14,867 research outputs found
Purifying noisy entanglement requires collective measurements
Known entanglement purification protocols for mixed states use collective
measurements on several copies of the state in order to increase the
entanglement of some of them. We address the question of whether it is possible
to purify the entanglement of a state by processing each copy separately. While
this is possible for pure states, we show that this is impossible, in general,
for mixed states. The importance of this result both conceptually and for
experimental realization of purification is discussed. We also give explicit
invariants of an entangled state of two qubits under local actions and
classical communication.Comment: 5 pages, Late
Bound Entanglement and Teleportation
Recently M. Horodecki, P. Horodecki and R. Horodecki have introduced a set of
density matrices of two spin-1 particles from which it is not possible to
distill any maximally entangled states, even though the density matrices are
entangled. Thus these density matrices do not allow reliable teleportation.
However it might nevertheless be the case that these states can be used for
teleportation, not reliably, but still with fidelity greater than that which
may be achieved with a classical scheme. We show that, at least for some of
these density matrices, teleportation cannot be achieved with better than
classical fidelity.Comment: 3 pages, RevTe
The biomechanical effects of the inclusion of a torque absorber and type of knee units on trans-femoral amputee gait
This paper reports on a pilot study
investigating the effects on the gait of two transfemoral
amputees of to the inclusion of a torque
absorber (TA) and its location relative to the
knee unit. Both subjects carried out gait tests
with a prosthesis with no TA with, a TA
proximal to the knee unit and with a TA distal to
the knee unit. Three-dimensional gait analysis
was carried out to establish the kinematic and
kinetic gait parameters of both the prosthetic and
sound side. It was found that the TA did not
significantly affect the sagittal kinetic and
kinematic parameters of the sound or the
prosthetic side. However, for one subject the
axial rotation of the socket relative to the foot
was significantly greater with the TA. It was
concluded that by using the methodology of this
pilot study, it is possible to investigate the
rotations in the transverse plane within the
prosthetic limb and pelvis. Further, including a
TA may reduce the relative motion between the
stump and the socket and therefore may decrease
skin breakdown due to diminished shear force
Non-local properties of multi-particle density matrices
As far as entanglement is concerned, two density matrices of particles
are equivalent if they are on the same orbit of the group of local unitary
transformations, (where the Hilbert space of
particle has dimension ). We show that for greater than or equal
to two, the number of independent parameters needed to specify an -particle
density matrix up to equivalence is . For
spin- particles we also show how to characterise generic
orbits, both by giving an explicit parametrisation of the orbits and by finding
a finite set of polynomial invariants which separate the orbits.Comment: 13 pages RevTe
Cross-country comparison of the replacement incentives of the EU ETS in 2008-12: the case of the power sector
In this paper, we conduct a cross-country quantitative analysis of the replacement incentives generated by the EU ETS for the power sector in 2008-12. In order to do so, the allocation rules of the Member States are applied to concrete reference power plants for three different fuel types (lignite, hard coal and gas). Based on these calculations, we compare installation-specific replacement in-centives across the Member States. Our analysis shows that replacement incentives vary significantly across Member States and typically deviate from the incentives provided in the reference case of full auctioning. Furthermore, the EU ETS allocation rules lead to perverse incentives in approximately 30% of the possible replacement options. Only 5 MS do not provide any perverse incentives. Finally, we explore the link between replacement incentives and allocation types. Based on our findings, we derive policy recommendations for the design of emission trading schemes emerging around the world. --EU emission trading scheme (EU ETS),replacement,adoption,diffusion,power sector,allocation rules
Bell inequalities for arbitrarily high dimensional systems
We develop a novel approach to Bell inequalities based on a constraint that
the correlations exhibited by local realistic theories must satisfy. This is
used to construct a family of Bell inequalities for bipartite quantum systems
of arbitrarily high dimensionality which are strongly resistant to noise. In
particular our work gives an analytic description of numerical results of D.
Kaszlikowski, P. Gnacinski, M. Zukowski, W. Miklaszewski, A. Zeilinger, Phys.
Rev. Lett. {\bf 85}, 4418 (2000) and T. Durt, D. Kaszlikowski, M. Zukowski,
quant-ph/0101084, and generalises them to arbitrarily high dimensionality.Comment: 6 pages, late
The Extent of Multi-particle Quantum Non-locality
It is well known that entangled quantum states can be nonlocal: the
correlations between local measurements carried out on these states cannot
always be reproduced by local hidden variable models. Svetlichny, followed by
others, showed that multipartite quantum states are even more nonlocal than
bipartite ones in the sense that nonlocal classical models with (super-luminal)
communication between some of the parties cannot reproduce the quantum
correlations. Here we study in detail the kinds of nonlocality present in
quantum states. More precisely we enquire what kinds of classical communication
patterns cannot reproduce quantum correlations. By studying the extremal points
of the space of all multiparty probability distributions, in which all parties
can make one of a pair of measurements each with two possible outcomes, we find
a necessary condition for classical nonlocal models to reproduce the statistics
of all quantum states. This condition extends and generalises work of
Svetlichny and others in which it was shown that a particular class of
classical nonlocal models, the ``separable'' models, cannot reproduce the
statistics of all multiparticle quantum states. Our condition shows that the
nonlocality present in some entangled multiparticle quantum states is much
stronger than previously thought. We also study the sufficiency of our
condition.Comment: 10 pages, 2 figures, journal versio
Interferometric near-field characterization of plasmonic slot waveguides in single- and poly-crystalline gold films
Plasmonic waveguides are a promising platform for integrated nanophotonic
circuits and nanoscale quantum optics. Their use is however often hampered by
the limited propagation length of the guided surface plasmon modes. A detailed
understanding of the influence of the material quality and the waveguide
geometry on the complex mode index is therefore crucial. In this letter, we
present interferometric near-field measurements at telecommunication wavelength
on plasmonic slot waveguides fabricated by focused ion beam milling in single-
and poly-crystalline gold films. We observe a significantly better performance
of the slot waveguides in the single-crystalline gold film for slot widths
below . In contrast for larger slot widths, both gold films
give rise to comparable mode propagation lengths. Our experimental observations
indicate that the nature of the dominant loss channel changes with increasing
gap size from Ohmic to leakage radiation. Our experimental findings are
reproduced by three dimensional numerical calculations.Comment: 4 figure
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