958 research outputs found
Entanglement monotones and maximally entangled states in multipartite qubit systems
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-,
five- and six-qubit entanglement.Comment: 7 pages, revtex4. Talk presented at the Workshop on "Quantum
entanglement in physical and information sciences", SNS Pisa, December 14-18,
200
Out of equilibrium correlation functions of quantum anisotropic XY models: one-particle excitations
We calculate exactly matrix elements between states that are not eigenstates
of the quantum XY model for general anisotropy. Such quantities therefore
describe non equilibrium properties of the system; the Hamiltonian does not
contain any time dependence. These matrix elements are expressed as a sum of
Pfaffians. For single particle excitations on the ground state the Pfaffians in
the sum simplify to determinants.Comment: 11 pages, no figures; revtex. Minor changes in the text; list of
refs. modifie
One-Dimensional Impenetrable Anyons in Thermal Equilibrium. II. Determinant Representation for the Dynamic Correlation Functions
We have obtained a determinant representation for the time- and
temperature-dependent field-field correlation function of the impenetrable
Lieb-Liniger gas of anyons through direct summation of the form factors. In the
static case, the obtained results are shown to be equivalent to those that
follow from the anyonic generalization of Lenard's formula.Comment: 16 pages, RevTeX
Excitation and Entanglement Transfer Near Quantum Critical Points
Recently, there has been growing interest in employing condensed matter
systems such as quantum spin or harmonic chains as quantum channels for short
distance communication. Many properties of such chains are determined by the
spectral gap between their ground and excited states. In particular this gap
vanishes at critical points of quantum phase transitions. In this article we
study the relation between the transfer speed and quality of such a system and
the size of its spectral gap. We find that the transfer is almost perfect but
slow for large spectral gaps and fast but rather inefficient for small gaps.Comment: submitted to Optics and Spectroscopy special issue for ICQO'200
Quantum many particle systems in ring-shaped optical lattices
In the present work we demonstrate how to realize 1d-optical closed lattice
experimentally, including a {\it tunable} boundary phase-twist. The latter may
induce ``persistent currents'', visible by studing the atoms' momentum
distribution. We show how important phenomena in 1d-physics can be studied by
physical realization of systems of trapped atoms in ring-shaped optical
lattices. A mixture of bosonic and/or fermionic atoms can be loaded into the
lattice, realizing a generic quantum system of many interacting particles.Comment: 10 pages, 5 figures. To be published in PR
Classification of qubit entanglement: SL(2,C) versus SU(2) invariance
The role of SU(2) invariants for the classification of multiparty
entanglement is discussed and exemplified for the Kempe invariant I_5 of pure
three-qubit states. It is found to being an independent invariant only in
presence of both W-type entanglement and threetangle. In this case, constant
I_5 admits for a wide range of both threetangle and concurrences. Furthermore,
the present analysis indicates that an SL^3 orbit of states with equal tangles
but continuously varying I_5 must exist. This means that I_5 provides no
information on the entanglement in the system in addition to that contained in
the tangles (concurrences and threetangle) themselves. Together with the
numerical evidence that I_5 is an entanglement monotone this implies that SU(2)
invariance or the monotone property are too weak requirements for the
characterization and quantification of entanglement for systems of three
qubits, and that SL(2,C) invariance is required. This conclusion can be
extended to general multipartite systems (including higher local dimension)
because the entanglement classes of three-qubit systems appear as subclasses.Comment: 9 pages, 10 figures, revtex
Mesoscopic BCS pairing in the repulsive 1d-Hubbard model
We study mesoscopic pairing in the one dimensional repulsive Hubbard model
and its interplay with the BCS model in the canonical ensemble. The key tool is
comparing the Bethe ansatz equations of the two models in the limit of small
Coulomb repulsion. For the ordinary Hubbard interaction the BCS Bethe equations
with infinite pairing coupling are recovered; a finite pairing is obtained by
considering a further density-dependent phase-correlation in the hopping
amplitude of the Hubbard model. We find that spin degrees of freedom in the
Hubbard ground state are arranged in a state of the BCS type, where the
Cooper-pairs form an un-condensed liquid on a ``lattice'' of single particle
energies provided by the Hubbard charge degrees of freedom; the condensation in
the BCS ground state corresponds to Hubbard excitations constituted by a sea of
spin singlets.Comment: 15 pages, 6 figures. To be published on Physical Review
One-Dimensional Impenetrable Anyons in Thermal Equilibrium. IV. Large Time and Distance Asymptotic Behavior of the Correlation Functions
This work presents the derivation of the large time and distance asymptotic
behavior of the field-field correlation functions of impenetrable
one-dimensional anyons at finite temperature. In the appropriate limits of the
statistics parameter, we recover the well-known results for impenetrable bosons
and free fermions. In the low-temperature (usually expected to be the
"conformal") limit, and for all values of the statistics parameter away from
the bosonic point, the leading term in the correlator does not agree with the
prediction of the conformal field theory, and is determined by the singularity
of the density of the single-particle states at the bottom of the
single-particle energy spectrum.Comment: 26 pages, RevTeX
Accretion Disks Around Young Objects. II. Tests of Well-Mixed Models with Ism Dust
We construct detailed vertical structure models of irradiated accretion disks
around T Tauri stars with interstellar medium dust uniformly mixed with gas.
The dependence of the structure and emission properties on mass accretion rate,
viscosity parameter, and disk radius is explored using these models. The
theoretical spectral energy distributions (SEDs) and images for all
inclinations are compared with observations of the entire population of
Classical T Tauri stars (CTTS) and Class I objects in Taurus. In particular, we
find that the median near-infrared fluxes can be explained within the errors
with the most recent values for the median accretion rates for CTTS. We further
show that the majority of the Class I sources in Taurus cannot be Class II
sources viewed edge-on because they are too luminous and their colors would be
consistent with disks seen only in a narrow range of inclinations. Our models
appear to be too geometrically thick at large radii, as suggested by: (a)
larger far-infrared disk emission than in the typical SEDs of T Tauri stars;
(b) wider dark dust lanes in the model images than in the images of HH30 and HK
Tau/c; and (c) larger predicted number of stars extincted by edge-on disks than
consistent with current surveys. The large thickness of the model is a
consequence of the assumption that dust and gas are well-mixed, suggesting that
some degree of dust settling may be required to explain the observations.Comment: 41 pages, 13 figures, accepted in Ap
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