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.
Quantum Key Distribution (QKD) is attracting much interest for the distribution of cryptographic keys using single photon signals. Currently QKD is often used to provide secure distribution of cryptographic keys for the encryption of data transmitted using conventional classical communication systems. This paper reports major field trials carried out over several months on the Cambridge UK Quantum Network showing the operation of QKD systems alongside high-speed classical transmission systems encrypted QKD derived with AES keys. Quantum Key transmission at record secure key rates of 3.3Mbps, 3.2Mbps and 2.5Mbps has been achieved over 5km, 9.5km and 10.5km long links respectively with corresponding average Quantum bit error rates (QBER) of 2.9%, 2.4% and 3%. Using a 33km link attached to the network with a loss of 7.5 dB, a secure key rate of 1.4 Mbps is achieved with an average QBER of 3.4%. Under loop back conditions this link provides a 66 km transmission path with a 16dB fibre loss, enabling a field trial using the QKD signals multiplexed with two wavelengths each transmitting 100Gb/s classical data to be carried out. This achieves an average secure key rate of 80.2 kbps and a mean QBER of 6.6%, in line with theoretical predictions. During the trial duration, the statistics of the QBER were found to be Gaussian distributed with a standard deviation of 0.5. The results of the field trial suggest that the system works stably and has considerable potential for applications in metropolitan networks. Further measurements will be reported at the conference.
We describe recent progress in quantum secured optical networks in the UK. The Cambridge Quantum Network has been operating for several years with 3 nodes separated by between 5-10 km of installed fibre. All links are secured by QKD systems operating with secure key rates in excess of 1 Mb/s, the highest recorded long term key rates in a deployed network. The network operates in the presence of 100Gb/s classical traffic with no significant reduction of secure key generation rate. In addition, the Bristol Quantum Network has four nodes 1-3km apart connected in a mesh protected by two pairs of QKD systems. The network is designed to be very dynamic, switching both QKD and WDM classical traffic to enable rapid reconfiguration and is used as a testbed for QKD protected dynamic applications. The two metropolitan networks are being connected by a 410 km QKD link, with 4 spans, the longest of which operates over 129km of fibre with an attenuation of 28dB achieving secure key rates of 2.7kb/s, the longest and highest loss QKD field trial to date. A 120km extension of the UK quantum network from Cambridge to BT Labs, Adastral Park operates with fully commercially available components and is an important testbed comprising 3 intermediate nodes and operates with 5 x 100Gb/s classical channels. This helps determine how to proceed with a large-scale commercial deployment of QKD.
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) is the first commercial quantum technology operating at the level of single quanta and is a leading light for quantum-enabled photonic technologies. However, controlling these quantum optical systems in real world environments presents significant challenges. For the first time, we have brought together three key concepts for future QKD systems: a simple high-speed protocol; high performance detection; and integration both, at the component level and for standard fibre network connectivity. The QKD system is capable of continuous and autonomous operation, generating secret keys in real time. Laboratory and field tests were performed and comparisons made with robust InGaAs avalanche photodiodes and superconducting detectors. We report the first real world implementation of a fully functional QKD system over a 43 dB-loss (150 km) transmission line in the Swisscom fibre optic network where we obtained average real-time distribution rates over 3 hours of 2.5 bps.