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Thermal State Quantum Key Distribution

Walton, A, Anne Ghesquière, Brumpton, G, Jennings, D, Benjamin T. H. Varcoe

Published Oct 13, 2021• 2 citations
OpenAlex

Abstract

This thesis covers two separate fields of research, first focusing on quantum key distribution before covering research on atomic clock synchronisation. Initially, a method of performing thermal state key distribution was analysed. A protocol for radio key distribution inspired by quantum methods which could be performed using easily accessible equipment was known, with some evidence that it could realistically be carried out, but this had yet to be confirmed in testing. In this writing, we first simulate the protocol in Python, and verify that the results of simulation suggests that a practical setup would be reasonable. After confirming that off-the-shelf broadcasting equipment may be used for the protocol, we performed both wired and wireless tests of the protocol and was able to distribute bit strings suitable for key distillation. This provides a method for performing quantum key distribution using only common radio equipment and open-source software -- a procedure which typically requires specialist equipment, is not compatible with current communication setups, and does not currently exist in a widely accessible form for public use. For clock synchronisation, current accessible methods of wireless synchronisation have poor precision, relying on GPS for timing information. Optical protocols for synchronisation offer far more precise results, but require either line of sight or a fibre connection, which is not always reasonable to implement. This can lead to issues in which accessible methods of synchronisation are not sufficient for parties which require higher precision, such as in market trading. Here, we devise and test a radio-based method of clock synchronisation with commonly-used broadcast protocols based on experimental results observed in the first section of the thesis. From this testing, we find that the protocol is able to reach precision approximately 10^3 times smaller than the minimum advertised by the clocks used, which itself was already more precise than can be reached using GPS synchronisation. Overall, this represents new methods of solving known problems; producing the security conferred by Quantum Key Distribution (QKD) and clock synchronisation, without requiring specialised equipment which may prevent other potential solutions from being viable in real-world applications. The ability to perform the experiments set up here with off-the-shelf radio equipment and open-source software provides a straightforward way to apply the protocols for any party interested in either of these fields. This will be shown to be especially true in the case of clock synchronisation, in which we find evidence that our protocol outperforms currently used methods by a significant margin while using similar equipment.

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This thesis covers two separate fields of research, first focusing on quantum key distribution before covering research on atomic clock synchronisation. Initially, a method of performing thermal state key distribution was analysed. A protocol for radio key distribution inspired by quantum methods which could be performed using easily accessible equipment was known, with some evidence that it could realistically be carried out, but this had yet to be confirmed in testing. In this writing, we first simulate the protocol in Python, and verify that the results of simulation suggests that a practical setup would be reasonable. After confirming that off-the-shelf broadcasting equipment may be used for the protocol, we performed both wired and wireless tests of the protocol and was able to distribute bit strings suitable for key distillation. This provides a method for performing quantum key distribution using only common radio equipment and open-source software -- a procedure which typically requires specialist equipment, is not compatible with current communication setups, and does not currently exist in a widely accessible form for public use. For clock synchronisation, current accessible methods of wireless synchronisation have poor precision, relying on GPS for timing information. Optical protocols for synchronisation offer far more precise results, but require either line of sight or a fibre connection, which is not always reasonable to implement. This can lead to issues in which accessible methods of synchronisation are not sufficient for parties which require higher precision, such as in market trading. Here, we devise and test a radio-based method of clock synchronisation with commonly-used broadcast protocols based on experimental results observed in the first section of the thesis. From this testing, we find that the protocol is able to reach precision approximately 10^3 times smaller than the minimum advertised by the clocks used, which itself was already more precise than can be reached using GPS synchronisation. Overall, this represents new methods of solving known problems; producing the security conferred by Quantum Key Distribution (QKD) and clock synchronisation, without requiring specialised equipment which may prevent other potential solutions from being viable in real-world applications. The ability to perform the experiments set up here with off-the-shelf radio equipment and open-source software provides a straightforward way to apply the protocols for any party interested in either of these fields. This will be shown to be especially true in the case of clock synchronisation, in which we find evidence that our protocol outperforms currently used methods by a significant margin while using similar equipment.
Precise time synchronisation underpins critical infrastructure from telecommunications and financial markets to power grids and scientific metrology. Several families of quantum protocols have been proposed and demonstrated for clock synchronisation and time distribution, exploiting entangled photon pairs, quantum key distribution (QKD) correlations, Hong-Ou-Mandel interference, and entangled clock networks. We critically assess these approaches, reviewing the main quantum time synchronisation (QTS) protocol families, quantifying the gap between theory and experiment, and identifying practical bottlenecks in sources, detectors, and channels. We survey the classical timing landscape from Network Time Protocol (NTP) and GPS to laboratory-grade optical frequency transfer, and compare quantum and classical methods at equivalent maturity. We examine use cases including financial trading, power grids, telecommunications, scientific metrology, and military applications, evaluating whether quantum timing offers a realistic advantage. We show that time transfer, not clock performance, is now the bottleneck for distributed optical timekeeping: the best demonstrated synchronisation uncertainty (2.46~ps) falls two to three orders of magnitude short of what optical clocks with fractional frequency uncertainties of 10−1810^{-18}10−18--10−1910^{-19}10−19 require. Our assessment is that quantum time synchronisation will not replace classical methods for most applications in the near-to-medium future. Its near-term value lies in physical-layer security against timing manipulation and integration with quantum communication networks, while closing the synchronisation gap for scientific metrology remains the most critical open challenge.
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 convergence of quantum cryptography with applications used in everyday life is a topic drawing attention from the industrial and academic worlds. The development of quantum electronics has led to the practical achievement of quantum devices that are already available on the market and waiting for their first application on a broader scale. A major aspect of quantum cryptography is the methodology of Quantum Key Distribution (QKD), which is used to generate and distribute symmetric cryptographic keys between two geographically separate users using the principles of quantum physics. In previous years, several successful QKD networks have been created to test the implementation and interoperability of different practical solutions. This article surveys previously applied methods, showing techniques for deploying QKD networks and current challenges of QKD networking. Unlike studies focusing on optical channels and optical equipment, this survey focuses on the network aspect by considering network organization, routing and signaling protocols, simulation techniques, and a software-defined QKD networking approach.
Quantum Key Distribution (QKD), based on the laws of physics rather than the computational \ncomplexity of mathematical problems, provides a secure way of establishing symmetrical binary \nkeys between two geographically distant users. The keys are secure from eavesdropping during \ntransmission and QKD ensures that any third party’s knowledge of the key is reduced to a \nminimum. In recent years, a noticeable progress in the development of quantum equipment has \nbeen reflected through a number of successful demonstrations of QKD technology. While they \nshow the great achievements of QKD, many practical difficulties still need to be resolved, such as \nto provide better service differentiation. These networks are characterized as being multihop in \nnature where the consumption key rate is often higher than the charging key rate, which means that \nthe links are available for a limited period of time only. Such features impose several challenges \non the effective modeling and evaluation of reliability as well as finding appropriate Quality \nof Service (QoS) solution. This thesis focuses on research in the field of QKD for securing \nreal-time communication by supporting QoS in QKD networks including a novel QoS model \nand novel distributed reactive routing protocol to achieve high-level scalability and minimize the \nconsumption of key material used for securing routing data. \nAs research in QKD networks grows larger and more complex, the need for highly accurate and \nscalable simulation technologies becomes important to assess the practical feasibility and foresee \ndifficulties in the practical implementation of theoretical achievements. Due to the specificity of \nQKD link which requires optical/quantum and Internet connection between the network nodes, it \nis very costly to deploy a complete testbed containing multiple network hosts and links to validate \nand verify a certain network algorithm or protocol. The network simulators in these circumstances \nsave a lot of money and time in accomplishing such task. A simulation environment offers the \ncreation of complex network topologies, a high degree of control and repeatable experiments, \nwhich in turn allows researchers to conduct exactly the same experiments and confirm their results. \nThis thesis describes the design and implementation of QKD network simulation module which \nwas developed in the network simulator of version 3 (NS-3). The module supports simulation \nof QKD network in overlay mode or in a single TCP/IP mode. Therefore, it can be used for \nsimulation of other network technologies regardless of QKD. \nImplemented simulation model was used for verification of proposed QoS solution. A number \nof simulations were performed. The obtained data have confirmed the primary thesis of this study, \nthat it is possible to use real-time applications in QKD networks.
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.
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.