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IEEE International Conference on Space Optical Systems and Applications (ICSOS)

Evaluation of integration concepts of Optical Ground Stations for satellite-based Quantum Key Distribution into a quantum network

Stefanie Häusler, Davide Orsucci, Andrew Reeves, Florian Moll

Published Oct 11, 2023• 6 citations
Publisher

Abstract

Quantum Key Distribution (QKD) is a promising method to guarantee future-proof, information theoretic security. Since optical fibers have an exponential loss with distance, satellite-based QKD solutions are being developed in order to realize long-distance links. Therefore, Optical Ground Stations for QKD (QKD-OGS) need to be designed to enable quantum communication with satellites. Different link configurations will result in different integration options of the QKD-OGS in the terrestrial fiber network and therefore impact its performance. Applicable integration options must be identified and trade-off analysis conducted at the architecture, system and sub-system level. The reference scenario is a Low Earth Orbit (LEO) downlink configuration employing a decoy-state BB84 protocol with polarization encoded qubits at 1550 nm wavelength. The satellite acts as trusted node for implementation of the key distribution between two parties on ground. Furthermore, only direct connections from the satellite to the end-users are considered, i.e. the QKD-OGS does not serve as a trusted-node relay. This results in the selection of three main integration concepts: first, a free-space, end-user QKD-OGS which is located directly at the end-user where the quantum signal is measured directly behind the receiver telescope in free-space; second, a fiber-coupled, end-user QKD-OGS where the quantum signal is coupled into a single-mode fiber and guided to a co-located server room hosting the QKD Receiver system; third, a fiber-coupled, provider QKD-OGS, which reaches several end-users by using a local fiber network. In order to evaluate these three concepts, the comparison parameter is defined to be an estimated factor of suppliable end-users. The estimated comparison factor shows that the fiber-coupled, enduser QKD-OGS can supply 6.7 times more end users than the free-space, end-user QKD-OGS. The fiber-coupled, end-user QKD-OGS can supply 129.7 times more end-users than the free-space, end-user QKD-OGS.

DOI
10.1109/icsos59710.2023.10491230

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Quantum Key Distribution (QKD) is a promising method to guarantee future-proof, information theoretic security. Since optical fibers have an exponential loss with distance, satellite-based QKD solutions are being developed in order to realize long-distance links. Therefore, Optical Ground Stations for QKD (QKD-OGS) need to be designed to enable quantum communication with satellites. Different link configurations will result in different integration options of the QKD-OGS in the terrestrial fiber network and therefore impact its performance. Applicable integration options must be identified and trade-off analysis conducted at the architecture, system and sub-system level. The reference scenario is a Low Earth Orbit (LEO) downlink configuration employing a decoy-state BB84 protocol with polarization encoded qubits at 1550 nm wavelength. The satellite acts as trusted node for implementation of the key distribution between two parties on ground. Furthermore, only direct connections from the satellite to the end-users are considered, i.e. the QKD-OGS does not serve as a trusted-node relay. This results in the selection of three main integration concepts: first, a free-space, end-user QKD-OGS which is located directly at the end-user where the quantum signal is measured directly behind the receiver telescope in free-space; second, a fiber-coupled, end-user QKD-OGS where the quantum signal is coupled into a single-mode fiber and guided to a co-located server room hosting the QKD Receiver system; third, a fiber-coupled, provider QKD-OGS, which reaches several end-users by using a local fiber network. In order to evaluate these three concepts, the comparison parameter is defined to be an estimated factor of suppliable end-users. The estimated comparison factor shows that the fiber-coupled, enduser QKD-OGS can supply 6.7 times more end users than the free-space, end-user QKD-OGS. The fiber-coupled, end-user QKD-OGS can supply 129.7 times more end-users than the free-space, end-user QKD-OGS.
Satellite-based optical communication enables quantum key distribution (QKD) on global scales, with realistic near-term architectures usually considering prepare-and-measure protocols that employ satellites in Low Earth Orbit (LEO) acting as trusted nodes. One major challenge is constituted by the high loss in the free-space link from the satellite to the optical ground station (OGS) due to the large distances involved. If the quantum signal is directly forwarded from the OGS to an end-user via a ground-based fiber connection, the additional losses limit the key generation to the vicinity of a few kilometers. For longer-distance links, instead, the OGS would have to act as another trusted node to relay the key, introducing a severe bottleneck in the practical security. Here we consider an alternative solution based on the use of Mode-Pairing (MP) QKD, employing the OGS as an untrusted intermediate node. Due to its improved scaling of the key rate with the channel transmission, MP QKD allows end-users up to hundreds of kilometers around the OGS to establish a secure key with the satellite, without having to trust the OGS provider. We discuss the effect of Doppler shifts on the performance of MP QKD and propose a possible implementation to mitigate its impact with sufficient precision.
Quantum key distribution (QKD) enables secure communication even in presence of quantum computers. Satellite based systems enable worldwide key distribution and connection of terrestrial quantum networks. In future, QKD satellite constellations can be operated to service ground users that are connected using an optical ground station. These QKD ground stations are assumed to be deployed in two scenarios. One is the option as end-user ground station, in which the ground user is directly connected to the satellite via the ground station. The other option is the use as a provider ground station, in which the ground user is connected via an intermediary fiber line. The performance of the satellite to ground QKD link depends on, amongst other, the environmental and infrastructural impacts of the site where it is located. Unlike in astronomical sites, the QKD Optical Ground Station (OGS) are not positioned in selected optimal environment, especially considering atmospheric turbulence and extinction. The QKD OGS mainly has to cope with the given environment instead. Knowledge of the site’s meteorological and infrastructural environment is important to assess performance of a QKD OGS when deployed. This paper discusses the most relevant aspects of a QKD OGS site characterization. These are atmospheric turbulence, atmospheric extinction, and background light. As a concrete example, the DLR Optical Ground Station Oberpfaffenhofen nearby Munich is used. Existing data is summarized, reviewed and needs for further data acquisition defined.
This paper presents a comprehensive feasibility analysis designed to model Low Earth Orbit (LEO) satellite-to-ground communications for Quantum Key Distribution (QKD), focusing on the 1550nm wavelength to facilitate the distribution of information through fiber segments. Discrete Variable Decoy State Polarization-based BB84 protocol is assumed whereas various detection setups and environmental conditions are simulated. Our analysis focuses on the receiver's design in terms of aperture and focal length to deliver a general framework by examining the coupling efficiency, a parameter strongly affecting the QKD satellite downlink. Through detailed simulation, we identify optimal configurations and operational parameters for various receiver designs, significantly enhancing the efficiency and reliability of satellite-to-ground QKD systems. The findings offer promising directions for extending the range of QKD systems, potentially revolutionizing secure communications by integrating satellite-transmitted quantum keys with terrestrial fiber networks. This study not only underscores the feasibility of advancing satellite-to-ground QKD technology but also lays the groundwork for future innovations in global quantum communications infrastructure.
As Quantum Key Distribution (QKD) seems to turn towards satellite communication infrastructure to interconnect distant nodes aiming to a global quantum secured network, the number of assisting satellites is expected to increase. In this work, a feasibility analysis of a large-scale Low Earth Orbit (LEO) satellite constellation supporting LEO to ground QKD links is presented. A software tool to simulate the physical properties of a large-scale satellite constellation and a link budget calculator tailored for LEO satellite to ground QKD links have been developed. A large-scale LEO satellite constellation with 100 satellites orbiting at 550km altitude with 53deg inclination angle in ten different orbital planes (10 satellites per orbital plane) has been simulated and subsatellite points for 1 year have been obtained. Assuming, an optical ground station network consisting of nine different optical ground stations located in observatories across Europe, the performance of this large-scale constellation is evaluated in terms of availability and Secure Key Rate (SKR) employing the prepare and measure Decoy-State BB84 QKD protocol, during nighttime, under different atmospheric conditions including cloud coverage and turbulence effects among others, while two different wavelengths (i.e., 800nm and 1550nm) are examined. For each Optical Ground Station (OGS) location, Gbits of distilled keys per year per ground station and key rates up to Kbps are reported.
Quantum repeaters and satellite-based free-space optical links are complementary technological approaches slated to overcome the exponential photon loss occurring in optical fibers and thus allow Quantum Key Distribution (QKD) at global scales. We analyse architectures where these approaches are combined and satellites are used as quantum repeater nodes to distribute entanglement to far-away optical ground stations. Here, we simulate dynamical, three-dimensional satellite-ground-station passes, going beyond previous investigations which typically consider static satellite links. Therefore, we numerically solve the equations of motion of the dynamic system consisting of three, low-Earth-orbit satellites separated in the along-track direction. The optical link model includes atmospheric attenuation, single mode fiber coupling, beam wandering and broadening, as well as adaptive optics effects. Analytical expressions for the Bell state measurement and associated error rates are derived for quantum memory assisted communications, accounting for exponentially decreasing retrieval efficiency and state coherence. We consider uplink and downlink architectures for continental and inter-continental connections and assess the influence of varying orbital elements on the expected. We also investigate alternative solutions for realising practically secure QKD networks in the near-term, whereby trusted nodes are only located in space and relatively inaccessible. By exploiting the newly introduced mode-pairing QKD protocol, we show that users located hundreds of kilometres from a ground station could establish a secure key with a satellite using well-established technologies and without requiring intermediate trusted nodes.
The development of quantum computers, which could potentially implement Shor’s algorithm and thereby pose a threat to classical encryption, has created the need for establishing ultra-secure communication systems. One solution is quantum key distribution (QKD), a method based on the principles of quantum mechanics that enables sharing an encryption key in a way that is fundamentally resilient to computational attacks, quantum or classical. QKD used together with a one-time-pad symmetric cryptography algorithm thus provides information-theoretically secure communication. In addition, QKD enables the detection of any eavesdropping attempt on communications lines. While classical optical communication systems allow signal amplification, the no-cloning theorem from the foundations of quantum mechanics prohibits the cloning of unknown states. This nogo theorem results in the practical limitation of quantum key distribution via optical fibers, prompting the need for free-space solutions. In this work, we present the proposed design of the first Optical Ground Station connected to the terrestrial Croatian Quantum Communication Infrastructure through a QKD node. The station’s function involves receiving prepare-and-measure photons from the low Earth orbit satellite and quantum key generation through the decoy-state BB84 protocol. By utilizing the satellite as a trusted node, this process ensures ultra-secure communication between two optical ground stations.