Photonic microwave signals with zeptosecond-level absolute timing noise

Photonic synthesis of radiofrequency (RF) revived the quest for unrivalled microwave purity because of its ability to convey the benefits of optics to the microwave world(1-11). In this work, we perform a high-fidelity transfer of frequency stability between an optical reference and a microwave signal via a low-noise fibre-based frequency comb and cutting-edge photodetection techniques. We demonstrate the generation of the purest microwave signal with a fractional frequency stability below 6.5 x 10(-16) at 1 s and a timing noise floor below 41 zs Hz(-1/2) (phase noise below -173 dBc Hz(-1) for a 12 GHz carrier). This outperforms existing sources and promises a new era for state-of-the-art microwave generation. The characterization is achieved through a heterodyne cross-correlation scheme with the lowermost detection noise. This unprecedented level of purity can impact domains such as radar systems(12), telecommunications(13) and time-frequency metrology(2,14). The measurement methods developed here can benefit the characterization of a broad range of signals.