We investigate a quantum key distribution (QKD) system with relay satellite for communication between two distant Earth-based parties, Alice and Bob. The satellite acts as a relay station which simply redirects the QKD signal. It has several advantages that can be decisive. The optical relay provides a high transparency to protocols and wavelengths. The relay node does not have to be trusted. Like entanglement-based or measurement-device-independent satellite QKD, relayassisted QKD suffers from a higher propagation loss than trusted-node scenarios. Challenges are expected when pointing the relay mirror precisely between Alice and Bob and dealing with significant point-ahead angles that result from the satellite velocity. We start our analysis by evaluating the point ahead angles (PAA) of Alice and Bob for scenarios of interest. The difference between the two PAA vectors tells us whether the relay mirror will be able to maintain a bidirectional transmission with beams sent from the ground. Considering a satellite altitude of 500 km and an Alice-Bob separation of 1000 km, observation of large PAA vector difference enjoins us to place additional beacon sources on the satellite. A conceptual design of the relay-tracking system is drafted. Onboard measurements of the beacons from Alice and Bob provide feedback to the relay-mirror positioning control loop. With a typical relay-mirror size of 0.2 m, propagation losses are calculated for different ground antennas. We conclude on the attractiveness and feasibility of satellite relay QKD.
We investigate a quantum key distribution (QKD) system with relay satellite for communication between two distant Earth-based parties, Alice and Bob. The satellite acts as a relay station which simply redirects the QKD signal. It has several advantages that can be decisive. The optical relay provides a high transparency to protocols and wavelengths. The relay node does not have to be trusted. Like entanglement-based or measurement-device-independent satellite QKD, relayassisted QKD suffers from a higher propagation loss than trusted-node scenarios. Challenges are expected when pointing the relay mirror precisely between Alice and Bob and dealing with significant point-ahead angles that result from the satellite velocity. We start our analysis by evaluating the point ahead angles (PAA) of Alice and Bob for scenarios of interest. The difference between the two PAA vectors tells us whether the relay mirror will be able to maintain a bidirectional transmission with beams sent from the ground. Considering a satellite altitude of 500 km and an Alice-Bob separation of 1000 km, observation of large PAA vector difference enjoins us to place additional beacon sources on the satellite. A conceptual design of the relay-tracking system is drafted. Onboard measurements of the beacons from Alice and Bob provide feedback to the relay-mirror positioning control loop. With a typical relay-mirror size of 0.2 m, propagation losses are calculated for different ground antennas. We conclude on the attractiveness and feasibility of satellite relay QKD.
Quantum key distribution (QKD), the technology to generate distant secure keys for later usage in secure communication, has become a mature topic since the 1980s. While terrestrial QKD over fiber networks is suitable for short-distance urban networks, QKD via satellites promises to gap the distance requirement on a global scale that limits terrestrial QKD. Satellite QKD requires the exchange of quantum states between a satellite and multiple ground stations. The goal is then to facilitate shared secret keys between the ground stations, which can then be relayed to users near the ground stations. The satellite usually carries the quantum sources as its payload and sends them to the ground station via a downlink. The QKD protocol is heavily influenced by the choice of the quantum source, with typically one of two types: approximate single photon source (such as a weak laser) or an entangled source. Similarly, the ground station must be equipped to receive, measure and process the quantum signals sent by the satellite. Although there are also reverse schemes, where the satellite acts as the receiver and the ground station as the sender, these schemes are less explored due to higher noise in the uplink and less engineering experience. As with any system containing a high heterogeneity, many different implementation schemes lead to a variety of optimized use cases based on different criteria. Satellite QKD carries the further complication that it must be adapted to international standards. In this paper we present different satellite and ground station architectures that lean on the developments in the projects EAGLE-1, QUBE, QuNET, RoGloQuaN, addressing different protocols, sources, satellite sizes and telescope sizes. We also discuss the need for agility and configuration of components in a heterogeneous environment.
Quantum key distribution (QKD) is a cryptographic method enabling two parties to establish a private encryption key. The range of communication of ground-based QKD is limited to an order of 100km, due to in-fibre attenuations and atmospheric losses, and the development of quantum repeaters remains technologically challenging. While trusted-node links make communication over large distances possible, satellite-QKD is required for communication over global distances. By using satellites equipped with high-quality optical links, satellite-QKD can achieve ultra-long-distance quantum communication in the 1000-km range. The significant potential of satellite-QKD for the creation of global quantum networks thus makes it a particularly interesting field of research. In this analysis, we begin with an overview of the technical parameters of performing satellite-QKD, including infrastructure and protocols. We continue with a high-level summary of advancements in satellite-QKD by analysing past, present and proposed satellite-QKD missions and initiatives around the world. We conclude by discussing the technical challenges currently faced in satellite-QKD, which can be tackled through future research in this area.
Quantum Key Distribution (QKD) uses quantum states in an optical channel to grow a key with guaranteed security. Satellite QKD (SatQKD) is a proposed topology for QKD which provides unlimited terrestrial range for links and allows mobile access. SatQKD (and generally satellite optical communications) Optical Ground Stations (OGSs) require an optical beacon, a beam from OGS to satellite to assist pointing the satellite’s telescope(s). Beaconing is challenging for SatQKD due to the single-photon sensitivity of the receiver to back-reflected beacon light and the need to point the outgoing beacon ahead of the downlink channel (the Point-Ahead Angle (PAA)). Hence, beacon light is often propagated through separate telescopes, increasing complexity. We describe a spatially separated beacon which reduces back-reflection errors and how its PAA is achieved. The far field beams of this and other beacon architectures are compared. Beaconing in this way can give greater up-time (due to less maintenance) and a lower cost barrier to development. A related system could have similar benefits for laser-guide-star telescopes.
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.
Various optical quantum key distribution protocols have been proposed and examined to implement a global quantum communication system both in fiber-based network and in free-space systems. Employing a satellite as a quantum relay is a possible solution to increase the secure covered distance, which is available for sharing secure keys between two remote ground stations. In this paper, combining with the idea of measurement device independent quantum key distribution, a modified free-space quantum communication protocol is proposed to build a QKD network with untrusted nodes, where the parties on the ground send quantum signals to the untrusted satellite relay that perform a joint measurement on the incoming signals. Our results demonstrate that though the modified protocol suffers higher link attentuation than the entanglement-based quantum communication protocol owing to higher effects of atmospheric turbulence for uplink, our modified protocol provides secure quantum communications with untrusted relays. Furthermore, the complex and non-space-qualified sources remain on the ground in our scheme , which makes our scheme more practical for bridging a seamless connection form ground to space.
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.