Synchronization of distant optical clocks across free-space optical links

The continued remarkable advances in optical clocks and oscillators has led to a parallel strong development of optical clock networks [1]. Such networks have the potential to support a wide range of applications from basic time/frequency dissemination, to clock-based geodesy, to tests of general relativity [1]. To support optical clocks/oscillators at their highest accuracy and precision, these networks must rely on optical signals between the nodes to carry the timing signals. Networks based on the reciprocal transmission across fiber optics have allowed frequency comparisons between remote clocks across Europe and Japan [2, 3]. However, there are many potential applications, both terrestrial and space-based, for which the clocks will not be connected via an optical fiber and the signals must be sent across free space optical links. To this end, we have been developing techniques for the free-space transfer of time and frequency [4-6]. Moreover, rather than pursuing only frequency comparison, we have developed an approach that allows time comparison and even real-time synchronization between nodes. Such a capability can support the wide variety of proposed applications of mobile optical clock networks.