768 research outputs found
Model for the low-temperature magnetic phases observed in doped YBa_2Cu_3O_{6+x}
A classical statistical model for the antiferromagnetic (AFM) ordering of the
Cu-spins in the CuO_2 planes of reduced YBa_2Cu_3O_{6+x} type materials is
presented. The magnetic phases considered are the experimentally observed
high-temperature AFI phase with ordering vector Q_I=(1/2,1/2,0), and the
low-temperature phases: AFII with Q_II=(1/2,1/2,1/2) and intermediate TA (Turn
Angle) phases TAI, TAII and TAIII with components of both ordering vectors. It
is shown that the AFII and TA phases result from an effective ferromagnetic
(FM) type coupling mediated by free spins in the CuO_x basal plane. Good
agreement with experimental data is obtained for realistic model parameters.Comment: 11 pages, 2 Postscript figures, Submitted to Phys.Rev.Let
High-speed noise-free optical quantum memory
Quantum networks promise to revolutionise computing, simulation, and
communication. Light is the ideal information carrier for quantum networks, as
its properties are not degraded by noise in ambient conditions, and it can
support large bandwidths enabling fast operations and a large information
capacity. Quantum memories, devices that store, manipulate, and release on
demand quantum light, have been identified as critical components of photonic
quantum networks, because they facilitate scalability. However, any noise
introduced by the memory can render the device classical by destroying the
quantum character of the light. Here we introduce an intrinsically noise-free
memory protocol based on two-photon off-resonant cascaded absorption (ORCA). We
consequently demonstrate for the first time successful storage of GHz-bandwidth
heralded single photons in a warm atomic vapour with no added noise; confirmed
by the unaltered photon statistics upon recall. Our ORCA memory platform meets
the stringent noise-requirements for quantum memories whilst offering technical
simplicity and high-speed operation, and therefore is immediately applicable to
low-latency quantum networks
Theory of noise suppression in {\Lambda}-type quantum memories by means of a cavity
Quantum memories, capable of storing single photons or other quantum states
of light, to be retrieved on-demand, offer a route to large-scale quantum
information processing with light. A promising class of memories is based on
far-off-resonant Raman absorption in ensembles of -type atoms. However
at room temperature these systems exhibit unwanted four-wave mixing, which is
prohibitive for applications at the single-photon level. Here we show how this
noise can be suppressed by placing the storage medium inside a moderate-finesse
optical cavity, thereby removing the main roadblock hindering this approach to
quantum memory.Comment: 10 pages, 3 figures. This paper provides the theoretical background
to our recent experimental demonstration of noise suppression in a
cavity-enhanced Raman-type memory ( arXiv:1510.04625 ). See also the related
paper arXiv:1511.05448, which describes numerical modelling of an atom-filled
cavity. Comments welcom
Venus-Solar Wind Interaction: Asymmetries and the Escape of O+ Ions
We study the interaction between Venus and the solar wind using a global
three-dimensional self-consistent quasi-neutral hybrid (QNH) model. The model
treats ions (H+, O+) as particles and electrons as a massless charge
neutralising fluid. In the analysed Parker spiral interplanetary magnetic field
(IMF) case (IMF = [8.09, 5.88, 0] nT) a notable north-south asymmetry of the
magnetic field and plasma exists, especially in the properties of escaping
planetary O+ ions. The asymmetry is associated with ion finite gyroradius
effects. Furthermore, the IMF x-component results in a dawn-dusk asymmetry.
Overall, the QNH model is found to reproduce the main observed plasma and
magnetic field regions (the bow shock, the magnetosheath, the magnetic barrier
and the magnetotail), implying the potential of the developed model to study
the Venusian plasma environment and especially the non-thermal ion escape.Comment: 10 pages, 22 figures, accepted for Planetary and Space Scienc
"Some people are born strange": A Brechtian theater pedagogy as philosophical ethnography
The article explores the role of a Brechtian theater pedagogy as âphilosophical ethnographyâ in four investigative drama based workshops, which took international studentsâ intercultural âstrangenessâ experiences as the starting point for aesthetic experimentation. It is argued that a Brechtian theater pedagogy allows for a productive rather than representational orientation in research, which is underpinned by a love for the aesthetic âre-entanglementâ of (dis-embodied) language and ethical concerns about mimetic representational acts. To show how a Brechtian research pedagogy functioned as philosophical ethnography, the article maps the aesthetic transformation of participant Jamalâs verbatim account in the drama workshopsâfrom (a) its emergence in a post-creative-writing discussion in Workshop 2, to (b) its enactment as a body sculpture in Workshop 3, and (c) to its translation into a rehearsal piece in Workshop 4.The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The author thanks the School of Education/University of Glasgow (Scotland, United Kingdom) for the PhD scholarship that made this research possibl
Optimal Coherent Filtering for Single Noisy Photons
We introduce a filter using a noise-free quantum buffer with large optical
bandwidth that can both filter temporal-spectral modes, as well as
inter-convert them and change their frequency. We show that such quantum
buffers optimally filter out temporal-spectral noise; producing identical
single-photons from many distinguishable noisy single-photon sources with the
minimum required reduction in brightness. We then experimentally demonstrate a
noise-free quantum buffer in a warm atomic system that is well matched to
quantum dots and can outperform all intensity (incoherent) filtering schemes
for increasing indistinguishability.Comment: 5 pages, 4 Figure
Towards an analytical framework of science communication models
This chapter reviews the discussion in science communication circles of models for public communication of science and technology (PCST). It questions the claim that there has been a large-scale shift from a âdeficit modelâ of communication to a âdialogue modelâ, and it demonstrates the survival of the deficit model along with the ambiguities of that model. Similar discussions in related fields of communication, including the critique of dialogue, are briefly sketched. Outlining the complex circumstances governing approaches to PCST, the author argues that communications models often perceived to be opposed can, in fact, coexist when the choices are made explicit. To aid this process, the author proposes an analytical framework of communication models based on deficit, dialogue and participation, including variations on each
Analysis of role-play in medical communication training using a theatrical device the fourth wall
BACKGROUND: Communication training is a central part of medical education. The aim of this article is to explore the positions and didactic functions of the fourth wall in medical communication training, using a role-play model basically similar to a theatrical performance. METHOD: The empirical data stem from a communication training model demonstrated at an international workshop for medical teachers and course organizers. The model involves an actress playing a patient, students alternating in the role of the doctor, and a teacher who moderates. The workshop was videotaped and analyzed qualitatively. RESULTS: The analysis of the empirical material revealed three main locations of the fourth wall as it moved and changed qualities during the learning session: 1) A traditional theatre location, where the wall was transparent for the audience, but opaque for the participants in the fiction. 2) A "timeout/reflection" location, where the wall was doubly opaque, for the patient on the one side and the moderator, the doctor and the audience on the other side and 3) an "interviewing the character" location where the wall enclosed everybody in the room. All three locations may contribute to the learning process. CONCLUSION: The theatrical concept 'the fourth wall' may present an additional tool for new understanding of fiction based communication training. Increased understanding of such an activity may help medical teachers/course organizers in planning and evaluating communication training courses
Antiferromagnetic ordering in a 90 K copper oxide superconductor
Using elastic neutron scattering, we evidence a commensurate
antiferromagnetic Cu(2) order (AF) in the superconducting (SC) high-
cuprate (y=0.013, =93 K). As
in the Co-free system, the spin excitation spectrum is dominated by a magnetic
resonance peak at 41 meV but with a reduced spectral weight. The substitution
of Co thus leads to a state where AF and SC cohabit showing that the CuO
plane is a highly antiferromagnetically polarizable medium even for a sample
where T remains optimum.Comment: 3 figure
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