Quantum computing poses a threat to the standard cryptographic primitives and requires the implementation of quantum-safe designs in the existing network infrastructures. Quantum Key Distribution (QKD) has been used to provide information-theoretic security but its compatibility with high capacity wavelength-division multiplexing (WDM) systems is a challenge to its deployment in metropolitan networks. In this paper, the architectural design and outcomes of a field trial assessing the concept of co-propagation of quantum and classical signals in a dense WDM metro environment are introduced. We discuss and measure the performance of multi-span fibres and measure quantum bit error rates in the presence of highlypower classical loading and the stability of the end-to-end key generation. A hybrid architecture that incorporates both QKD and classical post-quantum solutions is introduced to make them more resilient, optimized in terms of spectrum usage, and scalable to operator-grade services. The experimental findings show that QKD can be smoothly incorporated into the metro networks without affecting throughput and latency, indicating a way forward to practical quantum safe communications to future critical infrastructures.
Quantum computing poses a threat to the standard cryptographic primitives and requires the implementation of quantum-safe designs in the existing network infrastructures. Quantum Key Distribution (QKD) has been used to provide information-theoretic security but its compatibility with high capacity wavelength-division multiplexing (WDM) systems is a challenge to its deployment in metropolitan networks. In this paper, the architectural design and outcomes of a field trial assessing the concept of co-propagation of quantum and classical signals in a dense WDM metro environment are introduced. We discuss and measure the performance of multi-span fibres and measure quantum bit error rates in the presence of highlypower classical loading and the stability of the end-to-end key generation. A hybrid architecture that incorporates both QKD and classical post-quantum solutions is introduced to make them more resilient, optimized in terms of spectrum usage, and scalable to operator-grade services. The experimental findings show that QKD can be smoothly incorporated into the metro networks without affecting throughput and latency, indicating a way forward to practical quantum safe communications to future critical infrastructures.
Quantum Key Distribution (QKD) is currently receiving much attention as it provides a secure source of encryption keys. Discrete-Variable QKD (DV-QKD) is possible for single photon transmission in QKD to-coexist with and encode classical wavelength division multiplexed (WDM) data with appropriate system design. Nevertheless, previous QKD field trials adopted either or both of key relay via trusted nodes and transparent link via optical switching. The former requires guaranteed physical security of the relay nodes, but can expand key distribution distance arbitrarily. The latter can realize key establishment for more users with less complexity of key management over an untrusted network. To realise the adaption of the QKD system for future high speed and long distance metropolitan world exploitation at lower cost, there has to be investigations on existing fibre infrastructures. Prior to this work, previous researches over similar distances feature extremely low secure key rates. For example, the Swiss Quantum Network between three sites displayed secure bit rates of 2.5 kbps at a fibre length of 17km. Quantum Key distribution within the 25km Cambridge Quantum Network have demonstrated the highest long-term secure key rates yet demonstrated in a field trial of at least 2.5Mb/s which is the fastest and much higher than 0.8 kbps which was reached over the similar channel loss field trial up to date. Additional field trials have been performed on the UK Quantum Network using a 66km path having 16dB loss. Combined wavelength division multiplexed 2 x 100 Gb/s traffic encrypted using QKD co-existing on the same fibres has operated for several months, with a long-term key rate of 80kb/s that is also faster than any other similar long-term QKD trial systems. In addition to this advanced commercial QKD system, there have been secure key rate analysis comparisons between laboratory fibre coils and practical field trials more than field trials only conducted before.These comparisons help to identify factors that limit future QKD network scale in both quantity and quality aspects. Also, the limit for the highest secure key rate at longest fibre length QKD in the multiplexing environment is discussed and determined in this research thesis. Nevertheless, in this thesis, improvements have been made to minimise the corresponding negative effects by investigations on the dependence of temperature have been done in order to ensure system operation environment effects. It was found from the trial results that there exists a relationship between temperature and secure key rate and further study has been done to evaluate the system sensitivity to operating temperature. Although the conventional DV-QKD system, original BB84 coding scheme, was designed to exploit the quantum properties of single photon polarization states, the trial equipment operates based upon the phase coding schemes. These coding schemes are based on the properties of interferometers and the coding is implemented by changing the relative optical path lengths or phase between the internal arms of the interferometer, while in the real transmission environment, temperature or polarization variation happens unpredictably. The existing polarisation controllers operate at relative low speed align within the interferometer, which slows to operation environment such as a punch to fibre causing phase difference. Therefore, in this project, there has been an improvement in the QKD-WDM system performance by adding an external polarization controller to minimize the Raman noise and increase the secure key rate at the longest fibre length up to date. In Summary, transmitting quantum keys over a coil of fibre in the lab differs a lot from actually putting it in the ground. This work contrasts the world fastest QKD system at the longest distance in field trials with lab fibre reels and then characterises and identifies two of the key factors, temperature and polarizations, influencing performance in practical wavelength-multiplexed secure communication systems. This is a significant step towards the coexistence of the quantum and conventional data channels on the same fibre for metropolitan networks and paves a way for an information-secure communication infrastructure.
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.
The advent of cryptographically relevant quantum computers poses an existential threat to classical public-key infrastructure. Quantum Key Distribution (QKD) addresses this challenge by providing information-theoretic security for key establishment, independently of any computational hardness assumption. In this work, the deployment and experimental validation of a metropolitan-scale quantum-secure network between data centers in Milan is reported. The network operates over installed fiber infrastructure and implements a layered architecture integrating QKD hardware, standards-compliant Key Management (KM), and centralized Software-Defined Networking (SDN) orchestration. Dynamic path reconfiguration via active optical switching and trusted-node routing allow automated fail-over solutions. Application-layer validation across diverse protocols and workloads confirms the seamless interoperability of all system components. These results establish the technical and operational readiness of metropolitan QKD networks for production deployment, and offer a replicable blueprint for building quantum-secure communication infrastructure at metropolitan scale.
As quantum computing threatens to compromise classical cryptographic systems, the need for future-proof, information-theoretically secure methods of communication has become critical. Quantum Key Distribution (QKD) leverages the fundamental principles of quantum mechanics to enable the secure generation and exchange of encryption keys, immune to computational attacks. This paper investigates the integration of QKD into telecom infrastructures to enable ultra-secure communication. We examine the performance of key QKD protocols, such as BB84 and E91, over fiber-optic and free-space channels, and simulate their deployment within realistic metropolitan network topologies. Results show that secure key rates of up to 10 kbps can be maintained over 50–80 km fiber links with Quantum Bit Error Rates (QBER) below 5%. The study also explores architecture models based on trusted relay nodes and WDM coexistence, highlighting practical deployment considerations. Our findings suggest that QKD is a viable enhancement for telecom security and a necessary foundation for long-term quantum-resilient communication systems.
This paper reports a practical quantum-safe metro network, integrating optically-switched QKD systems with high speed reconfigurability to protect classical network traffic. Quantum signals are routed by millisecond optical switches and secure keys are shared between any two endpoints or network nodes via low-latency reconfigurable connections. Efficient quantum encryption topologies between different end-users are also presented. We show experimentally the feasibility of a rapidly reconfigured QKD transmission system between multiple users in the proposed network. Classical data and control signals coexist with the quantum signals in the same fibre. Proof-of-concept experiments are conducted over effective transmission distances of 30 km, 31.7 km, 33.1 km, and 44.6 km. Software controlled QKD transmission is established between four different transmitters (Alice) and one receiver (Bob) with a switching time of a few milliseconds. The quantum bit error rates for the four paths are proportional to the channel losses with values between 2.6% and 4.1%. Optimization of the reconciliation and clock distribution architecture is predicted to result in a maximum key generation delay of 20 s, far shorter than previously demonstrated.
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.