4,556 research outputs found
Ekpyrotic collapse with multiple fields
A scale invariant spectrum of isocurvature perturbations is generated during
collapse in the scaling solution in models where two or more fields have steep
negative exponential potentials. The scale invariance of the spectrum is
realised by a tachyonic instability in the isocurvature field. We show that
this instability is due to the fact that the scaling solution is a saddle point
in the phase space. The late time attractor is identified with a single field
dominated ekpyrotic collapse in which a steep blue spectrum for isocurvature
perturbations is found. Although quantum fluctuations do not necessarily to
disrupt the classical solution, an additional preceding stage is required to
establish classical homogeneity.Comment: 13 pages, 1 figur
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
Curvature perturbations from ekpyrotic collapse with multiple fields
A scale-invariant spectrum of isocurvature perturbations is generated during
collapse in the ekpyrotic scaling solution in models where multiple fields have
steep negative exponential potentials. The scale invariance of the spectrum is
realized by a tachyonic instability in the isocurvature field. This instability
drives the scaling solution to the late time attractor that is the old
ekpyrotic collapse dominated by a single field. We show that the transition
from the scaling solution to the single field dominated ekpyrotic collapse
automatically converts the initial isocurvature perturbations about the scaling
solution to comoving curvature perturbations about the late-time attractor. The
final amplitude of the comoving curvature perturbation is determined by the
Hubble scale at the transition.Comment: 15 pages, 3 figures, a reference added, to be published in CQG, a
remark on the comoving density perturbation correcte
Experimental observation of fractional topological phases with photonic qudits
Geometrical and topological phases play a fundamental role in quantum theory.
Geometric phases have been proposed as a tool for implementing unitary gates
for quantum computation. A fractional topological phase has been recently
discovered for bipartite systems. The dimension of the Hilbert space determines
the topological phase of entangled qudits under local unitary operations. Here
we investigate fractional topological phases acquired by photonic entangled
qudits. Photon pairs prepared as spatial qudits are operated inside a Sagnac
interferometer and the two-photon interference pattern reveals the topological
phase as fringes shifts when local operations are performed. Dimensions and were tested, showing the expected theoretical values.Comment: 6 pages, 4 figure
Topological phase for spin-orbit transformations on a laser beam
We investigate the topological phase associated with the double connectedness
of the SO(3) representation in terms of maximally entangled states. An
experimental demonstration is provided in the context of polarization and
spatial mode transformations of a laser beam carrying orbital angular momentum.
The topological phase is evidenced through interferometric measurements and a
quantitative relationship between the concurrence and the fringes visibility is
derived. Both the quantum and the classical regimes were investigated.Comment: 4 pages, 4 figure
Orbital angular momentum exchange in an optical parametric oscillator
We present a study of orbital angular momentum transfer from pump to
down-converted beams in a type-II Optical Parametric Oscillator. Cavity and
anisotropy effects are investigated and demostrated to play a central role in
the transverse mode dynamics. While the idler beam can oscillate in a
Laguerre-Gauss mode, the crystal birefringence induces an astigmatic effect in
the signal beam that prevents the resonance of such mode.Comment: 10 pages, 8 figures, regular articl
Dirac quantization of membrane in time dependent orbifold
We present quantum theory of a membrane propagating in the vicinity of a time
dependent orbifold singularity. The dynamics of a membrane, with the parameters
space topology of a torus, winding uniformly around compact dimension of the
embedding spacetime is mathematically equivalent to the dynamics of a closed
string in a flat FRW spacetime. The construction of the physical Hilbert space
of a membrane makes use of the kernel space of self-adjoint constraint
operators. It is a subspace of the representation space of the constraints
algebra. There exist non-trivial quantum states of a membrane evolving across
the singularity.Comment: 16 pages, no figures, version accepted for publication in Journal of
High Energy Physic
Cosmological Perturbations in a Big Crunch/Big Bang Space-time
A prescription is developed for matching general relativistic perturbations
across singularities of the type encountered in the ekpyrotic and cyclic
scenarios i.e. a collision between orbifold planes. We show that there exists a
gauge in which the evolution of perturbations is locally identical to that in a
model space-time (compactified Milne mod Z_2) where the matching of modes
across the singularity can be treated using a prescription previously
introduced by two of us. Using this approach, we show that long wavelength,
scale-invariant, growing-mode perturbations in the incoming state pass through
the collision and become scale-invariant growing-mode perturbations in the
expanding hot big bang phase.Comment: 47 pages, 4 figure
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