Quantum communications in a moderate-to-strong turbulent space

Since the invention of the laser in the 60s, one of the most fundamental communication channels has been the free-space optical channel. For this type of channel, a number of effects generally need to be considered, including diffraction, refraction, atmospheric extinction, pointing errors and, most importantly, turbulence. Because of all these adverse features, the free-space optical (FSO) channel is more difficult to study than a stable fiber-based link. For the same reasons, only recently it has been possible to establish the ultimate performances achievable in quantum communications via free-space channels, together with practical rates for continuous variable (CV) quantum key distribution (QKD). Differently from previous literature, mainly focused on the regime of weak turbulence, this work considers the FSO channel in the more challenging regime of moderate-to-strong turbulence, where effects of beam widening and breaking are more important than beam wandering. This regime may occur in long-distance free-space links on the ground, in uplink to high-altitude platform systems (HAPS) and, more interestingly, in downlink from near-horizon satellites. In such a regime we rigorously investigate ultimate limits for quantum communications and show that composable keys can be extracted using CV-QKD. As quantum communication networks mature and expand to global scale, free-space links will become increasingly important in bridging the distances involved. The authors provide a model for such links under conditions in which atmospheric turbulence is significant, showing that a finite key rate is possible even in challenging scenarios such as satellite operating at high zenith angle.

Automated summary

Since the invention of the laser in the 60s, the free-space optical channel has been a fundamental communication channel. However, studying this channel is more difficult than a stable fiber-based link due to effects such as diffraction, refraction, atmospheric extinction, pointing errors, and turbulence. Recent research has shown that it is possible to achieve ultimate performances in quantum communications and extract composable keys in the challenging regime of moderate-to-strong turbulence in free-space channels.