Phase-coherent microwave-to-optical link with a self-referenced microcomb

Precise measurements of the frequencies of light waves have become common with mode-locked laser frequency combs(1). Despite their huge success, optical frequency combs currently remain bulky and expensive laboratory devices. Integrated photonic microresonators are promising candidates for comb generators in out-of-the-lab applications, with the potential for reductions in cost, power consumption and size(2). Such advances will significantly impact fields ranging from spectroscopy and trace gas sensing(3) to astronomy(4), communications(5) and atomic time-keeping(6,7). Yet, in spite of the remarkable progress shown over recent years(8-10), microresonator frequency combs ('microcombs') have been without the key function of direct f-2f self-referencing(1), which enables precise determination of the absolute frequency of each comb line. Here, we realize this missing element using a 16.4 GHz microcomb that is coherently broadened to an octave-spanning spectrum and subsequently fully phase-stabilized to an atomic clock. We show phase-coherent control of the comb and demonstrate its low-noise operation.