206 research outputs found
Entangled photon apparatus for the undergraduate laboratory
We present detailed instructions for constructing and operating an apparatus
to produce and detect polarization-entangled photons. The source operates by
type-I spontaneous parametric downconversion in a two-crystal geometry. Photons
are detected in coincidence by single-photon counting modules and show strong
angular and polarization correlations. We observe more than 100 entangled
photon pairs per second. A test of a Bell inequality can be performed in an
afternoon.Comment: 6 pages, 9 figure
Entangled photons, nonlocality and Bell inequalities in the undergraduate laboratory
We use polarization-entangled photon pairs to demonstrate quantum nonlocality
in an experiment suitable for advanced undergraduates. The photons are produced
by spontaneous parametric downconversion using a violet diode laser and two
nonlinear crystals. The polarization state of the photons is tunable. Using an
entangled state analogous to that described in the Einstein-Podolsky-Rosen
``paradox,'' we demonstrate strong polarization correlations of the entanged
photons. Bell's idea of a hidden variable theory is presented by way of an
example and compared to the quantum prediction. A test of the Clauser, Horne,
Shimony and Holt version of the Bell inequality finds , in
clear contradiciton of hidden variable theories. The experiments described can
be performed in an afternoon.Comment: 10 pages, 6 figure
Caveats in Eliciting Mobile App Requirements
Factors such as app stores or platform choices heavily affect functional and
non-functional mobile app requirements. We surveyed 45 companies and
interviewed ten experts to explore how factors that impact mobile app
requirements are understood by requirements engineers in the mobile app
industry.
We observed a lack of knowledge in several areas. For instance, we observed
that all practitioners were aware of data privacy concerns, however, they did
not know that certain third-party libraries, usage aggregators, or advertising
libraries also occasionally leak sensitive user data. Similarly, certain
functional requirements may not be implementable in the absence of a
third-party library that is either banned from an app store for policy
violations or lacks features, for instance, missing desired features in ARKit
library for iOS made practitioners turn to Android.
We conclude that requirements engineers should have adequate technical
experience with mobile app development as well as sufficient knowledge in areas
such as privacy, security and law, in order to make informed decisions during
requirements elicitation.Comment: The 24th International Conference on Evaluation and Assessment in
Software Engineering (EASE 2020
Second harmonic generation on incommensurate structures: The case of multiferroic MnWO4
A comprehensive analysis of optical second harmonic generation (SHG) on an
incommensurate (IC) magnetically ordered state is presented using multiferroic
MnWO4 as model compound. Two fundamentally different SHG contributions coupling
to the primary IC magnetic order or to secondary commensurate projections of
the IC state, respectively, are distinguished. Whereas the latter can be
described within the formalism of the 122 commensurate magnetic point groups
the former involves a breakdown of the conventional macroscopic symmetry
analysis because of its sensitivity to the lower symmetry of the local
environment in a crystal lattice. Our analysis thus foreshadows the fusion of
the hitherto disjunct fields of nonlinear optics and IC order in
condensed-matter systems
On the driven Frenkel-Kontorova model: I. Uniform sliding states and dynamical domains of different particle densities
The dynamical behavior of a harmonic chain in a spatially periodic potential
(Frenkel-Kontorova model, discrete sine-Gordon equation) under the influence of
an external force and a velocity proportional damping is investigated. We do
this at zero temperature for long chains in a regime where inertia and damping
as well as the nearest-neighbor interaction and the potential are of the same
order. There are two types of regular sliding states: Uniform sliding states,
which are periodic solutions where all particles perform the same motion
shifted in time, and nonuniform sliding states, which are quasi-periodic
solutions where the system forms patterns of domains of different uniform
sliding states. We discuss the properties of this kind of pattern formation and
derive equations of motion for the slowly varying average particle density and
velocity. To observe these dynamical domains we suggest experiments with a
discrete ring of at least fifty Josephson junctions.Comment: Written in RevTeX, 9 figures in PostScrip
Periodic vacuum and particles in two dimensions
Different dynamical symmetry breaking patterns are explored for the two
dimensional phi4 model with higher order derivative terms. The one-loop saddle
point expansion predicts a rather involved phase structure and a new Gaussian
critical line. This vacuum structure is corroborated by the Monte Carlo method,
as well. Analogies with the structure of solids, the density wave phases and
the effects of the quenched impurities are mentioned. The unitarity of the time
evolution operator in real time is established by means of the reflection
positivity.Comment: Final version, additional references and the proof of reflection
positivity added, 41 pages, 16 figure
Coexistence of Weak Localization and a Metallic Phase in Si/SiGe Quantum Wells
Magnetoresistivity measurements on p-type Si/SiGe quantum wells reveal the
coexistence of a metallic behavior and weak localization. Deep in the metallic
regime, pronounced weak localization reduces the metallic behavior around zero
magnetic field without destroying it. In the insulating phase, a positive
magnetoresistivity emerges close to B=0, possibly related to spin-orbit
interactions.Comment: 4 pages, 3 figure
Analysis of the Metallic Phase of Two-Dimensional Holes in SiGe in Terms of Temperature Dependent Screening
We find that temperature dependent screening can quantitatively explain the
metallic behaviour of the resistivity on the metallic side of the so-called
metal-insulator transition in p-SiGe. Interference and interaction effects
exhibit the usual insulating behaviour which is expected to overpower the
metallic background at sufficiently low temperatures. We find empirically that
the concept of a Fermi-liquid describes our data in spite of the large r_s = 8.Comment: 4 pages, 3 figure
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