On-chip closed-loop optical circulation enabled by directional quantum squeezing

Optical circulators are key components for integrated photonic information processing and quantum networks, yet realizing compact and magnet-free closed-loop circulation remains challenging. We propose an on-chip four-port optical circulator consisting of three evanescently coupled microring resonators, with directional quantum squeezing introduced into the central nonlinear resonator. The squeezing selectively enhances the effective inter-resonator coupling and renormalizes the resonance frequency for one propagation direction, while leaving the counterpropagating channel unaffected. This directional asymmetry breaks optical reciprocity and enables closed-loop circulation along 1 → 2 → 3 → 4 → 1. At the optimal operating point, the circulator achieves a fidelity of approximately 0.99 and an average insertion loss of about 0.22 dB. In addition, pump-induced quantum noise can be suppressed by injecting a phase-matched broadband squeezed-vacuum field, allowing low-noise operation from the classical to the single-photon regime. The proposed scheme provides a compact, magnet-free, and chip-compatible route to closed-loop optical circulation, with potential applications in integrated optical routing and quantum photonic networks.