These devices seem to inherently require mirror symmetry breaking to separate backwards from forwards propagation, and thus existing implementations rely on magnetic materials or interactions driven by propagating waves. By contrast to past works, we exhibit a mirror-symmetric non-reciprocal device that comprises three coupled photonic resonators implemented in thin-film lithium niobate. Applying radiofrequency modulation, we drive conversion between the frequency eigenmodes of this system. We measure nearly 40 dB of isolation for approximately 75 mW of radiofrequency power near 1,550 nm. We simultaneously generate non-reciprocal conversion between all of the eigenmodes to demonstrate circulation. Mirror-symmetric circulation simplifies the fabrication and operation of non-reciprocal integrated devices. Finally, we consider applications of such on-chip isolators and circulators, such as full-duplex isolation within a single waveguide.