Global-scale quantum communication networks will require efficient long-distance distribution of ... more 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.
We report the in-orbit operation of a photon pair source aboard a 1.65 kg nanosatellite and demon... more We report the in-orbit operation of a photon pair source aboard a 1.65 kg nanosatellite and demonstrate pair generation and polarization correlation under space conditions. The in-orbit photon correlations exhibit a contrast of 97 ± 2%, matching ground-based tests. This pathfinding mission overcomes the challenge of demonstrating in-orbit performance for the components of future entangled photon experiments.
The recently described pushframe imager, a parallelized single pixel camera capturing with a push... more The recently described pushframe imager, a parallelized single pixel camera capturing with a pushbroom-like motion, is intrinsically suited to both remote-sensing and compressive sampling. It optically applies a 2D mask to the imaged scene, before performing light integration along a single spatial axis, but previous work has not made use of the architecture's potential for taking measurements sparsely. In this paper we develop a strongly performing static binarized noiselet compressive sampling mask design, tailored to pushframe hardware, allowing both a single exposure per motion time-step, and retention of 2D correlations in the scene. Results from simulated and real-world captures are presented, with performance shown to be similar to that of immobile-and hence inappropriate for satellite use-wholescene imagers. A particular feature of our sampling approach is that the degree of compression can be varied without altering the pattern, and we demonstrate the utility of this for efficiently storing and transmitting multi-spectral images. Index Terms-Compressive sampling, pushframe imaging, columnar block compressed sensing (BCS), parallel single pixel camera (SPC).
Global-scale quantum communication networks will require efficient long-distance distribution of ... more 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 is 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.
SIGLEAvailable from British Library Document Supply Centre- DSC:DN056650 / BLDSC - British Librar... more SIGLEAvailable from British Library Document Supply Centre- DSC:DN056650 / BLDSC - British Library Document Supply CentreGBUnited Kingdo
Quantum key distribution with solid-state single-photon emitters is gaining traction due to their... more 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 (2) (0) = 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.
An incomplete quantum measurement can induce non-trivial dynamics between degenerate subspaces. A... more An incomplete quantum measurement can induce non-trivial dynamics between degenerate subspaces. A closed sequence of such projections produces a non-abelian holonomy (generalized geometric phase). We show how to induce unitary evolution from such finite sequences and construct a near deterministic repeat-until-success protocol. We also prove necessary and sufficient criteria on subspace dimension required for unitary subspace discrete dynamics.
In satellite-based quantum key distribution (QKD), the number of secret bits that can be generate... more 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 finitesize 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.
Global-scale quantum networking faces significant technical and scientific obstacles. Quantum rep... more 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.
Secured global quantum communication networks sharing a private encryption key can be established... more Secured global quantum communication networks sharing a private encryption key can be established with entangled photon sources onboard satellites in Earth orbit. Even though optical transceivers for ground-tospace communication exist, no space capable source of entangled photons has been demonstrated. A faster and cost-effective way to reach the orbit is by means of CubeSats. This demands significant miniaturisation work on the photon source to fit within the size, weight and power restrictions of CubeSats. The first milestone in our programme is to deploy a source of correlated photon pairs in space as a pathfinder experiment to demonstrate and validate the technology readiness level of critical optical components required to build entangled sources. The first attempt was unsuccessful when the launch vehicle (CRS Orb-3) failed shortly after takeoff , although our payload was successfully recovered intact and found to be fully operational. We are pleased to report that the second attempt with a newly built payload, has been successful and the first milestone has been accomplished. The source was launched on board the Galassia CubeSat (PSLV C29) to an orbit of approximately 550 km and 15 0 inclination. We observed in-orbit generation of high quality photon pair correlations (with a contrast of 97 ± 2%). This performance is compatible with the baseline data collected prior to launch and shows no degradation after spending 140 days in orbit. The performance of the subsystems and the in orbit correlation generation will be presented with the base line data. We will also present plans for future missions.
Quantum key distribution (QKD) has matured in recent years from laboratory proof-of-principle dem... more 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 Hα 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.
OSA Imaging and Applied Optics Congress 2021 (3D, COSI, DH, ISA, pcAOP), 2021
The recently described pushframe imager, a parallelized single pixel camera capturing with a push... more The recently described pushframe imager, a parallelized single pixel camera capturing with a pushbroom-like motion, is intrinsically suited to both remote-sensing and compressive sampling. It optically applies a 2D mask to the imaged scene, before performing light integration along a single spatial axis, but previous work has not made use of the architecture's potential for taking measurements sparsely. In this paper we develop a strongly performing static binarized noiselet compressive sampling mask design, tailored to pushframe hardware, allowing both a single exposure per motion time-step, and retention of 2D correlations in the scene. Results from simulated and real-world captures are presented, with performance shown to be similar to that of immobile-and hence inappropriate for satellite use-wholescene imagers. A particular feature of our sampling approach is that the degree of compression can be varied without altering the pattern, and we demonstrate the utility of this for efficiently storing and transmitting multi-spectral images. Index Terms-Compressive sampling, pushframe imaging, columnar block compressed sensing (BCS), parallel single pixel camera (SPC).
We describe in detail a general strategy for implementing a conditional geometric phase between t... more We describe in detail a general strategy for implementing a conditional geometric phase between two spins. Combined with single-spin operations, this simple operation is a universal gate for quantum computation, in that any unitary transformation can be implemented with arbitrary precision using only single-spin operations and conditional phase shifts. Thus quantum geometrical phases can form the basis of any quantum computation. Moreover, as the induced conditional phase depends only on the geometry of the paths executed by the spins it is resilient to certain types of errors and offers the potential of a naturally fault-tolerant way of performing quantum computation.
Space based quantum technologies are essential building blocks for global quantum networks. Howev... more Space based quantum technologies are essential building blocks for global quantum networks. However, the optoelectronic components and devices used are susceptible to radiation damage. The SpooQy-1 CubeSat mission demonstrated polarization-based quantum entanglement correlations using avalanche photodiodes for single-photon detection. We report the increasing dark count rates of two silicon Geiger-mode avalanche photodiodes (GM-APD) observed throughout its 2 year orbital lifetime. As a means of diagnosing the unexpected trends in the increase of dark counts, we implement a high-fidelity radiation model combined with 3D computer aided design models of the SpooQy-1 CubeSat to estimate the accumulated displacement damage dose in each photodiode. Using these results, we were able to support the claim that differences in radiation shielding was a major contributor to the observed in-orbit data. This illustrates how radiation modelling can have applications beyond conventional lifetime estimates for low-earth orbit CubeSats.
Terrestrial free-space (FS) quantum key distribution (QKD) is ideally suited for deployment in de... more Terrestrial free-space (FS) quantum key distribution (QKD) 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 FS optics of a QKD systems and assist in their design.
Global-scale quantum communication links will form the backbone of the quantum internet. However,... more Global-scale quantum communication links will form the backbone of the quantum internet. However, exponential loss in optical fibres precludes any realistic application beyond few hundred kilometres. Quantum repeaters and space-based systems offer solutions to overcome this limitation. Here, we analyse the use of quantum memory (QM)-equipped satellites for quantum communication focussing on global range repeaters and memory-assisted (MA-) QKD, where QMs help increase the key rate by synchronising otherwise probabilistic detection events. We demonstrate that satellites equipped with QMs provide three orders of magnitude faster entanglement distribution rates than existing protocols based on fibre-based repeaters or space systems without QMs. We analyse how entanglement distribution performance depends on memory characteristics, determine benchmarks to assess the performance of different tasks and propose various architectures for light-matter interfaces. Our work provides a roadmap to realise unconditionally secure quantum communications over global distances with near-term technologies.
Global-scale quantum communication networks will require efficient long-distance distribution of ... more 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.
We report the in-orbit operation of a photon pair source aboard a 1.65 kg nanosatellite and demon... more We report the in-orbit operation of a photon pair source aboard a 1.65 kg nanosatellite and demonstrate pair generation and polarization correlation under space conditions. The in-orbit photon correlations exhibit a contrast of 97 ± 2%, matching ground-based tests. This pathfinding mission overcomes the challenge of demonstrating in-orbit performance for the components of future entangled photon experiments.
The recently described pushframe imager, a parallelized single pixel camera capturing with a push... more The recently described pushframe imager, a parallelized single pixel camera capturing with a pushbroom-like motion, is intrinsically suited to both remote-sensing and compressive sampling. It optically applies a 2D mask to the imaged scene, before performing light integration along a single spatial axis, but previous work has not made use of the architecture's potential for taking measurements sparsely. In this paper we develop a strongly performing static binarized noiselet compressive sampling mask design, tailored to pushframe hardware, allowing both a single exposure per motion time-step, and retention of 2D correlations in the scene. Results from simulated and real-world captures are presented, with performance shown to be similar to that of immobile-and hence inappropriate for satellite use-wholescene imagers. A particular feature of our sampling approach is that the degree of compression can be varied without altering the pattern, and we demonstrate the utility of this for efficiently storing and transmitting multi-spectral images. Index Terms-Compressive sampling, pushframe imaging, columnar block compressed sensing (BCS), parallel single pixel camera (SPC).
Global-scale quantum communication networks will require efficient long-distance distribution of ... more 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 is 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.
SIGLEAvailable from British Library Document Supply Centre- DSC:DN056650 / BLDSC - British Librar... more SIGLEAvailable from British Library Document Supply Centre- DSC:DN056650 / BLDSC - British Library Document Supply CentreGBUnited Kingdo
Quantum key distribution with solid-state single-photon emitters is gaining traction due to their... more 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 (2) (0) = 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.
An incomplete quantum measurement can induce non-trivial dynamics between degenerate subspaces. A... more An incomplete quantum measurement can induce non-trivial dynamics between degenerate subspaces. A closed sequence of such projections produces a non-abelian holonomy (generalized geometric phase). We show how to induce unitary evolution from such finite sequences and construct a near deterministic repeat-until-success protocol. We also prove necessary and sufficient criteria on subspace dimension required for unitary subspace discrete dynamics.
In satellite-based quantum key distribution (QKD), the number of secret bits that can be generate... more 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 finitesize 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.
Global-scale quantum networking faces significant technical and scientific obstacles. Quantum rep... more 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.
Secured global quantum communication networks sharing a private encryption key can be established... more Secured global quantum communication networks sharing a private encryption key can be established with entangled photon sources onboard satellites in Earth orbit. Even though optical transceivers for ground-tospace communication exist, no space capable source of entangled photons has been demonstrated. A faster and cost-effective way to reach the orbit is by means of CubeSats. This demands significant miniaturisation work on the photon source to fit within the size, weight and power restrictions of CubeSats. The first milestone in our programme is to deploy a source of correlated photon pairs in space as a pathfinder experiment to demonstrate and validate the technology readiness level of critical optical components required to build entangled sources. The first attempt was unsuccessful when the launch vehicle (CRS Orb-3) failed shortly after takeoff , although our payload was successfully recovered intact and found to be fully operational. We are pleased to report that the second attempt with a newly built payload, has been successful and the first milestone has been accomplished. The source was launched on board the Galassia CubeSat (PSLV C29) to an orbit of approximately 550 km and 15 0 inclination. We observed in-orbit generation of high quality photon pair correlations (with a contrast of 97 ± 2%). This performance is compatible with the baseline data collected prior to launch and shows no degradation after spending 140 days in orbit. The performance of the subsystems and the in orbit correlation generation will be presented with the base line data. We will also present plans for future missions.
Quantum key distribution (QKD) has matured in recent years from laboratory proof-of-principle dem... more 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 Hα 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.
OSA Imaging and Applied Optics Congress 2021 (3D, COSI, DH, ISA, pcAOP), 2021
The recently described pushframe imager, a parallelized single pixel camera capturing with a push... more The recently described pushframe imager, a parallelized single pixel camera capturing with a pushbroom-like motion, is intrinsically suited to both remote-sensing and compressive sampling. It optically applies a 2D mask to the imaged scene, before performing light integration along a single spatial axis, but previous work has not made use of the architecture's potential for taking measurements sparsely. In this paper we develop a strongly performing static binarized noiselet compressive sampling mask design, tailored to pushframe hardware, allowing both a single exposure per motion time-step, and retention of 2D correlations in the scene. Results from simulated and real-world captures are presented, with performance shown to be similar to that of immobile-and hence inappropriate for satellite use-wholescene imagers. A particular feature of our sampling approach is that the degree of compression can be varied without altering the pattern, and we demonstrate the utility of this for efficiently storing and transmitting multi-spectral images. Index Terms-Compressive sampling, pushframe imaging, columnar block compressed sensing (BCS), parallel single pixel camera (SPC).
We describe in detail a general strategy for implementing a conditional geometric phase between t... more We describe in detail a general strategy for implementing a conditional geometric phase between two spins. Combined with single-spin operations, this simple operation is a universal gate for quantum computation, in that any unitary transformation can be implemented with arbitrary precision using only single-spin operations and conditional phase shifts. Thus quantum geometrical phases can form the basis of any quantum computation. Moreover, as the induced conditional phase depends only on the geometry of the paths executed by the spins it is resilient to certain types of errors and offers the potential of a naturally fault-tolerant way of performing quantum computation.
Space based quantum technologies are essential building blocks for global quantum networks. Howev... more Space based quantum technologies are essential building blocks for global quantum networks. However, the optoelectronic components and devices used are susceptible to radiation damage. The SpooQy-1 CubeSat mission demonstrated polarization-based quantum entanglement correlations using avalanche photodiodes for single-photon detection. We report the increasing dark count rates of two silicon Geiger-mode avalanche photodiodes (GM-APD) observed throughout its 2 year orbital lifetime. As a means of diagnosing the unexpected trends in the increase of dark counts, we implement a high-fidelity radiation model combined with 3D computer aided design models of the SpooQy-1 CubeSat to estimate the accumulated displacement damage dose in each photodiode. Using these results, we were able to support the claim that differences in radiation shielding was a major contributor to the observed in-orbit data. This illustrates how radiation modelling can have applications beyond conventional lifetime estimates for low-earth orbit CubeSats.
Terrestrial free-space (FS) quantum key distribution (QKD) is ideally suited for deployment in de... more Terrestrial free-space (FS) quantum key distribution (QKD) 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 FS optics of a QKD systems and assist in their design.
Global-scale quantum communication links will form the backbone of the quantum internet. However,... more Global-scale quantum communication links will form the backbone of the quantum internet. However, exponential loss in optical fibres precludes any realistic application beyond few hundred kilometres. Quantum repeaters and space-based systems offer solutions to overcome this limitation. Here, we analyse the use of quantum memory (QM)-equipped satellites for quantum communication focussing on global range repeaters and memory-assisted (MA-) QKD, where QMs help increase the key rate by synchronising otherwise probabilistic detection events. We demonstrate that satellites equipped with QMs provide three orders of magnitude faster entanglement distribution rates than existing protocols based on fibre-based repeaters or space systems without QMs. We analyse how entanglement distribution performance depends on memory characteristics, determine benchmarks to assess the performance of different tasks and propose various architectures for light-matter interfaces. Our work provides a roadmap to realise unconditionally secure quantum communications over global distances with near-term technologies.
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Papers by Daniel Oi