403 research outputs found
Generation of Entanglement Outside of the Light Cone
The Feynman propagator has nonzero values outside of the forward light cone.
That does not allow messages to be transmitted faster than the speed of light,
but it is shown here that it does allow entanglement and mutual information to
be generated at space-like separated points. These effects can be interpreted
as being due to the propagation of virtual photons outside of the light cone or
as a transfer of pre-existing entanglement from the quantum vacuum. The
differences between these two interpretations are discussed.Comment: 25 pages, 7 figures. Additional references and figur
Single particle Green's function in the Calogero-Sutherland model for rational couplings
We derive an exact expression for the single particle Green function in the
Calogero-Sutherland model for all rational values of the coupling . The
calculation is based on Jack polynomial techniques and the results are given in
the thermodynamical limit. Two type of intermediate states contribute. The
firts one consists of a particle propagating out of the Fermi sea and the
second one consists of a particle propagating in one direction, q particles in
the opposite direction and p holes.Comment: 9 pages, RevTeX, epsf.tex, 4 figures, files uuencode
A Homological Approach to Belief Propagation and Bethe Approximations
We introduce a differential complex of local observables given a
decomposition of a global set of random variables into subsets. Its boundary
operator allows us to define a transport equation equivalent to Belief
Propagation. This definition reveals a set of conserved quantities under Belief
Propagation and gives new insight on the relationship of its equilibria with
the critical points of Bethe free energy.Comment: 14 pages, submitted for the 2019 Geometric Science of Information
colloquiu
Neutrino-driven Explosions
The question why and how core-collapse supernovae (SNe) explode is one of the
central and most long-standing riddles of stellar astrophysics. A solution is
crucial for deciphering the SN phenomenon, for predicting observable signals
such as light curves and spectra, nucleosynthesis, neutrinos, and gravitational
waves, for defining the role of SNe in the evolution of galaxies, and for
explaining the birth conditions and properties of neutron stars (NSs) and
stellar-mass black holes. Since the formation of such compact remnants releases
over hundred times more energy in neutrinos than the SN in the explosion,
neutrinos can be the decisive agents for powering the SN outburst. According to
the standard paradigm of the neutrino-driven mechanism, the energy transfer by
the intense neutrino flux to the medium behind the stagnating core-bounce
shock, assisted by violent hydrodynamic mass motions (sometimes subsumed by the
term "turbulence"), revives the outward shock motion and thus initiates the SN
blast. Because of the weak coupling of neutrinos in the region of this energy
deposition, detailed, multidimensional hydrodynamic models including neutrino
transport and a wide variety of physics are needed to assess the viability of
the mechanism. Owing to advanced numerical codes and increasing supercomputer
power, considerable progress has been achieved in our understanding of the
physical processes that have to act in concert for the success of
neutrino-driven explosions. First studies begin to reveal observational
implications and avenues to test the theoretical picture by data from
individual SNe and SN remnants but also from population-integrated observables.
While models will be further refined, a real breakthrough is expected through
the next Galactic core-collapse SN, when neutrinos and gravitational waves can
be used to probe the conditions deep inside the dying star. (abridged)Comment: Author version of chapter for 'Handbook of Supernovae,' edited by A.
Alsabti and P. Murdin, Springer. 54 pages, 13 figure
Massive stars as thermonuclear reactors and their explosions following core collapse
Nuclear reactions transform atomic nuclei inside stars. This is the process
of stellar nucleosynthesis. The basic concepts of determining nuclear reaction
rates inside stars are reviewed. How stars manage to burn their fuel so slowly
most of the time are also considered. Stellar thermonuclear reactions involving
protons in hydrostatic burning are discussed first. Then I discuss triple alpha
reactions in the helium burning stage. Carbon and oxygen survive in red giant
stars because of the nuclear structure of oxygen and neon. Further nuclear
burning of carbon, neon, oxygen and silicon in quiescent conditions are
discussed next. In the subsequent core-collapse phase, neutronization due to
electron capture from the top of the Fermi sea in a degenerate core takes
place. The expected signal of neutrinos from a nearby supernova is calculated.
The supernova often explodes inside a dense circumstellar medium, which is
established due to the progenitor star losing its outermost envelope in a
stellar wind or mass transfer in a binary system. The nature of the
circumstellar medium and the ejecta of the supernova and their dynamics are
revealed by observations in the optical, IR, radio, and X-ray bands, and I
discuss some of these observations and their interpretations.Comment: To be published in " Principles and Perspectives in Cosmochemistry"
Lecture Notes on Kodai School on Synthesis of Elements in Stars; ed. by Aruna
Goswami & Eswar Reddy, Springer Verlag, 2009. Contains 21 figure
Coordinate Representation of the Two-Spinon wavefunction and Spinon Interaction
By deriving and studying the coordinate representation for the two-spinon
wavefunction, we show that spinon excitations in the Haldane-Shastry model
interact. The interaction is given by a short-range attraction and causes a
resonant enhancement in the two-spinon wavefunction at short separations
between the spinons. We express the spin susceptibility for a finite lattice in
terms of the resonant enhancement, given by the two-spinon wavefunction at zero
separation. In the thermodynamic limit, the spinon attraction turns into the
square-root divergence in the dynamical spin susceptibility.Comment: 19 pages, 5 .eps figure
Formulas and Asymptotics for the Asymmetric Simple Exclusion Process
This is an expanded version of a series of lectures delivered by the second
author in June, 2009. It describes the results of three of the authors' papers
on ASEP, from the derivation of exact formulas for configuration probabilities,
through Fredholm determinant representation, to asymptotics for ASEP with step
initial condition establishing KPZ universality. Although complete proofs are
in general not given, at least the main elements of them are.Comment: 25 pages. Version 2 corrects an error in Section II.
Allowed and forbidden transitions in artificial hydrogen and helium atoms
The strength of radiative transitions in atoms is governed by selection
rules. Spectroscopic studies of allowed transitions in hydrogen and helium
provided crucial evidence for the Bohr's model of an atom. Forbidden
transitions, which are actually allowed by higher-order processes or other
mechanisms, indicate how well the quantum numbers describe the system. We apply
these tests to the quantum states in semiconductor quantum dots (QDs), which
are regarded as artificial atoms. Electrons in a QD occupy quantized states in
the same manner as electrons in real atoms. However, unlike real atoms, the
confinement potential of the QD is anisotropic, and the electrons can easily
couple with phonons of the material. Understanding the selection rules for such
QDs is an important issue for the manipulation of quantum states. Here we
investigate allowed and forbidden transitions for phonon emission in one- and
two-electron QDs (artificial hydrogen and helium atoms) by electrical
pump-and-probe experiments, and find that the total spin is an excellent
quantum number in artificial atoms. This is attractive for potential
applications to spin based information storage.Comment: slightly longer version of Nature 419, 278 (2002
Single-particle Green's functions of the Calogero-Sutherland model at couplings \lambda = 1/2, 1, and 2
At coupling strengths lambda = 1/2, 1, or 2, the Calogero-Sutherland model
(CSM) is related to Brownian motion in a Wigner-Dyson random matrix ensemble
with orthogonal, unitary, or symplectic symmetry. Using this relation in
conjunction with superanalytic techniques developed in mesoscopic conductor
physics, we derive an exact integral representation for the CSM two-particle
Green's function in the thermodynamic limit. Simple closed expressions for the
single-particle Green's functions are extracted by separation of points. For
the advanced part, where a particle is added to the ground state and later
removed, a sum of two contributions is found: the expected one with just one
particle excitation present, plus an extra term arising from fractionalization
of the single particle into a number of elementary particle and hole
excitations.Comment: 19 REVTeX page
Wideband THz time domain spectroscopy based on optical rectification and electro-optic sampling
We present an analytical model describing the full electromagnetic propagation in a THz time-domain spectroscopy (THz-TDS) system, from the THz pulses via Optical Rectification to the detection via Electro Optic-Sampling. While several investigations deal singularly with the many elements that constitute a THz-TDS, in our work we pay particular attention to the modelling of the time-frequency behaviour of all the stages which compose the experimental set-up. Therefore, our model considers the following main aspects: (i) pump beam focusing into the generation crystal; (ii) phase-matching inside both the generation and detection crystals; (iii) chromatic dispersion and absorption inside the crystals; (iv) Fabry-Perot effect; (v) diffraction outside, i.e. along the propagation, (vi) focalization and overlapping between THz and probe beams, (vii) electro-optic sampling. In order to validate our model, we report on the comparison between the simulations and the experimental data obtained from the same set-up, showing their good agreement
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