28,742 research outputs found

    Comment on "High Field Studies of Superconducting Fluctuations in High-Tc Cuprates. Evidence for a Small Gap distinct from the Large Pseudogap"

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    By using high magnetic field data to estimate the background conductivity, Rullier-Albenque and coworkers have recently published [Phys.Rev.B 84, 014522 (2011)] experimental evidence that the in-plane paraconductivity in cuprates is almost independent of doping. In this Comment we also show that, in contrast with their claims, these useful data may be explained at a quantitative level in terms of the Gaussian-Ginzburg-Landau approach for layered superconductors, extended by Carballeira and coworkers to high reduced-temperatures by introducing a total-energy cutoff [Phys.Rev.B 63, 144515 (2001)]. When combined, these two conclusions further suggest that the paraconductivity in cuprates is conventional, i.e., associated with fluctuating superconducting pairs above the mean-field critical temperature.Comment: 9 pages, 1 figur

    Entanglement in a second order quantum phase transition

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    We consider a system of mutually interacting spin 1/2 embedded in a transverse magnetic field which undergo a second order quantum phase transition. We analyze the entanglement properties and the spin squeezing of the ground state and show that, contrarily to the one-dimensional case, a cusp-like singularity appears at the critical point λc\lambda_c, in the thermodynamic limit. We also show that there exists a value λ0≥λc\lambda_0 \geq \lambda_c above which the ground state is not spin squeezed despite a nonvanishing concurrence.Comment: 4 pages, 4 EPS figures, minor corrections added and title change

    Classification of GHZ-type, W-type and GHZ-W-type multiqubit entanglements

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    We propose the concept of SLOCC-equivalent basis (SEB) in the multiqubit space. In particular, two special SEBs, the GHZ-type and the W-type basis are introduced. They can make up a more general family of multiqubit states, the GHZ-W-type states, which is a useful kind of entanglement for quantum teleporatation and error correction. We completely characterize the property of this type of states, and mainly classify the GHZ-type states and the W-type states in a regular way, which is related to the enumerative combinatorics. Many concrete examples are given to exhibit how our method is used for the classification of these entangled states.Comment: 16 pages, Revte

    Optimal quantum teleportation with an arbitrary pure state

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    We derive the maximum fidelity attainable for teleportation using a shared pair of d-level systems in an arbitrary pure state. This derivation provides a complete set of necessary and sufficient conditions for optimal teleportation protocols. We also discuss the information on the teleported particle which is revealed in course of the protocol using a non-maximally entangled state.Comment: 10 pages, REVTe

    Hydrogen column density evaluations toward Capella: consequences on the interstellar deuterium abundance

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    The deuterium abundance evaluation in the direction of Capella has for a long time been used as a reference for the local interstellar medium (ISM) within our Galaxy. We show here that broad and weak HI components could be present on the Capella line of sight, leading to a large new additional systematic uncertainty on the N(HI) evaluation. The D/H ratio toward Capella is found to be equal to 1.67 (+/-0.3)x10^-5 with almost identical chi^2 for all the fits (this range includes only the systematic error; the 2 sigma statistical one is almost negligible in comparison). It is concluded that D/H evaluations over HI column densities below 10^19 cm^-2 (even perhaps below 10^20 cm^-2 if demonstrated by additional observations) may present larger uncertainties than previously anticipated. It is mentionned that the D/O ratio might be a better tracer for DI variations in the ISM as recently measured by the Far Ultraviolet Spectroscopic Explorer (FUSE).Comment: Accepted for publication in the Astrophysical Journal Letter
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