Establishing secure communication links at a global scale is a major potential application of quantum information science but also extremely challenging for the underlying technology. Although milestone experiments using satellite-to-ground links and exploiting singe-photon encoding for implementing quantum key distribution have shown recently that this goal is achievable, it is still necessary to further investigate practical solutions compatible with classical optical communication systems. Here, we examine the feasibility of establishing secret keys in a satellite-to-ground downlink configuration using continuous-variable encoding, which can be implemented using standard telecommunication components certified for space environment and able to operate at high symbol rates. Considering a realistic channel model and state-of-the-art technology, and exploiting an orbit subdivision technique for mitigating fluctuations in the transmission efficiency, we find positive secret key rates for a low-Earth-orbit scenario, whereas finite-size effects can be a limiting factor for higher orbits. Our analysis determines regions of values for important experimental parameters where secret key exchange is possible and can be used as a guideline for experimental efforts in this direction.

在全球范围内建立安全的通信链接是量子信息科学的主要潜在应用，但对基础技术也极具挑战。尽管最近使用卫星到地面链路并利用单光子编码来实现量子密钥分配的里程碑实验表明可以实现此目标，但仍然有必要进一步研究与经典光通信系统兼容的实际解决方案。在这里，我们研究了使用连续变量编码在卫星到地面下行链路配置中建立秘密密钥的可行性，该变量可以使用经过空间环境认证并能够以高符号率运行的标准电信组件来实现。考虑到现实的渠道模型和最新技术，并利用轨道细分技术来缓解传输效率的波动，我们发现低地球轨道情景下的正秘密密钥率，而有限大小的影响可能是较高轨道的限制因素。我们的分析确定了可能进行秘密密钥交换的重要实验参数的值区域，并且可以用作朝该方向进行实验的准则。