6,735 research outputs found
Doubly infinite separation of quantum information and communication
We prove the existence of (one-way) communication tasks with a subconstant
versus superconstant asymptotic gap, which we call "doubly infinite," between
their quantum information and communication complexities. We do so by studying
the exclusion game [C. Perry et al., Phys. Rev. Lett. 115, 030504 (2015)] for
which there exist instances where the quantum information complexity tends to
zero as the size of the input increases. By showing that the quantum
communication complexity of these games scales at least logarithmically in ,
we obtain our result. We further show that the established lower bounds and
gaps still hold even if we allow a small probability of error. However in this
case, the -qubit quantum message of the zero-error strategy can be
compressed polynomially.Comment: 16 pages, 2 figures. v4: minor errors fixed; close to published
version; v5: financial support info adde
Conic approach to quantum graph parameters using linear optimization over the completely positive semidefinite cone
We investigate the completely positive semidefinite cone ,
a new matrix cone consisting of all matrices that admit a Gram
representation by positive semidefinite matrices (of any size). In particular
we study relationships between this cone and the completely positive and doubly
nonnegative cones, and between its dual cone and trace positive non-commutative
polynomials.
We use this new cone to model quantum analogues of the classical independence
and chromatic graph parameters and , which are roughly
obtained by allowing variables to be positive semidefinite matrices instead of
scalars in the programs defining the classical parameters. We can
formulate these quantum parameters as conic linear programs over the cone
. Using this conic approach we can recover the bounds in
terms of the theta number and define further approximations by exploiting the
link to trace positive polynomials.Comment: Fixed some typo
Thermally induced instability of a doubly quantized vortex in a Bose-Einstein condensate
We study the instability of a doubly quantized vortex topologically imprinted
on Na condensate, as reported in recent experiment [Phys. Rev. Lett.
\textbf{93}, 160406 (2004)]. We have performed numerical simulations using
three-dimensional Gross-Pitaevskii equation with classical thermal noise.
Splitting of a doubly quantized vortex turns out to be a process that is very
sensitive to the presence of thermal atoms. We observe that even ve ry small
thermal fluctuations, corresponding to 10 to 15% of thermal atoms, ca use the
decay of doubly quantized vortex into two singly quantized vortices in tens of
milliseconds. As in the experiment, the lifetime of doubly quantized vortex i s
a monotonic function of the interaction strength.Comment: 4 pages, 5 figure
Doubly excited ferromagnetic spin-chain as a pair of coupled kicked rotors
We show that the dynamics of a doubly-excited 1D Heisenberg ferromagnetic
chain, subject to short pulses from a parabolic magnetic field may be analyzed
as a pair of quantum kicked rotors. By focusing on the two-magnon dynamics in
the kicked XXZ model we investigate how the anisotropy parameter - which
controls the strength of the magnon-magnon interaction - changes the nature of
the coupling between the two "image" coupled Kicked Rotors. We investigate
quantum state transfer possibilities and show that one may control whether the
spin excitations are transmitted together, or separate from each other.Comment: 8 pages, 4 figures; extended appendix and corrected typo
Ab Initio Calculations on the H_(2)+D_(2)=2HD FourâCenter Exchange Reaction. I. Elements of the Reaction Surface
We present the results of ab initio calculations on some interesting regions of the reaction surface for the fourâcenter exchange reaction H_(2)+D_(2)=2HD. These calculations, which use a minimum basis set of Slater orbitals, indicate that for all geometries appropriate to the transition state of the reaction, a barrier height of at least 148 kcal/mole is present. This is far greater than the energy required to produce free radicals and more than three times the experimental energy of activation, 42 kcal/mole. Considering the sources and magnitudes for errors due to correlation and basis set restrictions, we estimate the barrier height for this exchange reaction to be 132â±â20 kcal/mole exclusive of zeroâpoint energies. In this paper we discuss the surface as determined by configuration interaction techniques. We find that the most favorable geometries for the exchange reactions are the square, rhombus, and kite configurations. However, all of these states are unstable with respect to H_(2)â+â2H. In addition we find no evidence of collision complexes for any of the likely transition state geometries. In the following paper we will examine the G1 wavefunctions for this system in order to obtain an understanding of the factors responsible for the shape of the surface
Generic spectral properties of right triangle billiards
This article presents a new method to calculate eigenvalues of right triangle
billiards. Its efficiency is comparable to the boundary integral method and
more recently developed variants. Its simplicity and explicitness however allow
new insight into the statistical properties of the spectra. We analyse
numerically the correlations in level sequences at high level numbers (>10^5)
for several examples of right triangle billiards. We find that the strength of
the correlations is closely related to the genus of the invariant surface of
the classical billiard flow. Surprisingly, the genus plays and important role
on the quantum level also. Based on this observation a mechanism is discussed,
which may explain the particular quantum-classical correspondence in right
triangle billiards. Though this class of systems is rather small, it contains
examples for integrable, pseudo integrable, and non integrable (ergodic,
mixing) dynamics, so that the results might be relevant in a more general
context.Comment: 18 pages, 8 eps-figures, revised: stylistic changes, improved
presentatio
The Skyrme Interaction in finite nuclei and nuclear matter
Self-consistent mean-field models are a powerful tool in the investigation of
nuclear structure and low-energy dynamics. They are based on effective
energy-density functionals, often formulated in terms of effective
density-dependent nucleon-nucleon interactions. The free parameters of the
functional are adjusted to empirical data. A proper choice of these parameters
requires a comprehensive set of constraints covering experimental data on
finite nuclei, concerning static as well as dynamical properties, empirical
characteristics of nuclear matter, and observational information on
nucleosynthesis, neutron stars and supernovae. This work aims at a
comprehensive survey of the performance of one of the most successful
non-relativistic self-consistent method, the Skyrme-Hartree-Fock model (SHF),
with respect to these constraints. A full description of the Skyrme functional
is given and its relation to other effective interactions is discussed. The
validity of the application of SHF far from stability and in dense environments
beyond the nuclear saturation density is critically assessed. The use of SHF in
models extended beyond the mean field approximation by including some
correlations is discussed. Finally, future prospects for further development of
SHF towards a more consistent application of the existing and promisingly newly
developing constraints are outlined.Comment: 71 pages, 22 figures. Accepted for publication in Prog.Part.Nucl.Phy
Magnetic-Moment Fragmentation and Monopole Crystallization
The Coulomb phase, with its dipolar correlations and pinch-point-scattering
patterns, is central to discussions of geometrically frustrated systems, from
water ice to binary and mixed-valence alloys, as well as numerous examples of
frustrated magnets. The emergent Coulomb phase of lattice-based systems has
been associated with divergence-free fields and the absence of long-range
order. Here, we go beyond this paradigm, demonstrating that a Coulomb phase can
emerge naturally as a persistent fluctuating background in an otherwise ordered
system. To explain this behavior, we introduce the concept of the fragmentation
of the field of magnetic moments into two parts, one giving rise to a magnetic
monopole crystal, the other a magnetic fluid with all the characteristics of an
emergent Coulomb phase. Our theory is backed up by numerical simulations, and
we discuss its importance with regard to the interpretation of a number of
experimental results
Classical capacity of a qubit depolarizing channel with memory
The classical product state capacity of a noisy quantum channel with memory
is investigated. A forgetful noise-memory channel is constructed by Markov
switching between two depolarizing channels which introduces non-Markovian
noise correlations between successive channel uses. The computation of the
capacity is reduced to an entropy computation for a function of a Markov
process. A reformulation in terms of algebraic measures then enables its
calculation. The effects of the hidden-Markovian memory on the capacity are
explored. An increase in noise-correlations is found to increase the capacity
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