55,784 research outputs found

    Planar Visibility: Testing and Counting

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    In this paper we consider query versions of visibility testing and visibility counting. Let SS be a set of nn disjoint line segments in R2\R^2 and let ss be an element of SS. Visibility testing is to preprocess SS so that we can quickly determine if ss is visible from a query point qq. Visibility counting involves preprocessing SS so that one can quickly estimate the number of segments in SS visible from a query point qq. We present several data structures for the two query problems. The structures build upon a result by O'Rourke and Suri (1984) who showed that the subset, VS(s)V_S(s), of R2\R^2 that is weakly visible from a segment ss can be represented as the union of a set, CS(s)C_S(s), of O(n2)O(n^2) triangles, even though the complexity of VS(s)V_S(s) can be Ω(n4)\Omega(n^4). We define a variant of their covering, give efficient output-sensitive algorithms for computing it, and prove additional properties needed to obtain approximation bounds. Some of our bounds rely on a new combinatorial result that relates the number of segments of SS visible from a point pp to the number of triangles in ⋃s∈SCS(s)\bigcup_{s\in S} C_S(s) that contain pp.Comment: 22 page

    Vulnerability analysis of three remote voting methods

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    This article analyses three methods of remote voting in an uncontrolled environment: postal voting, internet voting and hybrid voting. It breaks down the voting process into different stages and compares their vulnerabilities considering criteria that must be respected in any democratic vote: confidentiality, anonymity, transparency, vote unicity and authenticity. Whether for safety or reliability, each vulnerability is quantified by three parameters: size, visibility and difficulty to achieve. The study concludes that the automatisation of treatments combined with the dematerialisation of the objects used during an election tends to substitute visible vulnerabilities of a lesser magnitude by invisible and widespread vulnerabilities.Comment: 15 page

    Testing of quantum phase in matter wave optics

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    Various phase concepts may be treated as special cases of the maximum likelihood estimation. For example the discrete Fourier estimation that actually coincides with the operational phase of Noh, Fouge`res and Mandel is obtained for continuous Gaussian signals with phase modulated mean.Since signals in quantum theory are discrete, a prediction different from that given by the Gaussian hypothesis should be obtained as the best fit assuming a discrete Poissonian statistics of the signal. Although the Gaussian estimation gives a satisfactory approximation for fitting the phase distribution of almost any state the optimal phase estimation offers in certain cases a measurable better performance. This has been demonstrated in neutron--optical experiment.Comment: 8 pages, 4 figure

    Pedestrian demand modelling of large cities: an applied example from London

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    This paper introduces a methodology for the development of city wide pedestrian demand models and shows its application to London. The approach used for modelling is Multiple Regression Analysis of independent variables against the dependent variable of observed pedestrian flows. The test samples were from manual observation studies of average total pedestrian flow per hour on 237 sample sites. The model will provide predicted flow values for all 7,526 street segments in the 25 square kilometres of Central London. It has been independently validated by Transport for London and is being tested against further observation data. The longer term aim is to extend the model to the entire greater London area and to incorporate additional policy levers for use as a transport planning and evaluation tool

    Experimental Long-Distance Decoy-State Quantum Key Distribution Based On Polarization Encoding

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    We demonstrate the decoy-state quantum key distribution (QKD) with one-way quantum communication in polarization space over 102km. Further, we simplify the experimental setup and use only one detector to implement the one-way decoy-state QKD over 75km, with the advantage to overcome the security loopholes due to the efficiency mismatch of detectors. Our experimental implementation can really offer the unconditionally secure final keys. We use 3 different intensities of 0, 0.2 and 0.6 for the pulses of source in our experiment. In order to eliminate the influences of polarization mode dispersion in the long-distance single-mode optical fiber, an automatic polarization compensation system is utilized to implement the active compensation.Comment: 4 pages,3 figure

    The Mach-Zehnder and the Teleporter

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    We suggest a self-testing teleportation configuration for photon q-bits based on a Mach-Zehnder interferometer. That is, Bob can tell how well the input state has been teleported without knowing what that input state was. One could imagine building a "locked" teleporter based on this configuration. The analysis is performed for continuous variable teleportation but the arrangement could equally be applied to discrete manipulations.Comment: 4 pages, 5 figure

    Noiseless Linear Amplification and Distillation of Entanglement

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    The idea of signal amplification is ubiquitous in the control of physical systems, and the ultimate performance limit of amplifiers is set by quantum physics. Increasing the amplitude of an unknown quantum optical field, or more generally any harmonic oscillator state, must introduce noise. This linear amplification noise prevents the perfect copying of the quantum state, enforces quantum limits on communications and metrology, and is the physical mechanism that prevents the increase of entanglement via local operations. It is known that non-deterministic versions of ideal cloning and local entanglement increase (distillation) are allowed, suggesting the possibility of non-deterministic noiseless linear amplification. Here we introduce, and experimentally demonstrate, such a noiseless linear amplifier for continuous-variables states of the optical field, and use it to demonstrate entanglement distillation of field-mode entanglement. This simple but powerful circuit can form the basis of practical devices for enhancing quantum technologies. The idea of noiseless amplification unifies approaches to cloning and distillation, and will find applications in quantum metrology and communications.Comment: Submitted 10 June 200
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