Feng Liu, James Farmer, Grahame E. Faulkner, Abderrahmen Trichili, Jonathan J. D. McKendry, Enyuan Xie, Zhaoming Wang, Jianming Wang, Jordan Hill, Johannes Herrnsdorf, Sujan Rajbhandari, Martin D. Dawson, Dominic C. O'Brien
Quantum key distribution (QKD) has emerged as a promising technique for secure communication in the post-quantum era, offering information-theoretic security against eavesdroppers with unlimited computational power. While fiber-based and satellite QKD systems have demonstrated impressive distances and key rates, terrestrial free-space QKD, which shows potential for secure mobile and wireless payment applications, remains relatively under-investigated due to severe ambient light interference. Here, we present a QKD link operating in the ultraviolet-C (UV-C) region, which is, to the best of our knowledge, the first such example. This has the potential to leverage the “solar-blind” nature of the UV-C region and mitigate sunlight interference. By integrating high-speed AlGaN micro-LEDs with silicon photomultiplier (SiPM)-based single-photon detectors (SPDs), we achieve a secure key rate (SKR) exceeding 1 Mbit/s. Furthermore, we analyze the system’s robustness to ambient light and propose a strategy for full sunlight rejection. Our results establish UV-C as a promising spectral window for terrestrial free-space QKD, paving the way for QKD implementations in daylight conditions.
Quantum key distribution (QKD) has emerged as a promising technique for secure communication in the post-quantum era, offering information-theoretic security against eavesdroppers with unlimited computational power. While fiber-based and satellite QKD systems have demonstrated impressive distances and key rates, terrestrial free-space QKD, which shows potential for secure mobile and wireless payment applications, remains relatively under-investigated due to severe ambient light interference. Here, we present a QKD link operating in the ultraviolet-C (UV-C) region, which is, to the best of our knowledge, the first such example. This has the potential to leverage the “solar-blind” nature of the UV-C region and mitigate sunlight interference. By integrating high-speed AlGaN micro-LEDs with silicon photomultiplier (SiPM)-based single-photon detectors (SPDs), we achieve a secure key rate (SKR) exceeding 1 Mbit/s. Furthermore, we analyze the system’s robustness to ambient light and propose a strategy for full sunlight rejection. Our results establish UV-C as a promising spectral window for terrestrial free-space QKD, paving the way for QKD implementations in daylight conditions.
The rapid advancement of quantum technology increases the security risks of conventional encryption schemes, such as the Rivest-Shamir-Adleman (RSA) algorithm. Quantum key distribution (QKD) has emerged as a secure cryptographic solution, using Heisenberg’s uncertainty principle and the no-cloning theorem to ensure unconditional security in key exchange. Significant progress has been made in fibre-based and satellite QKD systems, achieving long transmission distances and high secure key rates (SKR). However, short-range wireless QKD remains largely unexplored, primarily due to severe ambient light interference, which degrades system performance and limits practical deployment. This thesis reports a short-range wireless QKD operating in the ultraviolet-C (UV-C) spectrum, utilizing the solar-blind region to mitigate ambient light interference. A high-sensitivity UV-C receiver was developed using a cooled silicon photomultiplier (SiPM), enabling a 1 Mbit/s optical wireless communication (OWC) link. Subsequently, ultralow photon flux links were demonstrated using a UV-C micro-LED and a single-photon detector (SPD), achieving a maximum data rate of 120 Mbit/s with a minimum transmitted photon number per bit of 7.7. Finally, the world’s first wireless QKD system operating in the UV-C region was implemented, inferring an SKR exceeding 1 Mbit/s. Additionally, the impact of LED-based ambient light and sunlight interference was analysed, and mitigation strategies were proposed to enhance system robustness. Overall, this work establishes the feasibility of UV-C wireless QKD and provides a foundation for future improvements in ambient light rejection and real-world deployment.
Quantum Key Distribution (QKD) has the potential to secure indoor optical wireless links. In a typical room scenario, an indoor optical wireless link will have a transmitter on the ceiling and a receiver on a desk. Ambient light from room (typically LED) lighting on the ceiling and sunlight coming through the windows present a challenging environment for free-space QKD links to operate, and a key challenge is to mitigate the noise induced by ambient light, particularly sunlight. A combination of spectral and spatial filtering can be used to reduce the effect of ambient light, with a narrowband optical filter typically used. Moreover, the wavelength of operation is key to further reduce the impact of ambient light. Wavelengths in ‘quiet’ regions of the solar spectrum, such as the atmospheric absorption bands, are promising candidates. We are currently working on a system that operates at 1370 nm, where water and carbon dioxide absorption band in the atmosphere attenuate the solar spectrum substantially. This paper reports the design and modelling of the system, with a series of validation measurements to characterise the effect of solar radiation on a typical photon-counting detector as would be used in a QKD system. The aim of this work is to show the feasibility of the wavelength region around 1370 nm as a necessary step towards a low noise QKD receiver for indoor optical wireless links in a practical environment.
Quantum key distribution (QKD) has matured in recent years from laboratory proof-of-principle demonstrations to commercially available systems. One of the major bottlenecks is the limited communication distance in fiber networks due to the exponential signal damping. To bridge intercontinental distances, low Earth orbit satellites transmitting quantum signals over the atmosphere can be used. These free-space links, however, can only operate during the night, as the sunlight otherwise saturates the detectors used to measure the quantum states. For applying QKD in a global quantum internet with continuous availability and high data rates, operation during daylight is required. In this work, we model a satellite-to-ground quantum channel for different quantum light sources to identify the optimal wavelength for free-space QKD under ambient conditions. Daylight quantum communication is possible within the Fraunhofer lines or in the near-infrared spectrum, where the intrinsic background from the sun is comparably low. The highest annual secret key length considering the finite key effect is achievable at the Hα Fraunhofer line. More importantly, we provide the fundamental model that can be adapted, in general, to any other specific link scenario taking into account the required modifications. We also propose a true single-photon source based on a color center in hexagonal boron nitride coupled to a microresonator that can implement such a scheme. Our results can also be applied in roof-to-roof scenarios and are, therefore, relevant for near-future quantum networks.
Free-space Quantum key distribution (QKD) has shown the potential for the practical production of cryptographic key for ultra-secure communications. The performance of any QKD system is ultimately limited by the signal to noise ratio on the single-photon channel, and over most useful communications links the resulting key rates are impractical for performing continuous one-time-pad encryption of today's broadband communications. We have adapted clock and data recovery techniques from modern telecommunications practice, combined with a synchronous classical free-space optical communications link operating in parallel, to increase the repetition rate of a free-space QKD system by roughly 2 orders of magnitude over previous demonstrations. We have also designed the system to operate in the H-alpha Fraunhofer window at 656.28 nm, where the solar background is reduced by roughly 7 dB. This system takes advantage of high efficiency silicon single-photon avalanche photodiodes with <50ps timing resolution that are expected to enable operation at a repetition rate of 2.5 GHz. We have identified scalable solutions for delivering sustained one-time-pad encryption at 10 Mbps, thus making it possible to integrate quantum cryptography into first-generation Ethernet protocols.
Due to the recent rapid growth of computing technologies, today's encrypted information based on classical computational complexities will soon no longer remain secure. Quantum cryptography, or quantum key distribution (QKD), relying on the physical laws of quantum mechanics, stands out as a more powerful cryptographic solution that enables the secret key to be shared securely over publicly unsecured communication channels in the presence of eavesdroppers. QKD can be possibly implemented over both optical fiber and free-space optical (FSO) links. Compared to optical fiber, FSO has the immediate advantage of flexibility and cost-effectiveness in terms of infrastructure deployment. More interestingly, recent advancements in satellite communications using FSO links promise the future of a global QKD network for highly secure applications. This paper reviews the development of QKD over FSO for both terrestrial and satellite channels, which highlights the most up-to-date QKD protocols. Owing to how the information is encoded, different QKD protocols are classified into technology-focused categories, where their properties are reviewed and compared. The feasibility of current QKD protocols is then investigated with respect to its specific application scenarios for terrestrial and satellite FSO links, equipped with results from both theoretical and experimental studies. Based on the latest free-space QKD achievements, novel perspectives for future developments and challenges to enhance the secrecy performance and reduce implementation cost are then summarized and comprehensively discussed. The potential combination of QKD and physical layer cryptography for the newly emerging wireless mobile communications networks involved with unmanned aerial vehicles (UAV) is also introduced.
Satellite-based quantum key distribution (QKD) holds the potential to establish global quantum communication networks. While satellite QKD has been demonstrated using polarization encoding, time-bin encoding offers distinct advantages, such as simplifying polarization-maintaining telescope designs and being insensitive to satellite-ground relative motion. In this work, we developed a 625-MHz QKD light source using a robust Sagnac-interferometer-based modulation scheme and subsequently demonstrated time-bin QKD over a 7 km urban terrestrial free-space channel. This experiment successfully operated over a channel traversing an equivalent atmospheric thickness and loss exceeding that of typical satellite-to-ground links, achieving a low quantum bit error rate of 0.87% and a secure key rate of 134 bps@51.4 dB. Furthermore, by using a half-wave plate to simulate the polarization-basis-rotation inherent in such links, we verified the robustness of time-bin encoding for satellite scenarios. The results validate time-bin encoding as a compelling alternative and lay the technical foundation for future satellite QKD applications.