Quantum-limited optical time transfer for future geosynchronous links.

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 1-3 , dark-matter searches 4 and gravitational-wave detection 5 . The ability to connect optical clocks to a distant satellite could enable space-based very long baseline interferometry 6,7 , advanced satellite navigation 8 , clock-based geodesy 2,9,10 and thousandfold improvements in intercontinental time dissemination 11,12 . Thus far, only optical clocks have pushed towards quantum-limited performance 13 . By 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 14-24 . Over 300 km between mountaintops in Hawaii with launched powers as low as 40 µW, distant sites are synchronized to 320 attoseconds. This nearly quantum-limited operation is critical for long-distance free-space links in which 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.