Photonic synthesis of radiofrequency (RF) waveforms revived the quest for unrivalled microwave purity because of its ability to convey the benefits of optics to the microwave world1,2,3,4,5,6,7,8,9,10,11. 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 × 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 systems12, telecommunications13 and time–frequency metrology2,14. The measurement methods developed here can benefit the characterization of a broad range of signals.
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We thank J. Pinto for help with the electronics and R. Le Targat for the reference-laser distribution. This work is funded by the Defense Advanced Research Projects Agency (DARPA) as a part of the Program in Ultrafast Laser Science and Engineering (PμreComb project) under contract no. W31P4Q-14-C-0050, by the Formation, Innovation, Recherche, Services et Transfert en Temps-Fréquence (FIRST-TF) Labex, by the Eurostar Eureka program (Stable Microwave Generation and Dissemination over Optical Fiber project) and by the EU FP7 Initial Training Network FACT (Future Atomic Clock Technology).
The authors declare no competing financial interests.
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Xie, X., Bouchand, R., Nicolodi, D. et al. Photonic microwave signals with zeptosecond-level absolute timing noise. Nature Photon 11, 44–47 (2017). https://doi.org/10.1038/nphoton.2016.215
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