Single-mode fiber coupling with a M-SPGD algorithm for long-range quantum communications

TL;DR

This paper presents an adaptive optics system based on a modal stochastic parallel gradient descent algorithm that enhances single-mode fiber coupling efficiency in free-space quantum communication over 8 km, addressing atmospheric turbulence challenges.

Contribution

The study introduces a novel M-SPGD adaptive optics system that significantly improves fiber coupling efficiency and stability under strong turbulence for long-range quantum links.

Findings

Achieved 3.7 dB improvement in coupling efficiency

Significant suppression of coupling fluctuation

Demonstrated effectiveness over 8 km urban free-space channel

Abstract

Satellite-based quantum communication is a promising approach for realizing global-scale quantum networks. For free-space quantum channel, single-mode fiber coupling is particularly important for improving signal-to-noise ratio of daylight quantum key distribution (QKD) and compatibility with standard fiber-based QKD. However, achieving a highly efficient and stable single-mode coupling efficiency under strong atmospheric turbulence remains experimentally challenging. Here, we develop a single-mode receiver with an adaptive optics (AO) system based on a modal version of the stochastic parallel gradient descent (M-SPGD) algorithm and test its performance over an 8 km urban terrestrial free-space channel. Under strong atmospheric turbulence, the M-SPGD AO system obtains an improvement of about 3.7 dB in the single-mode fiber coupling efficiency and a significant suppression of fluctuation, which can find its applications in free-space long-range quantum communications.