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SIGLEAvailable from British Library Document Supply Centre- DSC:DN056650 / BLDSC - British Library Document Supply CentreGBUnited Kingdo
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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... 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.
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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... 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.
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(September 23, 2011)We examine how to distinguish between unitary operators, when the exact form of thepossible operators is not known. Instead we are supplied with “programs” in the form ofunitary transforms, which can be used as... more
(September 23, 2011)We examine how to distinguish between unitary operators, when the exact form of thepossible operators is not known. Instead we are supplied with “programs” in the form ofunitary transforms, which can be used as references for identifying the unknown unitarytransform. All unitary transforms should be used as few times as possible. This situationis analoguous to programmable state discrimination. One difference, however, is that thequantum state to which we apply the unitary transforms may be entangled, leading to aricher variety of possible strategies. By suitable selection of an input state and generalizedmeasurement of the output state, both unambiguous and minimum-error discrimination canbe achieved. Pairwise comparison of operators, comparing each transform to be identifiedwith a program transform, is often a useful strategy. There are, however, situations in whichmore complicated strategies perform better. This is the case especially when the number ofallowed applications of program operations is different from the number of the transforms tobe identified.Keywords:unambiguous discrimination; optimum discrimination; operator comparison;generalized measurements
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We summarise the discussions at a virtual Community Workshop on Cold Atoms in Space concerning the status of cold atom technologies, the prospective scientific and societal opportunities offered by their deployment in space, and the... more
We summarise the discussions at a virtual Community Workshop on Cold Atoms in Space concerning the status of cold atom technologies, the prospective scientific and societal opportunities offered by their deployment in space, and the developments needed before cold atoms could be operated in space. The cold atom technologies discussed include atomic clocks, quantum gravimeters and accelerometers, and atom interferometers. Prospective applications include metrology, geodesy and measurement of terrestrial mass change due to, e.g., climate change, and fundamental science experiments such as tests of the equivalence principle, searches for dark matter, measurements of gravitational waves and tests of quantum mechanics. We review the current status of cold atom technologies and outline the requirements for their space qualification, including the development paths and the corresponding technical milestones, and identifying possible pathfinder missions to pave the way for missions to explo...
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The objective of the proposed macroscopic quantum resonators (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... more
The objective of the proposed macroscopic quantum resonators (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 at the interface with gravity. Developing the necessary technologies, achieving the required sensitivities and providing the necessary isolation of macroscopic quantum systems from their environment will lay the path for developing novel quantum sensors. Earlier studies showed that the proposal is feasible but that several critical challenges remain, and key technologies need to be developed. Recent scientific and technological developments since the original proposal of MAQRO promise 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 s...
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Most efforts in space-based global quantum communications focus on trusted networks. Here we analyse use of space-borne quantum memories and show that it provides much faster entanglement distribution rates than the existing hybrid... more
Most efforts in space-based global quantum communications focus on trusted networks. Here we analyse use of space-borne quantum memories and show that it provides much faster entanglement distribution rates than the existing hybrid architectures.
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College of Mechatronic Engineering and Automation, National University ofDefense Technology, Changsha 410073, ChinaAbstract. Engineering quantum systems o ers great opportunities both tech-nologically and scienti cally for communication,... more
College of Mechatronic Engineering and Automation, National University ofDefense Technology, Changsha 410073, ChinaAbstract. Engineering quantum systems o ers great opportunities both tech-nologically and scienti cally for communication, computation, and simulation.The construction and operation of large scale quantum information devicespresents a grand challenge and a major issue is the e ective control of coherentdynamics. This is often in the presence of decoherence which further complicatesthe task of determining the behaviour of the system. Here, we show how to de-termine open system Markovian dynamics of a quantum system with restrictedinitialisation and partial output state information.PACS numbers: 03.67.Lx
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Light’s bosonic nature leads to counterintuitive bunching effects. We describe a new experimentally-testable one in which single photons are conditionally sucked through a highly-reflecting beam splitter by a coherent state.
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Satellite-based platforms are currently the only feasible way of achieving intercontinental range for quantum communication, enabling thus the future global quantum internet. Recent demonstrations by the Chinese spacecraft Micius have... more
Satellite-based platforms are currently the only feasible way of achieving intercontinental range for quantum communication, enabling thus the future global quantum internet. Recent demonstrations by the Chinese spacecraft Micius have spurred an international space race and enormous interest in the development of both scientific and commercial systems. Research efforts so far have concentrated upon in-orbit demonstrations involving a single satellite and one or two ground stations. Ultimately satellite quantum key distribution should enable secure network communication between multiple nodes, which requires efficient scheduling of communication with the set of ground stations. Here we present a study of how satellite quantum key distribution can service many ground stations taking into account realistic constraints such as geography, operational hours, and most importantly, weather conditions. The objective is to maximise the number of keys a set of ground stations located in the Un...
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Photonic quantum memories are required in many applications in quantum information science with varying performance requirements depending on specific applications. Although classical light storage has been demonstrated in time scales of... more
Photonic quantum memories are required in many applications in quantum information science with varying performance requirements depending on specific applications. Although classical light storage has been demonstrated in time scales of minutes (Dudin et al., 2013; Heinze et al., 2013) to hours (Ma et al., 2021) in different systems, storing true single photons and single photon level coherent pulses are still limited to around a few seconds at most (Wang et al., 2021; Ortu et al., 2022; Hain et al., 2022; Stas et al., 2022). In this question, we would like to explore what the challenges for quantum memory storage for the purposes of quantum communication and the distribution of entanglement are, e.g. in quantum repeaters. Furthermore, recent work has proposed using quantum memories with hour-long storage times for quantum computation (Gouzien and Sangouard, 2021) and physically transporting single photons for astronomical interferometry (Bland-Hawthorn et al., 2021) and global qua...
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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... 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. While predicting instrument performance remains a challenging part of space missions, here, we report on how in-situ dark count rate trends of two silicon Geiger-mode avalanche photodiodes (GM-APD) may be predicted using high-fidelity radiation modelling techniques. This aids the diagnosis of unexpected trends in instrument performance as we were able to support the claim that differences in radiation shielding was a major contributor to the observed in-orbit data. While low-fidelity modelling is limited to lifetime predictions, implementing high-fidelity radiation modelling can have applications beyond this for low-earth orbit CubeSats.
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These data are measured and simulated from the experimental setup described in the publication: "Timing and Synchronisation for High-Loss Free-Space Quantum Communication with Hybrid de Bruijn Code"
CQT Principal Investigator Alexander Ling and Daniel Oi from the University of Strathclyde, UK, are collaborating to put an optical entanglement experiment into low Earth orbit. In this article, they describe the motivations and... more
CQT Principal Investigator Alexander Ling and Daniel Oi from the University of Strathclyde, UK, are collaborating to put an optical entanglement experiment into low Earth orbit. In this article, they describe the motivations and challenges. Quantum theory has survived all challenges so far: the pioneering experiments of Alain Aspect demonstrated quantum correlations violating classical predictions, and experiments have tested quantum entanglement – the “spooky action at a distance” that so disturbed Einstein – over ever greater separation. But these experiments have all been Earth-bound. Isn’t it time to test quantum theory in space? Science is driven by unexpected discoveries, discoveries often made at the limits of our understanding or in extreme physical environments. Going into space will stretch our tests of quantum theory, from observing entanglement over even longer distances to exploring relativistic effects.
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Institüt für Physik, Humboldt Universität zu Berlin Institute for Quantum Computing and Department of Physics and Astronomy, University of Waterloo Institute for Quantum Science and Technology, and Department of Physics and Astronomy,... more
Institüt für Physik, Humboldt Universität zu Berlin Institute for Quantum Computing and Department of Physics and Astronomy, University of Waterloo Institute for Quantum Science and Technology, and Department of Physics and Astronomy, University of Calgary Department of Physics, University of Alberta Institute of Applied Physics, Abbe Center of Photonics, Friedrich-Schiller-Universität Jena Cavendish Laboratory, University of Cambridge Fraunhofer Institute for Applied Optics and Precision Engineering SUPA Department of Physics, University of Strathclyde ICFO-Institut de Ciències Fotòniques, Barcelona Institute of Science and Technology Jet Propulsion Lab, California Institute of Technology Institut für Theoretische Physik, Freie Universität Berlin Department of Physics and Astronomy, University of Southampton Institute of Photonics and Quantum Sciences, SUPA, Heriot-Watt University Institute of Optical Sensor Systems, Deutsches Zentrum für Luftund Raumfahrt e.V. (DLR) Institut für O...
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We consider how to characterize the dynamics of a quantum system from a restricted set of initial states and measurements using Bayesian analysis. Previous work has shown that Hamiltonian systems can be well estimated from analysis of... more
We consider how to characterize the dynamics of a quantum system from a restricted set of initial states and measurements using Bayesian analysis. Previous work has shown that Hamiltonian systems can be well estimated from analysis of noisy data. Here we show how to generalize this approach to systems with moderate dephasing in the eigenbasis of the Hamiltonian. We illustrate the process for a range of three-level quantum systems. The results suggest that the Bayesian estimation of the frequencies and dephasing rates is generally highly accurate and the main source of errors are errors in the reconstructed Hamiltonian basis.
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Quantum dynamics can be driven by measurement. By constructing measurements that gain no information, effective unitary evolution can be induced on a quantum system, for example in ancilla driven quantum computation. In the non-ideal case... more
Quantum dynamics can be driven by measurement. By constructing measurements that gain no information, effective unitary evolution can be induced on a quantum system, for example in ancilla driven quantum computation. In the non-ideal case where a measurement does reveal some information about the system, it may be possible to "unlearn" this information and restore unitary evolution through subsequent measurements. Here we analyse two methods of quantum "unlearning" and present a simplified proof of the bound on the probability of successfully applying the required correction operators. We find that the probability of successful recovery is inversely related to the ability of the initial measurement to exclude the possibility of a state.
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A major challenge for quantum computation in ion trap systems is scalable integration of error correction and fault tolerance. We analyze a distributed architecture with rapid high fidelity local control within nodes and entangled links... more
A major challenge for quantum computation in ion trap systems is scalable integration of error correction and fault tolerance. We analyze a distributed architecture with rapid high fidelity local control within nodes and entangled links between nodes alleviating long-distance transport. We demonstrate fault-tolerant operator measurements which are used for error correction and non-local gates. This scheme is readily applied to linear ion traps which cannot be scaled up beyond a few ions per individual trap but which have access to a probabilistic entanglement mechanism. A proof-of-concept system is presented which is within the reach of current experiment.