  Free-space quantum cryptography plays an integral role in realizing a
global-scale quantum internet system. Compared to fiber-based communication
networks, free-space networks experience significantly less decoherence and
photon loss due to the absence of birefringent effects in the atmosphere.
However, the atmospheric turbulence contributes to deviation in transmittance
distribution, which introduces noise and channel loss. Several methods have
been proposed to overcome the low signal-to-noise ratio. Active research is
currently focused on establishing secure and practical quantum communication in
a high-loss channel, and enhancing the secure key rate by implementing bit
rejection strategies when the channel transmittance drops below a certain
threshold. By simulating the atmospheric turbulence using an
acousto-optical-modulator (AOM) and implementing the prefixed-threshold
real-time selection (P-RTS) method, our group performed finite-size decoy-state
Bennett-Brassard 1984 (BB84) quantum key distribution (QKD) protocol for 19 dB
channel loss. With better optical calibration and efficient superconducting
nano-wire single photon detector (SNSPD), we have extended our previous work to
40 dB channel loss characterizing the transmittance distribution of our system
under upper moderate turbulence conditions.
