37,796 research outputs found
Information Flow of quantum states interacting with closed timelike curves
Recently, the quantum information processing power of closed timelike curves
have been discussed. Because the most widely accepted model for quantum closed
timelike curve interactions contains ambiguities, different authors have been
able to reach radically different conclusions as to the power of such
interactions. By tracing the information flow through such systems we are able
to derive equivalent circuits with unique solutions, thus allowing an objective
decision between the alternatives to be made. We conclude that closed timelike
curves, if they exist and are well described by these simple models, would be a
powerful resource for quantum information processing.Comment: Now includes appendix proving Deutsch's maximum entropy conjectur
High-speed observation of sprite streamers
This article is distributed under the terms of the Creative Commons Attribution License
which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the
source are credited.Sprites are optical emissions in the mesosphere mainly at altitudes 50–90 km.
They are caused by the sudden re-distribution of charge due to lightning in the troposphere which can produce electric fields in the mesosphere in excess of the local breakdown field. The resulting optical displays can be spectacular and this has led to research into the physics and chemistry involved. Imaging at faster than 5,000 frames per second has revealed streamer discharges to be an important and very dynamic part of sprites, and this paper will review high-speed observations of sprite streamers. Streamers are initiated in the 65–85 km altitude range and observed to propagate both down and up at velocities normally in the 106–5 9 107 m/s range. Sprite streamer heads are small, typically less than a few hundreds of meters, but very bright and appear in images much like stars with signals up to that expected of a magnitude -6 star. Many details of streamer formation have been modeled and successfully compared with observations. Streamers frequently split into multiple sub-streamers. The splitting is very fast. To resolve details will require framing
rates higher than the maximum 32,000 fps used so far. Sprite streamers are similar to
streamers observed in the laboratory and, although many features appear to obey simple
scaling laws, recent work indicates that there are limits to the scaling.Research funding has been provided by
the US National Science Foundation grants to the University of Alaska Fairbanks, and the US Air Force Academy, and by DARPA through a grant to the University of Florida
Generation of Werner states via collective decay of coherently driven atoms
We show deterministic generation of Werner states as a steady state of the
collective decay dynamics of a pair of neutral atom coupled to a leaky cavity
and strong coherent drive. We also show how the scheme can be extended to
generate -particle analogue of the bipartite Werner states.Comment: 4 pages, 1 figur
Scattering fidelity in elastodynamics
The recent introduction of the concept of scattering fidelity, causes us to
revisit the experiment by Lobkis and Weaver [Phys. Rev. Lett. 90, 254302
(2003)]. There, the ``distortion'' of the coda of an acoustic signal is
measured under temperature changes. This quantity is in fact the negative
logarithm of scattering fidelity. We re-analyse their experimental data for two
samples, and we find good agreement with random matrix predictions for the
standard fidelity. Usually, one may expect such an agreement for chaotic
systems only. While the first sample, may indeed be assumed chaotic, for the
second sample, a perfect cuboid, such an agreement is more surprising. For the
first sample, the random matrix analysis yields a perturbation strength
compatible with semiclassical predictions. For the cuboid the measured
perturbation strength is much larger than expected, but with the fitted values
for this strength, the experimental data are well reproduced.Comment: 4 page
Detecting separable states via semidefinite programs
We introduce a new technique to detect separable states using semidefinite
programs. This approach provides a sufficient condition for separability of a
state that is based on the existence of a certain local linear map applied to a
known separable state. When a state is shown to be separable, a proof of this
fact is provided in the form of an explicit convex decomposition of the state
in terms of product states. All states in the interior of the set of separable
states can be detected in this way, except maybe for a set of measure zero.
Even though this technique is more suited for a numerical approach, a new
analytical criterion for separability can also be derived.Comment: 8 pages, accepted for publication in Physical Review
Discrimination between evolution operators
Under broad conditions, evolutions due to two different Hamiltonians are
shown to lead at some moment to orthogonal states. For two spin-1/2 systems
subject to precession by different magnetic fields the achievement of
orthogonalization is demonstrated for every scenario but a special one. This
discrimination between evolutions is experimentally much simpler than
procedures proposed earlier based on either sequential or parallel application
of the unknown unitaries. A lower bound for the orthogonalization time is
proposed in terms of the properties of the two Hamiltonians.Comment: 7 pages, 2 figures, REVTe
Optimal path for a quantum teleportation protocol in entangled networks
Bellman's optimality principle has been of enormous importance in the
development of whole branches of applied mathematics, computer science, optimal
control theory, economics, decision making, and classical physics. Examples are
numerous: dynamic programming, Markov chains, stochastic dynamics, calculus of
variations, and the brachistochrone problem. Here we show that Bellman's
optimality principle is violated in a teleportation problem on a quantum
network. This implies that finding the optimal fidelity route for teleporting a
quantum state between two distant nodes on a quantum network with bi-partite
entanglement will be a tough problem and will require further investigation.Comment: 4 pages, 1 figure, RevTeX
Entanglement generation resonances in XY chains
We examine the maximum entanglement reached by an initially fully aligned
state evolving in an XY Heisenberg spin chain placed in a uniform transverse
magnetic field. Both the global entanglement between one qubit and the rest of
the chain and the pairwise entanglement between adjacent qubits is analyzed. It
is shown that in both cases the maximum is not a monotonous decreasing function
of the aligning field, exhibiting instead a resonant behavior for low
anisotropies, with pronounced peaks (a total of [n/2] peaks in the global
entanglement for an -spin chain), whose width is proportional to the
anisotropy and whose height remains finite in the limit of small anisotropy. It
is also seen that the maximum pairwise entanglement is not a smooth function of
the field even in small finite chains, where it may exhibit narrow peaks above
strict plateaus. Explicit analytical results for small chains, as well as
general exact results for finite n-spin chains obtained through the
Jordan-Wigner mapping, are discussed
Quantum information processing with single photons and atomic ensembles in microwave coplanar waveguide resonators
We show that pairs of atoms optically excited to the Rydberg states can
strongly interact with each other via effective long-range dipole-dipole or van
der Waals interactions mediated by their non-resonant coupling to a common
microwave field mode of a superconducting coplanar waveguide cavity. These
cavity mediated interactions can be employed to generate single photons and to
realize in a scalable configuration a universal phase gate between pairs of
single photon pulses propagating or stored in atomic ensembles in the regime of
electromagnetically induced transparency
Dimension minimization of a quantum automaton
A new model of a Quantum Automaton (QA), working with qubits is proposed. The
quantum states of the automaton can be pure or mixed and are represented by
density operators. This is the appropriated approach to deal with measurements
and dechorence. The linearity of a QA and of the partial trace super-operator,
combined with the properties of invariant subspaces under unitary
transformations, are used to minimize the dimension of the automaton and,
consequently, the number of its working qubits. The results here developed are
valid wether the state set of the QA is finite or not. There are two main
results in this paper: 1) We show that the dimension reduction is possible
whenever the unitary transformations, associated to each letter of the input
alphabet, obey a set of conditions. 2) We develop an algorithm to find out the
equivalent minimal QA and prove that its complexity is polynomial in its
dimension and in the size of the input alphabet.Comment: 26 page
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