888 research outputs found

### Towards Microscopic Understanding of the Phonon Bottleneck

The problem of the phonon bottleneck in the relaxation of two-level systems
(spins) to a narrow group of resonant phonons via emission-absorption processes
is investigated from the first principles. It is shown that the kinetic
approach based on the Pauli master equation is invalid because of the narrow
distribution of the phonons exchanging their energy with the spins. This
results in a long-memory effect that can be best taken into account by
introducing an additional dynamical variable corresponding to the nondiagonal
matrix elements responsible for spin-phonon correlation. The resulting system
of dynamical equations describes the phonon-bottleneck plateau in the spin
excitation, as well as a gap in the spin-phonon spectrum for any finite
concentration of spins. On the other hand, it does not accurately render the
lineshape of emitted phonons and still needs improving.Comment: 13 Phys. Rev. pages, 5 figure captions (7 figures

### Infra-red Divergences in Light-Front QED and Coherent State Basis

We present a next to leading order calculation of electron mass
renormalization in Light-Front Quantum Electrodynamics (LFQED) using
old-fashioned time ordered perturbation theory (TOPT). We show that the true
infrared divergences in $\delta m^2$ get canceled up to $O(e^4)$ if one uses
coherent state basis instead of fock basis to calculate the transition matrix
elements.Comment: 44 pages,11 figure

### Phonon bottleneck in the low-excitation limit

The phonon-bottleneck problem in the relaxation of two-level systems (spins)
via direct phonon processes is considered numerically in the weak-excitation
limit where the Schroedinger equation for the spin-phonon system simplifies.
The solution for the relaxing spin excitation p(t), emitted phonons n_k(t),
etc. is obtained in terms of the exact many-body eigenstates. In the absence of
phonon damping Gamma_{ph} and inhomogeneous broadening, p(t) approaches the
bottleneck plateau p_\infty > 0 with strongly damped oscillations, the
frequency being related to the spin-phonon splitting Delta at the avoided
crossing. For any Gamma_{ph} > 0 one has p(t) -> 0 but in the case of strong
bottleneck the spin relaxation rate is much smaller than Gamma_{ph} and p(t) is
nonexponential. Inhomogeneous broadening exceeding Delta partially alleviates
the bottleneck and removes oscillations of p(t). The line width of emitted
phonons, as well as Delta, increase with the strength of the bottleneck, i.e.,
with the concentration of spins.Comment: 16 PR pages, 14 Figure captions, submitted to PRB. The whole text
does dot fit here. Please, get the correct file from
http://www.lehman.edu/faculty/dgaranin/Bottleneck2.pd

### Sub-Natural Linewidth Single Photons from a Quantum Dot

The observation of quantum dot resonance fluorescence enabled a new
solid-state approach to generating single photons with a bandwidth almost as
narrow as the natural linewidth of a quantum dot transition. Here, we operate
in the Heitler regime of resonance fluorescence to generate sub-natural
linewidth and high-coherence quantum light from a single quantum dot. The
measured single-photon bandwidth exhibits a 30-fold reduction with respect to
the radiative linewidth of the QD transition and the single photons exhibit
coherence properties inherited from the excitation laser. In contrast,
intensity-correlation measurements reveal that this photon source maintains a
high degree of antibunching behaviour on the order of the transition lifetime
with vanishing two-photon scattering probability. This light source will find
immediate applications in quantum cryptography, measurement-based quantum
computing and, in particular, deterministic generation of high-fidelity
distributed entanglement among independent and even disparate quantum systems

### Photon Orbital Angular Momentum in Astrophysics

Astronomical observations of the orbital angular momentum of photons, a
property of electromagnetic radiation that has come to the fore in recent
years, have apparently never been attempted. Here, I show that measurements of
this property of photons have a number of astrophysical applications.Comment: 17 pages plus two figure

### Three and Four-Body Interactions in Spin-Based Quantum Computers

In the effort to design and to construct a quantum computer, several leading
proposals make use of spin-based qubits. These designs generally assume that
spins undergo pairwise interactions. We point out that, when several spins are
engaged mutually in pairwise interactions, the quantitative strengths of the
interactions can change and qualitatively new terms can arise in the
Hamiltonian, including four-body interactions. In parameter regimes of
experimental interest, these coherent effects are large enough to interfere
with computation, and may require new error correction or avoidance techniques.Comment: 5 pages incl. 4 figures. To appear in Phys. Rev. Lett. For an
expanded version including detailed calculations see
http://xxx.lanl.gov/abs/cond-mat/030201

### Entanglement between the future and past in the quantum vacuum

We note that massless fields within the future and past light cone may be
quantized as independent systems. We show that the vacuum is an entangled state
of these systems, exactly mirroring the known entanglement between the
spacelike separated Rindler wedges. We describe a detector which exhibits a
thermal response to the vacuum when switched on at t=0. The feasibility of
experimentally detecting this effect is discussed.Comment: 4 pages, 1 figur

### Quantization Of Cyclotron Motion and Quantum Hall Effect

We present a two dimensional model of IQHE in accord with the cyclotron
motion. The quantum equation of the QHE curve and a new definition of filling
factor are also given.Comment: 13 Pages, Latex, 1 figure, to appear in Europhys. Lett. September
199

### New facts about muon production in Extended Air Shower simulations

Whereas air shower simulations are very valuable tools for interpreting
cosmic ray data, there is a long standing problem: is seems to be impossible to
accommodate at the same time the longitudinal development of air showers and
the number of muons measured at ground. Using a new hadronic interaction model
(EPOS) in air shower simulations produces considerably more muons, in agreement
with results from the HiRes-MIA experiment. We find that this is mainly due to
a better description of baryon-antibaryon production in hadronic interactions.
This is a new aspect of air shower physics which has never been considered so
far

### Quantum derivation of the use of classical electromagnetic potentials in relativistic Coulomb excitation

We prove that a relativistic Coulomb excitation calculation in which the
classical electromagnetic field of the projectile is used to induce transitions
between target states gives the same target transition amplitudes, to all
orders of perturbation theory, as would a calculation in which the interaction
between projectile and target is mediated by a quantized electromagnetic field.Comment: 1 .zip file containing LaTex source plus three figures as .eps file

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