733 research outputs found
Global topological k-defects
We consider global topological defects in symmetry breaking models with a
non-canonical kinetic term. Apart from a mass parameter entering the potential,
one additional dimensional parameter arises in such models -- a ``kinetic''
mass. The properties of defects in these models are quite different from
``standard'' global domain walls, vortices and monopoles, if their kinetic mass
scale is smaller than their symmetry breaking scale. In particular, depending
on the concrete form of the kinetic term, the typical size of such a defect can
be either much larger or much smaller than the size of a standard defect with
the same potential term. The characteristic mass of a non-standard defect,
which might have been formed during a phase transition in the early universe,
depends on both the temperature of a phase transition and the kinetic mass.Comment: 7 pages, 3 figures; v2: references added, matches the published
versio
Remote preparation of a single-mode photonic qubit by measuring field quadrature noise
An electromagnetic field quadrature measurement, performed on one of the
modes of the nonlocal single-photon state , collapses it into a
superposition of the single-photon and vacuum states in the other mode. We use
this effect to implement remote preparation of arbitrary single-mode photonic
qubits conditioned on observation of a preselected quadrature value. The
quantum efficiency of the prepared qubit can be higher than that of the initial
single photon
Interconvertibility of single-rail optical qubits
We show how to convert between partially coherent superpositions of a single
photon with the vacuum using linear optics and postselection based on homodyne
measurements. We introduce a generalized quantum efficiency for such states and
show that any conversion that decreases this quantity is possible. We also
prove that our scheme is optimal by showing that no linear optical scheme with
generalized conditional measurements, and with one single-rail qubit input can
improve the generalized efficiency.Comment: 3 pages, 2 figure
Efficiency limits for linear optical processing of single photons and single-rail qubits
We analyze the problem of increasing the efficiency of single-photon sources
or single-rail photonic qubits via linear optical processing and destructive
conditional measurements. In contrast to previous work we allow for the use of
coherent states and do not limit to photon-counting measurements. We conjecture
that it is not possible to increase the efficiency, prove this conjecture for
several important special cases, and provide extensive numerical results for
the general case.Comment: 10 pages, 4 figure
Quantum scissors: teleportation of single-mode optical states by means of a nonlocal single photon
We employ the quantum state of a single photon entangled with the vacuum
(|1,0>-|0,1>), generated by a photon incident upon a symmetric beam splitter,
to teleport single-mode quantum states of light by means of the Bennett
protocol. Teleportation of coherent states results in truncation of their Fock
expansion to the first two terms. We analyze the teleported ensembles by means
of homodyne tomography and obtain fidelities of up to 99 per cent for low
source state amplitudes. This work is an experimental realization of the
quantum scissors device proposed by Pegg, Phillips and Barnett (Phys. Rev.
Lett. 81, 1604 (1998)
Suppressing Quantum Fluctuations in Classicalization
We study vacuum quantum fluctuations of simple Nambu-Goldstone bosons -
derivatively coupled single scalar-field theories possessing shift-symmetry in
field space. We argue that quantum fluctuations of the interacting field can be
drastically suppressed with respect to the free-field case. Moreover, the
power-spectrum of these fluctuations can soften to become red for sufficiently
small scales. In quasiclassical approximation, we demonstrate that this
suppression can only occur for those theories that admit such classical static
backgrounds around which small perturbations propagate faster than light. Thus,
a quasiclassical softening of quantum fluctuations is only possible for
theories which classicalize instead of having a usual Lorentz invariant and
local Wilsonian UV- completion. We illustrate our analysis by estimating the
quantum fluctuations for the DBI-like theories.Comment: 6 pages, no figures, published version, more general discussion of
uncertainty relation in QFT, improved and more general derivation of the main
resul
Imperfect Dark Energy from Kinetic Gravity Braiding
We introduce a large class of scalar-tensor models with interactions
containing the second derivatives of the scalar field but not leading to
additional degrees of freedom. These models exhibit peculiar features, such as
an essential mixing of scalar and tensor kinetic terms, which we have named
kinetic braiding. This braiding causes the scalar stress tensor to deviate from
the perfect-fluid form. Cosmology in these models possesses a rich
phenomenology, even in the limit where the scalar is an exact Goldstone boson.
Generically, there are attractor solutions where the scalar monitors the
behaviour of external matter. Because of the kinetic braiding, the position of
the attractor depends both on the form of the Lagrangian and on the external
energy density. The late-time asymptotic of these cosmologies is a de Sitter
state. The scalar can exhibit phantom behaviour and is able to cross the
phantom divide with neither ghosts nor gradient instabilities. These features
provide a new class of models for Dark Energy. As an example, we study in
detail a simple one-parameter model. The possible observational signatures of
this model include a sizeable Early Dark Energy and a specific equation of
state evolving into the final de-Sitter state from a healthy phantom regime.Comment: 41 pages, 7 figures. References and some clarifying language added.
This version was accepted for publication in JCA
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