716 research outputs found
The "mirror box" illusion: effect of visual information on bimanual coordination in children with spastic hemiparetic cerebral palsy
The study examined symmetrical bimanual coordination of children with spastic hemiparetic cerebral palsy (SHCP) and a typically developing (TD) control group under conditions of visual feedback created by placing a glass screen, opaque screen or a mirror (“mirror box”) between the arms. The “mirror box” creates a visual illusion, which gives rise to a visual perception of a zero lag, symmetric movement between the two arms. Children with SHCP exhibited a similar mean coordination pattern as the TD control group, but had greater movement variability between the arms. Furthermore, movement variability in children with SHCP was significantly greater in the screen condition compared with the glass and mirror condition, which were similar to each other. The effects of the availability of visual feedback in individuals with hemiparesis are discussed with reference to central and peripheral mechanisms
First order phase transition with a logarithmic singularity in a model with absorbing states
Recently, Lipowski [cond-mat/0002378] investigated a stochastic lattice model
which exhibits a discontinuous transition from an active phase into infinitely
many absorbing states. Since the transition is accompanied by an apparent
power-law singularity, it was conjectured that the model may combine features
of first- and second-order phase transitions. In the present work it is shown
that this singularity emerges as an artifact of the definition of the model in
terms of products. Instead of a power law, we find a logarithmic singularity at
the transition. Moreover, we generalize the model in such a way that the
second-order phase transition becomes accessible. As expected, this transition
belongs to the universality class of directed percolation.Comment: revtex, 4 pages, 5 eps figure
Constraining Warm Inflation with the Cosmic Microwave Background
We discuss the spectrum of scalar density perturbations from warm inflation
when the friction coefficient in the inflaton equation is dependent on
the inflaton field. The spectral index of scalar fluctuations depends on a new
slow-roll parameter constructed from . A numerical integration of the
perturbation equations is performed for a model of warm inflation and gives a
good fit to the WMAP data for reasonable values of the model's parameters.Comment: 6 pages, 2 figures, RevTeX
Spectroscopy by frequency entangled photon pairs
Quantum spectroscopy was performed using the frequency-entangled broadband
photon pairs generated by spontaneous parametric down-conversion. An absorptive
sample was placed in front of the idler photon detector, and the frequency of
signal photons was resolved by a diffraction grating. The absorption spectrum
of the sample was measured by counting the coincidences, and the result is in
agreement with the one measured by a conventional spectrophotometer with a
classical light source.Comment: 11 pages, 5 figures, to be published in Phys. Lett.
A high-flux source of polarization-entangled photons from a periodically-poled KTP parametric downconverter
We have demonstrated a high-flux source of polarization-entangled photons
using a type-II phase-matched periodically-poled KTP parametric downconverter
in a collinearly propagating configuration. We have observed quantum
interference between the single-beam downconverted photons with a visibility of
99% and a measured coincidence flux of 300/s/mW of pump. The
Clauser-Horne-Shimony-Holt version of Bell's inequality was violated with a
value of 2.711 +/- 0.017.Comment: 7 pages submitted to Physical Review
Tackling Systematic Errors in Quantum Logic Gates with Composite Rotations
We describe the use of composite rotations to combat systematic errors in
single qubit quantum logic gates and discuss three families of composite
rotations which can be used to correct off-resonance and pulse length errors.
Although developed and described within the context of NMR quantum computing
these sequences should be applicable to any implementation of quantum
computation.Comment: 6 pages RevTex4 including 4 figures. Will submit to Phys. Rev.
First-order interference of nonclassical light emitted spontaneously at different times
We study first-order interference in spontaneous parametric down-conversion
generated by two pump pulses that do not overlap in time. The observed
modulation in the angular distribution of the signal detector counting rate can
only be explained in terms of a quantum mechanical description based on
biphoton states. The condition for observing interference in the signal channel
is shown to depend on the parameters of the idler radiation.Comment: 5 pages, two-column, submitted to PR
Does a Computer have an Arrow of Time?
In [Sch05a], it is argued that Boltzmann's intuition, that the psychological
arrow of time is necessarily aligned with the thermodynamic arrow, is correct.
Schulman gives an explicit physical mechanism for this connection, based on the
brain being representable as a computer, together with certain thermodynamic
properties of computational processes. [Haw94] presents similar, if briefer,
arguments. The purpose of this paper is to critically examine the support for
the link between thermodynamics and an arrow of time for computers. The
principal arguments put forward by Schulman and Hawking will be shown to fail.
It will be shown that any computational process that can take place in an
entropy increasing universe, can equally take place in an entropy decreasing
universe. This conclusion does not automatically imply a psychological arrow
can run counter to the thermodynamic arrow. Some alternative possible explana-
tions for the alignment of the two arrows will be briefly discussed.Comment: 31 pages, no figures, publication versio
Single photon emitters based on Ni/Si related defects in single crystalline diamond
We present investigations on single Ni/Si related color centers produced via
ion implantation into single crystalline type IIa CVD diamond. Testing
different ion dose combinations we show that there is an upper limit for both
the Ni and the Si dose 10^12/cm^2 and 10^10/cm^2 resp.) due to creation of
excess fluorescent background. We demonstrate creation of Ni/Si related centers
showing emission in the spectral range between 767nm and 775nm and narrow
line-widths of 2nm FWHM at room temperature. Measurements of the intensity
auto-correlation functions prove single-photon emission. The investigated color
centers can be coarsely divided into two groups: Drawing from photon statistics
and the degree of polarization in excitation and emission we find that some
color centers behave as two-level, single-dipole systems whereas other centers
exhibit three levels and contributions from two orthogonal dipoles. In
addition, some color centers feature stable and bright emission with saturation
count rates up to 78kcounts/s whereas others show fluctuating count rates and
three-level blinking.Comment: 7 pages, submitted to Applied Physics B, revised versio
Dynamical modelling of the elliptical galaxy NGC 2974
In this paper we analyse the relations between a previously described oblate
Jaffe model for an ellipsoidal galaxy and the observed quantities for NGC 2974,
and obtain the length and velocity scales for a relevant elliptical galaxy
model. We then derive the finite total mass of the model from these scales, and
finally find a good fit of an isotropic oblate Jaffe model by using the
Gauss-Hermite fit parameters and the observed ellipticity of the galaxy NGC
2974. The model is also used to predict the total luminous mass of NGC 2974,
assuming that the influence of dark matter in this galaxy on the image,
ellipticity and Gauss-Hermite fit parameters of this galaxy is negligible
within the central region, of radius Comment: 7 figure
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