6,343 research outputs found
Magnon squeezing in an antiferromagnet: reducing the spin noise below the standard quantum limit
At absolute zero temperature, thermal noise vanishes when a physical system
is in its ground state, but quantum noise remains as a fundamental limit to the
accuracy of experimental measurements. Such a limitation, however, can be
mitigated by the formation of squeezed states. Quantum mechanically, a squeezed
state is a time-varying superposition of states for which the noise of a
particular observable is reduced below that of the ground state at certain
times. Quantum squeezing has been achieved for a variety of systems, including
the electromagnetic field, atomic vibrations in solids and molecules, and
atomic spins, but not so far for magnetic systems. Here we report on an
experimental demonstration of spin wave (i.e., magnon) squeezing. Our method
uses femtosecond optical pulses to generate correlations involving pairs of
magnons in an antiferromagnetic insulator, MnF2. These correlations lead to
quantum squeezing in which the fluctuations of the magnetization of a
crystallographic unit cell vary periodically in time and are reduced below that
of the ground state quantum noise. The mechanism responsible for this squeezing
is stimulated second order Raman scattering by magnon pairs. Such squeezed
states have important ramifications in the emerging fields of spintronics and
quantum computing involving magnetic spin states or the spin-orbit coupling
mechanism
Observation of Surface-Avoiding Waves: A New Class of Extended States in Periodic Media
Coherent time-domain optical experiments on GaAs-AlAs superlattices reveal
the exis-tence of an unusually long-lived acoustic mode at ~ 0.6 THz, which
couples weakly to the environment by evading the sample boundaries. Classical
as well as quantum states that steer clear of surfaces are generally shown to
occur in the spectrum of periodic struc-tures, for most boundary conditions.
These surface-avoiding waves are associated with frequencies outside forbidden
gaps and wavevectors in the vicinity of the center and edge of the Brillouin
zone. Possible consequences for surface science and resonant cavity
ap-plications are discussed.Comment: 16 pages, 3 figure
Near-infrared spatially resolved spectroscopy of (136108) Haumea's multiple system
The transneptunian region of the solar system is populated by a wide variety
of icy bodies showing great diversity. The dwarf planet (136108) Haumea is
among the largest TNOs and displays a highly elongated shape and hosts two
moons, covered with crystalline water ice like Hamuea. Haumea is also the
largest member of the sole TNO family known to date. A catastrophic collision
is likely responsible for its unique characteristics. We report here on the
analysis of a new set of observations of Haumea obtained with SINFONI at the
ESO VLT. Combined with previous data, and using light-curve measurements in the
optical and far infrared, we carry out a rotationally resolved spectroscopic
study of the surface of Haumea. We describe the physical characteristics of the
crystalline water ice present on the surface of Haumea for both regions, in and
out of the Dark Red Spot (DRS), and analyze the differences obtained for each
individual spectrum. The presence of crystalline water ice is confirmed over
more than half of the surface of Haumea. Our measurements of the average
spectral slope confirm the redder characteristic of the spot region. Detailed
analysis of the crystalline water-ice absorption bands do not show significant
differences between the DRS and the remaining part of the surface. We also
present the results of applying Hapke modeling to our data set. The best
spectral fit is obtained with a mixture of crystalline water ice (grain sizes
smaller than 60 micron) with a few percent of amorphous carbon. Improvements to
the fit are obtained by adding ~10% of amorphous water ice. Additionally, we
used the IFU-reconstructed images to measure the relative astrometric position
of the largest satellite Hi`iaka and determine its orbital elements. An orbital
solution was computed with our genetic-based algorithm GENOID and our results
are in full agreement with recent results.Comment: Accepted for publication in A&
Generation and remote detection of THz sound using semiconductor superlattices
The authors introduce a novel approach to study the propagation of high
frequency acoustic phonons in which the generation and detection involves two
spatially separated superlattices apart. Propagating modes
of frequencies up to escape from the superlattice where they
are generated and reach the second superlattice where they are detected. The
measured frequency spectrum reveals finite size effects, which can be accounted
for by a continuum elastic model.Comment: Submitted to Applied Physics Letter
Investigating the diurnal cycle of precipitation over Central Africa
AbstractThe present study investigated the reliability of downscaling tool RegCM4.4 to simulate 2002–2006 June–September diurnal cycle precipitation characteristics. Besides their diurnal cycles, the spatial and temporal patterns in precipitation intensity, amount and frequency over Central Africa (CA) are investigated. Diurnal variance, phase and amplitude based on 3‐hourly model simulations are obtained by diurnal harmonics from each 24‐h period. Two statistical measures are used to evaluate model performance: the root mean square error and the index of agreement. The result shows that the RegCM outputs are well simulated compared with reference data in revealing the temporal and spatial patterns of precipitation amount and frequency over the continental area with some systematic wet biases over Cameroon highlands area. Diurnal variability of precipitation frequency and amount are properly well reproduced by the model with an afternoon peak around 1800 LST over entire domain except Atlantic Ocean sub‐region. The model does not properly describe the observed diurnal variation of precipitation intensity over the study area. One of the prominent results is that the pattern of precipitation frequency is quite similar to that of precipitation amount. This strong relationship between these two precipitation characteristics over the entire region of interest suggests that the diurnal precipitation variability is generally determined by how often it rains
T-PHOT version 2.0: improved algorithms for background subtraction, local convolution, kernel registration, and new options
We present the new release v2.0 of T-PHOT, a publicly available software
package developed to perform PSF-matched, prior-based, multiwavelength
deconfusion photometry of extragalactic fields. New features included in the
code are presented and discussed: background estimation, fitting using position
dependent kernels, flux prioring, diagnostical statistics on the residual
image, exclusion of selected sources from the model and residual images,
individual registration of fitted objects. These new options improve on the
performance of the code, allowing for more accurate results and providing
useful aids for diagnostics.Comment: 7 pages, 8 figure
Control of spin dynamics with laser pulses: Generation of entangled states of donor-bound electrons in a Cd1-xMnxTe quantum well
A quantum-mechanical many-particle system may exhibit non-local behavior in
that measurements performed on one of the particles can affect a second one
that is far apart. These so-called entangled states are crucial for the
implementation of quantum information protocols and gates for quantum
computation. Here, we use ultrafast optical pulses and coherent pump-probe
techniques to create and control spin entangled states in an ensemble of up to
three non-interacting electrons bound to donors in a Cd1-xMnxTe quantum well.
Our method, relying on the exchange interaction between optically-excited
excitons and the paramagnetic impurities, can in principle be applied to
entangle an arbitrarily large number of electrons. A microscopic theory of
impulsive stimulated Raman scattering and a model for multi-spin entanglement
are presented. The signature of entanglement is the observation of overtones of
donor spin-flips in the differential reflectivity of the probe pulse. Results
are shown for resonant excitation of localized excitons below the gap, and
above the gap where the signatures of entanglement are significantly enhanced.
Data is also presented on the generation of coherent excitations of
antiferromagnetically-coupled manganese pairs, folded acoustic phonons, exciton
Zeeman beats and entanglement involving two Mn2+ ions.Comment: Long version of quant-ph/020619
Filling of three-dimensional space by two-dimensional sheet growth.
Models of three-dimensional space filling based on growth of two-dimensional sheets are proposed. Beginning from planar Eden-style growth of sheets, additional growth modes are introduced. These enable the sheets to form layered or disordered structures. The growth modes can also be combined. An off-lattice kinetic Monte Carlo-based computer algorithm is presented and used to study the kinetics of the new models and the resulting structures. It is possible to study space filling by two-dimensional growth in a three-dimensional domain with arbitrarily oriented sheets; the results agree with previously published models where the sheets are only able to grow in a limited set of directions. The introduction of a bifurcation mechanism gives rise to complex disordered structures that are of interest as model structures for the mesostructure of calcium silicate hydrate in hardened cement paste.This research was supported by the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement 264448. AFR was supported by EPSRC grant EP/H035397/1.This is the author accepted manuscript. The final version is available from APS via http://dx.doi.org/10.1103/PhysRevE.92.04210
Optical probing of ultrafast electronic decay in Bi and Sb with slow phonons
Illumination with laser sources leads to the creation of excited electronic states of particular symmetries, which can drive isosymmetric vibrations. Here, we use a combination of ultrafast stimulated and cw spontaneous Raman scattering to determine the lifetime of A(1g) and E-g electronic coherences in Bi and Sb. Our results both shed new light on the mechanisms of coherent phonon generation and represent a novel way to probe extremely fast electron decoherence rates. The E-g state, resulting from an unequal distribution of carriers in three equivalent band regions, is extremely short lived. Consistent with theory, the lifetime of its associated driving force reaches values as small as 2 (6) fs for Bi (Sb) at 300 K. DOI: 10.1103/PhysRevLett.110.04740
Plasduino: an inexpensive, general purpose data acquisition framework for educational experiments
Based on the Arduino development platform, Plasduino is an open-source data
acquisition framework specifically designed for educational physics
experiments. The source code, schematics and documentation are in the public
domain under a GPL license and the system, streamlined for low cost and ease of
use, can be replicated on the scale of a typical didactic lab with minimal
effort. We describe the basic architecture of the system and illustrate its
potential with some real-life examples.Comment: 11 pages, 10 figures, presented at the XCIX conference of the
Societ\`a Italiana di Fisic
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