Optical frequency combs: Coherently uniting the electromagnetic spectrum

An optical timekeeper Optical clocks, based on optical transitions of atoms, operate at much higher frequency than the microwave atomic clocks currently used as our timing standards. They have been shown to exhibit better stability and are poised to redefine the second. The development of stable, self-referenced optical frequency combs that span the microwave to optical wavelengths has been key to these efforts. Diddams et al. reviewed developments and refinements of these optical combs over the past 20 years and provide an overview of where they are finding application, from precision timing to high-resolution spectroscopy and imaging, ranging, and navigation. Science this issue p. eaay3676