Super-efficient optical frequency division referenced to μHz Schawlow-Townes-linewidth quantum-noise-limited lasers
Optical frequency division (OFD) implements the conversion of ultra-stable optical frequencies into microwave frequencies through an optical frequency comb flywheel, generating microwave oscillators with record-low phase noise and time jitter. However, conventional OFD systems face significant trade-off between division complexity and noise suppression due to severe thermal noise and technical noise in the optical frequency references. Here, we address this challenge by generating common-cavity bi-color Brillouin lasers as the optical frequency references, which operate at the fundamental quantum noise limit with Schawlow-Townes linewidth on the 10 μHz level. Enabled by these ultra-coherent reference lasers, our OFD system uses a dramatically simplified comb divider with an unprecedented small division factor of 10, and generates 10 GHz microwave signal with exceptional phase noise of -65 dBc/Hz at 1Hz, -155 dBc/Hz at 10 kHz, and -172 dBc/Hz at 10 MHz offset. Moreover, to fully harness the spectral purity of the OFD technology, here we implement broadband frequency synthesis directly referenced to the OFD oscillator, covering 5 to 20 GHz with millisecond tuning time. Our work redefines the trade-off between noise suppression and division complexity in OFD, paving the way for compact, high-performance microwave synthesis for next-generation atomic clocks, quantum sensors, and low-noise radar systems.