16,671 research outputs found
Simulation of quantum dynamics with quantum optical systems
We propose the use of quantum optical systems to perform universal simulation
of quantum dynamics. Two specific implementations that require present
technology are put forward for illustrative purposes. The first scheme consists
of neutral atoms stored in optical lattices, while the second scheme consists
of ions stored in an array of micro--traps. Each atom (ion) supports a
two--level system, on which local unitary operations can be performed through a
laser beam. A raw interaction between neighboring two--level systems is
achieved by conditionally displacing the corresponding atoms (ions). Then,
average Hamiltonian techniques are used to achieve evolutions in time according
to a large class of Hamiltonians.Comment: 14 pages, 6 figure
Entanglement cost of mixed states
We compute the entanglement cost of several families of bipartite mixed
states, including arbitrary mixtures of two Bell states. This is achieved by
developing a technique that allows us to ascertain the additivity of the
entanglement of formation for any state supported on specific subspaces. As a
side result, the proof of the irreversibility in asymptotic local manipulations
of entanglement is extended to two-qubit systems.Comment: 4 pages, no figures, (v4) new results, including a new method to
determine E_c for more general mixed states, presentation changed
significantl
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
Magnetization vector in the reversible region of a highly anisotropic cuprate superconductor: anisotropy factor and the role of 2D vortex fluctuations
By using a high quality Tl2Ba2Ca2Cu3O10 (Tl-2223) single crystal as an
example, the magnetization vector was probed in the reversible region of highly
anisotropic cuprate superconductors. For that, we have measured its components
along and transverse to the applied magnetic field for different crystal
orientations. The analysis shows that the angular dependence of the
perpendicular component of the magnetization vector follows the one predicted
by a London-like approach which includes a contribution associated with the
thermal fluctuations of the 2D vortex positions. For the Tl-2223 crystal
studied here, a lower bound for the anisotropy factor was estimated to be about
190.Comment: 6 pages, 3 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&
Characterizing mixing and measurement in quantum mechanics
What fundamental constraints characterize the relationship between a mixture
of quantum states, the states being mixed,
and the probabilities ? What fundamental constraints characterize the
relationship between prior and posterior states in a quantum measurement? In
this paper we show that there are many surprisingly strong constraints on these
mixing and measurement processes that can be expressed simply in terms of the
eigenvalues of the quantum states involved. These constraints capture in a
succinct fashion what it means to say that a quantum measurement acquires
information about the system being measured, and considerably simplify the
proofs of many results about entanglement transformation.Comment: 12 page
Interaction induced delocalisation for two particles in a periodic potential
We consider two interacting particles evolving in a one-dimensional periodic
structure embedded in a magnetic field. We show that the strong localization
induced by the magnetic field for particular values of the flux per unit cell
is destroyed as soon as the particles interact. We study the spectral and the
dynamical aspects of this transition.Comment: 4 pages, 5 EPS figures, minor misprints correcte
Fidelity balance in quantum operations
I derive a tight bound between the quality of estimating the state of a
single copy of a -level system, and the degree the initial state has to be
altered in course of this procedure. This result provides a complete analytical
description of the quantum mechanical trade-off between the information gain
and the quantum state disturbance expressed in terms of mean fidelities. I also
discuss consequences of this bound for quantum teleportation using nonmaximally
entangled states.Comment: 4 pages, REVTeX. Revised versio
Deuterium Toward WD1634-573: Results from the Far Ultraviolet Spectroscopic Explorer (FUSE) Mission
We use Far Ultraviolet Spectrocopic Explorer (FUSE) observations to study
interstellar absorption along the line of sight to the white dwarf WD1634-573
(d=37.1+/-2.6 pc). Combining our measurement of D I with a measurement of H I
from Extreme Ultraviolet Explorer data, we find a D/H ratio toward WD1634-573
of D/H=(1.6+/-0.5)e-5. In contrast, multiplying our measurements of D I/O
I=0.035+/-0.006 and D I/N I=0.27+/-0.05 with published mean Galactic ISM gas
phase O/H and N/H ratios yields D/H(O)=(1.2+/-0.2)e-5 and
D/H(N)=(2.0+/-0.4)e-5, respectively. Note that all uncertainties quoted above
are 2 sigma. The inconsistency between D/H(O) and D/H(N) suggests that either
the O I/H I and/or the N I/H I ratio toward WD1634-573 must be different from
the previously measured average ISM O/H and N/H values. The computation of
D/H(N) from D I/N I is more suspect, since the relative N and H ionization
states could conceivably vary within the LISM, while the O and H ionization
states will be more tightly coupled by charge exchange.Comment: 23 pages, 5 figures; AASTEX v5.0 plus EPSF extensions in mkfig.sty;
accepted by ApJ Supplemen
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