Quantum key distribution (QKD) has gained a lot of attention over the past few years, but the implementation of quantum security applications is still challenging to accomplish with the current technology. Towards a global-scale quantum-secured network, satellite communications seem to be a promising candidate to successfully support the quantum communication infrastructure (QCI) by delivering quantum keys to optical ground terminals. In this research, we examined the feasibility of satellite-to-ground QKD under daylight and nighttime conditions using the decoy-state BB84 QKD protocol. We evaluated its performance on a hypothetical constellation with 10 satellites in sun-synchronous Low Earth Orbit (LEO) that are assumed to communicate over a period of one year with three optical ground stations (OGSs) located in Greece. By taking into account the atmospheric effects of turbulence as well as the background solar radiance, we showed that positive normalized secure key rates (SKRs) up to $3.9\times {10}^{-4}$ (bps/pulse) can be obtained, which implies that satellite-to-ground QKD can be feasible for various conditions, under realistic assumptions in an existing infrastructure.

We present a real-world Gigabit Passive Optical Network (GPON)-based Fiber to the Home (FTTH) network serving up to 32 users, installed in COSMOTE laboratories in Athens. The deployment relies on the use of fiber optics and GPON telecom equipment which are identical to those found in the currently deployed FTTH infrastructure. We describe real-life network designs that correspond to specific installations considered in Athens urban metropolitan area. We propose a practical upstream Quantum Access Network (QAN) over the installed FTTH network, relied on a centralized quantum receiver (Bob) station located at the operator's Central Office, which exploits the Time-Division Multiplexing (TDM) framing technique to receive single-photons from multiple independent quantum transmitters (Alice) located at users premises. This integration scheme is characterized by performing a series of noise measurements at single-photon level to evaluate the potential of the coexistence of classical/quantum links on the same fiber cores. The noise count rates are found to follow the Raman noise profile for specified values of effective Raman cross-sections of downstream and upstream GPON data channels. Subsequently, the experimental results are exploited to evaluate the performance of the widely-used Coherent One Way (COW) Quantum Key Distribution (QKD) protocol. The presented simulation results for the quantum layer suggest the feasibility of our integration scheme for short feeder fiber segments. Finally, the dependance of the Secure Key Rate (SKR) on the network capacity is also discussed.

Satellite-based QKD is currently being developed to revolutionize global cryptographic key exchange by facilitating secure communication among remote parties at a global scale. By overcoming the exponential loss of fiber transmission, satellite-to-Earth communication can seamlessly interconnect vast distances as the link budget of such links is sufficient to support QKD links. In terms of this direction, DV-QKD implementations seems to be technologically ahead since key exchange has been experimentally demonstrated to perform much more efficiently by providing key rates that are orders of magnitude higher compared to entanglement-based key exchange. However, the specific requirements to support effectively functional DV-QKD satellite-to-ground links are yet to be defined. This work attempts to define the satellite and ground segment system requirements needed in order to achieve functional QKD service for various satellites orbits (LEO, MEO, and GEO). Finite key size effects are being considered to determine the minimum block sizes that are required for secure key generation between a satellite node and a ground terminal for a single satellite pass. The atmospheric link channel is modeled with consideration of the most important degradation effects such as turbulence and atmospheric and pointing loss. Critical Tx and Rx system parameters, such as the source’s intrinsic Quantum Bit Error Rate (iQBER), the Rx telescope aperture size, and detection efficiency, were investigated in order to define the minimum requirements to establish an operation satellite-to-ground QKD link under specific assumptions. The performance of each downlink scenario was evaluated for the wavelength of 1550 nm in terms of link availability, link budget, and in the distilling of secure key volumes over time. Finally, the feasibility and requirements for distributing the collected space photons via terrestrial telecom fibers was also studied and discussed, leading to the proposal of a more futuristic WDM-enabled satellite QKD architecture. This comprehensive analysis aims to contribute to the advancement and implementation of effective satellite-based QKD systems, which can further exploit the ground fiber segment to realize converged space/terrestrial QKD networks.

This paper presents an in-depth satellite-to-ground Quantum Key Distribution (QKD) feasibility analysis between a Low Earth Orbit (LEO) satellite and a 0.28 m small-sized Optical Ground Stations (OGSs) located in urban areas, over nighttime. Both Prepare & Measure (P&M) and Entanglement (ENT)- based QKD protocols are employed and compared in terms of distilled key bits. The performance of various detection setups and environmental conditions including different wavelengths (1550 nm and 810 nm) as well as different types of detectors (Avalanche and Superconducting Nanowires) is simulated. The results are compared to those obtained when OGSs with much larger receiver apertures are placed in rural environment. Our analysis delivers a framework examining the parameters affecting the QKD satellite downlink, and also provides a thorough theoretical framework for the employment of small-sized OGSs in urban environments.

We propose two configurations for the coexistence of sensing and communication signals, exploiting Free Space Optical (FSO) links to transmit commercial 6G traffic and remotely interrogate sensors distributed along distances of 100 m. The sensors are fabricated on a Silicon Photonics (SiPh) platform, to achieve miniaturization and highly compact form of the required features. The first scheme consists of couplers and circulators, while the second utilizes polarization handling elements to multiplex both signals, while a 100-m outdoor Free Space Optical link is utilized for both schemes. The configurations exhibit high stability, with uninterrupted transmission of traffic along the duration of the experiments, while the sensors’ performance is evaluated, with their discrepancies measured below 2%.

The authors present a novel approach to Quantum Key Distribution (QKD) research, emphasising cost‐effectiveness and practicality using a single photon polarisation‐encoded system employing mainly commercial off‐the‐shelf components. This study diverges from previous high‐cost, high‐end setups by exploring the viability of QKD in more accessible and realistic settings. Our approach focuses on practical measurements of the signal‐to‐noise ratio by analysing polarisation‐encoded photonic qubits over various transmission scenarios. The authors introduce a simplified evaluation method that incorporates experimental measurements, such as noise sources and losses, into a semi‐empirical theoretical framework. This framework simulates the standard DS‐BB84 protocol to estimate Secure Key Rates (SKRs), offering an alternative approach on the evaluation of the practical implementation of QKD. Specifically, the authors examine the feasibility of QKD over a 2.2 km intra‐campus fibre link in coexistence scenarios, identifying optimal Wavelength‐Division Multiplexing allocations to minimise Raman noise, achieving an expected SKR of up to 300 bps. Additionally, the authors’ study extends to 40 m indoor and 100 m outdoor Free‐Space Optical (FSO) links using low‐cost components, where the authors recorded Quantum Bit Error Rate (QBER) values below 3.2%, allowing for possible SKRs up to 600 bps even in daylight operation. The converged fibre/FSO scenario demonstrated robust performance, with QBER values below 3.7% and an expected SKR of over 200 bps. Our research bridges the gap between high‐end and economical QKD solutions, providing valuable insights into the feasibility of QKD in everyday scenarios, especially within metropolitan fibre based and FSO links. By leveraging cost‐effective components and a simplified single photon exchange setup, the authors work paves the way for the effortless characterisation of deployed infrastructure, highlighting its potential in diverse settings and its accessibility for widespread implementation.