In this paper, we present a novel analysis of the feasibility of quantum key distribution between a LEO satellite and a ground station. First of all, we study signal propagation through a turbulent atmosphere for uplinks and downlinks, discussing the contribution of beam spreading and beam wandering. Then we introduce a model for the background noise of the channel during night-time and day-time, calculating the signal-to-noise ratio for different configurations. We also discuss the expected error-rate due to imperfect polarization compensation in the channel. Finally, we calculate the expected key generation rate of a secure key for different configurations (uplink, downlink) and for different protocols (BB84 with and without decoy states, entanglement-based Ekert91 protocol).
In this paper, we present a novel analysis of the feasibility of quantum key distribution between a LEO satellite and a ground station. First of all, we study signal propagation through a turbulent atmosphere for uplinks and downlinks, discussing the contribution of beam spreading and beam wandering. Then we introduce a model for the background noise of the channel during night-time and day-time, calculating the signal-to-noise ratio for different configurations. We also discuss the expected error-rate due to imperfect polarization compensation in the channel. Finally, we calculate the expected key generation rate of a secure key for different configurations (uplink, downlink) and for different protocols (BB84 with and without decoy states, entanglement-based Ekert91 protocol).
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.
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.
Satellite-based free-space quantum key distribution (QKD) provides a practical framework for achieving secure global communication beyond the limitations of optical fibers. In this work, the quantum bit error rate (QBER) and secure key rate of four representative protocols-BB84, B92, BBM92, and E91 are investigated over low earth orbit (LEO) links in both uplink and downlink configurations. The optical link is modeled using a Gaussian beam formalism, incorporating the effects of diffraction, pointing errors, atmospheric turbulence, and background noise contributions. The protocols are examined under day and night-time operating conditions, and their dependence on the zenith angle is analyzed. The findings show that downlink links generally exhibit lower QBER and higher secure key rates than uplinks, and among prepare-and-measure schemes, BB84 consistently outperforms B92, while in entanglement-based approaches, BBM92 achieves higher key rates than E91.
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.
In this work, we model and simulate the performance of a quantum key distribution (QKD) downlink from a low earth orbit (LEO) small satellite to an optical ground station (OGS), as integral part of the concept and preliminary design phase of the QuantSat-PT mission. By modelling and simulating in detail downlink transmission channel effects, with emphasis on turbulent and atmospheric losses for BB84 and E91 protocols, we find a consistent set of values for the performance envelope that resolves ambiguities of mission experimental data that had been reported previous to this work. We obtain for the 4-state BB84 protocol a sifted key rate and Quantum Bit Error Rate (QBER) of 32.1 kbit/s and 4%, respectively, for zenith at 750 km orbit. For the E91 protocol the Clauser, Horne, Shimony and Holt (CHSH) test was performed resulting in a correlation factor of S∈[−2.63±0.02,−1.91±0.03] for the mission. The consistency of these results with the state of the art simulators and its relevance on experimental satellite based QKD is discussed.
Experimental realization of entanglement-based quantum key distribution (QKD) in a daylight-uplink (ground-to-satellite) communication channel is highly challenging due to the very low signal-to-noise ratio (SNR) of this configuration, and it has yet to be successfully implemented to date. While current research efforts focus on weak coherent pulse (WCP) sources for daylight QKD, we present a proof-of-principle laboratory demonstration emulating daylight-uplink QKD in a free-space-fiber hybrid configuration using polarization-entangled photon pairs generated via a spontaneous parametric down-conversion (SPDC) process. The emulated uplink channel attenuation reaches up to 50 dB, equivalent to low-earth-orbit (LEO) distances. Furthermore, the detected noise level (up to a few MHz) in the receiver telescope is representative of daylight conditions. The enhancement of SNR is achieved by implementing rigorous filtering on spatial, spectral, and temporal modes. Our ultra-bright source of entangled photons, characterized by a narrow spectral bandwidth of 0.54 nm FWHM, ensures minimal signal attenuation. Using a theoretical model, we estimate the conditional feasibility of entanglement-based QKD for daylight-uplink LEO satellite links up to approximately 400 km, subject to the experimentally motivated system assumptions adopted in this work, including efficient single-mode fiber (SMF) coupling performance.