5,257 research outputs found
Connection between two forms of extra-dimensional metrics revisited
5D cosmological model with 3-brane with matter is considered. The brane
divides bulk in two AdS half spaces. Geometry of the model can be described by
two types of coordinates: in the first setting the metric is static and the
brane is moving in the bulk, in the second approach the metric is
time-dependent and the brane is located at a fixed position in the bulk.
Coordinate transformation connecting two coordinate systems is constructed.Comment: 7 page
Bouncing of gravitons emitted from the brane to the bulk back to the brane
Solutions of geodesic equations describing propagation of gravitons in the
bulk are studied in a cosmological model with one extra dimension. Brane with
matter is embedded in the bulk. It is shown that in the period of early
cosmology gravitons emitted from the brane to the bulk under certain conditions
can return back to the brane. The model is discussed in two alternative
approaches: (i) brane with static metric moving in the AdS space, and (ii)
brane located at a fixed position in extra dimension with non-static metric.
Transformation of coordinates from the one picture to another is performed. In
both approaches conditions for gravitons emitted to the bulk to come back to
the brane are found.Comment: LaTeX2e, 12 pages. Refs. added. arXiv admin note: text overlap with
arXiv:1204.235
Entanglement of hard-core bose gas in degenerate levels under local noise
Quantum entanglement properties of the pseudo-spin representation of the BCS model is investigated. In case of degenerate energy levels, where wave functions take a particularly simple form, spontaneous breaking of exchange symmetry under local noise is studied. Even if the Hamiltonian has the same symmetry, it is shown that there is a non-zero probability to end up with a non-symmetric final state. For small systems, total probability for symmetry breaking is found to be inversely proportional to the system size
Optimum Quantum Error Recovery using Semidefinite Programming
Quantum error correction (QEC) is an essential element of physical quantum
information processing systems. Most QEC efforts focus on extending classical
error correction schemes to the quantum regime. The input to a noisy system is
embedded in a coded subspace, and error recovery is performed via an operation
designed to perfectly correct for a set of errors, presumably a large subset of
the physical noise process. In this paper, we examine the choice of recovery
operation. Rather than seeking perfect correction on a subset of errors, we
seek a recovery operation to maximize the entanglement fidelity for a given
input state and noise model. In this way, the recovery operation is optimum for
the given encoding and noise process. This optimization is shown to be
calculable via a semidefinite program (SDP), a well-established form of convex
optimization with efficient algorithms for its solution. The error recovery
operation may also be interpreted as a combining operation following a quantum
spreading channel, thus providing a quantum analogy to the classical diversity
combining operation.Comment: 7 pages, 3 figure
Evolution equation of entanglement for general bipartite systems
We explore how entanglement of a general bipartite system evolves when one
subsystem undergoes the action of an arbitrary noisy channel. It is found that
the dynamics of entanglement for general bipartite systems under the influence
of such channel is determined by the channel's action on the maximally
entangled state, which includes as a special case the results for two-qubit
systems [Nature Physics 4, 99 (2008)]. In particular, for multi-qubit or
qubit-qudit systems, we get a general factorization law for evolution equation
of entanglement with one qubit being subject to a noisy channel. Our results
can help the experimental characterization of entanglement dynamics.Comment: 4 pages, 1 figur
Decoherence induced spontaneous symmetry breaking
We study time dependence of exchange symmetry properties of Bell states when
two qubits interact with local baths having identical parameters. In case of
classical noise, we consider a decoherence Hamiltonian which is invariant under
swapping the first and second qubits. We find that as the system evolves in
time, two of the three symmetric Bell states preserve their qubit exchange
symmetry with unit probability, whereas the symmetry of the remaining state
survives with a maximum probability of 0.5 at the asymptotic limit. Next, we
examine the exchange symmetry properties of the same states under local,
quantum mechanical noise which is modeled by two identical spin baths. Results
turn out to be very similar to the classical case. We identify decoherence as
the main mechanism leading to breaking of qubit exchange symmetry.Comment: 12 page
An Observational Perspective of Transitional Disks
archiveprefix: arXiv primaryclass: astro-ph.SR keywords: Astrophysics - Solar and Stellar Astrophysics adsurl: http://adsabs.harvard.edu/abs/2014arXiv1402.7103E adsnote: Provided by the SAO/NASA Astrophysics Data SystemarticleTransitional disks are objects whose inner disk regions have undergone substantial clearing. The Spitzer Space Telescope produced detailed spectral energy distributions (SEDs) of transitional disks that allowed us to infer their radial dust disk structure in some detail, revealing the diversity of this class of disks. The growing sample of transitional disks also opened up the possibility of demographic studies, which provided unique insights. There now exist (sub)millimeter and infrared images that confirm the presence of large clearings of dust in transitional disks. In addition, protoplanet candidates have been detected within some of these clearings. Transitional disks are thought to be a strong link to planet formation around young stars and are a key area to study if further progress is to be made on understanding the initial stages of planet formation. Here we provide a review and synthesis of transitional disk observations to date with the aim of providing timely direction to the field, which is about to undergo its next burst of growth as ALMA reaches its full potential. We discuss what we have learned about transitional disks from SEDs, color-color diagrams, and imaging in the (sub)mm and infrared. We then distill the observations into constraints for the main disk clearing mechanisms proposed to date (i.e., photoevaporation, grain growth, and companions) and explore how the expected observational signatures from these mechanisms, particularly planet-induced disk clearing, compare to actual observations. Lastly, we discuss future avenues of inquiry to be pursued with ALMA, JWST, and next generation of ground-based telescopes.NAS
A Magnetic Resonance Realization of Decoherence-Free Quantum Computation
We report the realization, using nuclear magnetic resonance techniques, of
the first quantum computer that reliably executes an algorithm in the presence
of strong decoherence. The computer is based on a quantum error avoidance code
that protects against a class of multiple-qubit errors. The code stores two
decoherence-free logical qubits in four noisy physical qubits. The computer
successfully executes Grover's search algorithm in the presence of arbitrarily
strong engineered decoherence. A control computer with no decoherence
protection consistently fails under the same conditions.Comment: 5 pages with 3 figures, revtex4, accepted by Physical Review Letters;
v2 minor revisions to conten
Cosmological constraints on parameters of one-brane models with extra dimension
We study some aspects of cosmologies in 5D models with one infinite extra
dimension. Matter is confined to the brane, gravity extends to the bulk. Models
with positive and negative tension of the brane are considered. Cosmological
evolution of the 4D world is described by warped solutions of the generalized
Friedmann equation. Cosmological solutions on the brane are obtained with the
input of the present-time observational cosmological parameters. We estimate
the age of the Universe and abundance of produced in primordial
nucleosynthesis in different models. Using these estimates we find constraints
on dimensionless combinations of the 5D gravitational scale, scale of the warp
factor and coupling at the 4D curvature term in the action.Comment: 21 pages, 4 figure
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