21,313 research outputs found
Possible Stabilization Mechanism with Bulk and Branes SO(N) Yang-Mills for Closed Universes
Widrawn: The solutions presented in this work are not compatible with the
equation of motion for g_{00}, which we did not properly verify. In the
framework of this work, the only bulk solutions compatible with periodicity of
the fifth dimension are constant fields. Also for a scalar field only constant
solutions are obtained.Comment: withdrawn: see abstrac
Directly accessible entangling gates for capacitively coupled singlet-triplet qubits
The recent experimental advances in capacitively coupled singlet-triplet
qubits, particularly the demonstration of entanglement, opens the question of
what type of entangling gates the system's Hamiltonian can produce directly via
a single square pulse. We address this question by considering the system's
Hamiltonian from first principles and using the representation of its nonlocal
properties in terms of local invariants. In the analysis we include the three
different ways in which the system can be biased and their effect on the
generation of entangling gates. We find that, in one of the possible biasing
modes, the Hamiltonian has an especially simple form, which can directly
generate a wide range of different entangling gates including the iSWAP gate.
Moreover, using the complete form of the Hamiltonian we find that, for any
biasing mode, a CNOT gate can be generated directly.Comment: 10 pages, 5 figure
Dynamically Correcting a CNOT Gate for any Systematic Logical Error
We derive a set of composite pulse sequences that generates CNOT gates and
correct all systematic errors within the logical subspace to arbitrary order.
These sequences are applicable for any two-qubit interaction Hamiltonian, and
make no assumptions about the underlying noise mechanism except that it is
constant on the timescale of the operation. We do assume access to error-free
single-qubit gates, so single-qubit gate imperfections eventually limit the
achievable fidelity. However, since single-qubit gates generally have much
higher fidelities than two-qubit gates in practice, these pulse sequences offer
useful dynamical correction for a wide range of coupled qubit systems.Comment: 4 + pages, and 2 pages of supplemental materia
Entanglement dynamics of two Ising-coupled qubits with nonperpendicular local driving fields
We present an approximate analytical solution to the dynamic equation of two
Ising-coupled qubits with oscillating classical control fields that are
nonperpendicular to the static drift fields. This is a situation that has
recently arisen in some solid-state experiments. With our solution we derive
the analytical expressions for the local invariants as well as the local
rotations needed to isolate a purely nonlocal gate. This determines the set of
parameters that are required to generate any entangling gate. Moreover, we use
our results to describe a recent experimental work on capacitively coupled
singlet-triplet qubits in GaAs and discuss possible differences for a similar
device in silicon.Comment: 6 pages, 3 figure
SAdS black holes and spacetime atoms: a heuristic approach
In this work, both extended phase space and holographic equipartition
approaches are employed to develop an exact Van der Waals description of
non--rotating SAdS black holes as an ensemble of spacetime atoms. After a
possible microscopical interaction model is introduced, statistical mechanics
techniques, with certain heuristic gravitational constraints, are used to
derive the equation of state and the Bekenstein--Hawking entropy. The procedure
is generalized to the charged SAdS black hole and to arbitrary
dimensions for the uncharged cases.Comment: Accepted for publication in Gen. Rel. Gra
Reply to the comment on the paper "Thermodynamics of two-dimensional magneto nanoparticles (P. Vargas, D. Altbir, M.Knobel and D. Laroze)" by H. Buettner and Yu. Gaididei
It is shown that there is bi-stability in a two dimensional system consisting
of non interacting magnetic nanoparticles with equal uniaxial anisotropies. It
is also shown that bi-stability still remains in three dimensions. The only
consideration is that the applied magnetic field has to be perpendicular to the
anisotropy axis.Comment: 3 pages, 1 figur
A detailed analysis of dipolar interactions and analytical approximations in arrays of magnetic nanowires
The investigation of the role of interactions in magnetic wire arrays is
complex and often subject to strong simplifications. In this paper we obtained
analytical expressions for the magnetostatic interactions between wires and
investigate the range of validity of dipole-dipole, first order and second
order approximations. We also analyze the extension of the interwire
magnetostatic interactions in a sample and found that the number of wires
required to reach energy convergence in the array strongly depends on the
relative magnetic orientation between the wires.Comment: 7 pages and 5 figure
Linear guided modes and Whitham-Boussinesq model for variable topogra
In this article we study two classical linear water wave problems, i) normal
modes of infinite straight channels of bounded constant cross-section, and ii)
trapped longitudinal modes in domains with unbounded constant cross-section.
Both problems can be stated using linearized free surface potential flow
theory, and our goal is to compare known analytic solutions in the literature
to numerical solutions obtained using an ad-hoc but simple approximation of the
non-local Dirichlet-Neumann operator for linear waves proposed in
[vargas2016whitham]. To study normal modes in channels with bounded
cross-section we consider special symmetric triangular cross-sections, namely
symmetric triangles with sides inclined at and to the
vertical, and compare modes obtained using the non-local Dirichlet-Neumann
operator to known semi-exact analytic expressions by Lamb
[lamb1932hydrodynamics], Macdonald [macdonald1893waves] , Greenhill
[greenhill1887wave], Packham [packham1980small], and Groves
[groves1994hamiltonian]. These geometries have slopping beach boundaries that
should in principle limit the applicability of our approximate
Dirichlet-Neumann operator. We nevertheless see that the operator gives
remarkably close results for even modes, while for odd modes we have some
discrepancies near the boundary. For trapped longitudinal modes in domains with
an infinite cross-section we consider a piecewise constant depth profile and
compare modes computed with the nonlocal operator modes to known analytic
solutions of linearized shallow water theory by Miles [miles1972wave], Lin,
Juang and Tsay [lin2001anomalous], see also [mei2005theory]. This is a problem
of significant geophysical interest, and the proposed model is shows to give
quantitatively similar results for the lowest trapped modes.Comment: Submitted to Wave Motion Journal, October, 201
Linear Whitham-Boussinesq modes in channels of constant cross-section
We study normal modes for the linear water wave problem in infinite straight
channels of bounded constant cross-section. Our goal is to compare semianalytic
normal mode solutions known in the literature for special triangular
cross-sections, namely isosceles triangles of equal angle of 45 and 60 degrees,
see Lamb [17], Macdonald [19] , Greenhill [11], Packham [23], and Groves [12],
to numerical solutions obtained using approximations of the non-local
Dirichlet-Neumann operator for linear waves, specifically an ad-hoc
approximation proposed in [25], and a first order truncation of the systematic
depth expansion by Craig, Guyenne, Nicholls, and Sulem [6]. We consider cases
of transverse (i.e. 2-D) modes and longitudinal modes, i.e. 3-D modes with
sinusoidal dependence in the longitudinal direction. The triangular geometries
considered have slopping beach boundaries that should in principle limit the
applicability of the approximate Dirichlet-Neumann operators. We nevertheless
see that the approximate operators give remarkably close results for transverse
even modes, while for odd transverse modes we have some discrepancies near the
boundary. In the case of longitudinal modes, where the theory only yields even
modes, the different approximate operators show more discrepancies for the
first two longitudinal modes and better agreement for higher modes. The ad-hoc
approximation is generally closer to exact modes away from the boundary.Comment: Submitted manuscript 2018. arXiv admin note: substantial text overlap
with arXiv:1710.0478
Quasi-SU(3) truncation scheme for odd-even sd-shell nuclei
The quasi-SU(3) symmetry, as found in shell model calculations, refers to the
dominance of the single particle plus quadrupole-quadrupole terms in the
Hamiltonian used to describe well deformed nuclei, and to the subspace relevant
in its diagonalization. It provides a very efficient basis truncation scheme.
It is shown that a small number of SU(3) coupled irreps, those with the largest
C_2 values within the direct product of the proton and neutron SU(3) irreps
with spin 0 and 1 (for even number of particles), and spin 1/2 and 3/2 for (for
odd number of nucleons), are enough to describe the low energy spectra and
B(E2) transition strengths of 21Ne, 23Na and 25Mg. A simple but realistic
Hamiltonian is employed. Results compare favorably both with experimental data
and with full shell model calculations. Limitations and possible improvements
of the schematic Hamiltonian are discussed.Comment: 29 pages and 9 postscript figures. Submited to Nucl. Phys.
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