66 research outputs found
Removal of a single photon by adaptive absorption
We present a method to remove, using only linear optics, exactly one photon
from a field-mode. This is achieved by putting the system in contact with an
absorbing environment which is under continuous monitoring. A feedback
mechanism then decouples the system from the environment as soon as the first
photon is absorbed. We propose a possible scheme to implement this process and
provide the theoretical tools to describe it
Dynamics of Gravity in a Higgs Phase
We investigate the universal low-energy dynamics of the simplest Higgs phase
for gravity, `ghost condensation.' We show that the nonlinear dynamics of the
`ghostone' field dominate for all interesting gravitational sources. Away from
caustic singularities, the dynamics is equivalent to the irrotational flow of a
perfect fluid with equation of state p \propto \rho^2, where the fluid
particles can have negative mass. We argue that this theory is free from
catastrophic instabilities due to growing modes, even though the null energy
condition is violated. Numerical simulations show that solutions generally have
singularities in which negative energy regions shrink to zero size. We exhibit
partial UV completions of the theory in which these singularities are smoothly
resolved, so this does not signal any inconsistency in the effective theory. We
also consider the bounds on the symmetry breaking scale M in this theory. We
argue that the nonlinear dynamics cuts off the Jeans instability of the linear
theory, and allows M \lsim 100MeV.Comment: 54 pages, 15 figures; postscript figures downloadable from
http://schwinger.harvard.edu/~wiseman/Ghost/ghostepsfigs.tar.gz ; v2:
substantial revision to section 5 on bound
Causality in quantum teleportation: information extraction and noise effects in entanglement distribution
Quantum teleportation is possible because entanglement allows a definition of
precise correlations between the non-commuting properties of a local system and
corresponding non-commuting properties of a remote system. In this paper, the
exact causality achieved by maximal entanglement is analyzed and the results
are applied to the transfer of effects acting on the entanglement distribution
channels to the teleported output state. In particular, it is shown how
measurements performed on the entangled system distributed to the sender
provide information on the teleported state while transferring the
corresponding back-action to the teleported quantum state.Comment: 14 pages, including three figures, discussion of fidelity adde
Decoherence control in microwave cavities
We present a scheme able to protect the quantum states of a cavity mode
against the decohering effects of photon loss. The scheme preserves quantum
states with a definite parity, and improves previous proposals for decoherence
control in cavities. It is implemented by sending single atoms, one by one,
through the cavity. The atomic state gets first correlated to the photon number
parity. The wrong parity results in an atom in the upper state. The atom in
this state is then used to inject a photon in the mode via adiabatic transfer,
correcting the field parity. By solving numerically the exact master equation
of the system, we show that the protection of simple quantum states could be
experimentally demonstrated using presently available experimental apparatus.Comment: 13 pages, RevTeX, 8 figure
Spherically Symmetric Braneworld Solutions with R_{4} term in the Bulk
An analysis of a spherically symmetric braneworld configuration is performed
when the intrinsic curvature scalar is included in the bulk action; the
vanishing of the electric part of the Weyl tensor is used as the boundary
condition for the embedding of the brane in the bulk. All the solutions outside
a static localized matter distribution are found; some of them are of the
Schwarzschild-(A)dS_{4} form. Two modified Oppenheimer-Volkoff interior
solutions are also found; one is matched to a Schwarzschild-(A)dS_{4} exterior,
while the other does not. A non-universal gravitational constant arises,
depending on the density of the considered object; however, the conventional
limits of the Newton's constant are recovered. An upper bound of the order of
TeV for the energy string scale is extracted from the known solar system
measurements (experiments). On the contrary, in usual brane dynamics, this
string scale is calculated to be larger than TeV.Comment: 23 pages, 1 figure, one minor chang
Homodyne Bell's inequalities for entangled mesoscopic superpositions
We present a scheme for demonstrating violation of Bell's inequalities using
a spin-1/2 system entangled with a pair of classically distinguishable wave
packets in a harmonic potential. In the optical domain, such wave packets can
be represented by coherent states of a single light mode. The proposed scheme
involves standard spin-1/2 projections and measurements of the position and the
momentum of the harmonic oscillator system, which for a light mode can be
realized by means of homodyne detection. We discuss effects of imperfections,
including non-unit efficiency of the homodyne detector, and point out a close
link between the visibility of interference and violation of Bell's
inequalities in the described scheme.Comment: 6 pages, 3 figures. Extended version, journal reference adde
On the critical behavior of disordered quantum magnets: The relevance of rare regions
The effects of quenched disorder on the critical properties of itinerant
quantum antiferromagnets and ferromagnets are considered. Particular attention
is paid to locally ordered spatial regions that are formed in the presence of
quenched disorder even when the bulk system is still in the paramagnetic phase.
These rare regions or local moments are reflected in the existence of spatially
inhomogeneous saddle points of the Landau-Ginzburg-Wilson functional. We derive
an effective theory that takes into account small fluctuations around all of
these saddle points. The resulting free energy functional contains a new term
in addition to those obtained within the conventional perturbative approach,
and it comprises what would be considered non-perturbative effects within the
latter. A renormalization group analysis shows that in the case of
antiferromagnets, the previously found critical fixed point is unstable with
respect to this new term, and that no stable critical fixed point exists at
one-loop order. This is contrasted with the case of itinerant ferromagnets,
where we find that the previously found critical behavior is unaffected by the
rare regions due to an effective long-ranged interaction between the order
parameter fluctuations.Comment: 16 pp., REVTeX, epsf, 2 figs, final version as publishe
Gravitational radiation from a particle in circular orbit around a black hole. V. Black-hole absorption and tail corrections
A particle of mass moves on a circular orbit of a nonrotating black
hole of mass . Under the restrictions and , where
is the orbital velocity, we consider the gravitational waves emitted by such a
binary system. We calculate , the rate at which the gravitational
waves remove energy from the system. The total energy loss is given by , where denotes that part of the
gravitational-wave energy which is carried off to infinity, while
denotes the part which is absorbed by the black hole. We show that the
black-hole absorption is a small effect: . We
also compare the wave generation formalism which derives from perturbation
theory to the post-Newtonian formalism of Blanchet and Damour. Among other
things we consider the corrections to the asymptotic gravitational-wave field
which are due to wave-propagation (tail) effects.Comment: ReVTeX, 17 page
Relativistic quantum clocks
The conflict between quantum theory and the theory of relativity is
exemplified in their treatment of time. We examine the ways in which their
conceptions differ, and describe a semiclassical clock model combining elements
of both theories. The results obtained with this clock model in flat spacetime
are reviewed, and the problem of generalizing the model to curved spacetime is
discussed, before briefly describing an experimental setup which could be used
to test of the model. Taking an operationalist view, where time is that which
is measured by a clock, we discuss the conclusions that can be drawn from these
results, and what clues they contain for a full quantum relativistic theory of
time.Comment: 12 pages, 4 figures. Invited contribution for the proceedings for
"Workshop on Time in Physics" Zurich 201
Gravitational Wave Emission From a Binary Black Hole System in Presence of an Accretion Disk
We study time evolution and gravitational wave emission properties of a black
hole orbiting {\it inside} an accretion disk surrounding a massive black hole.
We simultaneously solve the structure equations of the accretion disk in
presence of heating, cooling and viscosity as well as the equations governing
the companion orbit. The deviation from Keplerian distribution of angular
momentum of the disk due to pressure and advection effects causes a significant
exchange of angular momentum between the disk and the companion. This
significantly affects the gravitational wave emission properties from the
binary system. We show that when the companion is light, the effect is
extremely important and must be taken into account while interpreting
gravitational wave signals from such systems.Comment: 8 pages of Latex plus postscript fil
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