Such optical clock networks would enable redefinition of the second, a tremendous range of fundamental physics tests and chronometric geodesy.  In locations with sufficient infrastructure, these networks can be established using CW-laser-based frequency transfer across fiber links.  However, in the absence of this infrastructure, free-space approaches are required which can operate with intermittency, high link losses and residual timing uncertainties below that of the state-of-the-art optical clocks themselves.  Furthermore, as transportable optical clocks capable of operating outside a metrology laboratory become available, these time transfer solutions need to be operable in the same challenging environments.

Here, I will present our development of a quantum-limited approach to optical time transfer that relies upon an optical tracking oscillator approach using a time programmable frequency comb. Using frequency combs as optical tracking oscillators to reach the quantum limit for optical time transfer, we have been able to demonstrate sub-femtosecond time transfer across a 300-km terrestrial free-space link with greater than 100 dB of loss, a factor of 10,000 times lower received power threshold than previous frequency-comb-based approaches.  I will show results from this 300-km demonstration as well as more recent work connecting optical atomic clocks across open air paths.