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# Quantum-limited optical time transfer for future geosynchronous links

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Dec 2022

The combination of optical time transfer and optical clocks opens up thepossibility of large-scale free-space networks that connect both ground-basedoptical clocks and future space-based optical clocks. Such networks promisebetter tests of general relativity, dark matter searches, and gravitationalwave detection. The ability to connect optical clocks to a distant satellitecould enable space-based very long baseline interferometry (VLBI), advancedsatellite navigation, clock-based geodesy, and thousand-fold improvements inintercontinental time dissemination. Thus far, only optical clocks have pushedtowards quantum-limited performance. In contrast, optical time transfer has notoperated at the analogous quantum limit set by the number of received photons.Here, we demonstrate time transfer with near quantum-limited acquisition andtiming at 10,000 times lower received power than previous approaches. Over 300km between mountaintops in Hawaii with launched powers as low as 40 $\mu$W,distant timescales are synchronized to 320 attoseconds. This nearlyquantum-limited operation is critical for long-distance free-space links wherephotons are few and amplification costly -- at 4.0 mW transmit power, thisapproach can support 102 dB link loss, more than sufficient for future timetransfer to geosynchronous orbits.

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