Superefficient optical frequency division referenced to μHz Schawlow-Townes-linewidth quantum noise-limited lasers.

Optical frequency division (OFD) converts ultrastable optical frequencies to microwaves via an optical frequency comb, generating microwave oscillators with record-low phase noise and time jitter. However, conventional OFD systems face a notable trade-off between division complexity and noise suppression because of severe thermal and technical noise in optical references. Here, we address this challenge by using common-cavity bicolor Brillouin lasers as references, operating at the fundamental quantum noise limit with a 10-microhertz Schawlow-Townes linewidth. Enabled by these ultracoherent lasers, our OFD system uses a markedly simplified comb divider with an unprecedented division factor of 10, producing a 10-gigahertz microwave signal with exceptional phase noise of -65 decibels relative to the carrier per hertz at 1-hertz offset, -155 decibels relative to the carrier per hertz at 10-kilohertz offset, and -172 decibels relative to the carrier per hertz at 10-megahertz offset. Leveraging this purity, we implement broadband synthesis from 5 to 20 gigahertz with millisecond tuning. This 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.