3,047 research outputs found

    Optimal purification of a generic n-qudit state

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    We propose a quantum algorithm for the purification of a generic mixed state ρ\rho of a nn-qudit system by using an ancillary nn-qudit system. The algorithm is optimal in that (i) the number of ancillary qudits cannot be reduced, (ii) the number of parameters which determine the purification state Ψ>|\Psi> exactly equals the number of degrees of freedom of ρ\rho, and (iii) Ψ>|\Psi> is easily determined from the density matrix ρ\rho. Moreover, we introduce a quantum circuit in which the quantum gates are unitary transformations acting on a 2n2n-qudit system. These transformations are determined by parameters that can be tuned to generate, once the ancillary qudits are disregarded, any given mixed nn-qudit state.Comment: 8 pages, 9 figures, remarks adde

    Translationally invariant conservation laws of local Lindblad equations

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    We study the conditions under which one can conserve local translationally invariant operators by local translationally invariant Lindblad equations in one-dimensional rings of spin-1/2 particles. We prove that for any 1-local operator (e.g., particle density) there exist Lindblad dissipators that conserve that operator, while on the other hand we prove that among 2-local operators (e.g., energy density) only trivial ones of the Ising type can be conserved, while all the other cannot be conserved, neither locally nor globally, by any 2- or 3-local translationally invariant Lindblad equation. Our statements hold for rings of any finite length larger than some minimal length determined by the locality of Lindblad equation. These results show in particular that conservation of energy density in interacting systems is fundamentally more difficult than conservation of 1-local quantities.Comment: 15 pages, no fig

    Conservative chaotic map as a model of quantum many-body environment

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    We study the dynamics of the entanglement between two qubits coupled to a common chaotic environment, described by the quantum kicked rotator model. We show that the kicked rotator, which is a single-particle deterministic dynamical system, can reproduce the effects of a pure dephasing many-body bath. Indeed, in the semiclassical limit the interaction with the kicked rotator can be described as a random phase-kick, so that decoherence is induced in the two-qubit system. We also show that our model can efficiently simulate non-Markovian environments.Comment: 8 pages, 4 figure

    Pairing of Cooper pairs in a Josephson junction network containing an impurity

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    We show how to induce pairing of Cooper pairs (and, thus, 4e4e superconductivity) as a result of local embedding of a quantum impurity in a Josephson network fabricable with conventional junctions. We find that a boundary double Sine-Gordon model provides an accurate description of the dc Josephson current patterns, as well as of the stable phases accessible to the network. We point out that tunneling of pairs of Cooper pairs is robust against quantum fluctuations, as a consequence of the time reversal invariance, arising when the central region of the network is pierced by a dimensionless magnetic flux ϕ=π\phi = \pi. We find that, for ϕ=π\phi = \pi, a stable attractive finite coupling fixed point emerges and point out its relevance for engineering a two level quantum system with enhanced coherence.Comment: 5 Pages, 5 Figures. Small modifications, ref.[11] added. To appear in EP

    Teleportation in a noisy environment: a quantum trajectories approach

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    We study the fidelity of quantum teleportation for the situation in which quantum logic gates are used to provide the long distance entanglement required in the protocol, and where the effect of a noisy environment is modeled by means of a generalized amplitude damping channel. Our results demonstrate the effectiveness of the quantum trajectories approach, which allows the simulation of open systems with a large number of qubits (up to 24). This shows that the method is suitable for modeling quantum information protocols in realistic environments.Comment: 9 pages, 2 figure

    Frustration of decoherence in YY-shaped superconducting Josephson networks

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    We examine the possibility that pertinent impurities in a condensed matter system may help in designing quantum devices with enhanced coherent behaviors. For this purpose, we analyze a field theory model describing Y- shaped superconducting Josephson networks. We show that a new finite coupling stable infrared fixed point emerges in its phase diagram; we then explicitly evidence that, when engineered to operate near by this new fixed point, Y-shaped networks support two-level quantum systems, for which the entanglement with the environment is frustrated. We briefly address the potential relevance of this result for engineering finite-size superconducting devices with enhanced quantum coherence. Our approach uses boundary conformal field theory since it naturally allows for a field-theoretical treatment of the phase slips (instantons), describing the quantum tunneling between degenerate levels.Comment: 11 pages, 5 .eps figures; several changes in the presentation and in the figures, upgraded reference

    Persistent and transient productive inefficiency in a regulated industry: electricity distribution in New Zealand

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    The productive efficiency of a firm can be decomposed into two parts, one persistent and one transient. So far, most of the cost efficiency studies estimated frontier models that provide either the transient or the persistent part of productive efficiency. This distinction seems to be appealing also for regulators. During the last decades, public utilities such as water and electricity have witnessed a wave of regulatory reforms aimed at improving efficiency through incentive regulation. Most of these regulation schemes use benchmarking, namely measuring companies' efficiency and rewarding them accordingly. The purpose of this study is to assess the level of persistent and transient efficiency in an electricity sector and to investigate their implications under price cap regulation. Using a theoretical model, we show that an imperfectly informed regulator may not disentangle the two parts of the cost efficiency; therefore, they may fail in setting optimal efficiency targets. The introduction of minimum quality standards may not offer a valid solution. To provide evidence we use data on 28 New Zealand electricity distribution companies between 1996 and 2011. We estimate a total cost function using three stochastic frontier models for panel data. We start with the random effects model (RE) proposed by Pitt and Lee (1981) that provides information on the persistent part of the cost effciency. Then, we apply the true random effects model (TRE) proposed by Greene (2005a, 2005b) that provides information on the transient part. Finally, we use the generalized true random effects model (GTRE) that allows for the simultaneous estimation of both transient and persistent efficiency. We find weak evidence that persistent efficiency is associated to higher quality, and wrong efficiency targets are associated to lower quality compliance

    Thermodynamic Bounds on Efficiency for Systems with Broken Time-reversal Symmetry

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    We show that for systems with broken time-reversal symmetry the maximum efficiency and the efficiency at maximum power are both determined by two parameters: a "figure of merit" and an asymmetry parameter. In contrast to the time-symmetric case, the figure of merit is bounded from above; nevertheless the Carnot efficiency can be reached at lower and lower values of the figure of merit and far from the so-called strong coupling condition as the asymmetry parameter increases. Moreover, the Curzon-Ahlborn limit for efficiency at maximum power can be overcome within linear response. Finally, always within linear response, it is allowed to have simultaneously Carnot efficiency and non-zero power.Comment: Final version, 4 pages, 3 figure
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