Microresonator-referenced soliton microcombs with zeptosecond-level timing noise

Abstract

Optical frequency division relies on optical frequency combs to translate ultrastable optical frequency references coherently to the microwave domain. This technology has enabled the synthesis of microwave signals with ultralow timing noise; however, the necessary instrumentation remains too bulky for practical applications. Recently, efforts have focused on leveraging microphotonic technologies to enhance system compactness. Here we develop an optical frequency division system using microresonator-based frequency references and comb generators. The soliton microcomb formed in an integrated Si₃N₄ microresonator is stabilized to two lasers referenced to an ultrahigh-quality-factor MgF₂ microresonator. Photodetection of the soliton pulse train produces 25-GHz microwaves with an absolute phase noise of –141 dBc Hz^–1 (546 zs Hz^−1/2) at a 10-kHz offset frequency, which can be further referenced to an atomic clock for improved long-term stability. The synthesized microwave signals are evaluated as carrier waves in communication and radar applications, demonstrating enhanced fidelity and sensitivity against interference compared with those derived from electronic oscillators. Our work demonstrates unprecedented coherence in microphotonic microwave oscillators, providing key building blocks for next-generation timekeeping, navigation and satellite communication systems.