Daniel K.L. Oi

High brightness, low second-order correlation function single-photon sources (SPSs) are an alternative to commonly employed weak coherent pulse (WCP) sources for discrete variable quantum key distribution (QKD) and offer potential key-rate and finite-block scaling advantages. However, the loss tolerance of SPS-based QKD is compromised by photon number splitting (PNS) attacks against non-negligible multiphoton emissions. Decoy state (DS) techniques mitigate against PNS attacks, with WCP-DS QKD over several hundred km in fibre being demonstrated. DS QKD protocols for different source photon number statistics have been proposed, such as for binomial and thermal distributions. Here, we investigate the use of generalised DS techniques assuming we do not have access to the true photon number statistics of the SPS. Thus, we bound the source distribution using the mean photon number and the second-order correlation function, which provides us with enough partial knowledge to compute our decoy SPS protocols. Hence, we provide finite-key security bounds for an SPS-based Efficient BB84 for several decoy protocols with optimised parameters, and derive required SPS characteristics to achieve a key rate enhancement over DS WCPs and match their loss tolerance.

Global quantum communications will enable long-distance secure data transfer, networked distributed quantum information processing, and other entanglement-enabled technologies. Satellite quantum communication overcomes optical fibre range limitations, with the first realisations of satellite quantum key distribution (SatQKD) being rapidly developed. However, limited transmission times between satellite and ground station severely constrains the amount of secret key due to finite-block size effects. Here, we analyse these effects and the implications for system design and operation, utilising published results from the Micius satellite to construct an empirically-derived channel and system model for a trusted-node downlink employing efficient BB84 weak coherent pulse decoy states with optimised parameters. We quantify practical SatQKD performance limits and examine the effects of link efficiency, background light, source quality, and overpass geometries to estimate long-term key generation capacity. Our results provide a guide to the design and analysis of future SatQKD missions, and establishes performance benchmarks for both sources and detectors.

Quantum key distribution (QKD) with solid-state single-photon emitters is gaining traction due to their rapidly improving performance and compatibility with future quantum networks. We report a bright quantum dot based source of telecom photons by frequency converting a near- infrared InGaAs quantum dot to the telecom C-band (1). We implement polarisation encoded BB84 quantum key distribution (QKD), achieving a positive asymptotic key rate over 175 km of optical fibre. We also present finite key analysis optimised for typically non-ideal single- photon sources, achieving 8 orders of magnitude improvement with finite key rates of 40 kbps over 50 km in practical acquisition times of one hour (2). To extend the distances further, we take inspiration from decoy state QKD protocols – typically used to overcome photon- splitting attacks when using weak coherent states – and demonstrate a QD excitation scheme for implementing modulation of the photon number distribution. We show experimentally that the decoy state protocol enables the distribution of a secret key over more than 200 km of optical fibre.

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.

Global-scale quantum communication networks will require efficient long-distance distribution of quantum signals. While optical fibre communications are range-limited due to exponential losses in the absence of quantum memories and repeaters, satellites enable intercontinental quantum communications. However, the design of satellite quantum key distribution (SatQKD) systems has unique challenges over terrestrial networks. The typical approach to modelling 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 performance benchmarks to support the design of upcoming missions

High brightness, low-g2 single-photon sources (SPSs) are an alternative to commonly employed weak coherent pulse (WCP) sources for discrete variable quantum key distribution (QKD) and offer potential key-rate and finite-block scaling advantages. However, the loss tolerance of SPS-based QKD is compromised by photon number splitting (PNS) attacks against non-negligible multiphoton emissions. Decoy state (DS) techniques mitigate against PNS attacks, with WCP-DS QKD over several hundred km in fibre being demonstrated. Here, we adapt the DS method to any practical SPS that can easily generate multiple photon number distributions (PND) by attenuating its original photon emissions. Hence, we provide finite-key security bounds for a Multi-PND (adapted 2-Decoy) protocol using Efficient BB84 with optimised parameters. We use a particular true quantum dot source to compare its key rate generation with a Single-PND (adapted Non-Decoy) protocol for several finite block sizes. As expected, the Multi-PND gives higher key rates than the Single-PND, except for considerably small blocks. Moreover, the Multi-PND protocol goes beyond 200 km of tolerable fibre distance for high acquisition times. In this work, we set a generalised method to employ the DS techniques with any realistic SPS and further research may be done implementing distinct SPS characteristics.

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.

We demonstrate fibre-based quantum key distribution over 175 km using a bright frequency converted quantum dot single-photon source. The source is capable of producing count rates approaching 2 MHz at 1550 nm with second order correlations on the order of 3%. This allows for a measured key rate of 130 bps (100 kbps) at 175 km (50 km) in the asymptotic regime using static encoding and predicted positive key rate out to 188 km. This can be extended to 240 km using ultra-low loss fibre based on the measured source parameters.

Space-based quantum key distribution (QKD) overcomes the limits of distance between terrestrial users caused by losses in optical fibre. Thus, it is the most promising method to establish a global scale QKD network. While the first QKD platform in the space - “the Micius satellite” – was a ground-breaking proof of principle demonstration, it is not a commercially favourable solution. We present our Cube-Sat payload design which has a more economically viable key-rate. The whole system is consisting of two parts, a 2U transmitter payload in Cube Satellite and an Optical Ground Station (OGS) working as receiver. The system is designed for polarisation based BB84/Decoy-State protocol with 100Mhz key transmission rate. In order to avoid the light pollution near the metropolitan centres and provide flexibility, we present our progress towards a mobile OGS which will be able to act as a receiver for the quantum signal.