  In practical applications to free-space quantum communications, the
utilization of active beam coupling and stabilization techniques offers notable
advantages, particularly when dealing with limited detecting areas or coupling
into single-mode fibers(SMFs) to mitigate background noise. In this work, we
introduce highly-enhanced active beam-wander-correction technique, specifically
tailored to efficiently couple and stabilize beams into SMFs, particularly in
scenarios where initial optical alignment with the SMF is misaligned. To
achieve this objective, we implement a SMF auto-coupling algorithm and a
decoupled stabilization method, effectively and reliably correcting beam wander
caused by atmospheric turbulence effects. The performance of the proposed
technique is thoroughly validated through quantitative measurements of the
temporal variation in coupling efficiency(coincidence counts) of a laser
beam(entangled photons). The results show significant improvements in both mean
values and standard deviations of the coupling efficiency, even in the presence
of 2.6 km atmospheric turbulence effects. When utilizing a laser source, the
coupling efficiency demonstrates a remarkable mean value increase of over 50 %,
accompanied by a substantial 4.4-fold improvement in the standard deviation.
For the entangled photon source, a fine mean value increase of 14 % and an
approximate 2-fold improvement in the standard deviation are observed.
Furthermore,the proposed technique successfully restores the fidelity of the
polarization-entangled state, which has been compromised by atmospheric effects
in the free-space channel, to a level close to the fidelity measured directly
from the source. Our work will be helpful in designing spatial light-fiber
coupling system not only for free-space quantum communications but also for
high-speed laser communications.

Quantum Physics