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Quantum-limited optical time transfer for future geosynchronous links

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posted on 2023-01-10, 03:35 authored by Emily D. Caldwell, Jean-Daniel Deschenes, Jennifer Ellis, William C. Swann, Benjamin K. Stuhl, Hugo Bergeron, Nathan R. Newbury, Laura C. Sinclair
The combination of optical time transfer and optical clocks opens up the possibility of large-scale free-space networks that connect both ground-based optical clocks and future space-based optical clocks. Such networks promise better tests of general relativity, dark matter searches, and gravitational wave detection. The ability to connect optical clocks to a distant satellite could enable space-based very long baseline interferometry (VLBI), advanced satellite navigation, clock-based geodesy, and thousand-fold improvements in intercontinental time dissemination. Thus far, only optical clocks have pushed towards quantum-limited performance. In contrast, optical time transfer has not operated at the analogous quantum limit set by the number of received photons. Here, we demonstrate time transfer with near quantum-limited acquisition and timing at 10,000 times lower received power than previous approaches. Over 300 km between mountaintops in Hawaii with launched powers as low as 40 $\mu$W, distant timescales are synchronized to 320 attoseconds. This nearly quantum-limited operation is critical for long-distance free-space links where photons are few and amplification costly -- at 4.0 mW transmit power, this approach can support 102 dB link loss, more than sufficient for future time transfer to geosynchronous orbits.



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