7,936 research outputs found
Efficient generation of graph states for quantum computation
We present an entanglement generation scheme which allows arbitrary graph
states to be efficiently created in a linear quantum register via an auxiliary
entangling bus. The dynamics of the entangling bus is described by an effective
non-interacting fermionic system undergoing mirror-inversion in which qubits,
encoded as local fermionic modes, become entangled purely by Fermi statistics.
We discuss a possible implementation using two species of neutral atoms stored
in an optical lattice and find that the scheme is realistic in its requirements
even in the presence of noise.Comment: 4 pages, 3 figures, RevTex 4; v2 - Major changes and new result
Energy in an Expanding Universe in the Teleparallel Geometry
The main purpose of this paper is to explicitly verify the consistency of the
energy-momentum and angular momentum tensor of the gravitational field
established in the Hamiltonian structure of the Teleparallel Equivalent of
General Relativity (TEGR). In order to reach these objectives, we obtained the
total energy and angular momentum (matter plus gravitational field) of the
closed universe of the Friedmann-Lemaitre-Robertson-Walker (FLRW). The result
is compared with those obtained from the pseudotensors of Einstein and
Landau-Lifshitz. We also applied the field equations (TEGR) in an expanding
FLRW universe. Considering the stress energy-momentum tensor for a perfect
fluid, we found a teleparallel equivalent of Friedmann equations of General
Relativity (GR).Comment: 19 pages, no figures. Revised in view of Referee's comments. Version
to appear in the Brazilian Journal of Physic
Conditions for free magnetic monopoles in nanoscale square arrays of dipolar spin ice
We study a modified frustrated dipolar array recently proposed by M\"{o}ller
and Moessner [Phys. Rev. Lett. \textbf{96}, 237202 (2006)], which is based on
an array manufactured lithographically by Wang \emph{et al.} [Nature (London)
\textbf{439}, 303 (2006)] and consists of introducing a height offset
between islands (dipoles) pointing along the two different lattice directions.
The ground-states and excitations are studied as a function of . We have
found, in qualitative agreement with the results of M\"{o}ller and Moessner,
that the ground-state changes for , where ( is the
lattice parameter or distance between islands). In addition, the excitations
above the ground-state behave like magnetic poles but confined by a string,
whose tension decreases as increases, in such a way that for
its value is around 20 times smaller than that for . The system exhibits
an anisotropy in the sense that the string tension and magnetic charge depends
significantly on the directions in which the monopoles are separated. In turn,
the intensity of the magnetic charge abruptly changes when the monopoles are
separated along the direction of the longest axis of the islands. Such a gap is
attributed to the transition from the anti to the ferromagnetic ground-state
when .Comment: 6 pages, 7 figures. Published versio
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