Satellite-mediated quantum key distribution (QKD) has received considerable attention due to its potential to reach a global scale. Associated with this application is the atmospheric effects that set a notably restrict to the secret key rate. Here we demonstrate the atmospheric effects on satellite-mediated continuous-variable QKD with respect to the transmittance, fiber coupling, and excess noise. We consider the effects on both downlink and uplink while illustrating the discrepancy between them. Numerical analysis shows that downlink has more reliable transmittance than uplink while enduring a severe fiber coupling issue. It reveals that the discrepancy in transmittance is mainly due to the energy redistribution of the arriving beam. In addition, the time-of-arrival fluctuation notably reduces transmittance when the transmitted pulse half-width is sufficiently narrow (less than 10−13), and it introduces a controllable excess noise if the signal has a strong correlation with the local oscillator.

The current investigation on continuous-variable quantum key distribution (CV-QKD) is mainly based on the optical fibre or the free space atmosphere channel, while the ocean, which covers most of the Earth, has not been well utilized. In this paper, we analyze the influence of depth and wavelength on the attenuation effect of seawater on light by using a model based on the concentration of chlorophyll-a, and propose an improved four-state underwater CV-QKD protocol with a heralded hybrid linear amplifier (HLA). Both homodyne detection with a phase-sensitive amplifier and heterodyne detection with a phase-insensitive amplifier are considered. Simulation results show that the introduction of the heralded HLA can effectively extend the secure transmission distance of the key. Besides, when the transmission distance is relatively long, the secret key rate of the protocol is higher than that of the protocol without HLA.

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An accurate estimation of system parameters is of significance for the practical implementation of the simultaneous quantum key distribution and classical communication (SQCC) over a satellite-mediated link when considering the finite-size effect. In this paper, we propose a Kalman filter (KF)-enabled parameter estimation method for the SQCC over a satellite-mediated link. The fast and slow phase drift can be both estimated by using the improved vector KF carrier phase estimation algorithm, and thus the phase estimation error can be tracked in real time and be almost approximate to the theoretical mean square error limit. Taking advantage of the achieved phase estimation and the dual modulation of the SQCC scheme, the excess noise can be estimated with not only a higher precise but also a lower sacrificing rate of raw keys. Numerical simulations demonstrate the feasibility of the SQCC in both the downlink and uplink in terms of the finite-size effect. As a comparison of the Mth-power algorithm, we find that the secret key rate and achievable zenith angle perform better by using the vector KF algorithm. It paves the way of practical implementations for the SQCC system.