53,755 research outputs found
Sudden Critical Current Drops Induced in S/F Structures
In the search for new physical properties of S/F structures, we have found
that the superconductor critical current can be controlled by the domain state
of the neighboring ferromagnet. The superconductor is a thin wire of thickness
d_{s} ~ 2 xi_{S}. Nb/Co and Nb/Py (Permalloy Ni_{80}Fe_{20}) bilayer structures
were grown with a significant magnetic anisotropy. Critical current
measurements of Nb/Co structures with ferromagnet thickness d_{F} > 30nm show
sudden drops in two very defined steps when the measurements are made along the
hard axes direction (i.e. current track parallel to hard anisotropy axes
direction). These drops disappear when they are made along the easy axis
direction or when the ferromagnet thickness is below 30nm. The drops are
accompanied by vortex flux flow. In addition magnetorestistance measurements
close to Tc show a sharp increase near saturation fields of the ferromagnet.
Similar results are reproduced in Nb/Py bilayer structure with the ferromagnet
thickness d_{F} ~ 50nm along the easy anisotropy axes. These results are
explained as being due to spontaneous vortex formation and flow induced by
Bloch domain walls of the ferromagnet underneath. We argue these Bloch domain
walls produce a 2D vortex-antivortex lattice structure.Comment: 6 pages, 6 figure
Quantum interference and non-locality of independent photons from disparate sources
We quantitatively investigate the non-classicality and non-locality of a
whole new class of mixed disparate quantum and semiquantum photon sources at
the quantum-classical boundary. The latter include photon added thermal and
photon added coherent sources, experimentally investigated recently by Zavatta
et al. [Phys. Rev. Lett. 103, 140406 (2009)]. The key quantity in our
investigations is the visibility of the corresponding photon-photon correlation
function. We present explicit results on the violations of the Cauchy-Schwarz
inequality - which is a measure of nonclassicality - as well as of Bell-type
inequalities.Comment: 9 pages, 3 figure
Minimum Detection Efficiencies for a Loophole-Free Bell-type Test
We discuss the problem of finding the most favorable conditions for closing
the detection loophole in a test of local realism with a Bell inequality. For a
generic non-maximally entangled two-qubit state and two alternative measurement
bases we apply Hardy's proof of non-locality without inequality and derive an
Eberhard-like inequality. For an infinity of non-maximally entangled states we
find that it is possible to refute local realism by requiring perfect detection
efficiency for only one of the two measurements: the test is free from the
detection loophole for any value of the detection efficiency corresponding to
the other measurement. The maximum tolerable noise in a loophole-free test is
also evaluated.Comment: 4 pages, 2 figure
The joys of permutation symmetry: direct measurements of entanglement
So-called direct measurements of entanglement are collective measurements on
multiple copies of a (bipartite or multipartite) quantum system that directly
provide one a value for some entanglement measure, such as the concurrence for
bipartite states. Multiple copies are needed since the entanglement of a mixed
state is not a linear function of the density matrix. Unfortunately, so far all
experimental implementations of direct measurements made unverified assumptions
about the form of the states, and, therefore, do not qualify as entanglement
verification tests. I discuss how a direct measurement can be turned into a
quantitative entanglement verification test by exploiting a recent theorem by
Renner (R. Renner, Nature Physics 3, 645 (2007)).Comment: 4 pages, 3 figure
Constructing N-qubit entanglement monotones from anti-linear operators
We present a method to construct entanglement measures for pure states of
multipartite qubit systems. The key element of our approach is an antilinear
operator that we call {\em comb} in reference to the {\em hairy-ball theorem}.
For qubits (or spin 1/2) the combs are automatically invariant under
SL(2,\CC). This implies that the {\em filters} obtained from the combs are
entanglement monotones by construction. We give alternative formulae for the
concurrence and the 3-tangle as expectation values of certain antilinear
operators. As an application we discuss inequivalent types of genuine
four-qubit entanglement.Comment: 5 pages, revtex4; more detailed illustration of the metho
Hardy's proof of nonlocality in the presence of noise
We extend the validity of Hardy's nonlocality without inequalities proof to
cover the case of special one-parameter classes of non-pure statistical
operators. These mixed states are obtained by mixing the Hardy states with a
completely chaotic noise or with a colored noise and they represent a realistic
description of imperfect preparation processes of (pure) Hardy states in
nonlocality experiments. Within such a framework we are able to exhibit a
precise range of values of the parameter measuring the noise affecting the
non-optimal preparation of an arbitrary Hardy state, for which it is still
possible to put into evidence genuine nonlocal effects. Equivalently, our work
exhibits particular classes of bipartite mixed states whose constituents do not
admit any local and deterministic hidden variable model reproducing the quantum
mechanical predictions.Comment: 9 pages, 2 figures, RevTex, revised versio
Boat electrofishing survey of common smelt and common bullies in the Ohau Channel
We conducted a boat electrofishing survey of the Ohau Channel, which flows from Lake Rotorua to Lake Rotoiti, on 13 December 2007. The purpose of the survey was to investigate the longitudinal pattern in densities of common smelt (Retropinna retropinna) and common bullies (Gobiomorphus cotidianus) along the Ohau Channel. We caught 1,267 fish comprising three native fish species and two introduced fish species in 1.58 km of fished distance at a total of 10 sites. Native species caught were the common smelt, common bully and longfinned eel (Anguilla dieffenbachii) and introduced species were rainbow trout (Oncorhynchus mykiss) and goldfish (Carassius auratus). Assuming that the bow-mounted anodes effectively fished a 4 m swath then the total area fished was 6,328 m2 (0.632 ha).
Common smelt densities varied among the 10 different sites in the Ohau Channel ranging from 0 to 10.6 fish 100 m-2. Smelt density was higher at the upstream end of the channel near the weir at the Lake Rotorua outlet, decreasing with increasing distance from the weir. Smelt were found in the littoral zones but were not caught in mid-channel habitats. In the upstream reaches of the Ohau Channel, directly below the weir, a high number of juveniles (4.4 fish 100 m-2) were captured compared to the amount of juveniles captured at the other sites (0 â 1.2 fish 100 m-2).
Common bully densities varied among the 10 different sites in the Ohau Channel ranging from 0.2 to 58.3 fish 100 m-2. No longitudinal pattern in the distribution of common bullies was evident along the channel. The highest densities were found halfway along the Ohau Channel where there was an abundance of dense macrophyte beds. Common bully densities were found to be much higher in the edge habitats with macrophyte beds compared to the mid-channel habitats and the willow edge habitat where there were relatively low densities. Size frequency data shows that there is generally a higher proportion of small bullies than larger ones suggesting that recruitment is occurring.
Both adult and juvenile rainbow trout were observed in the Ohau Channel. Most of these individuals were found in the upstream section of the channel below the weir and ranged from a 75 mm juvenile to a fully grown adult about 500 mm long. Large longfinned eels were also captured and were only found in the downstream section of the Ohau Channel in willow-dominated edges. In the bottom third section of the channel, near the possible artificial embayment, goldfish were present
Greenberger-Horne-Zeilinger argument of nonlocality without inequalities for mixed states
We generalize the Greenberger-Horne-Zeilinger nonlocality without
inequalities argument to cover the case of arbitrary mixed statistical
operators associated to three-qubits quantum systems. More precisely, we
determine the radius of a ball (in the trace distance topology) surrounding the
pure GHZ state and containing arbitrary mixed statistical operators which
cannot be described by any local and realistic hidden variable model and which
are, as a consequence, noncompletely separable. As a practical application, we
focus on certain one-parameter classes of mixed states which are commonly
considered in the experimental realization of the original GHZ argument and
which result from imperfect preparations of the pure GHZ state. In these cases
we determine for which values of the parameter controlling the noise a
nonlocality argument can still be exhibited, despite the mixedness of the
considered states. Moreover, the effect of the imperfect nature of measurement
processes is discussed.Comment: 8 pages, RevTex; added references, corrected typo
The bimodal spiral galaxy surface brightness distribution
We have assessed the significance of Tully and Verheijen's (1997) bimodal
Ursa Major Cluster spiral galaxy near-infrared surface brightness distribution,
focussing on whether this bimodality is simply an artifact of small number
statistics. A Kolmogorov-Smirnov style of significance test shows that the
total distribution is fairly represented by a single-peaked distribution, but
that their isolated galaxy subsample (with no significant neighbours within a
projected distance of around 80 kpc) is bimodal at the 96 per cent level. We
have also investigated the assumptions underlying the isolated galaxy surface
brightness distribution, finding that the (often large) inclination corrections
used in the construction of this distribution reduce the significance of the
bimodality. We conclude that the Ursa Major Cluster dataset is insufficient to
establish the presence of a bimodal near-infrared surface brightness
distribution: an independent sample of around 100 isolated, low inclination
galaxies is required to establish bimodality at the 99 per cent level.Comment: 5 pages LaTeX; 2 embedded figures; re-submitted to MNRAS after
replying to referee's comment
Loophole-free Bell's experiment and two-photon all-versus-nothing violation of local realism
We introduce an all-versus-nothing proof of impossibility of
Einstein-Podolsky-Rosen's local elements of reality for two photons entangled
both in polarization and path degrees of freedom, which leads to a Bell's
inequality where the classical bound is 8 and the quantum prediction is 16. A
simple estimation of the detection efficiency required to close the detection
loophole using this proof gives eta > 0.69. This efficiency is lower than that
required for previous proposals.Comment: REVTeX4, 4 page
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