343 research outputs found
Quantum teleportation between light and matter
Quantum teleportation is an important ingredient in distributed quantum
networks, and can also serve as an elementary operation in quantum computers.
Teleportation was first demonstrated as a transfer of a quantum state of light
onto another light beam; later developments used optical relays and
demonstrated entanglement swapping for continuous variables. The teleportation
of a quantum state between two single material particles (trapped ions) has now
also been achieved. Here we demonstrate teleportation between objects of a
different nature - light and matter, which respectively represent 'flying' and
'stationary' media. A quantum state encoded in a light pulse is teleported onto
a macroscopic object (an atomic ensemble containing 10^12 caesium atoms).
Deterministic teleportation is achieved for sets of coherent states with mean
photon number (n) up to a few hundred. The fidelities are 0.58+-0.02 for n=20
and 0.60+-0.02 for n=5 - higher than any classical state transfer can possibly
achieve. Besides being of fundamental interest, teleportation using a
macroscopic atomic ensemble is relevant for the practical implementation of a
quantum repeater. An important factor for the implementation of quantum
networks is the teleportation distance between transmitter and receiver; this
is 0.5 metres in the present experiment. As our experiment uses propagating
light to achieve the entanglement of light and atoms required for
teleportation, the present approach should be scalable to longer distances.Comment: 23 pages, 8 figures, incl. supplementary informatio
Master Equation for the Motion of a Polarizable Particle in a Multimode Cavity
We derive a master equation for the motion of a polarizable particle weakly
interacting with one or several strongly pumped cavity modes. We focus here on
massive particles with complex internal structure such as large molecules and
clusters, for which we assume a linear scalar polarizability mediating the
particle-light interaction. The predicted friction and diffusion coefficients
are in good agreement with former semiclassical calculations for atoms and
small molecules in weakly pumped cavities, while the current rigorous quantum
treatment and numerical assessment sheds a light on the feasibility of
experiments that aim at optically manipulating beams of massive molecules with
multimode cavities.Comment: 30 pages, 5 figure
Elastic Spin Relaxation Processes in Semiconductor Quantum Dots
Electron spin decoherence caused by elastic spin-phonon processes is
investigated comprehensively in a zero-dimensional environment. Specifically, a
theoretical treatment is developed for the processes associated with the
fluctuations in the phonon potential as well as in the electron procession
frequency through the spin-orbit and hyperfine interactions in the
semiconductor quantum dots. The analysis identifies the conditions (magnetic
field, temperature, etc.) in which the elastic spin-phonon processes can
dominate over the inelastic counterparts with the electron spin-flip
transitions. Particularly, the calculation results illustrate the potential
significance of an elastic decoherence mechanism originating from the
intervalley transitions in semiconductor quantum dots with multiple equivalent
energy minima (e.g., the X valleys in SiGe). The role of lattice anharmonicity
and phonon decay in spin relaxation is also examined along with that of the
local effective field fluctuations caused by the stochastic electronic
transitions between the orbital states. Numerical estimations are provided for
typical GaAs and Si-based quantum dots.Comment: 57 pages, 14 figure
Heat transport by lattice and spin excitations in the spin chain compounds SrCuO_2 and Sr_2CuO_3
We present the results of measurements of the thermal conductivity of the
quasi one-dimensional spin S=1/2 chain compound SrCuO_2 in the temperature
range between 0.4 and 300 K along the directions parallel and perpendicular to
the chains. An anomalously enhanced thermal conductivity is observed along the
chains. The analysis of the present data and a comparison with analogous recent
results for Sr_2CuO_3 and other similar materials demonstrates that this
behavior is generic for cuprates with copper-oxygen chains and strong
intrachain interactions. The observed anomalies are attributed to the
one-dimensional energy transport by spin excitations (spinons), limited by the
interaction between spin and lattice excitations. The energy transport along
the spin chains has a non-diffusive character, in agreement with theoretical
predictions for integrable models.Comment: 12 pages (RevTeX), 8 figure
Anyonic interferometry and protected memories in atomic spin lattices
Strongly correlated quantum systems can exhibit exotic behavior called
topological order which is characterized by non-local correlations that depend
on the system topology. Such systems can exhibit remarkable phenomena such as
quasi-particles with anyonic statistics and have been proposed as candidates
for naturally fault-tolerant quantum computation. Despite these remarkable
properties, anyons have never been observed in nature directly. Here we
describe how to unambiguously detect and characterize such states in recently
proposed spin lattice realizations using ultra-cold atoms or molecules trapped
in an optical lattice. We propose an experimentally feasible technique to
access non-local degrees of freedom by performing global operations on trapped
spins mediated by an optical cavity mode. We show how to reliably read and
write topologically protected quantum memory using an atomic or photonic qubit.
Furthermore, our technique can be used to probe statistics and dynamics of
anyonic excitations.Comment: 14 pages, 6 figure
Transport anomaly in the low energy regime of spin chains
The anomalous thermal conductivity in spin chains observed in experiments is
studied for the low temperature regime. In the effective dynamics with most
realistic perturbations, the so-called Umklapp terms is irrelevant to reduce
mean free path in the energy transport at even finite temperatures. This is
consistent with large conductivities found in recent experiments. The Drude
weight which is the prefactor in the divergent conductivity is calculated, and
the temperature dependence is discussed.Comment: 4 pages, no figure. PRB, in pres
Measles IgG Antibody Index Correlates with T2 Lesion Load on MRI in Patients with Early Multiple Sclerosis
Background: B cells and humoral immune responses play an important role in the pathogenesis and diagnosis of multiple sclerosis (MS). A characteristic finding in patients with MS is a polyspecific intrathecal B cell response against neurotropic viruses, specifically against measles virus, rubella virus, and varicella zoster virus, also known as an MRZ reaction (MRZR). Here, we correlated from the routine clinical diagnostics individual IgG antibody indices (AIs) of MRZR with magnetic resonance imaging (MRI) findings in patients with first MS diagnosis. Methods/Results: MRZR was determined in 68 patients with a clinically isolated syndrome (CIS) or early relapsing-remitting MS (RRMS). Absolute AI values for measles virus, rubella virus, and varicella zoster virus were correlated with T2 lesion load and gadolinium enhancing lesions on cerebral MRI (cMRI) and cMRI combined with spinal MRI (sMRI). Measles virus AI correlated significantly with T2 lesion load on cMRI (p = 0.0312, Mann-Whitney U test) and the sum of lesions on cMRI and sMRI (p = 0.0413). Varicella zoster virus AI also showed a correlation with T2 lesion load on cMRI but did not reach statistical significance (p = 0.2893). Conclusion: The results confirm MRZR as part of the polyspecific immune reaction in MS with possible prognostic impact on MRI and clinical parameters. Furthermore, the data indicate that intrathecal measles virus IgG production correlates wit
Simulation of dimensionality effects in thermal transport
The discovery of nanostructures and the development of growth and fabrication
techniques of one- and two-dimensional materials provide the possibility to
probe experimentally heat transport in low-dimensional systems. Nevertheless
measuring the thermal conductivity of these systems is extremely challenging
and subject to large uncertainties, thus hindering the chance for a direct
comparison between experiments and statistical physics models. Atomistic
simulations of realistic nanostructures provide the ideal bridge between
abstract models and experiments. After briefly introducing the state of the art
of heat transport measurement in nanostructures, and numerical techniques to
simulate realistic systems at atomistic level, we review the contribution of
lattice dynamics and molecular dynamics simulation to understanding nanoscale
thermal transport in systems with reduced dimensionality. We focus on the
effect of dimensionality in determining the phononic properties of carbon and
semiconducting nanostructures, specifically considering the cases of carbon
nanotubes, graphene and of silicon nanowires and ultra-thin membranes,
underlying analogies and differences with abstract lattice models.Comment: 30 pages, 21 figures. Review paper, to appear in the Springer Lecture
Notes in Physics volume "Thermal transport in low dimensions: from
statistical physics to nanoscale heat transfer" (S. Lepri ed.
Phonon Transport in Graphene
Properties of phonons - quanta of the crystal lattice vibrations - in
graphene have attracted strong attention of the physics and engineering
communities. Acoustic phonons are the main heat carriers in graphene near room
temperature while optical phonons are used for counting the number of atomic
planes in Raman experiments with few-layer graphene. It was shown both
theoretically and experimentally that transport properties of phonons, i.e.
energy dispersion and scattering rates, are substantially different in the
quasi two-dimensional system such as graphene compared to basal planes in
graphite or three-dimensional bulk crystals. The unique nature of
two-dimensional phonon transport translates to unusual heat conduction in
graphene and related materials. In this review we outline different theoretical
approaches developed for phonon transport in graphene, discuss contributions of
the in-plane and cross-plane phonon modes and provide comparison with available
experimental thermal conductivity data. Particular attention is given to
analysis of recent theoretical results for the phonon thermal conductivity of
graphene and few-layer graphene, and the effects of the strain, defects and
isotopes on the phonon transport in these systems.Comment: invited review; 41 pages; 9 figures; 3 table
The crossover from propagating to strongly scattered acoustic modes of glasses observed in densified silica
Spectroscopic results on low frequency excitations of densified silica are
presented and related to characteristic thermal properties of glasses. The end
of the longitudinal acoustic branch is marked by a rapid increase of the
Brillouin linewidth with the scattering vector. This rapid growth saturates at
a crossover frequency Omega_co which nearly coincides with the center of the
boson peak. The latter is clearly due to additional optic-like excitations
related to nearly rigid SiO_4 librations as indicated by hyper-Raman
scattering. Whether the onset of strong scattering is best described by
hybridization of acoustic modes with these librations, by their elastic
scattering (Rayleigh scattering) on the local excitations, or by soft
potentials remains to be settled.Comment: 14 pages, 6 figures, to be published in a special issue of J. Phys.
Condens. Matte
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