5,633 research outputs found
Multiphoton path entanglement by non-local bunching
Multiphoton path entanglement is created without applying post-selection, by
manipulating the state of stimulated parametric down-conversion. A specific
measurement on one of the two output spatial modes leads to the non-local
bunching of the photons of the other mode, forming the desired multiphoton path
entangled state. We present experimental results for the case of a heralded
two-photon path entangled state and show how to extend this scheme to higher
photon numbers.Comment: 4 pages, 5 figures, published versio
The cosmic gravitational wave background in a cyclic universe
Inflation predicts a primordial gravitational wave spectrum that is slightly
``red,'' i.e., nearly scale-invariant with slowly increasing power at longer
wavelengths. In this paper, we compute both the amplitude and spectral form of
the primordial tensor spectrum predicted by cyclic/ekpyrotic models. The
spectrum is blue and exponentially suppressed compared to inflation on long
wavelengths. The strongest observational constraint emerges from the
requirement that the energy density in gravitational waves should not exceed
around 10 per cent of the energy density at the time of nucleosynthesis.Comment: 4 pages, 3 figuer
Theoretical investigation of moir\'e patterns in quantum images
Moir\'e patterns are produced when two periodic structures with different
spatial frequencies are superposed. The transmission of the resulting structure
gives rise to spatial beatings which are called moir\'e fringes. In classical
optics, the interest in moir\'e fringes comes from the fact that the spatial
beating given by the frequency difference gives information about details(high
spatial frequency) of a given spatial structure. We show that moir\'e fringes
can also arise in the spatial distribution of the coincidence count rate of
twin photons from the parametric down-conversion, when spatial structures with
different frequencies are placed in the path of each one of the twin beams. In
other words,we demonstrate how moir\'e fringes can arise from quantum images
Warped product approach to universe with non-smooth scale factor
In the framework of Lorentzian warped products, we study the
Friedmann-Robertson-Walker cosmological model to investigate non-smooth
curvatures associated with multiple discontinuities involved in the evolution
of the universe. In particular we analyze non-smooth features of the spatially
flat Friedmann-Robertson-Walker universe by introducing double discontinuities
occurred at the radiation-matter and matter-lambda phase transitions in
astrophysical phenomenology.Comment: 10 page
Designing Cyclic Universe Models
Recent advances in understanding the propagation of perturbations through the
transition from big crunch to big bang (esp. Tolley et al. hep-th/0306109) make
it possible for the first time to consider the full set of phenomenological
constraints on the scalar field potential in cyclic models of the universe. We
show that cyclic models require a comparable degree of tuning to that needed
for inflationary models. The constraints are reduced to a set of simple design
rules including "fast-roll" parameters analogous to the "slow-roll" parameters
in inflation.Comment: 4 pages, 2 figures. Minor typos and figure correcte
String production at the level of effective field theory
Pair creation of strings in time-dependent backgrounds is studied from an
effective field theory viewpoint, and some possible cosmological applications
are discussed. Simple estimates suggest that excited strings may have played a
significant role in preheating, if the string tension as measured in
four-dimensional Einstein frame falls a couple of orders of magnitude below the
four-dimensional Planck scale.Comment: 20 pages, latex2e. v2: a reference adde
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