KEYWORDS: Satellites, Quantum key distribution, Quantum modeling, Systems modeling, Satellite communications, Quantum bit errors, Quantum protocols, Free space optical communications, Quantum based satellite communication, Satellite to ground quantum key distribution
Quantum Key Distribution (QKD) is a potential mitigation against quantum computation advances that threaten currently deployed public key cryptosystems. QKD is also a precursor to large-scale quantum communications and the quantum internet. Terrestrial direct transmission of quantum photonic signals is restricted by optical fibre exponential absorption, hence space-based systems have been proposed for intercontinental distribution of quantum keys with lower-loss free-space transmission through vacuum. However, challenges remain for the practical deployment of satellite QKD (SatQKD) such as weather, background light, and terminal deployment. In particular, the short transmission time inherent in low Earth orbit satellite QKD and finite statistical uncertainty can limit secure key generation. Here we illustrate the analysis of these effects and some practical engineering design and operational constraints on key length using the Satellite Quantum Modelling and Analysis (SatQuMA) software package being developed for satellite quantum communications research.
Satellite based Quantum Key Distribution (QKD) in Low Earth Orbit (LEO) is currently the only viable technology to span thousands of kilometres. Since the typical overhead pass of a satellite lasts for a few minutes, it is crucial to increase the the signal rate to maximise the secret key length. For the QUARC CubeSat mission due to be launched within two years, we are designing a dual wavelength, weak-coherent-pulse decoy-state Bennett- Brassard ’84 (WCP DS BB84) QKD source. The optical payload is designed in a 12×9×5cm3 bespoke aluminium casing. The Discrete Variable QKD Source consists of two symmetric sources operating at 785 nm and 808 nm. The laser diodes are fixed to produce Horizontal,Vertical, Diagonal, and Anti-diagonal (H,V,D,A) polarisation respectively, which are combined and attenuated to a mean photon number of 0.3 and 0.5 photons/pulse. We ensure that the source is secure against most side channel attacks by spatially mode filtering the output beam and characterising their spectral and temporal characterstics. The extinction ratio of the source contributes to the intrinsic Qubit Error Rate(QBER) with 0.817±0.001%. This source operates at 200MHz, which is enough to provide secure key rates of a few kilo bits per second despite 40 dB of estimated loss in the free space channel.1
Medium-range terrestrial free-space quantum key distribution systems enable widespread secure networked communications in dense urban environments, where it would be infeasible to install a large number of short optical fibre links. Such networks need to perform over a wide range of conditions and their design has to balance key rate maximisation versus robust key generation over the greatest range of circumstances. Practicalities, such as manufacturability and deployment, further constrain the design space. Here, we examine challenges in translating experiment into engineering reality and identify efficient BB84 weak coherent pulse-decoy state protocol parameter regimes suitable for medium-range QKD systems considering likely system performance and environmental conditions.
Developing global quantum communication networks is integral to the realisation of the quantum internet, which
is expected to impart a similar revolutionary impact on the technological landscape as the classical internet.
Satellite-based quantum communications provides a practical route to global quantum networking. In this work,
we model finite statistics to determine the finite secret key length generation in SatQKD systems that implement
trusted-node downlink operation with weak coherent pulse sources. We optimise the finite key rate for different
practical operations and determine the key generation footprints. Our work provides an essential guide for future
satellite missions to establish performance benchmarks for both sources and detectors.
Progress in quantum computers and their threat to conventional public key infrastructure is driving new forms of encryption. Quantum Key Distribution (QKD) using entangled photons is a promising approach. A global QKD network can be achieved using satellites equipped with optical links. Despite numerous proposals, actual experimental work demonstrating relevant entanglement technology in space is limited due to the prohibitive cost of traditional satellite development. To make progress, we have designed a photon pair source that can operate on modular spacecraft called CubeSats. We report the in-orbit operation of the photon pair source on board an orbiting CubeSat and demonstrate pair generation and polarisation correlation under space conditions. The in-orbit polarisation correlations are compatible with ground-based tests, validating our design. This successful demonstration is a major experimental milestone towards a space-based quantum network. Our approach provides a cost-effective method for proving the space-worthiness of critical components used in entangled photon technology. We expect that it will also accelerate efforts to probe the overlap between quantum and relativistic models of physics.
To enable space-based quantum key distribution proposals the Centre for Quantum Technologies is developing
a source of entangled photons ruggedized to survive deployment in space and greatly miniaturised so that it
conforms to the strict form factor and power requirements of a 1U CubeSat. The Small Photon Entangling
Quantum System is an integrated instrument where the pump, photon pair source and detectors are combined
within a single optical tray and electronics package that is no larger than 10 cm x 10 cm x 3 cm. This footprint
enables the instrument to be placed onboard nanosatellites or the CubeLab structure aboard the International
Space Station. We will discuss the challenges and future prospects of CubeSat-based missions.
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