Quantum key distribution (QKD) has been demonstrated over a point-to-point 1.6-km atmospheric optical path in full daylight. This record transmission distance brings QKD a step closer to surface-to-satellite and other long-distance applications.

Daylight Quantum Key Distribution over 1.6 km

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.

Satellite Quantum Communications: Fundamental Bounds and Practical Security

The Micius satellite, launched from China in August 2016, is the first and only satellite dedicated entirely to quantum experiments. The ultralow loss transmission of photons on most of the path between ground and space enables quantum communication capabilities that are still far from being realized. This review details the commissioning of Micius as a full quantum communications system, and describes the achievement of global-scale quantum key distribution, entanglement distribution, and other fundamental studies, with this unique space-based system. 

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Micius quantum experiments in space

Satellite-based quantum communication is essential for both foundational quantum physics tests and scalable quantum networks. However, the practical implementation of such application is limited due to the turbulent atmosphere, which is especially challenging for the ground-to-satellite uplink scenario. Here we consider the configuration of the continuous-variable quantum teleportation protocol to overcome both the uplink limit and background-radiation constraints with an unconditional and deterministic transfer, where the propagation of quantum light is affected by the favorable satellite-to-ground downlink (including the effects of absorption, scattering, turbulence, pointing errors, and background light). In particular, we derive the turbulent-induced fidelity that is achievable by the protocol for both nighttime and daytime operations, showing the effectiveness of this approach for various squeezing levels and orbit conditions (satellite altitudes and zenith angles). Finally, several approaches for fidelity enhancement are suggested, in terms of optimizing the transmitted Gaussian entanglement in the free-space scenario.

Overcoming the uplink limit of satellite-based quantum communication with deterministic quantum teleportation

The Adaptive Optics Revolution: A History

Previous demonstrations of free-space quantum communication in daylight have been touted as significant for the development of global-scale quantum networks. Until now, no one has carefully tuned their atmospheric channel to reproduce the daytime sky radiance and slant-path turbulence conditions as they exist between space and Earth. In this article we report a quantum communication field experiment under conditions representative of daytime downlinks from space. Higher-order adaptive optics increased quantum channel efficiencies far beyond those possible with tip/tilt correction alone while spatial filtering at the diffraction limit rejected optical noise without the need for an ultra-narrow spectral filter. High signal-to-noise probabilities and low quantum-bit-error rates were demonstrated over a wide range of channel radiances and turbulence conditions associated with slant-path propagation in daytime. The benefits to satellite-based quantum key distribution are quantified and discussed.

Adaptive-optics-enabled quantum communication: A technique for daytime space-to-Earth links

Daytime quantum communication was demonstrated under field-site conditions representative of space-to-Earth optical links. Higher-order adaptive optics increased quantum channel efficiencies significantly beyond those that were possible with tracking alone while tight spatial filtering rejected daytime background light without the need for an ultra-narrow spectral filter. High signal-to-noise probabilities and low quantum-bit-error rates were demonstrated over a wide range of relevant channel radiances and turbulence conditions. The impact to satellite-based quantum key distribution is discussed.

Adaptive-optics-enabled daytime field experiment for satellite-Earth quantum networking

Robust quantum channels between satellites and Earth are essential for the anticipated quantum Internet, but remain challenging in daylight, when background photons vastly outnumber qubit photons. Insufficient understanding of atmospheric propagation and turbulence compensation has led to approaches based on prohibitively narrow spectral filtering and unnecessarily low channel efficiencies. This field experiment uses adaptive optics to maximize efficiency while spatially filtering sky noise at the theoretical limit, enabling quantum communication over the daytime sky hemisphere. Requirements for spectral filtering are relaxed enough to accommodate present-day sources of entangled photons.

Scott C. Newey

Papers

Adaptive-optics-enabled quantum communication: A technique for daytime space-to-Earth links

Adaptive-optics-enabled daytime field experiment for satellite-Earth quantum networking

Adaptive-Optics-Enabled Quantum Communication: A Technique for Daytime Space-To-Earth Links