Experimental Development of a Polarisation-Entanglement System for Application in Real-World Quantum Communications

Karolina P. Schatz

Published Mar 1, 2026• 0 citations

Abstract

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.

Related papers

The practical application of quantum key distribution networks

Joseph Pearse

Quantum Key Distribution (QKD) exploits the principles of quantum mechanics to enable information-theoretic security, offering significant advantages over classical cryptographic approaches. Despite extensive experimental demonstrations worldwide, many implementations remain confined to laboratory environments that may be impractical for large-scale deployment. This thesis addresses the critical challenge of making QKD viable within real-world telecommunications networks. Current field trials often prioritise metrics such as distance and key rate at the expense of practicality, employing ultra-low-loss fibres, custom components, and dedicated dark channels. These constraints could hinder integration with existing infrastructure. To overcome these limitations, this work introduces UKQNtel—a testbed designed to incorporate commercial-grade equipment and multiplex quantum and classical traffic. UKQNtel represents the first field trial of a coherent one-way QKD system integrated with a commercial-grade encrypted Dense Wavelength Division Multiplexing (DWDM) network, delivering 500 Gbps of encrypted data over 121 km of real-world fibre. The network comprises four trusted-node links, each achieving performance metrics suitable for practical deployment. Complementing the experimental work, novel simulations were developed to predict QKD performance using only fundamental parameters such as distance, wavelength, and optical power; marking a clear improvement over models that require broad assumptions or empirical measurements of an existing QKD link. This simulation develops new tools for simulating non-linear effects such as Raman scattering and four-wave mixing, as well as many other aspects of QKD noise, loss, and detection. Validation against UKQNtel measurements showed strong agreement in attenuation modelling and link-level performance. While secret key rate estimates demonstrate the need to account for additional losses, the model provides a robust foundation for network planning and scalability analysis. This research demonstrates a clear pathway toward practical QKD deployment, combining experimental integration with predictive modelling applicable to larger, more complex networks. The results underscore QKD’s potential to become a cornerstone of future secure communications.

Dec 1, 20240 citations

Optically Switched Quantum Key Distribution Network

Xinke Tang

Encrypted data transmission is becoming increasingly more important as information security is vital to modern communication networks. Quantum Key Distribution (QKD) is a promising method based on the quantum properties of light to generate and distribute unconditionally secure keys for use in classical data encryption. Significant progress has been achieved in the performance of QKD point-to-point transmission over a fibre link between two users. The transmission distance has exceeded several hundred kilometres of optical fibre in recent years, and the secure bit rate achievable has reached megabits per second, making QKD applicable for metro networks. To realize quantum encrypted data transmission over metro networks, quantum keys need to be regularly distributed and shared between multiple end users. Optical switching has been shown to be a promising technique for cost-effective QKD networking, enabling the dynamic reconfiguration of transmission paths with low insertion loss. In this thesis, the performance of optically switched multi-user QKD systems are studied using a mathematical model in terms of transmission distance and secure key rates. The crosstalk and loss limitations are first investigated theoretically and then experimentally. The experiment and simulation both show that negligible system penalties are observed with crosstalk of -20 dB or below. A practical quantum-safe metro network solution is then reported, integrating optically-switched QKD systems with high speed reconfigurability to protect classical network traffic. Quantum signals are routed by rapid optical switches between any two endpoints or network nodes via reconfigurable connections. Proof-of-concept experiments with commercial QKD systems are conducted. Secure keys are continuously shared between virtualised Alice-Bob pairs over effective transmission distances of 30 km, 31.7 km, 33.1 km and 44.6 km. The quantum bit error rates (QBER) for the four paths are proportional to the channel losses with values between 2.6% and 4.1%. Optimising the reconciliation and clock distribution architecture is predicted to result in an estimated maximum system reconfiguration time of 20 s, far shorter than previously demonstrated. In addition, Continuous Variable (CV) QKD has attracted much research interest in recent years, due to its compatibility with standard telecommunication techniques and relatively low cost in practical implementation. A wide band balanced homodyne detection system built from modified off-the-shelf components is experimentally demonstrated. Practical limits and benefits for high speed CVQKD key transmission are demonstrated based on an analysis of noise performance. The feasibility of an optically switched CV-QKD is also experimentally demonstrated using two virtualised Alice-Bob pairs for the first time. This work represents significant advances towards the deployment of CVQKD in a practical quantum-safe metro network. A method of using the classical equalization technique for Inter-symbol-interference mitigation in CVQKD detection is also presented and investigated. This will encourage further research to explore the applications of classical communication tools in quantum communications.

Sep 28, 20182 citations

Entanglement distribution over a 96-km-long submarine optical fiber

Sören Wengerowsky, Siddarth Koduru Joshi, Fabian Steinlechner +15 more

Quantum entanglement is one of the most extraordinary effects in quantum physics, with many applications in the emerging field of quantum information science. In particular, it provides the foundation for quantum key distribution (QKD), which promises a conceptual leap in information security. Entanglement-based QKD holds great promise for future applications owing to the possibility of device-independent security and the potential of establishing global-scale quantum repeater networks. While other approaches to QKD have already reached the level of maturity required for operation in absence of typical laboratory infrastructure, comparable field demonstrations of entanglement-based QKD have not been performed so far. Here, we report on the successful distribution of polarization-entangled photon pairs between Malta and Sicily over 96 km of submarine optical telecommunications fiber. We observe around 257 photon pairs per second, with a polarization visibility above 90%. Our results show that QKD based on polarization entanglement is now indeed viable in long-distance fiber links. This field demonstration marks the longest-distance distribution of entanglement in a deployed telecommunications network and demonstrates an international submarine quantum communication channel. This opens up myriad possibilities for future experiments and technological applications using existing infrastructure.

arXiv:1803.00583Proceedings of the National Academy of SciencesMar 14, 2019144 citations

Multi-node quantum key distribution network using existing underground optical fibre infrastructure

Mariella Minder, Andreas Siakolas, Stephanos Yerolatsitis +2 more

Quantum key distribution (QKD) offers unconditional information security by allowing two distant users to establish a common encryption key resilient to hacking. Resultingly, QKD networks interconnecting critical infrastructure and enabling the secure exchange of classified information, can provide a solution to the increasing number of successful cyberattacks. To efficiently deploy quantum networks, the technology must be integrated over existing communication infrastructure, such as optical fibre links. Yet, QKD poses stringent requirements on the conditions of the network over which it is deployed. This work demonstrates the first quantum communication network in Cyprus via the deployment of a multi-node quantum network, exploiting existing commercial underground optical fibre. The network employs bidirectional occupation of fibres and wavelength multiplexing in a ring architecture to achieve, with minimal use of dark fibres, high-rate QKD. Results obtained reveal consistent key generation rates across all nodes, confirming reliable operation in a real-world environment. This deployment highlights the feasibility of leveraging existing telecom infrastructure for quantum-secured communication, marking a significant step toward scalable and cost-effective quantum networks suited for critical applications.

quant-pharXiv:2512.02701Workshop on Specialty Optical Fibers and Their Applications (WSOF)Dec 3, 20250 citations

Preparing the deployment of quantum key distribution over a classical network infrastructure in Prague

Tomáš Novák, Elisabeth Andriantsarazo, Carlos Guerra-Yánez +2 more

Quantum Key Distribution (QKD) is a promising tool for secure communication in the near future. In combination with one-time-pad technique, it provides an unconditionally secure communication channel (meaning that it is secure against an adversary, even with unlimited computational power), at least in principle. However, a major implementation challenge for some schemes is the reliable creation, transportation, and measurement of entangled photon pairs over long-distance fiber networks. Our project aims to explore the possibilities for distributing quantum information on an existing network infrastructure while measuring the effects of real-world conditions. We characterized a commercial source of entangled photons. We measured its spectrum, brightness (1.6±0.3)×104 pairs/s/μW, and performed quantum state tomography (QST) to reconstruct the density matrix of the quantum state. Our implementation focuses on an all-fiber solution, which would enable a simplified QKD implementation. In laboratory conditions, we achieved the visibility equal to (0.957 ± 0.004) as a mean in both bases with a coincidence rate of (275 ± 4) counts/s and successfully ran QKD protocol with secret key rate of (86 ± 1) bits/s and average quantum bit error rate (QBER) of (4.8 ± 0.7) %.

Quantum Optics and Photon CountingJun 6, 20230 citations

Ultra-high bandwidth quantum secured data transmission

J. F. Dynes, Winci Tam, Alan Plews +8 more

Quantum key distribution (QKD) provides an attractive means for securing communications in optical fibre networks. However, deployment of the technology has been hampered by the frequent need for dedicated dark fibres to segregate the very weak quantum signals from conventional traffic. Up until now the coexistence of QKD with data has been limited to bandwidths that are orders of magnitude below those commonly employed in fibre optic communication networks. Using an optimised wavelength divisional multiplexing scheme, we transport QKD and the prevalent 100 Gb/s data format in the forward direction over the same fibre for the first time. We show a full quantum encryption system operating with a bandwidth of 200 Gb/s over a 100 km fibre. Exploring the ultimate limits of the technology by experimental measurements of the Raman noise, we demonstrate it is feasible to combine QKD with 10 Tb/s of data over a 50 km link. These results suggest it will be possible to integrate QKD and other quantum photonic technologies into high bandwidth data communication infrastructures, thereby allowing their widespread deployment.

Oct 13, 201661 citations

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.