Reducing The Impact Of Adaptive Optics Lag On Optical And Quantum Communications Rates From Rapidly Moving Sources

Abstract

Wavefront of light passing through turbulent atmosphere gets distorted. This causes signal loss in free-space optical communication as the light beam spreads and wanders at the receiving end. Frequency and/or time division multiplexing adaptive optics (AO) techniques have been used to conjugate this kind of wavefront distortion. However, if the signal beam moves relative to the atmosphere, the AO system performance degrades due to high temporal anisoplanatism. Here we solve this problem by adding a pioneer beacon that is spatially separated from the signal beam with time delay between spatially separated pulses. More importantly, our protocol works irrespective of the signal beam intensity and hence is also applicable to secret quantum communication. In particular, using semi-empirical atmospheric turbulence calculation, we show that for low earth orbit satellite-to-ground decoy state quantum key distribution with the satellite at zenith angle $< 30^\circ$, our method increases the key rate by at least $215\%$ and $40\%$ for satellite altitude $400$~km and $800$~km, respectively. Finally, we propose a modification of existing wavelength division multiplexing systems as an effective alternative solution to this problem.