# Demonstration of 4.8 × 10−17 stability at 1 s for two independent optical clocks

## Abstract

Optical atomic clocks require local oscillators with exceptional optical coherence owing to the challenge of performing spectroscopy on their ultranarrow-linewidth clock transitions. Advances in laser stabilization have thus enabled rapid progress in clock precision. A new class of ultrastable lasers based on cryogenic silicon reference cavities has recently demonstrated the longest optical coherence times to date. Here we utilize such a local oscillator with two strontium (Sr) optical lattice clocks to achieve an advance in clock stability. Through an anti-synchronous comparison, the fractional instability of both clocks is assessed to be $$4.8 \times 10^{ - 17}/\sqrt \tau$$ for an averaging time τ (in seconds). Synchronous interrogation enables each clock to average at a rate of $$3.5 \times 10^{ - 17}/\sqrt \tau$$, dominated by quantum projection noise, and reach an instability of 6.6 × 10−19 over an hour-long measurement. The ability to resolve sub-10−18-level frequency shifts in such short timescales will affect a wide range of applications for clocks in quantum sensing and fundamental physics.

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## Data availability

The data that support the findings of this study are available from the corresponding authors upon reasonable request.

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## Acknowledgements

This work is supported by the National Institute of Standards and Technology (NIST), the Defense Advanced Research Projects Agency (DARPA), the Air Force Office of Scientific Research Multidisciplinary University Research Initiative, the National Science Foundation (NSF) JILA Physics Frontier Center (NSF PHY-1734006), the Cluster of Excellence (EXC 2132 Quantum Frontiers) and Physikalisch-Technische Bundesanstalt (PTB). E.O. and C.J.K. are supported by a postdoctoral fellowship from the National Research Council, L.S. is supported by a National Defense Science and Engineering Graduate Fellowship, A.G. is supported by a fellowship from the Japan Society for the Promotion of Science and C.S. is supported by a fellowship from the Humboldt Foundation. T.L., D.G.M. and U.S. acknowledge support from the Quantum sensors (Q-SENSE) project, supported by the European Commission’s H2020 Marie Skodowska-Curie Actions Research and Innovation Staff Exchange (MSCA RISE) under grant agreement no. 69115. M.G. and R.H. acknowledge support from the EU FP7 initial training network FACT (Future Atomic Clock Technology) and the DARPA Program in Ultrafast Laser Science and Engineering (PμreComb project) under contract no. W31P4Q-14-C-0050. The authors thank J. Munez and J. Sherman for careful reading of this manuscript.

## Author information

E.O., R.B.H., C.J.K., T.B., L.S., C.S., D.K., A.G., J.M.R., G.E.M. and J.Y. contributed to the clock instability measurements. E.O., J.M.R., L.S., C.J.K., T.B., D.K., D.G.M., T.L., F.R., U.S. and J.Y. worked on the Si cavity. L.S. and E.O. commissioned the laser stability transfer set-up based on the Er frequency comb developed by M.G. and R.H. All authors contributed to scientific discussions and the writing of this manuscript.

Correspondence to E. Oelker or J. Ye.

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