3,592 research outputs found
Heralded Two-Photon Entanglement from Probabilistic Quantum Logic Operations on Multiple Parametric Down-Conversion Sources
An ideal controlled-NOT gate followed by projective measurements can be used
to identify specific Bell states of its two input qubits. When the input qubits
are each members of independent Bell states, these projective measurements can
be used to swap the post-selected entanglement onto the remaining two qubits.
Here we apply this strategy to produce heralded two-photon polarization
entanglement using Bell states that originate from independent parametric
down-conversion sources, and a particular probabilistic controlled-NOT gate
that is constructed from linear optical elements. The resulting implementation
is closely related to an earlier proposal by Sliwa and Banaszek
[quant-ph/0207117], and can be intuitively understood in terms of familiar
quantum information protocols. The possibility of producing a ``pseudo-demand''
source of two-photon entanglement by storing and releasing these heralded pairs
from independent cyclical quantum memory devices is also discussed.Comment: 5 pages, 4 figures; submitted to IEEE Journal of Selected Topics in
Quantum Electronics, special issue on "Quantum Internet Technologies
A comparison between conventional and LANDSAT based hydrologic modeling: The Four Mile Run case study
Models designed to support the hydrologic studies associated with urban water resources planning require input parameters that are defined in terms of land cover. Estimating the land cover is a difficult and expensive task when drainage areas larger than a few sq. km are involved. Conventional and LANDSAT based methods for estimating the land cover based input parameters required by hydrologic planning models were compared in a case study of the 50.5 sq. km (19.5 sq. mi) Four Mile Run Watershed in Virginia. Results of the study indicate that the LANDSAT based approach is highly cost effective for planning model studies. The conventional approach to define inputs was based on 1:3600 aerial photos, required 110 man-days and a total cost of 2,350. The conventional and LANDSAT based models gave similar results relative to discharges and estimated annual damages expected from no flood control, channelization, and detention storage alternatives
Large Effects of Electric Fields on Atom-Molecule Collisions at Millikelvin Temperatures
Controlling interactions between cold molecules using external fields can
elucidate the role of quantum mechanics in molecular collisions. We create a
new experimental platform in which ultracold rubidium atoms and cold ammonia
molecules are separately trapped by magnetic and electric fields and then
combined to study collisions. We observe inelastic processes that are faster
than expected from earlier field-free calculations. We use quantum scattering
calculations to show that electric fields can have a major effect on collision
outcomes, even in the absence of dipole-dipole interactions.Comment: 5 pages, 4 figure
A two-circle powder diffractometer for synchrotron radiation on Station 2.3 at the SRS
The two-circle powder diffractometer on Station 8.3 at the SRS has recently been relocated to station 2.3, some half the distance from a dipole radiation source with the same nominal 1.2T field. The purpose of this paper is to detail the changes and modifications to the diffractometer operation
Visualizing sound emission of elephant vocalizations: evidence for two rumble production types
Recent comparative data reveal that formant frequencies are cues to body size in animals, due to a close relationship between formant frequency spacing, vocal tract length and overall body size. Accordingly, intriguing morphological adaptations to elongate the vocal tract in order to lower formants occur in several species, with the size exaggeration hypothesis being proposed to justify most of these observations. While the elephant trunk is strongly implicated to account for the low formants of elephant rumbles, it is unknown whether elephants emit these vocalizations exclusively through the trunk, or whether the mouth is also involved in rumble production. In this study we used a sound visualization method (an acoustic camera) to record rumbles of five captive African elephants during spatial separation and subsequent bonding situations. Our results showed that the female elephants in our analysis produced two distinct types of rumble vocalizations based on vocal path differences: a nasally- and an orally-emitted rumble. Interestingly, nasal rumbles predominated during contact calling, whereas oral rumbles were mainly produced in bonding situations. In addition, nasal and oral rumbles varied considerably in their acoustic structure. In particular, the values of the first two formants reflected the estimated lengths of the vocal paths, corresponding to a vocal tract length of around 2 meters for nasal, and around 0.7 meters for oral rumbles. These results suggest that African elephants may be switching vocal paths to actively vary vocal tract length (with considerable variation in formants) according to context, and call for further research investigating the function of formant modulation in elephant vocalizations. Furthermore, by confirming the use of the elephant trunk in long distance rumble production, our findings provide an explanation for the extremely low formants in these calls, and may also indicate that formant lowering functions to increase call propagation distances in this species'
Photon-number distributions of twin beams generated in spontaneous parametric down-conversion and measured by an intensified CCD camera
The measurement of photon-number statistics of fields composed of photon
pairs, generated in spontaneous parametric down-conversion and detected by an
intensified CCD camera is described. Final quantum detection efficiencies,
electronic noises, finite numbers of detector pixels, transverse intensity
spatial profiles of the detected beams as well as losses of single photons from
a pair are taken into account in a developed general theory of photon-number
detection. The measured data provided by an iCCD camera with single-photon
detection sensitivity are analyzed along the developed theory. Joint
signal-idler photon-number distributions are recovered using the reconstruction
method based on the principle of maximum likelihood. The range of applicability
of the method is discussed. The reconstructed joint signal-idler photon-number
distribution is compared with that obtained by a method that uses superposition
of signal and noise and minimizes photoelectron entropy. Statistics of the
reconstructed fields are identified to be multi-mode Gaussian. Elements of the
measured as well as the reconstructed joint signal-idler photon-number
distributions violate classical inequalities. Sub-shot-noise correlations in
the difference of the signal and idler photon numbers as well as partial
suppression of odd elements in the distribution of the sum of signal and idler
photon numbers are observed.Comment: 14 pages, 14 figure
Spin-driven Phase Transitions in ZnCrSe and ZnCrS Probed by High Resolution Synchrotron X-ray and Neutron Powder Diffraction
The crystal and magnetic structures of the spinel compounds ZnCrS and
ZnCrSe were investigated by high resolution powder synchrotron and
neutron diffraction. ZnCrSe exhibits a first order phase transition at
K into an incommensurate helical magnetic structure. Magnetic
fluctuations above are coupled to the crystal lattice as manifested by
negative thermal expansion. Both, the complex magnetic structure and the
anomalous structural behavior can be related to magnetic frustration.
Application of an external magnetic field shifts the ordering temperature and
the regime of negative thermal expansion towards lower temperatures. Thereby,
the spin ordering changes into a conical structure. ZnCrS shows two
magnetic transitions at K and K that are accompanied by
structural phase transitions. The crystal structure transforms from the cubic
spinel-type (space group \={3}) at high temperatures in the paramagnetic
state, via a tetragonally distorted intermediate phase (space group /
) for into a low temperature orthorhombic phase
(space group ) for . The cooperative displacement of
sulfur ions by exchange striction is the origin of these structural phase
transitions. The low temperature structure of ZnCrS is identical to the
orthorhombic structure of magnetite below the Verwey transition. When applying
a magnetic field of 5 T the system shows an induced negative thermal expansion
in the intermediate magnetic phase as observed in ZnCrSe.Comment: 11 pages, 13 figures, to be published in PR
Local lattice distortions vs. structural phase transition in NdFeAsO1-xFx
The lattice properties at low temperatures of two samples of NdFeAsO1-xFx
(x=0.05 and 0.25) have been examined in order to investigate possible
structural phase transition that may occur in the optimally doped
superconducting sample with respect to the non-superconducting low-F
concentration compound. In order to detect small modifications in the ion
displacements with temperature micro-Raman and high resolution synchrotron
powder diffraction measurements were carried out. No increase of the width of
the (220) or (322) tetragonal diffraction peaks and microstrains could be found
in the superconducting sample from synchrotron XRD measurements. On the other
hand, the atomic displacement parameters deviate from the expected behavior, in
agreement with modifications in the phonon width, as obtained by Raman
scattering. These deviations occur around 150 K for both F dopings, with
distinct differences among the two compounds, i.e., they decrease at low doping
and increase for the superconducting sample. The data do not support a hidden
phase transition to an orthorhombic phase in the superconducting compound, but
point to an isostructural lattice deformation. Based on the absence of magnetic
effects in this temperature range for the superconducting sample, we attribute
the observed lattice anomalies to the formation of local lattice distortions
that, being screened by the carriers, can only acquire long-range coherence by
means of a structural phase transition at low doping levels
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