526 research outputs found
Double-lambda microscopic model for entangled light generation by four-wave-mixing
Motivated by recent experiments, we study four-wave-mixing in an atomic
double-{\Lambda} system driven by a far-detuned pump. Using the
Heisenberg-Langevin formalism, and based on the microscopic properties of the
medium, we calculate the classical and quantum properties of seed and conju-
gate beams beyond the linear amplifier approximation. A continuous variable
approach gives us access to relative-intensity noise spectra that can be
directly compared to experiments. Restricting ourselves to the cold-atom
regime, we predict the generation of quantum-correlated beams with a
relative-intensity noise spectrum well below the standard quantum limit (down
to -6 dB). Moreover entanglement between seed and conjugate beams measured by
an inseparability down to 0.25 is expected. This work opens the way to the
generation of entangled beams by four-wave mixing in a cold atomic sample.Comment: 11 pages, 6 figures, submitted to PR
Metal artefact reduction sequences for a piezoelectric bone conduction implant using a realistic head phantom in MRI
Industry standards require medical device manufacturers to perform
implant-induced artefact testing in phantoms at a pre-clinical stage to define
the extent of artefacts that can be expected during MRI. Once a device is
commercially available, studies on volunteers, cadavers or patients are
performed to investigate implant-induced artefacts and artefact reduction
methods more in-depth. This study describes the design and evaluation of a
realistic head phantom for pre-clinical implant-induced artefact testing in a
relevant environment. A case study is performed where a state-of-the-art
piezoelectric bone conduction implant is used in the 1.5 T and 3 T MRI
environments. Images were acquired using clinical and novel metal artefact
reducing (MARS) sequences at both field strengths. Artefact width and length
were measured in a healthy volunteer and compared with artefact sizes obtained
in the phantom. Artefact sizes are reported that are similar in shape between
the phantom and a volunteer, yet with dimensions differing up to 20% between
both. When the implant magnet is removed, the artefact size can be reduced
below a diameter of 5 cm, whilst the presence of an implant magnet and splint
creates higher artefacts up to 20 cm in diameter. Pulse sequences have been
altered to reduce the scan time up to 7 minutes, while preserving the image
quality. These results show that the anthropomorphic phantom can be used at a
preclinical stage to provide clinically relevant images, illustrating the
impact of the artefact on important brain structures.Comment: 17 pages, 5 figure
Gradient echo memory in an ultra-high optical depth cold atomic ensemble
Quantum memories are an integral component of quantum repeaters - devices
that will allow the extension of quantum key distribution to communication
ranges beyond that permissible by passive transmission. A quantum memory for
this application needs to be highly efficient and have coherence times
approaching a millisecond. Here we report on work towards this goal, with the
development of a Rb magneto-optical trap with a peak optical depth of
1000 for the D2 transition using spatial and temporal
dark spots. With this purpose-built cold atomic ensemble to implement the
gradient echo memory (GEM) scheme. Our data shows a memory efficiency of % and coherence times up to 195 s, which is a factor of four greater
than previous GEM experiments implemented in warm vapour cells.Comment: 15 pages, 5 figure
Estimated glomerular filtration rate is a poor predictor of the concentration of middle molecular weight uremic solutes in chronic kidney disease
Background: Uremic solute concentration increases as Glomerular Filtration Rate (GFR) declines. Weak associations were demonstrated between estimated GFR (eGFR) and the concentrations of several small water-soluble and protein-bound uremic solutes (MW500Da).
Materials and Methods: In 95 CKD-patients (CKD-stage 2-5 not on dialysis), associations between different eGFR-formulae (creatinine, CystatinC-based or both) and the natural logarithm of the concentration of several LMWP's were analyzed: i.e. parathyroid hormone (PTH), Cystatin C (CystC), interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-alpha), leptin, retinol binding protein (RbP), immunoglobin light chains kappa and lambda (Ig-kappa and Ig-lambda), beta-2-microglobulin (beta M-2), myoglobin and fibroblast growth factor-23 (FGF-23)).
Results: The regression coefficients (R-2) between eGFR, based on the CKD-EPI-Crea-CystC-formula as reference, and the examined LMWP's could be divided into three groups. Most of the LMWP's associated weakly (R-2 0.7). Almost identical R-2-values were found per LMWP for all eGFR-formulae, with exception of CystC and beta M-2 which showed weaker associations with creatinine-based than with CystC-based eGFR.
Conclusion: The association between eGFR and the concentration of several LMWP's is inconsistent, with in general low R-2-values. Thus, the use of eGFR to evaluate kidney function does not reflect the concentration of several LMWP's with proven toxic impact in CKD
Nonlinear modeling of thermoacoustic systems
Thermoacoustic systems convert energy from heat to acoustic power and vice versa. These systems have commercial interest due to the high potential efficiency and low number of moving parts. To numerically predict the performance of a thermoacoustic device inherent nonlinearities in the system, such as thermoacoustic streaming and generation of harmonics need to be taken into account. We present a nonlinear frequency domain method with which these nonlinearities in thermoacoustic systems are modeled in a computationally efficient manner. Using this method, the nonlinear periodic steady state of a thermoacoustic engine can directly be computed, without computing the long initial transient of the system. In this publication, the developed method is applied to compute the periodic steady state of an experimental standing wave engine. The results obtained match well with experimental data
Localization of a Virtual Wall by Means of Active Echolocation by Untrained Sighted Persons
The active sensing and perception of the environment by auditory means is
typically known as echolocation and it can be acquired by humans, who can
profit from it in the absence of vision. We investigated the ability of twentyone
untrained sighted participants to use echolocation with self-generated oral
clicks for aligning themselves within the horizontal plane towards a virtual wall,
emulated with an acoustic virtual reality system, at distances between 1 and 32
m, in the absence of background noise and reverberation. Participants were able
to detect the virtual wall on 61% of the trials, although with large di↵erences
across individuals and distances. The use of louder and shorter clicks led to an
increased performance, whereas the use of clicks with lower frequency content
allowed for the use of interaural time di↵erences to improve the accuracy of
reflection localization at very long distances. The distance of 2 m was the most
difficult to detect and localize, whereas the furthest distances of 16 and 32 m
were the easiest ones. Thus, echolocation may be used e↵ectively to identify
large distant environmental landmarks such as buildings
Accessing temperature waves: A dispersion relation perspective
In order to account for non-Fourier heat transport, occurring on short time and length scales, the often-praised Dual-Phase-Lag (DPL) model was conceived, introducing a causality relation between the onset of heat flux and the temperature gradient. The most prominent aspect of the first-order DPL model is the prediction of wave-like temperature propagation, the detection of which still remains elusive. Among the challenges to make further progress is the capability to disentangle the intertwining of the parameters affecting wave-like behaviour. This work contributes to the quest, providing a straightforward, easy-to-adopt, analytical mean to inspect the optimal conditions to observe temperature wave oscillations. The complex-valued dispersion relation for the temperature scalar field is investigated for the case of a localised temperature pulse in space, and for the case of a forced temperature oscillation in time. A modal quality factor is introduced showing that, for the case of the temperature gradient preceding the heat flux, the material acts as a bandpass filter for the temperature wave. The bandpass filter characteristics are accessed in terms of the relevant delay times entering the DPL model. The optimal region in parameters space is discussed in a variety of systems, covering nine and twelve decades in space and time-scale respectively. The here presented approach is of interest for the design of nanoscale thermal devices operating on ultra-fast and ultra-short time scales, a scenario here addressed for the case of quantum materials and graphite
Характеристика структурних змін в яєчку при хронічній гіпертермії
Воздействие хронической гипертермии на яички экспериментальных животных
приводит к изменению микроциркуляторного русла в виде венозного полнокровия
и интерстициального отека. Появляются морфологические признаки замедления
процессов сперматогенеза, которые сопровождаются изменениями структур
извитых семенных канальцев.Laboratory animal testes influenced by chronic hyperthermia show changes in
hemomicrocirculation bed structure manifested as venous hyperemia and interstitial
edema. Morphological signs of spermatogenesis slowdown accompanied by changes in
convoluted somniferous tubules are observed
Bronchopulmonary Dysplasia
Hospitalizations for respiratory syncytial virus bronchioliti
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