A comprehensive design and performance analysis of low Earth orbit satellite quantum communication
J-P Bourgoin, Evan Meyer-Scott, Brendon L. Higgins, Bassam Helou, Chris Erven, Hannes Hübel, Balaji Shankar Kumar, Danya Hudson, I D'Souza, R. Girard, Raymond Laflamme, Thomas D. Jennewein
Optical quantum communication utilizing satellite platforms has the potential to extend the reach of quantum key distribution (QKD) from terrestrial limits of ∼200 km to global scales. We have developed a thorough numerical simulation using realistic simulated orbits and incorporating the effects of pointing error, diffraction, atmosphere and telescope design, to obtain estimates of the loss and background noise which a satellite-based system would experience. Combining with quantum optics simulations of sources and detection, we determine the length of secure key for QKD, as well as entanglement visibility and achievable distances for fundamental experiments. We analyse the performance of a low Earth orbit satellite for downlink and uplink scenarios of the quantum optical signals. We argue that the advantages of locating the quantum source on the ground justify a greater scientific interest in an uplink as compared to a downlink. An uplink with a ground transmitter of at least 25 cm diameter and a 30 cm receiver telescope on the satellite could be used to successfully perform QKD multiple times per week with either an entangled photon source or with a weak coherent pulse source, as well as perform long-distance Bell tests and quantum teleportation. Our model helps to resolve important design considerations such as operating wavelength, type and specifications of sources and detectors, telescope designs, specific orbits and ground station locations, in view of anticipated overall system performance.
Optical quantum communication utilizing satellite platforms has the potential to extend the reach of quantum key distribution (QKD) from terrestrial limits of ∼200 km to global scales. We have developed a thorough numerical simulation using realistic simulated orbits and incorporating the effects of pointing error, diffraction, atmosphere and telescope design, to obtain estimates of the loss and background noise which a satellite-based system would experience. Combining with quantum optics simulations of sources and detection, we determine the length of secure key for QKD, as well as entanglement visibility and achievable distances for fundamental experiments. We analyse the performance of a low Earth orbit satellite for downlink and uplink scenarios of the quantum optical signals. We argue that the advantages of locating the quantum source on the ground justify a greater scientific interest in an uplink as compared to a downlink. An uplink with a ground transmitter of at least 25 cm diameter and a 30 cm receiver telescope on the satellite could be used to successfully perform QKD multiple times per week with either an entangled photon source or with a weak coherent pulse source, as well as perform long-distance Bell tests and quantum teleportation. Our model helps to resolve important design considerations such as operating wavelength, type and specifications of sources and detectors, telescope designs, specific orbits and ground station locations, in view of anticipated overall system performance.
Information security is a growing concern in our modern world, where almost everything can be done online. To protect security, classical encryption protocols, such as RSA, are used. These encryption protocols are almost always based on mathematical problems that are computational difficult. Therefore, the security is only valid under limited computational resources, and therefore do not provide provable security. An exception is the one-time pad protocol, which is provably secure but requires an existing shared key that is as long as the information it must encrypt. The exchange of such a key can be challenging, often requiring the two partied to physically meet to exchange the key or the use a trusted courier to physically carry the key on a hard drive. \n \nQuantum key distribution (QKD) offers a solution by utilizing quantum mechanics to grow a secure cryptographic key shared between two distant parties. The quantum nature of the particles used in the exchange ensure that any eavesdropper would leave signs of their presence, allowing the users to precisely quantify the security of the key that is produced. This allows secure generation of a random key which can be used for the one-time pad encryption protocol which, unlike most encryption protocols, does not rely on computational assumptions, and is secure regardless of available computational power. \n \nCurrent implementations of QKD are limited to a single link distance of ≈200 km, preventing implementation of QKD on a global scale, or even between distant cities within a single country, without some additional techniques. One promising solution is the use of orbiting low Earth orbit satellite platforms as trusted nodes in a quantum communications network. The main purpose of this work has been to show the viability of this solution, trough theoretical performance modeling and experimental demonstrations. \n \nThorough numerical simulations have been developed to evaluate the performance and challenges in implementing QKD using a low Earth orbit satellite platform and determine an optimal approach to its implementation. The simulations include a realistic satellite orbit analysis, all expected loss mechanisms, estimates of background contributions and realistic simulation of quantum optical processes. This work addresses the questions of optimal wavelength and beam waist, the effect of the telescope design and pointing error, and the impact of detector degradation due to exposure to radiation in the space environment. The simulation is used to determine the length of secure key for QKD and the performance of fundamental quantum optics experiments such as Bell tests and quantum teleportation. We compare the advantages and disadvantages of uplink and dowlink scenarios, and show that an uplink, despite having reduced performance compared to a downlink, offers more scientific freedom, by allowing changes to the quantum source, and benefits from a simpler satellite design with reduced pointing requirement. This work provides a theoretical foundation for ongoing design and development of quantum systems for satellite implementation of QKD. \n \nIn addition to theoretical analyses, experiments where developed and conducted corresponding to the challenges likely to be faced by a satellite uplink. Two main challenges where identified and experimentally overcome. First, the difficulty of operating in the high loss regime of a satellite uplink, which typically experience losses in the range of 40-50 dB when above 40° of elevation from the horizon, an elevation only reached by half of the satellite passes. This challenge was addressed by demonstrating full QKD protocols at losses exceeding 50 dB. Also, the ability to perform full QKD during the short duration of a satellite pass was shown by replicating the varying loss of archetypal passes and successfully extracting secure keys. Secondly, the difficulty of accurately tracking and pointing to a receiver platform traveling at the high angular speed of a satellite was overcome by successfully exchange quantum signals to a truck traveling at angular speeds exceeding that of a low Earth orbit satellite. This required the design, construction, and implementation of a transmitter system and a quantum receiver system capable of active pointing using a custom pointing system. \n \nAn additional experiment was also performed where we experimentally investigate the feasibility of performing QKD using light scattered by a diffusive screen. A system capable of doing so could be used to create QKD hot-spots, where a QKD source could be aimed at a diffusive screen to allow multiple users to simultaneously exchange secure keys with the source without the need for high precision pointing. \n \nThrough these analyses and experimental demonstrations we evaluated the performance of QKD using a satellite and have shown its technological readiness to be implemented. Implementation of such a system would allow QKD to be performed on a global scale, enabling communications security that is not based on computational assumptions without the need to physically transport the key from one party to the other.
Quantum Key Distribution (QKD) has emerged as a fundamentally secure approach to communication in the era of quantum computing, offering protection against threats posed to classical cryptographic schemes such as RSA and Diffie-Hellman. This thesis presents a comprehensive performance analysis of satellite-based QKD protocols, focusing on both prepare-and-measure and entanglement-based schemes under realistic atmospheric and operational conditions. The study begins by introducing the theoretical foundations of quantum communication, including qubits, entanglement, and quantum entropy, and motivates the need for satellite-based QKD to overcome the distance limitations of fiber-based systems. Subsequently, the thesis evaluates four prominent QKD protocols-BB84, B92, BBM92, and E91-using a circular beam propagation model that incorporates atmospheric effects such as diffraction, turbulence, attenuation, and pointing errors, along with environmental noise contributions for uplink and downlink. Comparative numerical simulations reveal that protocol performance is strongly influenced by channel asymmetries, beam propagation characteristics, and noise, providing guidance on optimal protocol selection for low Earth orbit (LEO) satellite links. The research further investigates high-dimensional (HD) QKD protocols, specifically HD-BB84 and HD-Extended B92, using the elliptic-beam approximation to account for turbulence-induced distortions for both uplink and downlink. Simulations under vary ing system dimensions, weather conditions, and zenith angles demonstrate that HD-BB84 achieves higher key rates, superior noise tolerance, and more favorable probability distributions of the key rate compared to HD-Extended B92, highlighting the advantages of high-dimensional encoding for robust satellite-based QKD.
The analysis of link loss is one of the first and most important steps for the design of an optical communication system. This is particularly vital in quantum communications systems where the information is encoded at the single photon level, and the quantum optical signal cannot be amplified deterministically. In most cases, the desired quantum bit error rate and secure key rate can only be achieved by minimizing the link attenuation and the background noise level in the quantum communication system. In optical fiber implementations, the transmission distance is inherently limited by the loss per unit distance of the optical fiber, meaning fully global coverage is not readily achievable with the current optical fiber backbone networks. To overcome the terrestrial link limitations for quantum communications, long-distance free-space links, using low-Earth orbit satellites are being proposed and implemented. Due to the optical link length, the main contributors to link losses are geometric loss, atmospheric attenuation, and losses associated with pointing and tracking errors. The total link loss is dominated by the geometric loss, therefore, it is important to analyze its importance in relation to the quantum communications link. In this paper, the loss of a low-Earth orbit satellite-to-ground (downlink) quantum communication link is analyzed. The analysis includes losses associate with the channel (geometric and atmospheric) and the receiver system. This paper also compares the data of a known satellite quantum communications mission, highlighting trade-offs in investment for satellite platform and optical ground station. Based on the link loss analysis, decoy state BB84 and E91 protocols were chosen to demonstrate the link performance under an example scenario. The work contributes to the design of the optical ground station for a CubeSat mission.
This study aims at extending the concept of QKD to the optical free-space quantum communications domain. Moreover, the feasibility of a global satellite based quantum key distribution, in the low earth orbit (LEO) is investigated. Inline with this, three different link scenarios, including ground-to-ground, ground-to-satellite and satellite-to-satellite are considered. Moreover, system performance is analysed to show the sensitivity of the link to effect such as atmospheric attenuation, turbulence and detector noise. Generally, these factors are assumed to impair system performance by causing an overall increase in the systems quantum bit error rate (QBER), let alone limiting the communication bit rate and transmission distance of the link. The obtained results reveal the feasibility of satellite-based quantum key distribution in the LEO platform, a concept which could be potentially extended to geostationary earth orbit (GEO).
Establishing information-theoretically secure communication between arbitrary locations on Earth is a central goal of quantum networking, for which space-based distribution of quantum entanglement is a leading approach. Modelling and analysis of the performance of satellite entanglement pair distribution is therefore important for the architecture and design of future constellations and space systems. However, entanglement-based quantum key distribution in the absence of quantum repeaters, is especially prone to finite-key effects due to low coincident count rates compared to trusted-node single-path links. Quantifying these effects under realistic orbital conditions is therefore essential for the design of practical satellite quantum communication systems. Here, we develop a high-fidelity model of pair distribution from a low-Earth-orbit satellite that captures orbital dynamics, elevation-dependent loss, background noise, and extraneous detector effects. Integrating this with a finite-key security framework for the BBM92 protocol, we optimise the secret key length across different overpass geometries, orbital altitudes, and optical ground station (OGS) separations. We show that secure key generation remains feasible in early clear daytime conditions using baseline parameters representative of current technologies, while showing that modest system improvements could further extend daytime operation. Finally, we evaluate the annual secret key performance for representative intercity-scale OGS pairings. Our results provide quantitative performance bounds and practical design guidelines for near-term entanglement-based satellite QKD, enabling informed trade-offs between satellite payload complexity, ground infrastructure, and achievable secure key throughput.
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.