257 research outputs found
Exact ground state and kink-like excitations of a two dimensional Heisenberg antiferromagnet
A rare example of a two dimensional Heisenberg model with an exact dimerized
ground state is presented. This model, which can be regarded as a variation on
the kagome lattice, has several features of interest: it has a highly (but not
macroscopically) degenerate ground state; it is closely related to spin chains
studied by earlier authors; in particular, it is probably the first genuinely
two-dimensional quantum system to exhibit domain-wall-like ``kink'' excitations
normally found only in one-dimensional systems. In some limits it decouples
into non-interacting chains, purely dynamically and not because of weakening of
interchain couplings: indeed, paradoxically, this happens in the limit of
strong coupling of the chains.Comment: 4 pages, revtex, 5 figures included via epsfi
Motion of Bound Domain Walls in a Spin Ladder
The elementary excitation spectrum of the spin-
antiferromagnetic (AFM) Heisenberg chain is described in terms of a pair of
freely propagating spinons. In the case of the Ising-like Heisenberg
Hamiltonian spinons can be interpreted as domain walls (DWs) separating
degenerate ground states. In dimension , the issue of spinons as
elementary excitations is still unsettled. In this paper, we study two
spin- AFM ladder models in which the individual chains are
described by the Ising-like Heisenberg Hamiltonian. The rung exchange
interactions are assumed to be pure Ising-type in one case and Ising-like
Heisenberg in the other. Using the low-energy effective Hamiltonian approach in
a perturbative formulation, we show that the spinons are coupled in bound
pairs. In the first model, the bound pairs are delocalized due to a four-spin
ring exchange term in the effective Hamiltonian. The appropriate dynamic
structure factor is calculated and the associated lineshape is found to be
almost symmetric in contrast to the 1d case. In the case of the second model,
the bound pair of spinons lowers its kinetic energy by propagating between
chains. The results obtained are consistent with recent theoretical studies and
experimental observations on ladder-like materials.Comment: 12 pages, 7 figure
Conjectures about certain parabolic Kazhdan--Lusztig polynomials
Irreducibility results for parabolic induction of representations of the
general linear group over a local non-archimedean field can be formulated in
terms of Kazhdan--Lusztig polynomials of type . Spurred by these results and
some computer calculations, we conjecture that certain alternating sums of
Kazhdan--Lusztig polynomials known as parabolic Kazhdan--Lusztig polynomials
satisfy properties analogous to those of the ordinary ones.Comment: final versio
Output state in multiple entanglement swapping
The technique of quantum repeaters is a promising candidate for sending
quantum states over long distances through a lossy channel. The usual
discussions of this technique deals with only a finite dimensional Hilbert
space. However the qubits with which one implements this procedure will "ride"
on continuous degrees of freedom of the carrier particles. Here we analyze the
action of quantum repeaters using a model based on pulsed parametric down
conversion entanglement swapping. Our model contains some basic traits of a
real experiment. We show that the state created, after the use of any number of
parametric down converters in a series of entanglement swappings, is always an
entangled (actually distillable) state, although of a different form than the
one that is usually assumed. Furthermore, the output state always violates a
Bell inequality.Comment: 11 pages, 6 figures, RevTeX
Macroscopic entanglement jumps in model spin systems
In this paper, we consider some frustrated spin models for which the ground
states are known exactly. The concurrence, a measure of the amount of
entanglement can be calculated exactly for entangled spin pairs. Quantum phase
transitions involving macroscopic magnetization changes at critical values of
the magnetic field are accompanied by macroscopic jumps in the (T=0)
entanglement. A specific example is given in which magnetization plateaus give
rise to a plateau structure in the amount of entanglement associated with
nearest-neighbour bonds. We further show that macroscopic entanglement changes
can occur in quantum phase transitions brought about by the tuning of exchange
interaction strengths.Comment: 11 pages, 4 figures, Latex, communicated to Phys. Rev.
The spectral gap for some spin chains with discrete symmetry breaking
We prove that for any finite set of generalized valence bond solid (GVBS)
states of a quantum spin chain there exists a translation invariant
finite-range Hamiltonian for which this set is the set of ground states. This
result implies that there are GVBS models with arbitrary broken discrete
symmetries that are described as combinations of lattice translations, lattice
reflections, and local unitary or anti-unitary transformations. We also show
that all GVBS models that satisfy some natural conditions have a spectral gap.
The existence of a spectral gap is obtained by applying a simple and quite
general strategy for proving lower bounds on the spectral gap of the generator
of a classical or quantum spin dynamics. This general scheme is interesting in
its own right and therefore, although the basic idea is not new, we present it
in a system-independent setting. The results are illustrated with an number of
examples.Comment: 48 pages, Plain TeX, BN26/Oct/9
Irrelevance of photon events distinguishability in a class of Bell experiments
We show that the possibility of distinguishing between single- and two-photon detection events, usually not met in the actual experiments, is not a necessary requirement for proof that the experiments of Alley and Shih [Phys. Rev. Lett. 61, 2921 (1988)] and Ou and Mandel [Phys. Rev. Lett. 61, 50 (1988)] are modulo a fair sampling assumption, valid tests of local realism. We also give the critical parameters for the experiments to be unconditional tests of local realism, and show that some other interesting phenomena (involving bosonic-type particle indistinguishability) can be observed during such tests
Cavity QED and quantum information processing with "hot" trapped atoms
We propose a method to implement cavity QED and quantum information
processing in high-Q cavities with a single trapped but non-localized atom. The
system is beyond the Lamb-Dick limit due to the atomic thermal motion. Our
method is based on adiabatic passages, which make the relevant dynamics
insensitive to the randomness of the atom position with an appropriate
interaction configuration. The validity of this method is demonstrated from
both approximate analytical calculations and exact numerical simulations. We
also discuss various applications of this method based on the current
experimental technology.Comment: 14 pages, 8 figures, Revte
Sudden switch of generalized Lieb-Robinson velocity in a transverse field Ising spin chain
The Lieb-Robinson theorem states that the speed at which the correlations
between two distant nodes in a spin network can be built through local
interactions has an upper bound, which is called the Lieb-Robinson velocity.
Our central aim is to demonstrate how to observe the Lieb-Robinson velocity in
an Ising spin chain with a strong transverse field. We adopt and compare four
correlation measures for characterizing different types of correlations, which
include correlation function, mutual information, quantum discord, and
entanglement of formation. We prove that one of correlation functions shows a
special behavior depending on the parity of the spin number. All the
information-theoretical correlation measures demonstrate the existence of the
Lieb-Robinson velocity. In particular, we find that there is a sudden switch of
the Lieb-Robinson speed with the increasing of the number of spin
Search for the standard model Higgs boson decaying into two photons in pp collisions at sqrt(s)=7 TeV
A search for a Higgs boson decaying into two photons is described. The
analysis is performed using a dataset recorded by the CMS experiment at the LHC
from pp collisions at a centre-of-mass energy of 7 TeV, which corresponds to an
integrated luminosity of 4.8 inverse femtobarns. Limits are set on the cross
section of the standard model Higgs boson decaying to two photons. The expected
exclusion limit at 95% confidence level is between 1.4 and 2.4 times the
standard model cross section in the mass range between 110 and 150 GeV. The
analysis of the data excludes, at 95% confidence level, the standard model
Higgs boson decaying into two photons in the mass range 128 to 132 GeV. The
largest excess of events above the expected standard model background is
observed for a Higgs boson mass hypothesis of 124 GeV with a local significance
of 3.1 sigma. The global significance of observing an excess with a local
significance greater than 3.1 sigma anywhere in the search range 110-150 GeV is
estimated to be 1.8 sigma. More data are required to ascertain the origin of
this excess.Comment: Submitted to Physics Letters
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