107 research outputs found

    Integration of highly probabilistic sources into optical quantum architectures: perpetual quantum computation

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    In this paper we introduce a design for an optical topological cluster state computer constructed exclusively from a single quantum component. Unlike previous efforts we eliminate the need for on demand, high fidelity photon sources and detectors and replace them with the same device utilised to create photon/photon entanglement. This introduces highly probabilistic elements into the optical architecture while maintaining complete specificity of the structure and operation for a large scale computer. Photons in this system are continually recycled back into the preparation network, allowing for a arbitrarily deep 3D cluster to be prepared using a comparatively small number of photonic qubits and consequently the elimination of high frequency, deterministic photon sources.Comment: 19 pages, 13 Figs (2 Appendices with additional Figs.). Comments welcom

    Surface code quantum computing by lattice surgery

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    In recent years, surface codes have become a leading method for quantum error correction in theoretical large scale computational and communications architecture designs. Their comparatively high fault-tolerant thresholds and their natural 2-dimensional nearest neighbour (2DNN) structure make them an obvious choice for large scale designs in experimentally realistic systems. While fundamentally based on the toric code of Kitaev, there are many variants, two of which are the planar- and defect- based codes. Planar codes require fewer qubits to implement (for the same strength of error correction), but are restricted to encoding a single qubit of information. Interactions between encoded qubits are achieved via transversal operations, thus destroying the inherent 2DNN nature of the code. In this paper we introduce a new technique enabling the coupling of two planar codes without transversal operations, maintaining the 2DNN of the encoded computer. Our lattice surgery technique comprises splitting and merging planar code surfaces, and enables us to perform universal quantum computation (including magic state injection) while removing the need for braided logic in a strictly 2DNN design, and hence reduces the overall qubit resources for logic operations. Those resources are further reduced by the use of a rotated lattice for the planar encoding. We show how lattice surgery allows us to distribute encoded GHZ states in a more direct (and overhead friendly) manner, and how a demonstration of an encoded CNOT between two distance 3 logical states is possible with 53 physical qubits, half of that required in any other known construction in 2D.Comment: Published version. 29 pages, 18 figure

    Simulating chemistry efficiently on fault-tolerant quantum computers

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    Quantum computers can in principle simulate quantum physics exponentially faster than their classical counterparts, but some technical hurdles remain. Here we consider methods to make proposed chemical simulation algorithms computationally fast on fault-tolerant quantum computers in the circuit model. Fault tolerance constrains the choice of available gates, so that arbitrary gates required for a simulation algorithm must be constructed from sequences of fundamental operations. We examine techniques for constructing arbitrary gates which perform substantially faster than circuits based on the conventional Solovay-Kitaev algorithm [C.M. Dawson and M.A. Nielsen, \emph{Quantum Inf. Comput.}, \textbf{6}:81, 2006]. For a given approximation error Ï”\epsilon, arbitrary single-qubit gates can be produced fault-tolerantly and using a limited set of gates in time which is O(logâĄÏ”)O(\log \epsilon) or O(log⁥logâĄÏ”)O(\log \log \epsilon); with sufficient parallel preparation of ancillas, constant average depth is possible using a method we call programmable ancilla rotations. Moreover, we construct and analyze efficient implementations of first- and second-quantized simulation algorithms using the fault-tolerant arbitrary gates and other techniques, such as implementing various subroutines in constant time. A specific example we analyze is the ground-state energy calculation for Lithium hydride.Comment: 33 pages, 18 figure

    Towards Quantum Repeaters with Solid-State Qubits: Spin-Photon Entanglement Generation using Self-Assembled Quantum Dots

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    In this chapter we review the use of spins in optically-active InAs quantum dots as the key physical building block for constructing a quantum repeater, with a particular focus on recent results demonstrating entanglement between a quantum memory (electron spin qubit) and a flying qubit (polarization- or frequency-encoded photonic qubit). This is a first step towards demonstrating entanglement between distant quantum memories (realized with quantum dots), which in turn is a milestone in the roadmap for building a functional quantum repeater. We also place this experimental work in context by providing an overview of quantum repeaters, their potential uses, and the challenges in implementing them.Comment: 51 pages. Expanded version of a chapter to appear in "Engineering the Atom-Photon Interaction" (Springer-Verlag, 2015; eds. A. Predojevic and M. W. Mitchell

    Wellbeing: The Challenge of ‘Operationalising’ an Holistic Concept within a Reductionist Public Health Programme

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    Background Wellbeing is a concept that, whilst contested, recognises individual and wider social, economic, political and environmental contextual influences – and is of growing interest and relevance locally and globally. In this paper, we report on one aspect of an evaluative research study conducted on a public health programme in North West England. Aims Within the context of a process evaluation that explored the delivery of a public health programme and sought to increase understanding of how and why different approaches worked well or not so well, this paper focuses specifically on the concept of wellbeing, examining perceptions of multiple stakeholders. Methods Interviews and focus groups were undertaken with 52 stakeholders involved in managing and facilitating the programme and its composite projects and with 90 community members involved as project participants. Data were subjected to thematic analysis, cross‐check and refining. Findings Findings highlight stakeholders’ diverse understandings of wellbeing, the complex relationship between health and wellbeing, and the perceived dissonance between the holistic concept of wellbeing and the reductionist design of the programme. Conclusions Wellbeing was understood to be ‘more than health’ and ‘more than happiness’, concerned with effective functioning, sense of purpose and flourishing. Essentially holistic, wellbeing offers opportunities to transcend clinical/pathogenic conceptions of ‘health’ and resonate with individuals, communities and local authorities. This raises concerns about how wellbeing can be meaningfully realised without compromising the concept, particularly when programmes are structured in reductionist ways requiring monitoring against discrete outcomes. Implications for practice include: utilising wellbeing as a driver for cross‐cutting public health in challenging economic and organisational contexts; acknowledging that wellbeing is essentially social as well as individual; appreciating that wellbeing is experienced in relation to contexts and surroundings; and recognising that wellbeing defined in terms of individual happiness risks compromising the future wellbeing of societies and the planet

    Convolutional Neural Networks Applied to Neutrino Events in a Liquid Argon Time Projection Chamber

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    We present several studies of convolutional neural networks applied to data coming from the MicroBooNE detector, a liquid argon time projection chamber (LArTPC). The algorithms studied include the classification of single particle images, the localization of single particle and neutrino interactions in an image, and the detection of a simulated neutrino event overlaid with cosmic ray backgrounds taken from real detector data. These studies demonstrate the potential of convolutional neural networks for particle identification or event detection on simulated neutrino interactions. We also address technical issues that arise when applying this technique to data from a large LArTPC at or near ground level

    Determination of muon momentum in the MicroBooNE LArTPC using an improved model of multiple Coulomb scattering

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    We discuss a technique for measuring a charged particle's momentum by means of multiple Coulomb scattering (MCS) in the MicroBooNE liquid argon time projection chamber (LArTPC). This method does not require the full particle ionization track to be contained inside of the detector volume as other track momentum reconstruction methods do (range-based momentum reconstruction and calorimetric momentum reconstruction). We motivate use of this technique, describe a tuning of the underlying phenomenological formula, quantify its performance on fully contained beam-neutrino-induced muon tracks both in simulation and in data, and quantify its performance on exiting muon tracks in simulation. Using simulation, we have shown that the standard Highland formula should be re-tuned specifically for scattering in liquid argon, which significantly improves the bias and resolution of the momentum measurement. With the tuned formula, we find agreement between data and simulation for contained tracks, with a small bias in the momentum reconstruction and with resolutions that vary as a function of track length, improving from about 10% for the shortest (one meter long) tracks to 5% for longer (several meter) tracks. For simulated exiting muons with at least one meter of track contained, we find a similarly small bias, and a resolution which is less than 15% for muons with momentum below 2 GeV/c. Above 2 GeV/c, results are given as a first estimate of the MCS momentum measurement capabilities of MicroBooNE for high momentum exiting tracks

    Being an English academic:a social domains account

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    This paper considers the differential placements of social actors in the contemporary English university, as practices consistent with neoliberal ideologies become increasingly influential. It uses Layder's theory of ‘social domains’ and the first-hand experiences of the author to explore how the options available–to students, those on precarious conditions of employment, and those occupying more influential roles in the institution–change in relation to alterations in structured social relations. Examples are provided of interventions by people opposed to both the rhetoric and the reality of developments in the sector, and reasons for their limited effectiveness are also discussed

    Systematic Reviews in Educational Research: Methodology, Perspectives and Application

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    This chapter explores the processes of reviewing literature as a research method. The logic of the family of research approaches called systematic review is analysed and the variation in techniques used in the different approaches explored using examples from existing reviews. The key distinctions between aggregative and configurative approaches are illustrated and the chapter signposts further reading on key issues in the systematic review process
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