19,602 research outputs found
Relativistic Quantum Information in Detectors-Field Interactions
We review Unruh-DeWitt detectors and other models of detector-field
interaction in a relativistic quantum field theory setting as a tool for
extracting detector-detector, field-field and detector-field correlation
functions of interest in quantum information science, from entanglement
dynamics to quantum teleportation. We in particular highlight the contrast
between the results obtained from linear perturbation theory which can be
justified provided switching effects are properly accounted for, and the
nonperturbative effects from available analytic expressions which incorporate
the backreaction effects of the quantum field on the detector behaviour.Comment: 21 pages, 3 figures. Prepared for the special focus issue on RQ
Entanglement creation between two causally-disconnected objects
We study the full entanglement dynamics of two uniformly accelerated
Unruh-DeWitt detectors with no direct interaction in between but each coupled
to a common quantum field and moving back-to-back in the field vacuum. For two
detectors initially prepared in a separable state our exact results show that
quantum entanglement between the detectors can be created by the quantum field
under some specific circumstances, though each detector never enters the
other's light cone in this setup. In the weak coupling limit, this entanglement
creation can occur only if the initial moment is placed early enough and the
proper acceleration of the detectors is not too large or too small compared to
the natural frequency of the detectors. Once entanglement is created it lasts
only a finite duration, and always disappears at late times. Prior result by
Reznik derived using the time-dependent perturbation theory with extended
integration domain is shown to be a limiting case of our exact solutions at
some specific moment. In the strong coupling and high acceleration regime,
vacuum fluctuations experienced by each detector locally always dominate over
the cross correlations between the detectors, so entanglement between the
detectors will never be generated.Comment: 16 pages, 8 figures; added Ref.[7] and related discussion
No observational constraints from hypothetical collisions of hypothetical dark halo primordial black holes with galactic objects
It was suggested by several authors that hypothetical primordial black holes
(PBHs) may contribute to the dark matter in our Galaxy. There are strong
constraints based on the Hawking evaporation that practically exclude PBHs with
masses m~1e15-1e16g and smaller as significant contributors to the Galactic
dark matter. Similarly, PBHs with masses greater than about 1e26g are
practically excluded by the gravitational lensing observation. The mass range
between 10e16g<m<10e26g is unconstrained. In this paper, we examine possible
observational signatures in the unexplored mass range, investigating
hypothetical collisions of PBHs with main sequence stars, red giants, white
dwarfs, and neutron stars in our Galaxy. This has previously been discussed as
possibly leading to an observable photon eruption due to shock production
during the encounter. We find that such collisions are either too rare to be
observed (if the PBH masses are typically larger than about 1e20g), or produce
too little power to be detected (if the masses are smaller than about 1e20g).Comment: Accepted for publication in The Astrophysical Journa
An example of a uniformly accelerated particle detector with non-Unruh response
We propose a scalar background in Minkowski spacetime imparting constant
proper acceleration to a classical particle. In contrast to the case of a
constant electric field the proposed scalar potential does not create
particle-antiparticle pairs. Therefore an elementary particle accelerated by
such field is a more appropriate candidate for an "Unruh-detector" than a
particle moving in a constant electric field. We show that the proposed
detector does not reveal the universal thermal response of the Unruh type.Comment: 12 pages, 1 figur
Comparing Image Quality in Phase Contrast sub X-Ray Tomography -- A Round-Robin Study
How to evaluate and compare image quality from different sub-micrometer
(sub) CT scans? A simple test phantom made of polymer microbeads is used
for recording projection images as well as 13 CT scans in a number of
commercial and non-commercial scanners. From the resulting CT images, signal
and noise power spectra are modeled for estimating volume signal-to-noise
ratios (3D SNR spectra). Using the same CT images, a time- and
shape-independent transfer function (MTF) is computed for each scan, including
phase contrast effects and image blur (). The SNR spectra
and MTF of the CT scans are compared to 2D SNR spectra of the projection
images. In contrary to 2D SNR, volume SNR can be normalized with respect to the
object's power spectrum, yielding detection effectiveness (DE) a new measure
which reveals how technical differences as well as operator-choices strongly
influence scan quality for a given measurement time. Using DE, both
source-based and detector-based sub CT scanners can be studied and their
scan quality can be compared. Future application of this work requires a
particular scan acquisition scheme which will allow for measuring 3D
signal-to-noise ratios, making the model fit for 3D noise power spectra
obsolete
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