Satellite-to-aircraft quantum key distribution performance estimation with boundary layer effects

Advancements in Non-Terrestrial Network (NTN) technology facilitate ubiquitous network access for users, whereas satellite-based Quantum Key Distribution (QKD) offers a viable solution for long-distance quantum key exchange in scenarios lacking terrestrial network infrastructure. This study explores the feasibility and practical utility of integrating NTN technology with satellite-based QKD and proposes a novel quantum-enhanced security framework for next-generation space–terrestrial networks. We have developed and deployed the first-of-its-kind 5G-enabled (fifth generation mobile communication) NTN prototype system leveraging satellite-based QKD key encryption. This system comprises a quantum satellite system, a communication satellite system, a 5G network infrastructure, and end-to-end encryption/decryption modules, aiming to validate the feasibility and usability of the proposed quantum-encrypted NTN security framework. Comprehensive tests and performance evaluations were carried out on the testbed constructed based on this prototype system, which collected critical Quality of Experience (QoE) metrics, including Round-Trip Time (RTT) and jitter, during user-plane ping measurements. Experimental results demonstrate that the integration of quantum encryption capabilities incurs an RTT overhead of 5 ms (0.75%), a necessary trade-off for systems incorporating supplementary quantum-encrypted transmission. Concurrently, the deployment of Virtual Private Network (VPN) infrastructure mitigates network jitter by 50%. These results hold critical theoretical and practical implications for the development of next-generation NTN security frameworks enabled by satellite-based QKD.

With the substantial progress of terrestrial fiber-based quantum networks and satellite-based quantum nodes, airborne quantum key distribution (QKD) is now becoming a flexible bond between terrestrial fiber and satellite, which is an efficient solution to establish a mobile, on-demand and real-time coverage quantum network. However, the boundary layer (BL) normally adhere to the surface of the aircraft when its speed is higher than Mach 0.3. The BL would change local refractive index and energy flux density drastically, thus lowering the coupling efficiency and infidelity of quantum states. Here, we investigate the airborne QKD performance with the BL effects, which has been rarely mentioned in existing research. Through simulations and modeling, we present the relation between divergence angle and secure key rate. With the increase of flight speed v, relative flight altitude h and the shortest projection distance d, the key-rate curve is obviously jitter, and the QKD performance is continuously reduced. Simulation results show that, under several typical circumstances, the BL will affect the communicating distance, the transmission efficiency and the generation of secure key rate in varying degrees, which is helpful for future airborne experimental designs.

Recent advances in pointing and tracking capabilities of small satellite platforms have enabled adoption of capabilities such as high-resolution Earth Observation (EO), inter-satellite laser communications and, more recently, quantum communications. Quantum communications requires unusually narrow optical beams and tight pointing performance (on the order of ten microradians) to close an inherently brightness-limited quantum link. This limit is due to quantum communication protocols such as quantum key distribution and teleportation requiring individual quantum states to be transmitted with photon number restrictions. We examine an opportunity to combine quantum communications with laser communications in sharing an optical link. We discuss a combined quantum and laser communication terminal capable of performing space-to-ground entanglement-distribution and high data rate communications on a 12U CubeSat with a 95mm beam expander and an 60 cm aperture optical ground telescope. Photon pairs produced by the quantum terminal are entangled in polarization so the polarization must be maintained throughout the optical link. We discuss active and passive compensation methods in space and polarization reference frame correction using a polarized reference beacon at the ground station. The combined quantum and laser communication terminal approach enables secure communications over an optical channel with rates of 100 Mbps and sub-nanosecond time transfer.

Membrane mass transport by nanofiltration: Coupled effect of the polarization and membrane layers

As an important support for quantum communication, quantum key distribution (QKD) networks have achieved a relatively mature level of development, and they face higher requirements for multi-user end-to-end networking capabilities. Thus, QKD networks need an effective management plane to control and coordinate with the QKD resources. As a promising technology, software defined networking (SDN) can separate the control and management of QKD networks from the actual forwarding of the quantum keys. This paper systematically introduces QKD networks enabled by SDN, by elaborating on its overall architecture, related interfaces, and protocols. Then, three-use cases are provided as important paradigms with their corresponding schemes and simulation performances.

The usage of QKD protocols through the current terrestrial infrastructure is limited to a hundred kilometers due to attenuation problems with fiber. This implies that the satellite will be necessary for the transmission of quantum keys in order to avoid these terrestrial limitations. Therefore, the current debate does not lie in the need for the satellite but rather in what type of infrastructure will be best suited, and specifically the usage of GEO vs LEO satellites. This analysis is not trivial and should not consider only the link budgets and the physical layer as it has been done up to date. It should also consider even more important factors, as the service KPIs that the end user will request such as key rate, availability, security of the keys and their maintenance, etc., as well as the possible communications architectures to provide the service. Therefore, the analysis should introduce the market oriented component, and provide the optimal solutions based on the scenarios and requirements requested by the customer. In this frame, and under ESA ARTES funding, a Spanish consortium led by Hispasat is defining an operational GEO QKD mission that shall fly as hosted payload in 2025. The quantum communications payload proposed in this mission represents a step beyond the state of the art since it is the first implementation of a quantum key distribution channel from geostationary orbit. This new service will involve the implementation of a complete communications system not currently available that will require the development of new technological elements and the adaptation for their operation in space of technologies such as photon sources, telescopes for optical communications compatible with quantum state transmission or processors for the distillation of quantum keys, as well as a whole new ground segment to control and manage the new quantum key distribution service and its integration into a future European secure quantum communications network. Details of the challenges and approach of this mission shall be presented

Covert communication methods are used in the communication with high security level. When it turns to quantum communication, covertness is also an important concern which is firstly discussed by Arrazola and Scarani (Phys Rev Lett, 117:250503, 2016). To make quantum key distribution (QKD) protocol more suitable in the scenarios need high security, we propose a covert QKD protocol with decoy-state method in this paper. The secure key rate and covertness of the covert decoy-state QKD are proved. We compare the performance of the covert decoy-state QKD with those of the original decoy-state QKD and covert QKD without decoy states in numerical simulations. It shows that (1) the covert decoy-state QKD can have a performance comparable to the original decoy-state QKD protocol besides its covertness; (2) the covert decoy-state QKD can have a considerable improvement of transmission distance over covert QKD without decoy states at the cost of a small change of covertness parameter. Furthermore, the statistical fluctuation due to the finite length of data is also taken into account based on the Gaussian analysis method.

The relative permeability is a critical property to predict field performance by reservoir numerical simulation. However, determining the relative permeability curve by experiments is time consuming and difficult for tight oil reservoirs due to the extremely low permeability. Therefore, many studies focus on the analytical solution to determine the relative permeability curve. However, no exiting solutions take the effect of boundary layer, Jamin and wettability into account. This results in overestimated results because boundary layer is even up to 60 % of the throat and throat to pore ratio reaches up to 400 in tight oil reservoirs. Therefore, an improved analytical solution is desirable to solve these two problems. In this paper, based on the collected experimental results, the Purcell’s solution is modified and an improved solution considering the effect of boundary layer, Jamin and wettability is developed. Compared to the results of the relative permeability experiments, the improved solution is more accurate and reliable. Because the throat distribution of the tight oil reservoir in Daqing Field is narrower than that in Changqing Field, the productivity of the tight oil reservoir in Daqing Field is less than that in Changqing Field at first. However, the breakthrough time of the tight oil reservoir in Daqing Field is longer than that in Changqing Field. This improved solution can only be applied to tight oil reservoirs, because boundary layer and Jamin have little effect on the flow behavior in conventional reservoirs.

In recent years, quantum-based techniques have attracted significant research attention because of its unique advantages on satellite communications, especially for security problem. Security guarantee is one of the most important requirements in QoS-driven 3D satellite networks. Quantum key distribution (QKD) is a methodology for generating and distributing random encryption keys using the principles of quantum physics, which enables two distant communications parties to securely communicate in a way that cannot be eavesdropped on without being detected. Although the QKD method can ensure the absolute security transmission over 3D satellite networks, it imposes many new implementation challenges due to the various limitations on quantum communication over long distances via 3D free space, including quantum channel attenuation, photon-state disruption and vulnerability to noise/interference, laser-beam widening, and constrained security-key generation rate. These problems get even more challenging when QoS provisioning is required for the applications over the 3D satellite networks. To overcome the aforementioned difficulties, we propose the framework to efficiently implement the QKD for security guarantees over quantum-repeater-based QoS-driven 3D satellite networks. First, we develop the quantum-repeater-based QKD satellite network architecture. Then, we design the quantum repeater including the purification scheduling algorithm and the optimal QoS-based repeating-router selection scheme in quantum-repeater-based QKD satellite networks. Finally, the obtained simulations evaluation validate and evaluate our proposed algorithms and schemes.

In this paper we show evidence of aero-optic effects on the measured beacon beam as the gimbal angle of a nosemounted turret changes from 0 to 90 degrees and greater with respect to the line of flight. Data from the beacon beam was collected with a new technology 3-aperture scintillometer over a 82km to 104km air-to-ground downlink during field testing of the ORCA system in Nevada in May 2009. In this paper we present data analysis on the impact of an aero-optic boundary layer on a laser link between an aircraft and a ground-based stationary node. Particularly we look at the impact of an aero-optic boundary layer on the mean, variance, scintillation, probability density function (PDF), power spectral density (PSD), and fading of the received irradiance. We find that the most compelling argument for the presence of strong aero-optic effects comes from calculating the PSD of the received beacon intensity. We also find the cumulative effect of the aero-optic boundary layer differs depending on the transmitted beam parameters, i.e. collimated or divergent.

<sec>Quantum key distribution (QKD) is a pivotal technology in the field of secure communication by using the principles of quantum mechanics to implement theoretically unbreakable encryption. However, QKD faces significant challenges in achieving large-scale deployment. The primary hurdle lies in the scarcity of quantum resources, especially entangled photon pairs, which are fundamental to protocols such as Ekert91. In traditional QKD implementations, only a small potion of the generated entanglement pairs contribute to generating the original key, resulting in lower efficiency and resource waste. Resolving this limitation is crucial to the advancement and scalability of QKD networks.</sec><sec>This paper introduces an innovative approach to QKD by integrating the multiscale entanglement renormalization ansatz (MERA), a technique which is originally developed for many-body quantum systems. By utilizing MERA’s hierarchical structure, the proposed method not only improves the efficiency of entanglement distribution but also reduces the consumption of quantum resources. Specifically, MERA compresses many-body quantum states into lower-dimensional representations, allowing for the transmission and storage of entanglement in a more efficient manner. This compression significantly reduces the number of qubits required, optimizing both entanglement utilization and storage capacity in quantum networks.</sec><sec>To evaluate the performance of this method, we conduct simulations under standardized conditions. In the simulation, a 1024-bit encryption request, an 8% error rate, an average path length of 4 hops in the quantum network, and a 95% success rate for link entanglement generation and entanglement swapping operations are assumed. These parameters reflect the real physical conditions in contemporary QKD networks. The results demonstrate that compared with traditional QKD protocols, the MERA-based approach saves 124151 entangled pairs, which is impressive. This significant reduction in resource consumption indicates the potential application of MERA in improving the efficiency of QKD systems without sacrificing security. Importantly, the security of the key exchange process remains intact, for the method inherently adheres to the principles of quantum mechanics, particularly the no-cloning theorem and the use of randomness in the decompression layer.</sec><sec>Some conclusions can be drawn below. The MERA not only enhances the scalability of QKD by optimizing quantum resource allocation, but also maintains the necessary security guarantees for practical cryptographic applications. By integrating MERA into existing QKD frameworks, we can significantly reduce the resource overhead and make large-scale, secure quantum communication more feasible. These findings contribute a new dimension to the field of quantum cryptography, indicating that advanced quantum many-body techniques like MERA have the potential to unlock the full potential of quantum networks in real world.</sec>

Recent advancements in reactor designs could offer new revolutionary capabilities, including remote monitoring, increased flexibility, and reduced operation and maintenance costs. Embracing new digital technologies would allow for operational concepts such as semiautonomous or near-autonomous control, and two-way communications for real-time integration with the upcoming smart electric grid. However, such continuous data transmission from and toward a reactor site could potentially introduce new challenges and vulnerabilities, necessitating the prioritization of cybersecurity. Conventional information technology–based encryption schemes, which rely mostly on computational complexity, have been shown to be vulnerable to cyberattacks. With the advent of quantum computing, practically any asymmetric encryption could be potentially compromised. For example, it has been shown that a RSA-2048 bit key could be broken in 8 h. To address this challenge, we explore the feasibility of quantum key distribution (QKD) to secure communications. QKD is a physical layer security scheme relying on the laws of quantum mechanics instead of mathematical complexity. QKD promises not only unconditional security but also detection of potential intrusion, as it allows the communication parties to become aware of eavesdropping. To test the proposed hypothesis, a novel simulation tool (NuQKD) was developed to allow for real-time simulation of the BB84 QKD protocol between two remote terminals. A reference scenario is proposed, generic enough to cover various internal and external communication links to a reactor site. Using NuQKD, the internal and external data links were modeled, and receiver operating characteristic curves were calculated for various QKD configurations. A performance analysis was conducted, demonstrating that QKD can provide adequate secret key rates with low false alarms and has the potential of addressing the nuclear industry’s high standards of confidentiality for distance lengths up to 75 km of fiberoptics. Using a conservative estimation, QKD can provide up to 21.5 kbps of secret key rate for a distance of 1 km and 14.4 kbps at 10 km. The target secret key rates for the corresponding links were estimated at 16 kbps and 80 bps, respectively, based on the analysis of real data from a PUR-1 fully digital reactor. Consequently, QKD is shown to be compatible with existing and future point-to-point reactor communication architectures. These results motivate further study of quantum communications for nuclear reactors.

The current efficiency η of a planar membrane bearing nonuniformly distributed fixed charges is analyzed theoretically. In particular, the effect of the convective boundary layer near the outer boundary of the membrane is discussed. Two types of fixed charge distribution are considered: a class of linear distribution and a class of exponential distribution. We show that, if the flow velocity in the membrane phase is small, increasing the inhomogeneity of fixed charge distribution is capable of raising η. However, if it is appreciable, the reverse is true. If a boundary is present, η decreases with the increase in current density; the reverse is true if it is absent. The former is due to the fact that the degree of concentration polarization in the boundary layer increases with current density. Although the existence of a boundary layer has the effect of reducing η, the convective flow of fluid perpendicular to membrane has a positive effect on η. If a boundary layer is absent and the fixed charges are distributed homogeneously, η is independent of current density I; if the boundary layer is present, η will vary with I.

Boundary layer effects in He and Ar driven disk MHD generators

In this study we tested the hypothesis that metal uptake by unicellular algae may be affected by changes in metal speciation in the boundary layer surrounding the algal cells. The freshwater alga Chlamydomonas reinhardtii was preacclimated to different N nutrition regimes; changes in N nutrition are known to change the nature of extracellular metabolites (e.g., reactive oxygen species "ROS", and OH-) and thus boundary layer chemical conditions. Specifically, at a constant bulk free Cd2+ concentration, Cd uptake by N-starved algae in cysteine-buffered solution was significantly higher than that in NTA-buffered solution. This enhancement was likely due to an increase of the free Cd2+ concentration in the boundary layer, resulting from localized cysteine oxidation by ROS released from these algae. On the other hand, Cd uptake was markedly lower when the free Cd2+ concentration near cell surface decreased as a result of an increase in the boundary layer pH of nitrate-acclimated algae or enhanced localized metal complexation. The results imply that redox, acid-base and metal complexation processes in the boundary layer differ from those in bulk water, even under chemically stable bulk conditions, and the boundary layer effect may well be of significance to phytoplankton acquisition of other trace metals.