3,710 research outputs found
Entangling spins by measuring charge: a parity-gate toolbox
The parity gate emerged recently as a promising resource for performing
universal quantum computation with fermions using only linear interactions.
Here we analyse the parity gate (P-gate) from a theoretical point of view in
the context of quantum networks. We present several schemes for entanglement
generation with P-gates and show that native networks simplify considerably the
resources required for producing multi-qubit entanglement, like n-GHZ states.
Other applications include a Bell-state analyser and teleportation. We also
show that cluster state fusion can be performed deterministically with parity
measurements. We then extend this analysis to hybrid quantum networks
containing spin and mode qubits. Starting from an easy-to-prepare resource
(spin-mode entanglement of single electrons) we show how to produce a spin
n-GHZ state with linear elements (beam-splitters and local spin-flips) and
charge-parity detectors; this state can be used as a resource in a spin quantum
computer or as a precursor for constructing cluster states. Finally, we
construct a novel spin CZ-gate by using the mode degrees of freedom as
ancillae.Comment: updated to the published versio
Topological restrictions for circle actions and harmonic morphisms
Let be a compact oriented smooth manifold which admits a smooth circle
action with isolated fixed points which are isolated as singularities as well.
Then all the Pontryagin numbers of are zero and its Euler number is
nonnegative and even. In particular, has signature zero. Since a
non-constant harmonic morphism with one-dimensional fibres gives rise to a
circle action we have the following applications: (i) many compact manifolds,
for example , surfaces, () where
is the closed surface of genus can never be the domain of a
non-constant harmonic morphism with one-dimensional fibres whatever metrics we
put on them; (ii) let be a compact orientable four-manifold and
a non-constant harmonic morphism. Suppose that one of
the following assertions holds: (1) is half-conformally flat and its
scalar curvature is zero, (2) is Einstein and half-conformally flat,
(3) is Hermitian-Einstein. Then, up to homotheties and Riemannian
coverings, is the canonical projection between flat tori.Comment: 18 pages; Minor corrections to Proposition 3.1 and small changes in
Theorem 2.8, proof of Theorem 3.3 and Remark 3.
Skyrmions around Kerr black holes and spinning BHs with Skyrme hair
We study solutions of the Einstein-Skyrme model. Firstly we consider test
field Skyrmions on the Kerr background. These configurations -- hereafter
dubbed Skerrmions -- can be in equilibrium with a Kerr black hole (BH) by
virtue of a synchronisation condition. We consider two sectors for Skerrmions.
In the sector with non-zero baryon charge, Skerrmions are akin to the known
Skyrme solutions on the Schwarzschild background. These `topological'
configurations reduce to flat spacetime Skyrmions in a vanishing BH mass limit;
moreoever, they never become "small" perturbations on the Kerr background: the
non-linearities of the Skyrme model are crucial for all such Skerrmions. In the
non-topological sector, on the other hand, Skerrmions have no analogue on the
Schwarzschild background. Non-topological Skerrmions carry not baryon charge
and bifurcate from a subset of Kerr solutions defining an existence line.
Therein the appropriate truncation of the Skyrme model yield a linear scalar
field theory containing a complex plus a real field, both massive and
decoupled, and the Skerrmions reduce to the known stationary scalar clouds
around Kerr BHs. Moreover, non-topological Skerrmions trivialise in the
vanishing BH mass limit. We then discuss the backreaction of these Skerrmions,
that yield rotating BHs with synchronised Skyrme hair, which continously
connect to the Kerr solution (self-gravitating Skyrmions) in the
non-topological (topological) sector. In particular, the non-topological hairy
BHs provide a non-linear realisation, within the Skyrme model, of the
synchronous stationary scalar clouds around Kerr.Comment: 23 pages, 7 figures; to appear in JHE
Divergence of the Magnetic Gr\"{u}neisen Ratio at the Field-Induced Quantum Critical Point in YbRhSi
The heavy fermion compound YbRhSi is studied by low-temperature
magnetization and specific-heat measurements at magnetic fields
close to the quantum critical point ( T, ). Upon
approaching the instability, is more singular than , leading to a
divergence of the magnetic Gr\"uneisen ratio .
Within the Fermi liquid regime, with
and T which is consistent with
scaling behavior of the specific-heat coefficient in
YbRh(SiGe). The field-dependence of indicates
an inflection point of the entropy as a function of magnetic field upon passing
the line previously observed in Hall- and thermodynamic
measurements.Comment: 4 pages, 3 Figure
Is Territorial Cohesion Necessary for the Sustainable Development of the European Regions?
The Regional policy of the EU is pursuing the harmonious development of the European territory; this is a necessary condition for the creation of an environment that is favourable to the convergence of the Union’s policies. But, for these policies to converge, it is necessary that there are common objectives and these can arise only from the existence of common needs. It is obvious that a very different level of development leads to different needs for people and territories and therefore, to the pursuit of different objectives. The introduction of the territorial cohesion as an objective of the European Union through the Treaty of Lisbon has concluded many years of debate over the essence and future of the EU. But is this a prerequisite for the sustainable development of the EU regions? To answer this question, in this paper we will try to show the importance of the territorial cohesion in the EU. In order to do this, we will place the evolution of this concept in parallel with that of development and also with the process of enlargement and of deepening of the EU. We will thus try to determine the influence that territorial cohesion has on regions and on the European construction so that in the end we should be able to explain the effects that it has on their sustainable development
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