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 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) can guarantee security for practical indoor optical wireless environments. The key challenges are to mitigate artificial lighting and ambient light at the receiver. A new spectral region for QKD is proposed and an ideal QKD link model is simulated with experimental ambient light power measurements. Simulation, modelling, and analysis indicates that the carbon dioxide and water absorption band (1370 nm) is a new wavelength region for QKD operation in indoor optical wireless environments. For a feasible QKD link, approximately 20 dB of signal to noise ratio (SNR) is required and a maximum quantum bit error rate (QBER) of 11% when using the BB84 protocol. Links in the new spectral region with a FOV of several degrees are feasible, depending on available components.
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 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.
Among all emerging quantum information technologies, quantum key distribution (QKD) is one of the most developed techniques. QKD harnesses the intrinsic laws of quantum mechanics to provide a method for distributing secret random keys, which can be used for data encryption and decryption between two intended users. QKD has already been demonstrated in different scenarios over optical fibre and in atmospheric channels. QKD has also been used for security assurance in several network settings, in addition of being commercially available today. Despite remarkable progress in QKD systems, convenient access to the developing quantum communications networks is still missing. Adopting QKD in mobile devices would enable such a service, particularly, in indoor environments. This is in line with the recent advancement in fabricating microchip-scale QKD devices, which would ease this incorporation into mobile devices. This work focuses on the access networks, and, in particular, it addresses the wireless mode of access in indoor environments for QKD networks. We find a practical regime of operation, where, in the presence of external light sources and loss, secret keys can be exchanged. We then propose practical configurations that would enable wireless access to hybrid quantum-classical networks. The proposed setups would allow an indoor wireless user, equipped with a QKD-enabled mobile device, to communicate securely with a remote party on the other end of the access network. We account for adverse effects of the background noise induced by Raman scattered light on the QKD receivers due to the transmission of both quantum and classical signals over the same fibre. In addition, we consider the loss and the background noise that arise from indoor environments. We consider a number of discrete and continuous-variable QKD protocols and study their performance in different scenarios. In our analysis we consider the asymptotic scenario, as well as the finite-size key effects. In the former case, an infinite number of signals are assumed to be exchanged between the sender and the recipient, whereas in the latter, which represents the
practical scenario, a finite number of signals are exchanged between the two users. Our results indicate that a feasible regime of operation for wireless QKD exists. This makes the QKD technologies available to end users of a communications network.