13,089 research outputs found
Scalar Aharonov-Bohm effect with longitudinally polarized neutrons
In the scalar Aharonov-Bohm effect, a charged particle (electron) interacts with the scalar electrostatic potential U in the field-free (i.e., force-free) region inside an electrostatic cylinder (Faraday cage). Using a perfect single-crystal neutron interferometer we have performed a “dual” scalar Aharonov-Bohm experiment by subjecting polarized thermal neutrons to a pulsed magnetic field. The pulsed magnetic field was spatially uniform, precluding any force on the neutrons. Aligning the direction of the pulsed magnetic field to the neutron magnetic moment also rules out any classical torque acting to change the neutron polarization. The observed phase shift is purely quantum mechanical in origin. A detailed description of the experiment, performed at the University of Missouri Research Reactor, and its interpretation is given in this paper
The group approach to AdS space propagators: A fast algorithm
In this letter we show how the method of [4] for the calculation of two-point
functions in d+1-dimensional AdS space can be simplified. This results in an
algorithm for the evaluation of the two-point functions as linear combinations
of Legendre functions of the second kind. This algorithm can be easily
implemented on a computer. For the sake of illustration, we displayed the
results for the case of symmetric traceless tensor fields with rank up to l=4.Comment: 14 pages, comment adde
Quantum Channels with Memory
We present a general model for quantum channels with memory, and show that it
is sufficiently general to encompass all causal automata: any quantum process
in which outputs up to some time t do not depend on inputs at times t' > t can
be decomposed into a concatenated memory channel. We then examine and present
different physical setups in which channels with memory may be operated for the
transfer of (private) classical and quantum information. These include setups
in which either the receiver or a malicious third party have control of the
initializing memory. We introduce classical and quantum channel capacities for
these settings, and give several examples to show that they may or may not
coincide. Entropic upper bounds on the various channel capacities are given.
For forgetful quantum channels, in which the effect of the initializing memory
dies out as time increases, coding theorems are presented to show that these
bounds may be saturated. Forgetful quantum channels are shown to be open and
dense in the set of quantum memory channels.Comment: 21 pages with 5 EPS figures. V2: Presentation clarified, references
adde
The ovary learns to ovulate
Sensory stimuli from the external environment (e.g. visual and olfactory stimuli, stress) or internal stimuli cause brain nerve fibres to release neurotransmitters (catecholamines, indolamines and cholinergic agents). These neurotransmitters regulate the secretion of gonadotrophin-releasing hormone (GnRH) from neurosecretory cells of the hypothalamus (Kamberi, 1975). It would seem that under the proper steroid environment, catecholamines (dopamine, norepinephrine or epinephrine) and the cholinergic agent acetylcholine exert a stimulatory influence, whereas indolamines (serotonin or its metabolic product, melatonin) have an opposite effect. In turn, GnRH reaches the anterior pituitary via the hypophyseal portal system, and, through the mediation of cAMP [though may be not as an obligatory intermediate (Naor et al., 1975)], controls gonadotrophin secretion (Labrie et al., 1974
Neutron-irradiation effects in LaO0.9F0.1FeAs superconductor
The effect of atomic disorder induced by neutrons irradiation on
superconducting and normal state properties of polycrystalline LaFeAsO_0.9F_0.1
was investigated. The irradiation of the sample by a moderate neutron fluence F
= 1.6*1019 cm^-2 at Tirr = 50 +- 10 C leads to the suppression of
superconductivity which recovers almost completely after annealing at
temperatures Tann < 750 C. It is shown that the reduction of superconducting
transition temperature Tc under atomic disordering is not determined solely by
the value of Hall concentration nH, i.e. doping level, but is governed by the
reduction of electronic relaxation time. This behavior can be described
qualitatively by universal Abrikosov-Gorkov equation which presents evidence on
the anomalous type of electrons pairing in Fe-based superconductors.Comment: 8 pages, 11 figure
Non-local correlations as an information theoretic resource
It is well known that measurements performed on spatially separated entangled
quantum systems can give rise to correlations that are non-local, in the sense
that a Bell inequality is violated. They cannot, however, be used for
super-luminal signalling. It is also known that it is possible to write down
sets of ``super-quantum'' correlations that are more non-local than is allowed
by quantum mechanics, yet are still non-signalling. Viewed as an information
theoretic resource, super-quantum correlations are very powerful at reducing
the amount of communication needed for distributed computational tasks. An
intriguing question is why quantum mechanics does not allow these more powerful
correlations. We aim to shed light on the range of quantum possibilities by
placing them within a wider context. With this in mind, we investigate the set
of correlations that are constrained only by the no-signalling principle. These
correlations form a polytope, which contains the quantum correlations as a
(proper) subset. We determine the vertices of the no-signalling polytope in the
case that two observers each choose from two possible measurements with d
outcomes. We then consider how interconversions between different sorts of
correlations may be achieved. Finally, we consider some multipartite examples.Comment: Revtex. 12 pages, 6 figure
Gas perturbations in cool cores of galaxy clusters: effective equation of state, velocity power spectra and turbulent heating
We present the statistical analysis of X-ray surface brightness and gas density fluctuations in cool cores of ten, nearby and bright galaxy clusters that have deep Chandra observations and show observational indications of radio-mechanical AGN feedback. Within the central parts of cool cores the total variance of fluctuations is dominated by isobaric and/or isothermal fluctuations on spatial scales ~ 10-60 kpc, which are likely associated with slow gas motions and bubbles of relativistic plasma. Adiabatic fluctuations associated with weak shocks constitute less than 10 per cent of the total variance in all clusters. The typical amplitude of density fluctuations is small, ~ 10 per cent or less on scales of ~ 10-15 kpc. Subdominant contribution of adiabatic fluctuations and small amplitude of density fluctuations support a model of gentle AGN feedback as opposed to periodically explosive scenarios which are implemented in some numerical simulations. Measured one-component velocities of gas motions are typically below 100-150 km/s on scales < 50 kpc, and can be up to ~ 300 km/s on ~ 100 kpc scales. The non-thermal energy is < 12 per cent of the thermal energy. Regardless of the source that drives these motions the dissipation of the energy in such motions provides heat that is sufficient to balance radiative cooling on average, albeit the uncertainties are large. Presented results here support previous conclusions based on the analysis of the Virgo and Perseus Clusters, and agree with the Hitomi measurements. With next generation observatories like Athena and Lynx, these techniques will be yet more powerful
Entanglement transmission and generation under channel uncertainty: Universal quantum channel coding
We determine the optimal rates of universal quantum codes for entanglement
transmission and generation under channel uncertainty. In the simplest scenario
the sender and receiver are provided merely with the information that the
channel they use belongs to a given set of channels, so that they are forced to
use quantum codes that are reliable for the whole set of channels. This is
precisely the quantum analog of the compound channel coding problem. We
determine the entanglement transmission and entanglement-generating capacities
of compound quantum channels and show that they are equal. Moreover, we
investigate two variants of that basic scenario, namely the cases of informed
decoder or informed encoder, and derive corresponding capacity results.Comment: 45 pages, no figures. Section 6.2 rewritten due to an error in
equation (72) of the old version. Added table of contents, added section
'Conclusions and further remarks'. Accepted for publication in
'Communications in Mathematical Physics
Particle formation and interaction
A wide variety of experiments can be conducted on the Space Station that involve the physics of small particles of planetary significance. Processes of interest include nucleation and condensation of particles from a gas, aggregation of small particles into larger ones, and low velocity collisions of particles. All of these processes could be investigated with a general purpose facility on the Space Station. The microgravity environment would be necessary to perform many experiments, as they generally require that particles be suspended for periods substantially longer than are practical at 1 g. Only experiments relevant to planetary processes will be discussed in detail here, but it is important to stress that a particle facility will be useful to a wide variety of scientific disciplines, and can be used to address many scientific problems
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