Abstract Quantum key distribution (QKD) is a method for securely exchanging keys based on quantum mechanics. QKD has been thoroughly examined for terrestrial and near-Earth satellite applications; however, forthcoming deep-space links are anticipated to experience considerable relativistic effects. Quantum states can be altered in these circumstances by relative motion between terminals and by gravitational effects, which may affect entan-glement, measurement outcomes, and overall security. The effectiveness of entanglement-based quantum key distribution (QKD) in relativistic motion is investigated in this work, with particular attention to deep-space optical communication. The quantum channel model includes Wigner rotations due to Lorentz transformations, polarization mixing due to Doppler shift, and redshift due to gravity. This method gives us new ways to analyze the quantum bit error rate (QBER), quantum entanglement fidelity, Bell inequality violation , and secret key rate. Our numerical simulations show that when the relative velocity becomes high enough, the polarization correlations start to break down significantly. This leads to more errors in the quantum bits, making it harder to generate a secure secret key. Even though quantum nonlocality persists at these speeds, relativistic effects make it much harder to actually extract a secure key.
Abstract Quantum key distribution (QKD) is a method for securely exchanging keys based on quantum mechanics. QKD has been thoroughly examined for terrestrial and near-Earth satellite applications; however, forthcoming deep-space links are anticipated to experience considerable relativistic effects. Quantum states can be altered in these circumstances by relative motion between terminals and by gravitational effects, which may affect entan-glement, measurement outcomes, and overall security. The effectiveness of entanglement-based quantum key distribution (QKD) in relativistic motion is investigated in this work, with particular attention to deep-space optical communication. The quantum channel model includes Wigner rotations due to Lorentz transformations, polarization mixing due to Doppler shift, and redshift due to gravity. This method gives us new ways to analyze the quantum bit error rate (QBER), quantum entanglement fidelity, Bell inequality violation , and secret key rate. Our numerical simulations show that when the relative velocity becomes high enough, the polarization correlations start to break down significantly. This leads to more errors in the quantum bits, making it harder to generate a secure secret key. Even though quantum nonlocality persists at these speeds, relativistic effects make it much harder to actually extract a secure key.
There are substantial problems in sending quantum information safely over free-space channels because of air turbulence, diffraction, beam divergence, and noise. This study offers an extensive performance evaluation of quantum computing-based security methods utilised in free-space communication systems, specifically emphasising Quantum Secure Direct Communication (QSDC) and Quantum Key Distribution (QKD) protocols. The purpose of this work is to examine the attainable secret-key generation rates, assess the robustness of quantum security protocols in practical free-space environments, and contrast their effectiveness in preventing eavesdropping and signal degradation. This research employs quantum optics, turbulence theory, and quantum information theory to examine the performance constraints of continuous-variable (CV) coherent-state protocols amid fluctuating turbulence intensities. The suggested analytical approach simulates both fading and non-fading atmospheric channels to calculate essential rates and quantum bit error rates (QBER) for free-space quantum secure direct communication (QSDC) systems. Experimental findings and simulation outcomes indicate that free-space quantum channels can provide high-rate and long-distance secure communication when optimized using pilot-guided coherent-state protocols. The review shows how QSDC is better than regular QKD since it lets messages be sent directly without pre-shared keys. It also talks about how useful it may be for secure communication networks between satellites and between satellites and the earth. The paper closes by delineating the primary research deficiencies and suggesting avenues for improving protocol efficiency, error rectification, and turbulence mitigation in forthcoming global quantum communication systems.
Quantum key distribution (QKD) enables tap-proof exchange of cryptographic keys guaranteed by the very laws of physics. One of the last remaining roadblocks on the way towards widespread deployment of QKD is the high loss experienced during terrestrial distribution of photons, which limits the distance between the communicating parties. A viable solution to this problem is to avoid the terrestrial distribution of photons via optical fibers altogether and instead transmit them via satellite links, where the loss is dominated by diffraction instead of absorption and scattering. First dedicated satellite missions have demonstrated the feasibility of this approach, albeit with relatively low secure key rates. In order for QKD to become commercially viable, the design of future satellite missions must be focused on achieving higher key rates at lower system costs. Current satellite missions are already operating at almost optimal system parameters, which leaves little room for enhancing the key rates with currently deployed technology. Instead, fundamentally new techniques are required to drastically reduce the costs per secret bit shared between two distant parties. Entanglement-based protocols provide the highest level of security and offer several pathways for increasing the key rate by exploiting the underlying quantum correlations. In this contribution, we review the most relevant advances in entanglement-based QKD which are implementable over free-space links and thus enable distribution of secure keys from orbit. The development of satellite missions is notoriously lengthy. Possible candidates for a new generation of quantum payloads should therefore be scrutinized as early as possible in order to advance the development of quantum technologies for space applications.
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.
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.
The goal of cryptography is to encode data in such a way that no one can decode it unless they are the intended recipient. With the emergence of quantum computing, classical cryptography is not reliable enough to protect sensitive information. Nevertheless, there are alternatives. Quantum Key Distribution (QKD) is a method to secure communication based on the laws of quantum physics. Due to being impossible to measure the quantum state of a system without disturbing it, QKD provides a way to communicate between two parties while being, in theory, immune to eavesdropping. In this work, a free-space high-speed QKD setup is proposed, with the objective of implementing the polarization three-state one-decoy variant of the BB84 protocol. An assessment of the best type of laser diode to use for free-space QKD is made between several options. The system’s performance was assessed, incorporating a quantitative analysis of system losses and an estimation of the number of photons traversing the quantum channel. The system achieved a Quantum Bit Error Rate (QBER) of 1.58% for the Z basis and 1.46% for the X basis, for an average reception of two photons per pulse. Preliminary QBER measurements were also undertaken in the context of Quantum Keyless Private Communication protocols, focusing on the average number of photons per pulse.
Abstract: Terrestrial fiber-optic quantum networks are subject to exponential photon attenuation (≈ 0.2 dB/km at 1550 nm), constraining unrepeatered ground quantum key distribution (QKD) and entanglement distribution to distances below a few hundred kilometers. While quantum repeaters remain technologically nascent, intercontinental sovereign security mandates unhackable, physical-layer cryptographic connectivity across transoceanic baselines. Satellite-based quantum communications operating in Low Earth Orbit (LEO, altitude h ≈ 500–800 km) provide a transformative solution by exploiting near-vacuum propagation for the vast majority of the optical path, bypassing terrestrial exponential loss. However, high-velocity LEO satellite platforms (v ≈ 7.6 km/s) introduce severe relativistic and atmospheric distortions that rapidly destroy quantum coherence: (1) kinematic Doppler shifts (Δν/ν ≈ ± 2.5 × 10^-5) and general relativistic gravitational redshifts alter photon wavepackets and relative phase trajectories; (2) dynamic spacetime curvature induces Wigner phase rotations on polarization-entangled states; and (3) atmospheric turbulence creates beam wandering, wavefront aberration, and photon arrival time jitter across the troposphere (0–15 km). This monograph establishes the theoretical framework, mathematical formulation, and solid-state engineering architecture for the PHOENIX_ORBIT space-to-ground quantum communications network. We derive: (1) an exact relativistic transformation matrix incorporating Kerr-Schwarzschild spacetime metric perturbations and orbital kinematics, coupled to an ultra-fast electro-optic dynamic phase compensator operating with sub-picosecond latency; (2) an adaptive optics (AO) closed-loop architecture combining high-bandwidth tip-tilt mirrors and deformable piezo-actuators calibrated to the local Fried parameter r_0 and Greenwood frequency f_G; (3) an entanglement routing protocol linking space-borne spontaneous parametric down-conversion (SPDC) sources to ground-based PHOENIX_ROUTER quantum data centers; and (4) an asymptotic security proof verifying Clauser-Horne-Shimony-Holt (CHSH) Bell inequality violation (S ≥ 2√2 − ε) under non-inertial reference frame drift. Our architecture achieves a continuous secret key rate exceeding 1.2 Mbps per satellite pass with a quantum bit error rate QBER < 1.4%, providing an unassailable physical-layer foundation for global sovereign data sovereignty.