Ultra-Low Power Time Transfer: 300 Attosecond Synchronization with 300 fW Over 300 km of Air

SummaryWe demonstrate free-space time transfer across a 300 km atmospheric link with greater than 100 dB of loss. With received powers of only a few hundred femtowatts we synchronize two optical oscillators to 300 attoseconds and reach a fractional frequency instability of 3.1×10
^-19
, all without the use of large telescope apertures or adaptive optics. This work demonstrates a viable path for future time transfer between ground and satellite nodes in a global optical clock network with performance commensurate with state-of-the-art optical clocks and a low power-aperture product design. Work of the US government, not subject to copyright.