6,727 research outputs found
Bell's inequality with Dirac particles
We study Bell's inequality using the Bell states constructed from four
component Dirac spinors. Spin operator is related to the Pauli-Lubanski pseudo
vector which is relativistic invariant operator. By using Lorentz
transformation, in both Bell states and spin operator, we obtain an observer
independent Bell's inequality, so that it is maximally violated as long as it
is violated maximally in the rest frame.Comment: 7 pages. arXiv admin note: text overlap with arXiv:quant-ph/0308156
by other author
Complete light absorption in graphene-metamaterial corrugated structures
We show that surface-plasmon polaritons excited in negative permittivity
metamaterials having shallow periodic surface corrugation profiles can be
explored to push the absorption of single and continuous sheets of graphene up
to 100%. In the relaxation regime, the position of the plasmonic resonances of
the hybrid system is determined by the plasma frequency of the metamaterial,
allowing the frequency range for enhanced absorption to be set without the need
of engineering graphene.Comment: 6 pages, 4 figures; published version: text revised and references
adde
Weak ferromagnetism and spiral spin structures in honeycomb Hubbard planes
Within the Hartree Fock- RPA analysis, we derive the spin wave spectrum for
the weak ferromagnetic phase of the Hubbard model on the honeycomb lattice.
Assuming a uniform magnetization, the polar (optical) and acoustic branches of
the spin wave excitations are determined. The bipartite lattice geometry
produces a q-dependent phase difference between the spin wave amplitudes on the
two sub-lattices. We also find an instability of the uniform weakly magnetized
configuration to a weak antiferromagnetic spiraling spin structure, in the
lattice plane, with wave vector Q along the Gamma-K direction, for electron
densities n>0.6. We discuss the effect of diagonal disorder on both the
creation of electron bound states, enhancement of the density of states, and
the possible relevance of these effects to disorder induced ferromagnetism, as
observed in proton irradiated graphite.Comment: 13 pages, 7 figure
Thermal entanglement in the nanotubular system Na_2V_3O_7
Macroscopic entanglement witnesses have been put forward recently to reveal
nonlocal quantum correlations between individual constituents of the solid at
nonzero temperatures. Here we apply a recently proposed universal entanglement
witness, the magnetic susceptibility [New J. Phys. {\bf 7}, 258 (2005)] for the
estimation of the critical temperature in the nanotubular system below which thermal entanglement is present. As a result of an
analysis based on the experimental data for dc-magnetic susceptibility, we show
that K, which is approximately three times higher than the
critical temperature corresponding to the bipartite entanglement.Comment: 6 pages, 3 figures, REVTeX
Complementarity of Entanglement and Interference
A complementarity relation is shown between the visibility of interference
and bipartite entanglement in a two qubit interferometric system when the
parameters of the quantum operation change for a given input state. The
entanglement measure is a decreasing function of the visibility of
interference. The implications for quantum computation are briefly discussed.Comment: Final version, to appear on IJMPC; minor revision
Quantum mechanics explained
The physical motivation for the mathematical formalism of quantum mechanics
is made clear and compelling by starting from an obvious fact - essentially,
the stability of matter - and inquiring into its preconditions: what does it
take to make this fact possible?Comment: 29 pages, 5 figures. v2: revised in response to referee comment
A Schmidt number for density matrices
We introduce the notion of a Schmidt number of a bipartite density matrix,
characterizing the minimum Schmidt rank of the pure states that are needed to
construct the density matrix. We prove that Schmidt number is nonincreasing
under local quantum operations and classical communication. We show that
-positive maps witness Schmidt number, in the same way that positive maps
witness entanglement. We show that the family of states which is made from
mixing the completely mixed state and a maximally entangled state have
increasing Schmidt number depending on the amount of maximally entangled state
that is mixed in. We show that Schmidt number {\it does not necessarily
increase} when taking tensor copies of a density matrix ; we give an
example of a density matrix for which the Schmidt numbers of and are both 2.Comment: 5 pages RevTex, 1 typo in Proof Lemma 1 correcte
On Coulomb drag in double layer systems
We argue, for a wide class of systems including graphene, that in the low
temperature, high density, large separation and strong screening limits the
drag resistivity behaves as d^{-4}, where d is the separation between the two
layers. The results are independent of the energy dispersion relation, the
dependence on momentum of the transport time, and the wave function structure
factors. We discuss how a correct treatment of the electron-electron
interactions in an inhomogeneous dielectric background changes the theoretical
analysis of the experimental drag results of Ref. [1]. We find that a
quantitative understanding of the available experimental data [1] for drag in
graphene is lacking.Comment: http://iopscience.iop.org/0953-8984/24/33/335602
Wigner's little group and Berry's phase for massless particles
The ``little group'' for massless particles (namely, the Lorentz
transformations that leave a null vector invariant) is isomorphic to
the Euclidean group E2: translations and rotations in a plane. We show how to
obtain explicitly the rotation angle of E2 as a function of and we
relate that angle to Berry's topological phase. Some particles admit both signs
of helicity, and it is then possible to define a reduced density matrix for
their polarization. However, that density matrix is physically meaningless,
because it has no transformation law under the Lorentz group, even under
ordinary rotations.Comment: 4 pages revte
Confined magneto-optical waves in graphene
The electromagnetic mode spectrum of single-layer graphene subjected to a
quantizing magnetic field is computed taking into account intraband and
interband contributions to the magneto-optical conductivity. We find that a
sequence of weakly decaying quasi-transverse-electric modes, separated by
magnetoplasmon polariton modes, emerge due to the quantizing magnetic field.
The characteristics of these modes are tuneable, by changing the magnetic field
or the Fermi energy.Comment: 9 pages, 7 figures. published version: text and figures revised and
updated + new references and one figure adde
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