The emergence of a digital communications infrastructure over recent decades has fuelled the parallel development of advanced cryptographic techniques, to secure the ever increasing quantities of digital infonnation. From its foundation, quantum key distribution has generated significant theoretical and experimental research interest since it offers what ./ is currently the only method of verifiably secure cryptographic key distribution. By using the Heisenberg Uncertainty Principle, quantum key distribution offers a technique by which authorised parties can detect the potential presence of an eavesdropping unauthorised party and take appropriate action. Previous demonstrations of quantum key distribution have typically concentrated on extending the maximum transmission distance over which communication may be performed at the expense of bit-rates. This thesis investigates a quantum key distribution system operating at a wavelength of 850 nm in standard telecommunications fibre using commercially available silicon single-photon avalanche diodes to achieve clock rates in excess of 1 GHz, with a corresponding increase in received bit-rates. The principles of currently implemented passive optical networks are then applied to the system to demonstrate multi-user quantum key distribution systems. In addition, the point-to-point (single-user) system is demonstrated at a clock-rate of 3.3 GHz using low timing jitter niobium nitride nanowire superconducting detectors to demonstrate the highest clock-rate transmission in standard telecommunications optical fibre and the highest channel loss to date. Finally, single-photon sources based on quantum dot microcavity resonators are also examined for use in quantum key distribution.
The emergence of a digital communications infrastructure over recent decades has fuelled the parallel development of advanced cryptographic techniques, to secure the ever increasing quantities of digital infonnation. From its foundation, quantum key distribution has generated significant theoretical and experimental research interest since it offers what ./ is currently the only method of verifiably secure cryptographic key distribution. By using the Heisenberg Uncertainty Principle, quantum key distribution offers a technique by which authorised parties can detect the potential presence of an eavesdropping unauthorised party and take appropriate action. Previous demonstrations of quantum key distribution have typically concentrated on extending the maximum transmission distance over which communication may be performed at the expense of bit-rates. This thesis investigates a quantum key distribution system operating at a wavelength of 850 nm in standard telecommunications fibre using commercially available silicon single-photon avalanche diodes to achieve clock rates in excess of 1 GHz, with a corresponding increase in received bit-rates. The principles of currently implemented passive optical networks are then applied to the system to demonstrate multi-user quantum key distribution systems. In addition, the point-to-point (single-user) system is demonstrated at a clock-rate of 3.3 GHz using low timing jitter niobium nitride nanowire superconducting detectors to demonstrate the highest clock-rate transmission in standard telecommunications optical fibre and the highest channel loss to date. Finally, single-photon sources based on quantum dot microcavity resonators are also examined for use in quantum key distribution.
In recent years quantum information research has lead to the discovery of a number of remarkable new paradigms for information processing and communication. These developments include quantum cryptography schemes that offer unconditionally secure information transport guaranteed by quantum-mechanical laws. Such potentially disruptive security technologies could be of high strategic and economic value in the future. Two major issues confronting researchers in this field are the transmission range (typically <100km) and the key exchange rate, which can be as low as a few bits per second at long optical fiber distances. This paper describes further research of an approach to significantly enhance the key exchange rate in an optical fiber system at distances in the range of 1-20km. We will present results on a number of application scenarios, including point-to-point links and multi-user networks. Quantum key distribution systems have been developed, which use standard telecommunications optical fiber, and which are capable of operating at clock rates of up to 2GHz. They implement a polarization-encoded version of the B92 protocol and employ vertical-cavity surface-emitting lasers with emission wavelengths of 850 nm as weak coherent light sources, as well as silicon single-photon avalanche diodes as the single photon detectors. The point-to-point quantum key distribution system exhibited a quantum bit error rate of 1.4%, and an estimated net bit rate greater than 100,000 bits-1 for a 4.2 km transmission range.
This paper describes progress in gigahertz-clocked quantum key distribution systems. It details current advances in both point-to-point and network applications. We will discuss possibilities for practical quantum key distribution using single-photon sources, and discuss the experimental system performance of GHz-clocked quantum key distribution systems focusing on issues of quantum bit error rate, net bit rate and transmission distance with different detector structures, concentrating on single-photon avalanche diode detectors, but also examining superconducting nanowire-based structures. The quantum key distribution system is designed to be environmentally robust and an examination of long-term system operation will be presented.
This paper describes progress in gigahertz-clocked quantum key distribution systems. It details current advances in both point-to-point and network applications. We will discuss possibilities for practical quantum key distribution using single-photon sources, and discuss the experimental system performance of GHz-clocked quantum key distribution systems focusing on issues of quantum bit error rate, net bit rate and transmission distance with different detector structures, concentrating on single-photon avalanche diode detectors, but also examining superconducting nanowire-based structures. The quantum key distribution system is designed to be environmentally robust and an examination of long-term system operation will be presented.
Quantum key distribution (QKD) is a technology that allows two users to communicate with theoretically perfect security using standard optical fibres. This is possible by transmitting the key on single photons, meaning a measurement by an eavesdropper disturbs the system in a way observable to the legitimate parties. The technology has advanced since the first protocol proposed in 1984, to the stage where there are now many protocols that can be experimentally implemented. These protocols have allowed secure keys to be generated over distances greater than 400 km and with secure key rates over 10 Mbit/s. In a metropolitan QKD network, it would be desirable for as many users to be connected as possible. Unfortunately, each protocol comes with different requirements on the transmitter and receiver. Even within a single protocol, different clock rates can require individualised transmitter and receiver hardware. This prohibits users from communicating with all receivers, unless they have complex transmitters with hardware for many protocols. This thesis develops a transmitter for practical QKD that is able to adapt to a number of different protocols with no changes to the hardware. The transmitter works using optical injection locking, where a pulse preparation laser adopts the phase of a phase preparation laser. Controlling the phase and intensity of the pulses in this way removes the side channel ordinarily present with direct modulation, in that the phase, intensity and frequency simultaneously change in response to an applied current. The cavity-enhanced electro-optic effect allows for the first demonstration of sub-volt half-wave phase modulation at high clock rates. The transmitter successfully demonstrates phase encoding, intensity encoding and on-demand phase randomisation. This allows for the experimental realisation and direct comparison of different QKD protocols, including one that has never before been implemented due to experimental complexity. A stable intensity modulator is also developed, based on a Sagnac interferometer. This removes a side channel in QKD systems and integrates well with the directly-modulated quantum transmitter. This development also means that the transmitter can implement all current two-party QKD protocols based on weak coherent pulses. The transmitter has the potential to become the standard transmitter for future quantum communication networks due to its stability, versatility and power efficiency. The design could also be demonstrated on a photonic chip, making it compact enough to fit in small transmitter units.
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.
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.