18,708 research outputs found
Polarization-controlled single photons
Vacuum-stimulated Raman transitions are driven between two magnetic substates
of a rubidium-87 atom strongly coupled to an optical cavity. A magnetic field
lifts the degeneracy of these states, and the atom is alternately exposed to
laser pulses of two different frequencies. This produces a stream of single
photons with alternating circular polarization in a predetermined
spatio-temporal mode. MHz repetition rates are possible as no recycling of the
atom between photon generations is required. Photon indistinguishability is
tested by time-resolved two-photon interference.Comment: 4 pages, 3 figure
Hearing the grass grow. Emotional and epistemological challenges of practice-near research
This paper discusses the concept of practice-near research in terms of the emotional and epistemological challenges that arise from the researcher coming 'near' enough to other people for psychological processes to ensue. These may give rise in the researcher to confusion, anxiety and doubt about who is who and what is what; but also to the possibility of real emotional and relational depth in the research process. Using illustrations from three social work doctoral research projects undertaken by students at the Tavistock Clinic and the University of East London the paper examines four themes that seem to the author to be central to meaningful practice-near research undertaken in a spirit of true emotional and epistemological open-mindedness: the smell of the real; losing our minds; the inevitability of personal change; and the discovery of complex particulars
Universality class of quantum criticality for strongly repulsive spin-1 bosons with antiferromagnetic spin-exchange interaction
Using the thermodynamic Bethe ansatz equations we study the quantum phase
diagram, thermodynamics and criticality of one-dimensional spin-1 bosons with
strongly repulsive density-density and antiferromagnetic spin-exchange
interactions. We analytically derive a high precision equation of state from
which the Tomonaga-Luttinger liquid physics and quantum critical behavior of
the system are computed. We obtain explicit forms for the scaling functions
near the critical points yielding the dynamical exponent and correlation
length exponent for the quantum phase transitions driven by either
the chemical potential or the magnetic field. Consequently, we further
demonstrate that quantum criticality of the system can be mapped out from the
finite temperature density and magnetization profiles of the 1D trapped gas.
Our results provide the physical origin of quantum criticality in a 1D
many-body system beyond the Tomonaga-Luttinger liquid description.Comment: 12 pages, 11 figure
Morphological stability of electromigration-driven vacancy islands
The electromigration-induced shape evolution of two-dimensional vacancy
islands on a crystal surface is studied using a continuum approach. We consider
the regime where mass transport is restricted to terrace diffusion in the
interior of the island. In the limit of fast attachment/detachment kinetics a
circle translating at constant velocity is a stationary solution of the
problem. In contrast to earlier work [O. Pierre-Louis and T.L. Einstein, Phys.
Rev. B 62, 13697 (2000)] we show that the circular solution remains linearly
stable for arbitrarily large driving forces. The numerical solution of the full
nonlinear problem nevertheless reveals a fingering instability at the trailing
end of the island, which develops from finite amplitude perturbations and
eventually leads to pinch-off. Relaxing the condition of instantaneous
attachment/detachment kinetics, we obtain non-circular elongated stationary
shapes in an analytic approximation which compares favorably to the full
numerical solution.Comment: 12 page
Validation of the sulfo-phospho-vanillin (SPV) method for the determination of lipid content in oleaginous microorganisms
A colorimetric sulfo-phospho-vanillin (SPV) method was used to validate a high throughput method for total lipids analysis in fresh and lyophilized oleaginous microorganisms. This method uses a small amount of biological material, does not require a lot of sample manipulation, can be automated, is reproducible and easy to implement. A 96-well microplate SPV assay allows one to determine quickly total lipids in lyophilized cells of oleaginous yeast and microalgae. The new assay method possesses many advantages compared to the others described in the literature: requires a small amount sample, less time (around 1 h) and less labor and does not require organic reagents like chloroform in the reaction3411927CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTĂFICO E TECNOLĂGICO - CNPQCOORDENAĂĂO DE APERFEIĂOAMENTO DE PESSOAL DE NĂVEL SUPERIOR - CAPESFUNDAĂĂO DE AMPARO Ă PESQUISA DO ESTADO DE SĂO PAULO - FAPESPsem informaçã
A Single-Photon Server with Just One Atom
Neutral atoms are ideal objects for the deterministic processing of quantum
information. Entanglement operations have been performed by photon exchange or
controlled collisions. Atom-photon interfaces were realized with single atoms
in free space or strongly coupled to an optical cavity. A long standing
challenge with neutral atoms, however, is to overcome the limited observation
time. Without exception, quantum effects appeared only after ensemble
averaging. Here we report on a single-photon source with one-and-only-one atom
quasi permanently coupled to a high-finesse cavity. Quasi permanent refers to
our ability to keep the atom long enough to, first, quantify the
photon-emission statistics and, second, guarantee the subsequent performance as
a single-photon server delivering up to 300,000 photons for up to 30 seconds.
This is achieved by a unique combination of single-photon generation and atom
cooling. Our scheme brings truly deterministic protocols of quantum information
science with light and matter within reach.Comment: 4 pages, 3 figure
QCD Coherence and the Top Quark Asymmetry
Coherent QCD radiation in the hadroproduction of top quark pairs leads to a
forward--backward asymmetry that grows more negative with increasing transverse
momentum of the pair. This feature is present in Monte Carlo event generators
with coherent parton showering, even though the production process is treated
at leading order and has no intrinsic asymmetry before showering. In addition,
depending on the treatment of recoils, showering can produce a positive
contribution to the inclusive asymmetry. We explain the origin of these
features, compare them in fixed-order calculations and the Herwig++, Pythia and
Sherpa event generators, and discuss their implications.Comment: 28 pages, 11 figures, 2 table
Impact of phonons on dephasing of individual excitons in deterministic quantum dot microlenses
Optimized light-matter coupling in semiconductor nanostructures is a key to
understand their optical properties and can be enabled by advanced fabrication
techniques. Using in-situ electron beam lithography combined with a
low-temperature cathodoluminescence imaging, we deterministically fabricate
microlenses above selected InAs quantum dots (QDs) achieving their efficient
coupling to the external light field. This enables to perform four-wave mixing
micro-spectroscopy of single QD excitons, revealing the exciton population and
coherence dynamics. We infer the temperature dependence of the dephasing in
order to address the impact of phonons on the decoherence of confined excitons.
The loss of the coherence over the first picoseconds is associated with the
emission of a phonon wave packet, also governing the phonon background in
photoluminescence (PL) spectra. Using theory based on the independent boson
model, we consistently explain the initial coherence decay, the zero-phonon
line fraction, and the lineshape of the phonon-assisted PL using realistic
quantum dot geometries
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