2,509 research outputs found
Spontaneous excitation of an accelerated multilevel atom in dipole coupling to the derivative of a scalar field
We study the spontaneous excitation of an accelerated multilevel atom in
dipole coupling to the derivative of a massless quantum scalar field and
separately calculate the contributions of the vacuum fluctuation and radiation
reaction to the rate of change of the mean atomic energy of the atom. It is
found that, in contrast to the case where a monopole like interaction between
the atom and the field is assumed, there appear extra corrections proportional
to the acceleration squared, in addition to corrections which can be viewed as
a result of an ambient thermal bath at the Unruh temperature, as compared with
the inertial case, and the acceleration induced correction terms show
anisotropy with the contribution from longitudinal polarization being four
times that from the transverse polarization for isotropically polarized
accelerated atoms. Our results suggest that the effect of acceleration on the
rate of change of the mean atomic energy is dependent not only on the quantum
field to which the atom is coupled, but also on the type of the interaction
even if the same quantum scalar field is considered.Comment: 11 pages, no figure
Decay of accelerated particles
We study how the decay properties of particles are changed by acceleration.
It is shown that under the influence of acceleration (1) the lifetime of
particles is modified and (2) new processes (like the decay of the proton)
become possible. This is illustrated by considering scalar models for the decay
of muons, pions, and protons. We discuss the close conceptual relation between
these processes and the Unruh effect.Comment: Latex2e, 12 pages, 6 Postscript figures included with epsfig, to
appear in Phys. Rev.
Aharonov-Bohm scattering of charged particles and neutral atoms: the role of absorption
The Aharonov-Bohm scattering of charged particles by the magnetic field of an
infinitely long and infinitely thin solenoid (magnetic string) in an absorbing
medium is studied. We discuss the partial-wave approach to this problem and
show that standard partial-wave method can be adjusted to this case. The effect
of absorption leads to oscillations of the AB cross section.
Based on this we investigate the scattering of neutral atoms with induced
electric dipole moments by a charge wire of finite radius which is placed in an
uniform magnetic field. The physical realistic and practically important case
that all atoms which collide with the wire are totally absorbed at its surface,
is studied in detail. The dominating terms of the scattering amplitude are
evaluated analytically for different physical constellations. The rest terms
are written in a form suitable for a numerical computation. We show that if the
magnetic field is absent, the absorbing charged wire causes oscillations of the
cross section. In the presence of the magnetic field the cross section
increases and the dominating Aharonov--Bohm peak appears in the forward
direction, suppressing the oscillations.Comment: 15 pages, LaTeXfile, 2 figure
Quantum reflection of massless neutrinos from a torsion-induced potential
In the context of the Einstein-Cartan-Dirac model, where the torsion of the
space-time couples to the axial currents of the fermions, we study the effects
of this quantum-gravitational interaction on a massless neutrino beam crossing
through a medium with high number density of fermions at rest. We calculate the
reflection amplitude and show that a specific fraction of the incident
neutrinos reflects from this potential if the polarization of the medium is
different from zero. We also discuss the order of magnitude of the fermionic
number density in which this phenomenon is observable, in other theoretical
contexts, for example the strong-gravity regime and the effective field theory
approach.Comment: 8 pages, LaTe
The power spectrum of the circular noise
The circular noise is important in connection to Mach's principle, and also
as a possible probe of the Unruh effect. In this letter the power spectrum of
the detector following the Trocheries-Takeno motion in the Minkowski vacuum is
analytically obtained in the form of an infinite series. A mean distribution
function and corresponding energy density are obtained for this particular
detected noise. The analogous of a non constant temperature distribution is
obtained. And in the end, a brief discussion about the equilibrium
configuration is given.Comment: accepted for publication in GR
Evolution of a qubit under the influence of a succession of unsharp measurements
We investigate the evolution of a single qubit subject to a continuous
unitary dynamics and an additional interrupting influence which occurs
periodically. One may imagine a dynamically evolving closed quantum system
which becomes open at certain times. The interrupting influence is represented
by an operation, which is assumed to equivalently describe a non-selective
unsharp measurement. It may be decomposed into a positive operator, which in
case of a measurement represents the pure measurement part, followed by an
unitary back-action operator. Equations of motion for the state evolution are
derived in the form of difference equations. It is shown that the 'free'
Hamiltonian is completed by an averaged Hamiltonian, which goes back to the
back-action. The positive operator specifies a decoherence rate and results in
a decoherence term. The continuum limit to a master equation is performed. The
selective evolution is discussed and correcting higher order terms are worked
out in an Appendix.Comment: 19 pages, no figure
States insensitive to the Unruh effect in multi-level detectors
We give a general treatment of the spontaneous excitation rates and the
non-relativistic Lamb shift of constantly accelerated multi-level atoms as a
model for multi-level detectors. Using a covariant formulation of the dipole
coupling between the atom and the electromagnetic field we show that new
Raman-like transitions can be induced by the acceleration. Under certain
conditions these transitions can lead to stable ground and excited states which
are not affected by the non inertial motion. The magnitude of the Unruh effect
is not altered by multi-level effects. Both the spontaneous excitation rates
and the Lamb shift are not within the range of measurability.Comment: 9 Pages, late
Collapse and revival of ultracold atoms in a microwave cavity and of photons in parametric down-conversion
We present a new theoretical method to study a trapped gas of bosonic
two-level atoms interacting with a single mode of a microwave cavity. This
interaction is described by a trilinear Hamiltonian which is formally
completely equivalent to the one describing parametric down-conversion in
quantum optics. A system of differential equations describing the evolution,
including the long-time behaviour, of not only the mean value but also the
variance of the number of excited atoms is derived and solved analytically. For
different initial states the mean number of excited atoms exhibits periodically
reappearing dips, with an accompanying peak in the variance, or fractional
collapses and revivals. Closed expressions for the period and the revival time
are obtained.Comment: 5 pages, RevTeX, 3 figure
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