Abstract
Atomic clocks based on optical transitions are the most stable, and therefore precise, timekeepers available. These clocks operate by alternating intervals of atomic interrogation with the ‘dead’ time required for quantum state preparation and readout. This non-continuous interrogation of the atom system results in the Dick effect, an aliasing of frequency noise from the laser interrogating the atomic transition1,2. Despite recent advances in optical clock stability that have been achieved by improving laser coherence, the Dick effect has continually limited the performance of optical clocks. Here we implement a robust solution to overcome this limitation: a zero-dead-time optical clock that is based on the interleaved interrogation of two cold-atom ensembles3. This clock exhibits vanishingly small Dick noise, thereby achieving an unprecedented fractional frequency instability assessed to be 
for an averaging time τ in seconds. We also consider alternate dual-atom-ensemble schemes to extend laser coherence and reduce the standard quantum limit of clock stability, achieving a spectroscopy line quality factor of Q > 4 × 1015.
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Acknowledgements
The authors acknowledge the Defense Advanced Research Projects Agency (DARPA) Quantum Assisted Sensing and Readout (QuASAR) programme, the NASA Fundamental Physics programme and the National Institute of Standards and Technology for financial support. R.C.B. acknowledges support from the National Research Council Research Associateship programme. We also thank T. Fortier, F. Quinlan and S. Diddams for femtosecond optical frequency comb measurements.
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M.S., R.C.B., W.F.M., R.J.F., G.M., D.N. and A.D.L. carried out the instability measurements. M.S. and A.D.L. constructed the clock laser. W.F.M., T.H.Y and A.D.L. contributed to the optimization of the clock laser performance. J.A.S. constructed the DDS system for precise cavity drift compensation. R.C.B., N.H., T.H.Y., W.F.M., R.J.F., G.M. and A.D.L. were responsible for the operation of Yb-1 and Yb-2 systems and the phase noise cancellation. K.B. contributed to the evaluation of the instability budget. C.W.O. and A.D.L. supervised this work. All authors contributed to the final manuscript.
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Schioppo, M., Brown, R., McGrew, W. et al. Ultrastable optical clock with two cold-atom ensembles. Nature Photon 11, 48–52 (2017). https://doi.org/10.1038/nphoton.2016.231
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DOI: https://doi.org/10.1038/nphoton.2016.231
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