509 research outputs found
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
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)
Stationary Configurations Imply Shift Symmetry: No Bondi Accretion for Quintessence / k-Essence
In this paper we show that, for general scalar fields, stationary
configurations are possible for shift symmetric theories only. This symmetry
with respect to constant translations in field space should either be manifest
in the original field variables or reveal itself after an appropriate field
redefinition. In particular this result implies that neither k-Essence nor
Quintessence can have exact steady state / Bondi accretion onto Black Holes. We
also discuss the role of field redefinitions in k-Essence theories. Here we
study the transformation properties of observables and other variables in
k-Essence and emphasize which of them are covariant under field redefinitions.
Finally we find that stationary field configurations are necessarily linear in
Killing time, provided that shift symmetry is realized in terms of these field
variables.Comment: 8 page
Neutron stars in generalized f(R) gravity
Quartic gravity theory is considered with the Einstein-Hilbert Lagrangean
being Ricci\'s tensor and R
the curvature scalar. The parameters and are taken of order 1 km
Arguments are given which suggest that the effective theory so obtained may be
a plausible approximation of a viable theory. A numerical integration is
performed of the field equations for a free neutron gas. As in the standard
Oppenheimer-Volkoff calculation the star mass increases with increasing central
density until about 1 solar mass and then decreases. However a dramatic
difference exists in the behaviour of the baryon number, which increases
monotonically. The calculation suggests that the theory allows stars in
equilibrium with arbitrary baryon number, no matter how large.Comment: Keywords: stars, neutron stars; gravity; modified gravity Accepted in
Astrophysics and Space Scienc
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
Hairy black holes in theories with massive gravitons
This is a brief survey of the known black hole solutions in the theories of
ghost-free bigravity and massive gravity. Various black holes exist in these
theories, in particular those supporting a massive graviton hair. However, it
seems that solutions which could be astrophysically relevant are the same as in
General Relativity, or very close to them. Therefore, the no-hair conjecture
essentially applies, and so it would be hard to detect the graviton mass by
observing black holes.Comment: References added. 20 pages, 3 figures, based on the talk given at the
7-th Aegean Summer School "Beyond Einstein's theory of gravity", September
201
Dimuon production by laser-wakefield accelerated electrons
We analyze pair production generated by high-energy electrons
emerging from a laser-wakefield accelerator. The pairs are created
in a solid thick high- target, following the electron accelerating plasma
region. Numerical estimates are presented for electron beams obtained presently
in the LBL TW laser experiment \cite{C2} and possible future developments.
Reactions induced by the secondary bremsstrahlung photons dominate the dimuon
production. According to our estimates, a 20 pC electron bunch with energy of 1
(10) GeV may create about 200 (6000) muon pairs. The produced can be
used in studying various aspects of muon-related physics in table top
installations. This may be considered as an important step towards the
investigation of more complicated elementary processes induced by laser driven
electrons.Comment: 14 pages, 5 figure
Single-qubit optical quantum fingerprinting
We analyze and demonstrate the feasibility and superiority of linear optical
single-qubit fingerprinting over its classical counterpart. For one-qubit
fingerprinting of two-bit messages, we prepare `tetrahedral' qubit states
experimentally and show that they meet the requirements for quantum
fingerprinting to exceed the classical capability. We prove that shared
entanglement permits 100% reliable quantum fingerprinting, which will
outperform classical fingerprinting even with arbitrary amounts of shared
randomness.Comment: 4 pages, one figur
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