Abstract 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.
Abstract 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.
Current Free-Space Optical Quantum Key Distribution (FSO-QKD) systems are mainly focusing on the development of critical components needed towards the realization of satellite-based QKD, relying on bulky optical telescopes equipped with expensive optical systems. The cost and complexity of these systems hinder the establishment of a practical FSO-QKD infrastructure deployed as the wireless last-mile segment of Quantum Communication Infrastructure for applications in urban environments. In this work, we experimentally demonstrate successful transmission of polarization encoded qubits over 0.5 km free space metropolitan link with enhanced noise suppression for both nighttime and daylight operation. Βy deploying two low-cost, Single Mode Fiber-coupled, optical terminals along with a prototype quantum sender station operating in C-band, Quantum Bit Error Rate (QBER) values below 3.5% were achieved, with expected Secure Key Rates up to 1 kbps during nighttime. The FSO link was integrated with existing in-building fiber infrastructure, demonstrating stable operation and acceptable QBER values below 5.5%, even when coexisting with intense C-band light signals (> 10 mW) propagating alongside the photonic qubits. The field trial and its comprehensive verification paves the way for practical FSO-QKD in city-scale environments, enabling end users to share quantum keys over kilometer-scale wireless links between building rooftops in last-mile connectivity scenarios.
Abstract Quantum key distribution (QKD) provides a method of ensuring security using the laws of physics, avoiding the risks inherent in cryptosystems protected by computational complexity. Here, the authors investigate the feasibility of satellite‐based quantum key exchange using low‐cost compact nano‐satellites. The first prototype of system level quantum key distribution aimed at the Cube satellite scenario is demonstrated. It consists of a transmitter payload, a ground receiver and simulated free space channel to verify the timing and synchronisation (T&S) scheme designed for QKD and the required high loss tolerance of both QKD and T&S channels. The transmitter is designed to be deployed on various up‐coming nano‐satellite missions in the UK and internationally. The effects of channel loss, background noise, gate width and mean photon number on the secure key rate (SKR) and quantum bit error rate (QBER) are discussed. The authors also analyse the source of QBER and establish the relationship between effective signal noise ratio (ESNR) and noise level, signal strength, gating window and other parameters as a reference for SKR optimisation. The experiment shows that it can tolerate the 40 dB loss expected in space to ground QKD and with small adjustment decoy states can be achieved. The discussion offers valuable insight not only for the design and optimisation of miniature low‐cost satellite‐based QKD systems but also any other short or long range free space QKD on the ground or in the air.
Quantum Key Distribution (QKD) is an emerging technology that enables secure data encryption by basing key generation on the transmission of quantum states rather than computational complexity, thereby anchoring security in the laws of physics and offering resilience against quantum computing attacks. Current research efforts focus on transitioning QKD from laboratory-based demonstrations to real-world deployment, thereby establishing it as a practical communication technology. Quantum entanglement plays a central role in QKD by enabling robust security assurance and providing a path towards scalable quantum networks. This thesis presents a polarisation-entanglement system designed for operation in practical research scenarios, comprising state preparation from a commercial source, transmission, reception and data analysis. The system's capabilities are demonstrated through measurement of polarisation correlation visibility, violation of CHSH-type inequalities, and quantum state tomography, culminating in the generation of the first discrete-variable secret key generated within the Experimental Quantum Communication Group at the University of York. Using the developed system, the integration of quantum communication into existing information infrastructure is experimentally investigated in two distinct scenarios. First, the coexistence of quantum and classical signals in the shared optical fibre is explored in the context of a White Rabbit time-distribution system, recovering entanglement visibilities above 96\% over co-propagation distances of up to 1~km. Second, the transmission of photon pairs from the entanglement source is demonstrated over a 224~km deployed submarine fibre link, connecting the UK and Ireland used for classical data traffic. Together, these efforts provide a versatile, robust research tool for future experimental studies of polarisation-based QKD and demonstrate the viability of integrating quantum communication into existing optical infrastructure, paving the way for practical QKD systems.
As the demand for better security and higher capacity grows, quantum key distribution (QKD) using free-space optics (FSO) presents an innovative solution for 5G networks and beyond. The Innovate-UK AirQKD project explores how FSO–QKD could enable quantum-secure connections at a metropolitan scale, focusing on the critical “last-mile” of connectivity. However, implementing FSO–QKD in the real world brings several engineering challenges, spanning software, hardware, system integration, and security. This paper dives into the complexities of system architecture and integration, QKD transmission over FSO, and the post-processing of keys. We showcase a cohesive system design that blends the quantum and classical domains, incorporating cutting-edge FSO–QKD hardware and software. The system’s functionality is demonstrated for key processes such as transmission, post-processing, network control, and key management. As a case study, we tested an FSO–QKD link over 135 m at BT’s Adastral Park in Suffolk, UK. The setup achieved a photon count rate of up to 585 kcps through a channel with 15.9 dB of loss and 12 kHz of background noise, resulting in a security key rate (SKR) of up to 84.3 kbps using two detectors. We also conducted simulations to estimate the SKR and compared them with the experimental results. Both approaches confirm the system’s ability to generate keys and successfully transmit orthogonal polarization states, highlighting its potential for practical application.
ABSTRACT This study presents a comprehensive feasibility analysis of low Earth orbit (LEO) satellite‐to‐ground downlink communication for quantum key distribution (QKD), focusing on 1550 nm wavelength. This wavelength is selected for its compatibility with terrestrial fibre‐optic networks, allowing the distribution of quantum keys over extended distances. The analysis models a discrete variable decoy state BB84 protocol, emphasising the system's design requirements for the optical ground station (OGS) receiver, including critical parameters that affect single‐mode fibre (SMF) coupling efficiency. Through extended simulations, we assess atmospheric conditions, turbulence effects and adaptive optics (AO) correction to evaluate their impact on light coupling efficiency and achievable key rates. Results highlight the advantage of AO in sustaining secure key rates, particularly under high‐elevation satellite passes, and quantify the key volumes attainable across varying levels of turbulence and additional fibre transmission losses. Our findings demonstrate that LEO satellite‐based QKD downlinks can be feasibly integrated with terrestrial fibre systems, supporting secure key distribution over potentially large geographic areas without requiring OGSs to be trusted nodes. The results of this work provide practical insights into the technical requirements and configurations needed to realise robust satellite‐to‐ground QKD links, laying a foundation for future advancements in global quantum‐secure communication infrastructure.
This study introduces an advanced discrete-variable (DV) polarization-state quantum key distribution (QKD) transmitter tailored for the BB84 protocol with decoy states at 1550 nm. Demonstrating an intrinsic quantum bit error rate (QBER) of 0.15 % at a 615 MHz repetition rate and 0.34 % across a 50-km standard single-mode fiber link, our implementation marks a significant stride towards practical quantum-secured communication. The system's reliance on commercial off-the-shelf (COTS) components and low-voltage operation, coupled with a self-stable design, ensures resilience against environmental perturbations. This research pioneers the evaluation of a polarization-state QKD transmitter's performance in scenarios that involve copropagation with high classical powers within the telecom C Band on shared optical links, observing the impact of spontaneous Raman scattering on the quantum states.