Optical Frequency Transfer for 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 have residual instabilities 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 residual
instability of 2.5x10^-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.