We investigate the optimization of direct measurement twin beam (signal-idler) quantum illumination. We focus on each of the three main components of such a system in turn: the detectors, the light beam parameters and the information processing protocol. Surprisingly, there can be an advantage to having a signal detector whose quantum efficiency is significantly less than perfect. This advantage does not vanish in the quantum lidar limits of low object reflectivities or signal strengths. We show that decreasing the pulse separation, while keeping the photon flux fixed to retain the same degree of covertness, can improve target detection performance dramatically. Finally we show that post-selecting on the idler detector firing is sub-optimal for target detection.
We investigate the optimization of direct measurement twin beam (signal-idler) quantum illumination. We focus on each of the three main components of such a system in turn: the detectors, the light beam parameters and the information processing protocol. Surprisingly, there can be an advantage to having a signal detector whose quantum efficiency is significantly less than perfect. This advantage does not vanish in the quantum lidar limits of low object reflectivities or signal strengths. We show that decreasing the pulse separation, while keeping the photon flux fixed to retain the same degree of covertness, can improve target detection performance dramatically. Finally, we show that post-selecting on the idler detector firing is sub-optimal for target detection.
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.
We describe simple quantum lidar and show it provides the best, most rapid identification of high reflectivity nearby targets. For lower reflectivities or more distant targets quantum lidar is impractical so we describe a protocol that mimics the relevant feature of quantum lidar via random intensity modulation of a classical beam. This provides a degree of covertness together with many of the other useful properties of a quantum lidar without the complication of producing quantum states and crucially, without the limit on mean photon number. Hence it is useful for identifying much lower reflectivity targets than is practical with quantum states.
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.
Quantum cryptography was designed to provide a new approach to the problem of distributing keys for private-key
cryptography. The principal idea is that security can be ensured by exploiting the laws of quantum physics and, in
particular, by the fact that any attempt to measure a quantum state will change it uncontrollably. This change can be
detected by the legitimate users of the communication channel and so reveal to them the presence of an eavesdropper. In this paper I explain (briefly) how quantum key distribution works and some of the progress that has been made towards making this a viable technology. With the principles of quantum communication and quantum key distribution firmly established, it is perhaps time to consider how efficient it can be made. It is interesting to ask, in particular, how many bits of information might reasonably be encoded securely on each photon. The use of photons entangled in their time of arrival might make it possible to achieve data rates in excess of 10 bits per photon.
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