arXiv subjects

Daniel K. L. Oi

Publications and source records attributed to Daniel K. L. Oi.

At least 19 recordsLinked to original sources

arXiv2026

Roadmap on UV-C photodetectors: materials, applications and industry perspectives

UV-C photodetectors are poised to play an increasingly important role in future photonic technologies, driven by the rapid emergence of UV-C light sources and new wide bandgap semiconductors. These advances are enabling new levels of spectral selectivity, radiation hardness, sensitivity, and device integration, while opening opportunities across a broad range of applications. This roadmap provides a comprehensive overview of the current landscape of UV-C photodetection, spanning established and emerging material platforms (Ga2O3, AlGaN, BN, diamond, MgZnO, 2-dimensional materials, metal halide perovskites, micro-electromechanical systems), and their applications in metrology, astronomy, communications, environmental monitoring, fire detection, missile warning, gas sensing, and medical diagnostics. By identifying opportunities, bottlenecks, and future directions, this roadmap aims to support both newcomers and established researchers, with the aim of accelerating the translation of UV-C photodetectors into impactful technologies.

physics.app-ph↗

arXiv2026

Single-Satellite Quantum Repeater Performance Analysis

Space-based entanglement distribution has the potential to extend the range of quantum communication beyond that achievable through optical fibres that are constrained by exponential losses. Quantum repeaters have been proposed to mitigate the effects of channel losses for both fibre and satellite networks. Although quantum repeaters can improve entanglement distribution efficiency, the rate is constrained by classical communication latency in the entanglement swapping process. Direct dual downlink entangled pair distribution does not suffer such a latency restriction, hence can ``brute force'' the problem of high dual channel loss through increased source rate. Hence, the comparative requirements of direct pair distribution versus quantum repeater satellites are important for the design and deployment of space-based entanglement distribution systems. Here, we consider the simplest case of a single satellite establishing entanglement between two ground stations, comparing the performance of direct dual downlink to that of a space-based quantum repeater for general overpass geometries. We also study the long-term entanglement distribution performance for different ground station pairs and determine altitudinal dependence. Finally, we study the fidelity distribution of a satellite repeater system through Monte Carlo modelling of waiting times and rate statistics, exploring the effect of quantum memory capacity, decoherence rates, and operational policies. These results will inform mission design for future space-borne quantum repeater nodes, as well as requirements on space-based memory platforms.

quant-ph↗

arXiv2026

Operational limits to entanglement-based satellite quantum key distribution

Space-based distribution of quantum entanglement will be essential for global quantum networking and secure communications. Modelling and analysis of the performance of satellite entanglement pair distribution is important for the architecture and design of constellations and space systems. Entanglement-based quantum key distribution, in the absence of quantum repeaters, is especially prone to finite key effects due to low coincident count rates compared to trusted node single-path links. Therefore, there is a need for a comprehensive study of finite-key effects in the context of direct dual downlink quantum key distribution taking into account the characteristics of the overpass geometries. We develop a high-fidelity model of pair distribution from a low Earth orbit satellite that captures orbital dynamics, elevation-dependent loss, background noise, and extraneous detector effects. We integrate this with a rigorous finite-key security framework for the BBM92 protocol to optimise secret key length across different overpass geometries, orbital altitudes, and optical ground station (OGS) separations. These results provide quantitative performance bounds and design guidelines for near-term SatQKD missions, enabling informed trade-offs between satellite payload complexity, ground infrastructure, and achievable secure key throughput.

quant-ph↗

arXiv2025

Decoy-state quantum key distribution over 227 km with a frequency-converted telecom single-photon source

We implement a decoy-state quantum key distribution scheme using a telecom C-band single-emitter source. The decoy states are created by varying the optical excitation of the quantum emitter to modulate the photon number distribution. We provide an analysis of our scheme based on existing security proofs, allowing the calculation of secret key rates including finite key effects. This enables us to demonstrate, with a realistic single-photon source, positive secret key rates using our scheme over 227 km of optical fiber, equivalent to a loss tolerance one order of magnitude greater than non-decoy schemes. This work broadens the scope of single-photon sources in future quantum networks by enabling long-distance QKD with realistic levels of single-photon purity.

quant-ph↗

arXiv2025

A Decoy-like Protocol for Quantum Key Distribution: Enhancing the Performance with Imperfect Single Photon Sources

Quantum key distribution (QKD) relies on single photon sources (SPSs), e.g. from solid-state systems, as flying qubits, where security strongly requires sub-Poissonian photon statistics with low second-order correlation values (\$g^{(2)}(0)\$). However, achieving such low \$g^{(2)}(0)\$ remains experimentally challenging. We therefore propose a decoy-like QKD protocol that relaxes this constraint while maintaining security. This enables the use of many SPSs with \$g^{(2)}(0) > \$0.1, routinely achieved in experiments but rarely considered viable for QKD. Monte Carlo simulations and our experiment from defects in hexagonal boron nitride show that, under linear loss, \$g^{(2)}(0)\$ remains constant, whereas photon-number-splitting (PNS) attacks introduce nonlinear effects that modify the measured \$g^{(2)}(0)\$ statistics. Exploiting this \$g^{(2)}(0)\$ variation as a diagnostic tool, our protocol detects PNS attacks analogously to decoy-state methods. Both single- and two-photon pulses consequently securely contribute to the secret key rate. Our protocol outperforms the Gottesman--Lo--Lutkenhaus--Preskill (GLLP) framework under high channel loss across various solid-state SPSs and is applicable to the satellite-based communication. Since \$g^{(2)}(0)\$ can be extracted from standard QKD experiments, no additional hardware is required. The relaxed \$g^{(2)}(0)\$ requirement simplifies the laser system for SPS generation. This establishes a practical route toward high-performance QKD without the need for ultra-pure SPSs.

quant-ph↗

arXiv2025

Satellites promise global-scale quantum networks

Academia, governments, and industry around the world are on a quest to build long-distance quantum communication networks for a future quantum internet. Using air and fiber channels, quantum communication quickly faced the daunting challenge of exponential photon loss with distance. Quantum repeaters were invented to solve the loss problem by probabilistically establishing entanglement over short distances and using quantum memories to synchronize the teleportation of such entanglement to long distances. However, due to imperfections and complexities of quantum memories, ground-based proof-of-concept repeater demonstrations have been restricted to metropolitan-scale distances. In contrast, direct photon transmission from satellites through empty space faces almost no exponential absorption loss and only quadratic beam divergence loss. A single satellite successfully distributed entanglement over more than 1,200 km. It is becoming increasingly clear that quantum communication over large intercontinental distances (e.g. 4,000-20,000 km) will likely employ a satellite-based architecture. This could involve quantum memories and repeater protocols in satellites, or memory-less satellite-chains through which photons are simply reflected, or some combination thereof. Rapid advancements in the space launch and classical satellite communications industry provide a strong tailwind for satellite quantum communication, promising economical and easier deployment of quantum communication satellites.

quant-ph↗

arXiv2025

Secure Quantum Key Distribution Using a Room-Temperature Quantum Emitter

On-demand generation of single photons from solid-state quantum emitters is essential to build practical quantum networks and QKD systems by potentially enabling higher secure key rates (SKR) and lower quantum bit error rates (QBER) in short-range distances. Room-temperature operation is particularly important as it eliminates the need for bulky cryogenic setups, reducing complexity and cost for real-world applications. In this work, we showcase the versatility of defects in hexagonal boron nitride (hBN) at room temperature by implementing the B92 protocol. Our experiments yield a sifted key rate (SiKR) of 17.5 kbps with a QBER of 6.49% at a dynamic polarization encoding rate of 40 MHz, and finite-key analysis provides a SKR of 7 kbps, one of the highest achieved for a room-temperature single photon source. We analyzed the non-decoy efficient BB84 using our hBN emitter and other promising quantum dot source for QKD, and compare their key performance with a single quantum repeater scenario. We also explore potential applications of hBN defects beyond QKD and analyze scenarios that could outperform conventional point-to-point QKD schemes. These results underscore the promise of hBN emitters for advancing quantum communication technologies.

quant-ph↗

arXiv2025

Compact and stable source of polarization-entangled photon-pairs based on a folded linear displacement interferometer

The realization of quantum networks requires the development of robust low size, weight and power (SWaP) systems suitable for operation under harsh environments in remote and mobile nodes such as satellites. We present a source of polarization-entangled photon-pairs in a folded linear displacement interferometer based on spontaneous parametric down conversion using a Type-0 periodically poled potassium titanyl phosphate crystal. Featuring a compact and stable double-pass geometry using a corner-cube retroreflector, the source has a detected pair rate of 2.5 M pairs/s/mW with a Bell state fidelity of 94.1% +/- 2.1%. The qualities and demonstrated performance of the source make it suitable for deployment in entanglement-based quantum networks.

quant-ph↗

arXiv2025

Comparing a radiation damage model for avalanche photodiodes through in-situ observation of CubeSat based devices

Space-based quantum technologies are essential building blocks for global quantum networks. However, the optoelectronic components used can be susceptible to radiation damage. Predicting long-term instrument performance in the presence of radiation remains a challenging part of space missions. We present a model that accounts for differences in radiation shielding and can predict the trends for dark count rates of space-based silicon Geiger-mode avalanche photodiodes (GM-APD). We find that the predicted trends are correlated with in-situ observations from GM-APDs on-board the SpooQy-1 CubeSat mission.

quant-ph↗

arXiv2024

Comparison of non-decoy single-photon source and decoy weak coherent pulse in quantum key distribution

Advancements in practical single-photon sources (SPS) exhibiting high brightness and low $g^{(2)}(0)$ have garnered significant interest for their application in quantum key distribution (QKD). To assess their QKD performance, it is essential to compare them with the widely employed weak coherent pulses (WCPs) in the decoy state method. In this work, we analyze the non-decoy efficient BB84 protocol for an SPS, partially characterising its photon statistics by its $g^{(2)}(0)$ and mean photon number. We compare it to the 2-decoy efficient BB84 with WCPs within the finite-key analysis framework while optimizing the parameters of both protocols. Our findings indicate that the non-decoy SPS with a mean photon number of $\langle n \rangle = 0.5$ and $g^{(2)}(0) = 3.6\%$ can enhance the secure key generation over the 2-decoy WCP for block sizes under $4.66 \cdot 10^9$ sent signals ($29$ seconds of acquisition time) at a channel loss of $10$ dB ($52.5$ km of optical fibre). Additionally, we demonstrate an increase in the maximum tolerable channel loss for SPSs with mean photon number $\langle n \rangle \geq 0.0142$ at block sizes below $10^8$ sent signals ($0.62$ seconds of acquisition time). These results suggest that SPSs hold potential for key rate enhancement in short-range QKD networks, though further research is required to evaluate their key generation capabilities when integrated into the decoy method.

quant-ph↗

arXiv2024

Finite resource performance of small satellite-based quantum key distribution missions

In satellite-based quantum key distribution (QKD), the number of secret bits that can be generated in a single satellite pass over the ground station is severely restricted by the pass duration and the free-space optical channel loss. High channel loss may decrease the signal-to-noise ratio due to background noise, reduce the number of generated raw key bits, and increase the quantum bit error rate (QBER), all of which have detrimental effects on the output secret key length. Under finite-size security analysis, higher QBER increases the minimum raw key length necessary for non-zero secret key length extraction due to less efficient reconciliation and post-processing overheads. We show that recent developments in finite key analysis allow three different small-satellite-based QKD projects CQT-Sat, UK-QUARC-ROKS, and QEYSSat to produce secret keys even under very high loss conditions, improving on estimates based on previous finite key bounds. This suggests that satellites in low Earth orbit can satisfy finite-size security requirements, but remains challenging for satellites further from Earth. We analyse the performance of each mission to provide an informed route toward improving the performance of small-satellite QKD missions. We highlight the short and long-term perspectives on the challenges and potential future developments in small-satellite-based QKD and quantum networks. In particular, we discuss some of the experimental and theoretical bottlenecks, and improvements necessary to achieve QKD and wider quantum networking capabilities in daylight and at different altitudes.

quant-ph↗

arXiv2023

Time-delayed single satellite quantum repeater node for global quantum communications

Global-scale quantum networking faces significant technical and scientific obstacles. Quantum repeaters (QRs) have been proposed to overcome the inherent direct transmission range limit through optical fibre. However, QRs are typically limited to a total distance of a few thousand kilometres and/or require extensive hardware overhead. Recent proposals suggest that strings of space-borne QRs with on-board quantum memories (QMs) are able to provide global coverage. Here, we propose an alternative to such repeater constellations using a single satellite with two QMs that effectively acts as a time-delayed version of a single QR node. Using QKD as a benchmark, we estimate the amount of finite secure key generated and demonstrate an improvement of at least three orders of magnitude over prior single-satellite methods that rely on a single QM, while simultaneously reducing the necessary memory capacity similarly. We propose an experimental platform to realise this scheme based on rare-Earth ion doped crystals with appropriate performance parameters.

quant-ph↗

arXiv2023

The ideal wavelength for daylight free-space quantum key distribution

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 the 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 in 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 H\textalpha\ Fraunhofer line. More importantly, we provide the full model that can be adapted in general to any other specific link scenario. 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.

quant-ph↗

arXiv2023

Finite key performance of satellite quantum key distribution under practical constraints

Global-scale quantum communication networks will require efficient long-distance distribution of quantum signals. Optical fibre communication channels have range constraints due to exponential losses in the absence of quantum memories and repeaters. Satellites enable intercontinental quantum communication by exploiting more benign inverse square free-space attenuation and long sight lines. However, the design and engineering of satellite quantum key distribution (QKD) systems are difficult and characteristic differences to terrestrial QKD networks and operations pose additional challenges. The typical approach to modelling satellite QKD (SatQKD) has been to estimate performances with a fully optimised protocol parameter space and with few payload and platform resource limitations. Here, we analyse how practical constraints affect the performance of SatQKD for the Bennett-Brassard 1984 (BB84) weak coherent pulse decoy state protocol with finite key size effects. We consider engineering limitations and trade-offs in mission design including limited in-orbit tunability, quantum random number generation rates and storage, and source intensity uncertainty. We quantify practical SatQKD performance limits to determine the long-term key generation capacity and provide important performance benchmarks to support the design of upcoming missions.

quant-ph↗

arXiv2022

Responsive Operations for Key Services (ROKS): A Modular, Low SWaP Quantum Communications Payload

Quantum key distribution (QKD) is a theoretically proven future-proof secure encryption method that inherits its security from fundamental physical principles. Craft Prospect, working with a number of UK organisations, has been focused on miniaturising the technologies that enable QKD so that they may be used in smaller platforms including nanosatellites. The significant reduction of size, and therefore the cost of launching quantum communication technologies either on a dedicated platform or hosted as part of a larger optical communications will improve potential access to quantum encryption on a relatively quick timescale. The ROKS mission seeks to be among the first to send a QKD payload on a CubeSat into low Earth orbit, demonstrating the capabilities of newly developed modular quantum technologies. The ROKS payload comprises a quantum source module that supplies photons randomly in any of four linear polarisation states fed from a quantum random number generator; an acquisition, pointing, and tracking system to fine-tune alignment of the quantum source beam with an optical ground station; an imager that will detect cloud cover autonomously; and an onboard computer that controls and monitors the other modules, which manages the payload and assures the overall performance and security of the system. Each of these modules have been developed with low SWaP for CubeSats, but with interoperability in mind for other satellite form factors. We present each of the listed components, together with the initial test results from our test bench and the performance of our protoflight models prior to initial integration with the 6U CubeSat platform systems. The completed ROKS payload will be ready for flight at the end of 2022, with various modular components already being baselined for flight and integrated into third party communication missions.

quant-ph↗

arXiv2022

Single-emitter quantum key distribution over 175 km of fiber with optimised finite key rates

Quantum key distribution with solid-state single-photon emitters is gaining traction due to their rapidly improving performance and compatibility with future quantum network architectures. In this work, we perform fibre-based quantum key distribution with a quantum dot frequency-converted to telecom wavelength, achieving count rates of 1.6 MHz with $g^{\left(2\right)}\left(0\right) = 3.6 \%$. We demonstrate positive key rates up to 175 km in the asymptotic regime. We then show that the community standard analysis for non-decoy state QKD drastically overestimates the acquisition time required to generate secure finite keys. Our improved analysis using the multiplicative Chernoff bound reduces the required number of received signals by a factor of $10^8$ over existing work, with the finite key rate approaching the asymptotic limit at all achievable distances for acquisition times of one hour. Over a practical distance of 100 km we achieve a finite key rate of 13 kbps after one minute of integration time. This result represents major progress towards the feasibility of long-distance single-emitter QKD networks.

quant-ph↗

arXiv2022

Modelling efficient BB84 with applications for medium-range, terrestrial free-space QKD

Terrestrial free-space quantum key distribution is ideally suited for deployment in dense urban environments. The transition from laboratory to commercial deployment, however, raises a number of important engineering and deployment issues. Here, we investigate these issues for efficient BB84 using a weak coherent pulse-decoy state protocol. We calculate expected key lengths for different environmental conditions and when the scope for optimisation of protocol parameters is restricted due to practical considerations. In particular, we find that for a fixed receiver basis choice probability, it can be advantageous to allow the transmitter to have a different basis choice probability depending on varying channel loss and background light levels. Finally, we examine the effects of pulse intensity uncertainty finding that they can dramatically reduce the key length. These results can be used to determine the loss budget for the free-space optics of a QKD systems and assist in their design.

quant-ph↗

arXiv2022

MAQRO -- BPS 2023 Research Campaign Whitepaper

The objective of the proposed MAQRO mission is to harness space for achieving long free-fall times, extreme vacuum, nano-gravity, and cryogenic temperatures to test the foundations of physics in macroscopic quantum experiments. This will result in the development of novel quantum sensors and a means to probe the foundations of quantum physics at the interface with gravity. Earlier studies showed that the proposal is feasible but that several critical challenges remain, and key technologies need to be developed. These new technologies will open up the potential for achieving additional science objectives. The proposed research campaign aims to advance the state of the art and to perform the first macroscopic quantum experiments in space. Experiments on the ground, in micro-gravity, and in space will drive the proposed research campaign during the current decade to enable the implementation of MAQRO within the subsequent decade.

quant-ph↗