23,240 research outputs found
Optimal distillation of a GHZ state
We present the optimal local protocol to distill a
Greenberger-Horne-Zeilinger (GHZ) state from a single copy of any pure state of
three qubits.Comment: RevTex, 4 pages, 2 figures. Published version, some references adde
Entanglement renormalization and gauge symmetry
A lattice gauge theory is described by a redundantly large vector space that
is subject to local constraints, and can be regarded as the low energy limit of
an extended lattice model with a local symmetry. We propose a numerical
coarse-graining scheme to produce low energy, effective descriptions of lattice
models with a local symmetry, such that the local symmetry is exactly preserved
during coarse-graining. Our approach results in a variational ansatz for the
ground state(s) and low energy excitations of such models and, by extension, of
lattice gauge theories. This ansatz incorporates the local symmetry in its
structure, and exploits it to obtain a significant reduction of computational
costs. We test the approach in the context of the toric code with a magnetic
field, equivalent to Z2 lattice gauge theory, for lattices with up to 16 x 16
sites (16^2 x 2 = 512 spins) on a torus. We reproduce the well-known ground
state phase diagram of the model, consisting of a deconfined and spin polarized
phases separated by a continuous quantum phase transition, and obtain accurate
estimates of energy gaps, ground state fidelities, Wilson loops, and several
other quantities.Comment: reviewed version as published in PRB; this version includes a new
section about the accuracy of the results several corrections and added
citation
Fine-grained entanglement loss along renormalization group flows
We explore entanglement loss along renormalization group trajectories as a
basic quantum information property underlying their irreversibility. This
analysis is carried out for the quantum Ising chain as a transverse magnetic
field is changed. We consider the ground-state entanglement between a large
block of spins and the rest of the chain. Entanglement loss is seen to follow
from a rigid reordering, satisfying the majorization relation, of the
eigenvalues of the reduced density matrix for the spin block. More generally,
our results indicate that it may be possible to prove the irreversibility along
RG trajectories from the properties of the vacuum only, without need to study
the whole hamiltonian.Comment: 5 pages, 3 figures; minor change
Efficient classical simulation of slightly entangled quantum computations
We present a scheme to efficiently simulate, with a classical computer, the
dynamics of multipartite quantum systems on which the amount of entanglement
(or of correlations in the case of mixed-state dynamics) is conveniently
restricted. The evolution of a pure state of n qubits can be simulated by using
computational resources that grow linearly in n and exponentially in the
entanglement. We show that a pure-state quantum computation can only yield an
exponential speed-up with respect to classical computations if the entanglement
increases with the size n of the computation, and gives a lower bound on the
required growth.Comment: 4 pages. Major changes. Significantly improved simulation schem
Modelling the spinning dust emission from LDN 1780
We study the anomalous microwave emission (AME) in the Lynds Dark Nebula
(LDN) 1780 on two angular scales. Using available ancillary data at an angular
resolution of 1 degree, we construct an SED between 0.408 GHz to 2997 GHz. We
show that there is a significant amount of AME at these angular scales and the
excess is compatible with a physical spinning dust model. We find that LDN 1780
is one of the clearest examples of AME on 1 degree scales. We detected AME with
a significance > 20. We also find at these angular scales that the
location of the peak of the emission at frequencies between 23-70 GHz differs
from the one on the 90-3000 GHz map. In order to investigate the origin of the
AME in this cloud, we use data obtained with the Combined Array for Research in
Millimeter-wave Astronomy (CARMA) that provides 2 arcmin resolution at 30 GHz.
We study the connection between the radio and IR emissions using morphological
correlations. The best correlation is found to be with MIPS 70m, which
traces warm dust (T50K). Finally, we study the difference in radio
emissivity between two locations within the cloud. We measured a factor
of difference in 30 GHz emissivity. We show that this variation can
be explained, using the spinning dust model, by a variation on the dust grain
size distribution across the cloud, particularly changing the carbon fraction
and hence the amount of PAHs.Comment: 14 pages, 11 figures, submitted to MNRA
One Dimensional 1H, 2H and 3H
The ground-state properties of one-dimensional electron-spin-polarized
hydrogen H, deuterium H, and tritium H are obtained by means of
quantum Monte Carlo methods. The equations of state of the three isotopes are
calculated for a wide range of linear densities. The pair correlation function
and the static structure factor are obtained and interpreted within the
framework of the Luttinger liquid theory. We report the density dependence of
the Luttinger parameter and use it to identify different physical regimes:
Bogoliubov Bose gas, super-Tonks-Girardeau gas, and quasi-crystal regimes for
bosons; repulsive, attractive Fermi gas, and quasi-crystal regimes for
fermions. We find that the tritium isotope is the one with the richest
behaviour. Our results show unambiguously the relevant role of the isotope mass
in the properties of this quantum system.Comment: 19 pages, 7 figures, contribution to special issue in NJP in memory
of Marvin Girardea
Entanglement in a first order quantum phase transition
The phase diagram of spins 1/2 embedded in a magnetic field mutually
interacting antiferromagnetically is determined. Contrary to the ferromagnetic
case where a second order quantum phase transition occurs, a first order
transition is obtained at zero field. The spectrum is computed for a large
number of spins and allows one to study the ground state entanglement
properties which displays a jump of its concurrence at the critical point.Comment: 4 pages, 3 EPS figure
Electromagnetic dipole moments of charged baryons with bent crystals at the LHC
We propose a unique program of measurements of electric and magnetic dipole
moments of charm, beauty and strange charged baryons at the LHC, based on the
phenomenon of spin precession of channeled particles in bent crystals. Studies
of crystal channeling and spin precession of positively- and negatively-charged
particles are presented, along with feasibility studies and expected
sensitivities for the proposed experiment using a layout based on the LHCb
detector.Comment: 19 pages, 13 figure
VLBI observations of SN2011dh: imaging of the youngest radio supernova
We report on the VLBI detection of supernova SN2011dh at 22GHz using a subset
of the EVN array. The observations took place 14 days after the discovery of
the supernova, thus resulting in a VLBI image of the youngest radio-loud
supernova ever. We provide revised coordinates for the supernova with
milli-arcsecond precision, linked to the ICRF. The recovered flux density is a
factor 2 below the EVLA flux density reported by other authors at the same
frequency and epoch of our observations. This discrepancy could be due to
extended emission detected with the EVLA or to calibration problems in the VLBI
and/or EVLA observations.Comment: Letter. Accepted in A&
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