Abstract
Satellite quantum communications are emerging within the panorama of quantum technologies as a more effective strategy to distribute completely-secure keys at very long distances, therefore playing an important role in the architecture of a large-scale quantum network. In this work, we apply and extend recent results in free-space quantum communications to determine the ultimate limits at which secret (and entanglement) bits can be distributed via satellites. Our study is comprehensive of the various practical scenarios, encompassing both downlink and uplink configurations, with satellites at different altitudes and zenith angles. It includes effects of diffraction, extinction, background noise and fading, due to pointing errors and atmospheric turbulence (appropriately developed for slant distances). Besides identifying upper bounds, we also discuss lower bounds, i.e., achievable rates for key generation and entanglement distribution. In particular, we study the composable finite-size secret key rates that are achievable by protocols of continuous variable quantum key distribution, for both downlink and uplink, showing the feasibility of this approach for all configurations. Finally, we present a study with a sun-synchronous satellite, showing that its key distribution rate is able to outperform a ground chain of ideal quantum repeaters.
Highlights
Our study extends the free-space analysis of Ref. [14], there developed for ground-to-ground free-space communications, to the more general setting of ground-satellite communications, where the Published by the American Physical Society optical signals travel slant distances with variable altitudes and zenith angles
Once we have clarified the ultimate limits for distributing keys with satellites, we study the rates that are achievable by practical continuous variable (CV)-quantum key distribution (QKD) protocols, where we explicitly account for finite-size and composable aspects
In the following we investigate the performances achievable with an local LO” (LLO), but we stress that both approaches of transmitted LO” (TLO) and LLO are encompassed by our theory
Summary
Satellite quantum communications ([1], Sec. VI) represent a new collective endeavour of the scientific community, with pioneering experiments already demonstrated. An alternative strategy relies in the adoption of nodes able to distribute entanglement, which is swapped to the remote users This solution is rather expensive because it involves the development of quantum repeaters with long coherence times and distillation capabilities. We show that the number of secret key bits per day that can be distributed between two stations by a sunsynchronous satellite can be much larger than what achievable by a standard fiber connection between these stations, even when a substantial number of repeaters are employed in the middle and assumed to operate at their capacity level [16] This analysis proves the potential advantages of satellite links over ground networks and strongly corroborates their role for near-future realization of large-scale quantum communications
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