2,077 research outputs found
Subjective method of refractometry and depth of focus
AbstractPurposeTo study the impact of the depth of focus on subjective refraction and distribution of myopic and hyperopic refractions.MethodsA total of 450 eyes of 305 subjects in the age range of 23–34 years were recruited for the study. A distribution of refractions was examined using a traditional method of the subjective refractometry on the basis of point-like posterior focus notion. Correction of the results was made on the assumption that the emmetropic eye retains high visual acuity when applying convex lenses with values which are fewer or equal to the depth of focus values. The following values of the depth of focus were used: ±0.55D, ±0.35D and ±0.2D for visual acuity 1.0, 1.5 and 2.0, respectively.ResultsApplication of the traditional method of refractometry produced the following occurrence of refractions: hypermetropia 59.3%, myopia 22% and emmetropia 18.7%. After correction of the initial results of values of the depth of focus the distribution of refractions was as follows: hypermetropia 12.7%, myopia 22% and emmetropia 65.3%.ConclusionThe traditional method of subjective refractometry with application of trial lenses was developed on the basis of data of large optical aberrations and significant depth of focus which values should be taken into account during interpretation of results of subjective refractometry. Our data regarding to prevalence of emmetropic refraction falls in line with basic science provisions in respect of the physiology of the eye
EPR experiment and 2-photon interferometry: Report of a 2-photon interference experiment
After a very brief review of the historical Einstein, Podolsky, and Rosen (EPR) experiments, a new two-photon interference type EPR experiment is reported. A two-photon state was generated by optical parametric down conversion. Pairs of light quanta with degenerate frequency but divergent directions of propagation were sent to two independent Michelson interferometers. First and second order interference effectors were studied. Different than other reports, we observed that the second order interference visibility vanished when the optical path difference of the interferometers were much less than the coherence length of the pumping laser beam. However, we also observed that the second order interference behaved differently depending on whether the interferometers were set at equal or different optical path differences
Bi-photon propagation control with optimized wavefront by means of Adaptive Optics
We present an efficient method to control the spatial modes of entangled
photons produced through SPDC process. Bi-photon beam propagation is controlled
by a deformable mirror, that shapes a 404nm CW diode laser pump interacting
with a nonlinear BBO type-I crystal. Thanks to adaptive optical system, the
propagation of 808nm SPDC light produced is optimized over a distance of 2m.
The whole system optimization is carried out by a feedback between deformable
mirror action and entangled photon coincidence counts. We also demonstrated the
improvement of the two-photon coupling into single mode fibers
Performance of Photon-Pair Quantum Key Distribution Systems
We analyze the quantitative improvement in performance provided by a novel
quantum key distribution (QKD) system that employs a correlated photon source
(CPS) and a photon-number resolving detector (PNR). Our calculations suggest
that given current technology, the CPR implementation offers an improvement of
several orders of magnitude in secure bit rate over previously described
implementations
Roles of Bond Alternation in Magnetic Phase Diagram of RMnO3
In order to investigate nature of the antiferromagnetic structures in
perovskite RMnO3, we study a Heisenberg J1-J2 model with bond alternation using
analytical and numerical approaches. The magnetic phase diagram which includes
incommensurate spiral states and commensurate collinear states is reproduced.
We discuss that the magnetic structure with up-up-down-down spin configuration
(E-type structure) and the ferroelectricity emerge cooperatively to stabilize
this phase. Magnetoelastic couplings are crucial to understand the magnetic and
electric phase diagram of RMnO3.Comment: 5 pages, 6 figure
Aberration cancellation in quantum interferometry
We report the first experimental demonstration of even-order aberration
cancellation in quantum interferometry. The effect is a spatial counterpart of
the spectral group velocity dispersion cancellation, which is associated with
spectral entanglement. It is manifested in temporal interferometry by virtue of
the multi-parameter spatial-spectral entanglement. Spatially-entangled photons,
generated by spontaneous parametric down conversion, were subjected to spatial
aberrations introduced by a deformable mirror that modulates the wavefront. We
show that only odd-order spatial aberrations affect the quality of quantum
interference
Absolute calibration of Analog Detectors using Stimulated Parametric Down Conversion
Spontaneous parametric down conversion has been largely exploited as a tool
for absolute calibration of photon counting detectors, photomultiplier tubes or
avalanche photodiodes working in Geiger regime. In this work we investigate the
extension of this technique from very low photon flux of photon counting regime
to the absolute calibration of analog photodetectors at higher photon flux.
Moving toward higher photon rate, i.e. at high gain regime, with the
spontaneous parametric down conversion shows intrinsic limitations of the
method, while the stimulated parametric down conversion process, where a seed
beam properly injected into the crystal in order to increase the photon
generation rate in the conjugate arm, allows us to work around this problem. A
preliminary uncertainty budget is discussed
Metallic "Ferroelectricity" in the Pyrochlore Cd2Re2O7
A class of materials known as ``ferroelectric metals'' was discussed
theoretically by Anderson and Blount in 1965 [Phys. Rev. Lett. 14, 217 (1965)],
but to date no examples of this class have been reported. Here we present
measurements of the elastic moduli of Cd2Re2O7 through the 200 K
cubic-to-tetragonal phase transition. A Landau analysis of the moduli reveals
that the transition is consistent with Cd2Re2O7 being classified as a
``ferroelectric metal'' in the weaker sense described by Anderson and Blount
(loss of a center of symmetry). First-principles calculations of the lattice
instabilities indicate that the dominant lattice instability corresponds to a
two-fold degenerate mode with Eu symmetry, and that motions of the O ions
forming the O octahedra dominate the energetics of the transition.Comment: 4 pages, 2 figure
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