26,959 research outputs found
A balanced homodyne detector for high-rate Gaussian-modulated coherent-state quantum key distribution
We discuss excess noise contributions of a practical balanced homodyne
detector in Gaussian-modulated coherent-state (GMCS) quantum key distribution
(QKD). We point out the key generated from the original realistic model of GMCS
QKD may not be secure. In our refined realistic model, we take into account
excess noise due to the finite bandwidth of the homodyne detector and the
fluctuation of the local oscillator. A high speed balanced homodyne detector
suitable for GMCS QKD in the telecommunication wavelength region is built and
experimentally tested. The 3dB bandwidth of the balanced homodyne detector is
found to be 104MHz and its electronic noise level is 13dB below the shot noise
at a local oscillator level of 8.5*10^8 photon per pulse. The secure key rate
of a GMCS QKD experiment with this homodyne detector is expected to reach
Mbits/s over a few kilometers.Comment: 22 pages, 11 figure
A new observational and numerical study of tidal interactions in M81-M82-NGC3077 system
A nearby system of interacting galaxies M81-M82-NGC3077 triplet (D = 3.3 Mpc; Freeman & Madore 1988) has been studied using multi-wavelength observations and numerical simulations to obtain a comprehensive understanding on the dynamics and the consequences of tidal interactions in a group environment. The VLA 12-field Mosaic H I observations of 2 x 1.5 deg. region have revealed a vast array of H I filaments which suggests that the severity and extent of tidal disruptions far exceed the previous estimates. A tidal remnant of the former H I disk of M82 extending up to 30 kpc (in projection) is identified for the first time, and the pervasive effects of the tidal disruption are traced into the inner disk by optical and CO observations, including a kinematic trace of a large scale bar potential (Yun, Ho, & Lo 1992). The H I disk of M81 is traced out to 40 kpc in radius, and a large scale (l approx. 20 kpc) velocity anomaly ('High Velocity Trough'), which may be a remnant of a gaseous collision, is found within the disk of M81. The large H I bridge between M81 and NGC 3077 (van der Hulst 1979) is also found to extend approx. greater than 50 kpc further, bending around NGC 3077, toward M82. The total H I detected in this experiment, 5.6 x 10(exp 9) solar mass, represents the majority of the single-dish flux (Appleton, Davies, & Stephenson 1981) and suggests that the bulk of H I found in the region belongs to the three galaxies and the tidal filaments. The impact and details of the tidal interactions have been further examined through the use of numerical techniques. The 'restricted 3-body' approach was used to simulate the observed distribution of tidal H I streamers connecting the three galaxies, and the success of the simulation is further strengthened by the accurate predictions on the gas kinematics
Wireless Wearable Photoplethysmography Sensors for Continuous Blood Pressure Monitoring
Blood Pressure (BP) is a crucial vital sign taken into consideration for the general assessment of patient’s condition: patients with hypertension or hypotension are advised to record their BP routinely. Particularly, hypertension is emphasized by stress, diabetic neuropathy and coronary heart diseases and could lead to stroke. Therefore, routine and long-term monitoring can enable early detection of symptoms and prevent life-threatening events. The gold standard method for measuring BP is the use of a stethoscope and sphygmomanometer to detect systolic and diastolic pressures. However, only discrete measurements are taken. To enable pervasive and continuous monitoring of BP, recent methods have been proposed: pulse arrival time (PAT) or PAT difference (PATD) between different body parts are based on the combination of electrocardiogram (ECG) and photoplethysmography (PPG) sensors. Nevertheless, this technique could be quite obtrusive as in addition to at least two contacts/electrodes to measure the differential voltage across the left arm/leg/chest and the right arm/leg/chest, ECG measurements are easily corrupted by motion artefacts. Although such devices are small, wearable and relatively convenient to use, most devices are not designed for continuous BP measurements. This paper introduces a novel PPG-based pervasive sensing platform for continuous measurements of BP. Based on the principle of using PAT to estimate BP, two PPG sensors are used to measure the PATD between the earlobe and the wrist to measure BP. The device is compared with a gold standard PPG sensor and validation of the concept is conducted with a preliminary study involving 9 healthy subjects. Results show that the mean BP and PATD are correlated with a 0.3 factor. This preliminary study shows the feasibility of continuous monitoring of BP using a pair of PPG placed on the ear lobe and wrist with PATD measurements is possible
A size of ~1 AU for the radio source Sgr A* at the centre of the Milky Way
Although it is widely accepted that most galaxies have supermassive black
holes (SMBHs) at their centers^{1-3}, concrete proof has proved elusive.
Sagittarius A* (Sgr A*)^4, an extremely compact radio source at the center of
our Galaxy, is the best candidate for proof^{5-7}, because it is the closest.
Previous Very Long Baseline Interferometry (VLBI) observations (at 7mm) have
detected that Sgr A* is ~2 astronomical unit (AU) in size^8, but this is still
larger than the "shadow" (a remarkably dim inner region encircled by a bright
ring) arising from general relativistic effects near the event horizon^9.
Moreover, the measured size is wavelength dependent^{10}. Here we report a
radio image of Sgr A* at a wavelength of 3.5mm, demonstrating that its size is
\~1 AU. When combined with the lower limit on its mass^{11}, the lower limit on
the mass density is 6.5x10^{21} Msun pc^{-3}, which provides the most stringent
evidence to date that Sgr A* is an SMBH. The power-law relationship between
wavelength and intrinsic size (The size is proportional to wavelength^{1.09}),
explicitly rules out explanations other than those emission models with
stratified structure, which predict a smaller emitting region observed at a
shorter radio wavelength.Comment: 18 pages, 4 figure
Kerncraft: A Tool for Analytic Performance Modeling of Loop Kernels
Achieving optimal program performance requires deep insight into the
interaction between hardware and software. For software developers without an
in-depth background in computer architecture, understanding and fully utilizing
modern architectures is close to impossible. Analytic loop performance modeling
is a useful way to understand the relevant bottlenecks of code execution based
on simple machine models. The Roofline Model and the Execution-Cache-Memory
(ECM) model are proven approaches to performance modeling of loop nests. In
comparison to the Roofline model, the ECM model can also describes the
single-core performance and saturation behavior on a multicore chip. We give an
introduction to the Roofline and ECM models, and to stencil performance
modeling using layer conditions (LC). We then present Kerncraft, a tool that
can automatically construct Roofline and ECM models for loop nests by
performing the required code, data transfer, and LC analysis. The layer
condition analysis allows to predict optimal spatial blocking factors for loop
nests. Together with the models it enables an ab-initio estimate of the
potential benefits of loop blocking optimizations and of useful block sizes. In
cases where LC analysis is not easily possible, Kerncraft supports a cache
simulator as a fallback option. Using a 25-point long-range stencil we
demonstrate the usefulness and predictive power of the Kerncraft tool.Comment: 22 pages, 5 figure
Nonequilibrium Phase Transition in the Kinetic Ising model: Critical Slowing Down and Specific-heat Singularity
The nonequilibrium dynamic phase transition, in the kinetic Ising model in
presence of an oscillating magnetic field, has been studied both by Monte Carlo
simulation and by solving numerically the mean field dynamic equation of motion
for the average magnetisation. In both the cases, the Debye 'relaxation'
behaviour of the dynamic order parameter has been observed and the 'relaxation
time' is found to diverge near the dynamic transition point. The Debye
relaxation of the dynamic order parameter and the power law divergence of the
relaxation time have been obtained from a very approximate solution of the mean
field dynamic equation. The temperature variation of appropiately defined
'specific-heat' is studied by Monte Carlo simulation near the transition point.
The specific-heat has been observed to diverge near the dynamic transition
point.Comment: Revtex, Five encapsulated postscript files, submitted to Phys. Rev.
A multiwavelength survey of interacting galaxies
Galaxy-galaxy collisions are known to produce drastic changes in morphology and, in many cases, enhance the level of star formation activity in galaxies. In order to better quantify the effects that interactions have on the star formation characteristics of galaxies the authors undertook a multiwavelength survey of a large sample of interacting disk-type galaxies. The sample is optically-selected, the inclusion of systems having been based upon the presence of unusual morphological features--such as tidal tails, plumes, rings, warped disks--suggestive of tidal interaction. The sample is composed of about 115 systems, most of which are spiral-spiral pairs, with a few spiral-elliptical pairs and a few merging systems (see Bushouse 1986 for more details of the sample selection). This sample has now been studied in the optical, infrared, and radio regimes, including optical spectra and H alpha images, near-infrared photometry and imaging, far-infrared photometry, H I 21 cm emission-line measurements, Very Large Array (VLA) 20 cm maps, and CO emission-line measurements. This paper presents an overview and comparison of the results of the optical, infrared and CO surveys. With these data the authors can compare the far-infrared and CO properties of the galaxies with the classic optical and radio indicators of star formation activity and thereby determine what, if any, relationships exist between star formation activity and the far-infrared and CO properties of the galaxies
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