Towards optical frequency geopotential difference measurements via a flying drone

Abstract

Geopotential and orthometric height differences between distant points can be measured via timescale comparisons between atomic clocks. Modern optical atomic clocks achieve statistical uncertainties on the order of 10−18, allowing height differences of around 1 cm to be measured. Frequency transfer via free-space optical links will be needed for measurements where linking the clocks via optical fiber is not possible, but requires line of sight between the clock locations, which is not always practical due to local terrain or over long distances. We present an active optical terminal, phase stabilization system, and phase compensation processing method robust enough to enable optical frequency transfer via a flying drone, greatly increasing the flexibility of free-space optical clock comparisons. We demonstrate a statistical uncertainty of 2.5×10−18 after 3 s of integration, corresponding to a height difference of 2.3 cm, suitable for applications in geodesy, geology, and fundamental physics experiments.

Robust free-space optical frequency transfer in time-varying link distances conditions

“…Frequency stabilities are below 10 −15 at 1 s and 10 −18 at 200 s of integration time, for free-space links of 0 km, 2 km, and 4 km when the UAV flew at ±24 m/s. [19] Reference [20] also gave a solution for optical frequency transfer via a flying drone with a speed of approximately 0.3 m/s, increasing the flexibility of comparisons between free-space optical clocks. The statistical uncertainty of optical frequency is 2.5 × 10 −18 after 3 s of integration time.…”