911 research outputs found
Relations for classical communication capacity and entanglement capability of two-qubit operations
Bipartite operations underpin both classical communication and entanglement
generation. Using a superposition of classical messages, we show that the
capacity of a two-qubit operation for error-free entanglement-assisted
bidirectional classical communication can not exceed twice the entanglement
capability. In addition we show that any bipartite two-qubit operation can
increase the communication that may be performed using an ensemble by twice the
entanglement capability.Comment: 4 page
Perfect state distinguishability and computational speedups with postselected closed timelike curves
Bennett and Schumacher's postselected quantum teleportation is a model of
closed timelike curves (CTCs) that leads to results physically different from
Deutsch's model. We show that even a single qubit passing through a
postselected CTC (P-CTC) is sufficient to do any postselected quantum
measurement, and we discuss an important difference between "Deutschian" CTCs
(D-CTCs) and P-CTCs in which the future existence of a P-CTC might affect the
present outcome of an experiment. Then, based on a suggestion of Bennett and
Smith, we explicitly show how a party assisted by P-CTCs can distinguish a set
of linearly independent quantum states, and we prove that it is not possible
for such a party to distinguish a set of linearly dependent states. The power
of P-CTCs is thus weaker than that of D-CTCs because the Holevo bound still
applies to circuits using them regardless of their ability to conspire in
violating the uncertainty principle. We then discuss how different notions of a
quantum mixture that are indistinguishable in linear quantum mechanics lead to
dramatically differing conclusions in a nonlinear quantum mechanics involving
P-CTCs. Finally, we give explicit circuit constructions that can efficiently
factor integers, efficiently solve any decision problem in the intersection of
NP and coNP, and probabilistically solve any decision problem in NP. These
circuits accomplish these tasks with just one qubit traveling back in time, and
they exploit the ability of postselected closed timelike curves to create
grandfather paradoxes for invalid answers.Comment: 15 pages, 4 figures; Foundations of Physics (2011
Efficient Quantum Computation using Coherent States
Universal quantum computation using optical coherent states is studied. A
teleportation scheme for a coherent-state qubit is developed and applied to
gate operations. This scheme is shown to be robust to detection inefficiency.Comment: 6 pages, 5 figures, extended and modified (in print, PRA
Off-Diagonal Hyperfine Interaction and Parity Non-conservation in Cesium
We have performed relativistic many-body calculations of the hyperfine
interaction in the and states of Cs, including the off-diagonal
matrix element. The calculations were used to determine the accuracy of the
semi-empirical formula for the electromagnetic transition amplitude
induced by the hyperfine interaction. We have found that even
though the contribution of the many-body effects into the matrix elements is
very large, the square root formula remains valid to the accuracy of a fraction of .
The result for the M1-amplitude is used in the interpretation of the
parity-violation measurement in the transition in Cs which claims a
possible deviation from the Standard model.Comment: 13 pages, 4 figures, LaTeX, Submitted to Phys. Rev.
Strong subadditivity inequality for quantum entropies and four-particle entanglement
Strong subadditivity inequality for a three-particle composite system is an
important inequality in quantum information theory which can be studied via a
four-particle entangled state. We use two three-level atoms in
configuration interacting with a two-mode cavity and the Raman adiabatic
passage technique for the production of the four-particle entangled state.
Using this four-particle entanglement, we study for the first time various
aspects of the strong subadditivity inequality.Comment: 5 pages, 3 figures, RevTeX4, submitted to PR
Detecting genuine multipartite continuous-variable entanglement
We derive necessary conditions in terms of the variances of position and
momentum linear combinations for all kinds of separability of a multi-party
multi-mode continuous-variable state. Their violations can be sufficient for
genuine multipartite entanglement, provided the combinations contain both
conjugate variables of all modes. Hence a complete state determination, for
example by detecting the entire correlation matrix of a Gaussian state, is not
needed.Comment: 13 pages, 3 figure
Transverse oscillations of coronal loops
On 14 July 1998 TRACE observed transverse oscillations of a coronal loop generated by an external disturbance most probably caused by a solar flare. These oscillations were interpreted as standing fast kink waves in a magnetic flux tube. Firstly, in this review we embark on the discussion of the theory of waves and oscillations in a homogeneous straight magnetic cylinder with the particular emphasis on fast kink waves. Next, we consider the effects of stratification, loop expansion, loop curvature, non-circular cross-section, loop shape and magnetic twist.
An important property of observed transverse coronal loop oscillations is their fast damping. We briefly review the different mechanisms suggested for explaining the rapid damping phenomenon. After that we concentrate on damping due to resonant absorption. We describe the latest analytical results obtained with the use of thin transition layer approximation, and then compare these results with numerical findings obtained for arbitrary density variation inside the flux tube.
Very often collective oscillations of an array of coronal magnetic loops are observed. It is natural to start studying this phenomenon from the system of two coronal loops. We describe very recent analytical and numerical results of studying collective oscillations of two parallel homogeneous coronal loops.
The implication of the theoretical results for coronal seismology is briefly discussed. We describe the estimates of magnetic field magnitude obtained from the observed fundamental frequency of oscillations, and the estimates of the coronal scale height obtained using the simultaneous observations of the fundamental frequency and the frequency of the first overtone of kink oscillations.
In the last part of the review we summarise the most outstanding and acute problems in the theory of the coronal loop transverse oscillations
Measurement of the 6s - 7p transition probabilities in atomic cesium and a revised value for the weak charge Q_W
We have measured the 6s - 7p_{1/2,3/2} transition probabilities in atomic
cesium using a direct absorption technique. We use our result plus other
previously measured transition rates to derive an accurate value of the vector
transition polarizability \beta and, consequently, re-evaluate the weak charge
Q_W. Our derived value Q_W=-72.65(49) agrees with the prediction of the
standard model to within one standard deviation.Comment: 4 pages, 2 figure
Entanglement preparation using symmetric multiports
We investigate the entanglement produced by a multi-path interferometer that
is composed of two symmetric multiports, with phase shifts applied to the
output of the first multiport. Particular attention is paid to the case when we
have a single photon entering the interferometer. For this situation we derive
a simple condition that characterize the types of entanglement that one can
generate. We then show how one can use the results from the single photon case
to determine what kinds of multi-photon entangled states one can prepare using
the interferometer.Comment: 6 pages, 2 figures, accepted for publication in European Journal of
Physics
Revisiting consistency conditions for quantum states of systems on closed timelike curves: an epistemic perspective
There has been considerable recent interest in the consequences of closed
timelike curves (CTCs) for the dynamics of quantum mechanical systems. A vast
majority of research into this area makes use of the dynamical equations
developed by Deutsch, which were developed from a consistency condition that
assumes that mixed quantum states uniquely describe the physical state of a
system. We criticise this choice of consistency condition from an epistemic
perspective, i.e., a perspective in which the quantum state represents a state
of knowledge about a system. We demonstrate that directly applying Deutsch's
condition when mixed states are treated as representing an observer's knowledge
of a system can conceal time travel paradoxes from the observer, rather than
resolving them. To shed further light on the appropriate dynamics for quantum
systems traversing CTCs, we make use of a toy epistemic theory with a strictly
classical ontology due to Spekkens and show that, in contrast to the results of
Deutsch, many of the traditional paradoxical effects of time travel are
present.Comment: 10 pages, 6 figures, comments welcome; v2 added references and
clarified some points; v3 published versio
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