806 research outputs found
Optimality and Complexity in Measured Quantum-State Stochastic Processes
If an experimentalist observes a sequence of emitted quantum states via
either projective or positive-operator-valued measurements, the outcomes form a
time series. Individual time series are realizations of a stochastic process
over the measurements' classical outcomes. We recently showed that, in general,
the resulting stochastic process is highly complex in two specific senses: (i)
it is inherently unpredictable to varying degrees that depend on measurement
choice and (ii) optimal prediction requires using an infinite number of
temporal features. Here, we identify the mechanism underlying this
complicatedness as generator nonunifilarity -- the degeneracy between sequences
of generator states and sequences of measurement outcomes. This makes it
possible to quantitatively explore the influence that measurement choice has on
a quantum process' degrees of randomness and structural complexity using
recently introduced methods from ergodic theory. Progress in this, though,
requires quantitative measures of structure and memory in observed time series.
And, success requires accurate and efficient estimation algorithms that
overcome the requirement to explicitly represent an infinite set of predictive
features. We provide these metrics and associated algorithms, using them to
design informationally-optimal measurements of open quantum dynamical systems.Comment: 31 pages, 6 appendices, 22 figures;
http://csc.ucdavis.edu/~cmg/compmech/pubs/qdic.ht
Collapse of the ESR fine structure throughout the coherent temperature of the Gd-doped Kondo Semiconductor
Experiments on the Electron Spin Resonance (ESR) in the filled
skutterudite (), at temperatures
where the host resistivity manifests a smooth insulator-metal crossover,
provides evidence of the underlying Kondo physics associated with this system.
At low temperatures (below ), behaves
as a Kondo-insulator with a relatively large hybridization gap, and the
ESR spectra displays a fine structure with lorentzian line shape,
typical of insulating media. The electronic gap is attributed to the large
hybridization present in the coherent regime of a Kondo lattice, when Ce
4f-electrons cooperate with band properties at half-filling. Mean-field
calculations suggest that the electron-phonon interaction is fundamental at
explaining the strong 4f-electron hybridization in this filled skutterudite.
The resulting electronic structure is strongly temperature dependent, and at
about the system undergoes an insulator-to-metal
transition induced by the withdrawal of 4f-electrons from the Fermi volume, the
system becoming metallic and non-magnetic. The ESR fine structure
coalesces into a single dysonian resonance, as in metals. Still, our
simulations suggest that exchange-narrowing via the usual Korringa mechanism,
alone, is not capable of describing the thermal behavior of the ESR spectra in
the entire temperature region ( - K). We propose that temperature
activated fluctuating-valence of the Ce ions is the missing ingredient that,
added to the usual exchange-narrowing mechanism, fully describes this unique
temperature dependence of the ESR fine structure observed in
.Comment: 19 pages, 6 figure
Nonlinear dynamics of coupled transverse-rotational waves in granular chains
The nonlinear dynamics of coupled waves in one-dimensional granular chains with and without a substrate
is theoretically studied accounting for quadratic nonlinearity. The multiple time scale method is used to derive
the nonlinear dispersion relations for infinite granular chains and to obtain the wave solutions for semiinfinite
systems. It is shown that the sum-frequency and difference-frequency components of the coupled
transverse-rotational waves are generated due to their nonlinear interactions with the longitudinal wave.
Nonlinear resonances are not present in the chain with no substrate where these frequency components have
low amplitudes and exhibit beating oscillations. In the chain positioned on a substrate two types of nonlinear
resonances are predicted. At resonance, the fundamental frequency wave amplitudes decrease and the
generated frequency component amplitudes increase along the chain, accompanied by the oscillations due to
the wave numbers asynchronism. The results confirm the possibility of a highly efficient energy transfer
between the waves of different frequencies, which could find applications in the design of acoustic devices
for energy transfer and energy rectification
Controlled Skyrmion Ratchet in Linear Protrusion Defects
Using atomistic simulations, we investigate the dynamical behavior of a
single skyrmion interacting with an asymmetric linear protrusion array under
external ac driving. When the ac drive is applied along the direction, the
skyrmion moves along the hard direction of the substrate asymmetry in three
phases: a pinned phase with localized skyrmion orbits, a constant velocity
phase where the orbits become delocalized, and a reentrant pinned phase with
larger localized orbits. We measure the dependence of the skyrmion velocity on
the frequency and amplitude of the ac drive. All three phases appear for all
frequency values, and in the constant velocity phase the skyrmion velocity
depends only on the frequency and not on the amplitude of the ac drive. When ac
driving is applied in the direction, the skyrmion moves along the easy
direction of the substrate asymmetry and exhibits the same three phases as for
direction driving along with a fourth phase which, at high driving
frequencies, consists of a series of constant velocity phases, each with
different average skyrmion velocities. For low frequencies, the constant
velocity phase is lost and the skyrmion speed increases linearly with
increasing ac drive amplitude due to a Magnus boost effect. Our findings
suggest new ways to create reliable data transport for spintronic devices using
skyrmions as information carriers, where the skyrmion direction and speed can
be controlled by varying only the ac drive amplitude and frequency.Comment: 12 pages, 11 figure
Skyrmion Transport and Annihilation in Funnel Geometries
Using atomistic simulations, we have investigated the transport and
annihilation of skyrmions interacting with a funnel array under a current
applied perpendicular to the funnel axis. We find that transport without
annihilation is possible at low currents, when the motion is dominated by
skyrmion-skyrmion interactions and skyrmions push each other through the funnel
opening. Skyrmion annihilation occurs for higher currents when skyrmions in the
upper half of the sample exert pressure on skyrmions in the bottom half of the
sample due to the external current. Upon interacting with the funnel wall, the
skyrmions undergo a size reduction that makes it easier for them to pass
through the funnel opening. We find five phases as a function of the applied
current and the size of the funnel opening: (i) pinned, (ii) transport without
annihilation, (iii) transport with annihilation, (iv) complete annihilation,
and (v) a reentrant pinning phase that only occurs for very narrow openings.
Our findings provide insight into how to control skyrmion transport using
funnel arrays by delineating regimes in which transport of skyrmions is
possible as well as the conditions under which annihilation occurs.Comment: 17 pages, 10 figure
Asymptotic entanglement in a two-dimensional quantum walk
The evolution operator of a discrete-time quantum walk involves a conditional
shift in position space which entangles the coin and position degrees of
freedom of the walker. After several steps, the coin-position entanglement
(CPE) converges to a well defined value which depends on the initial state. In
this work we provide an analytical method which allows for the exact
calculation of the asymptotic reduced density operator and the corresponding
CPE for a discrete-time quantum walk on a two-dimensional lattice. We use the
von Neumann entropy of the reduced density operator as an entanglement measure.
The method is applied to the case of a Hadamard walk for which the dependence
of the resulting CPE on initial conditions is obtained. Initial states leading
to maximum or minimum CPE are identified and the relation between the coin or
position entanglement present in the initial state of the walker and the final
level of CPE is discussed. The CPE obtained from separable initial states
satisfies an additivity property in terms of CPE of the corresponding
one-dimensional cases. Non-local initial conditions are also considered and we
find that the extreme case of an initial uniform position distribution leads to
the largest CPE variation.Comment: Major revision. Improved structure. Theoretical results are now
separated from specific examples. Most figures have been replaced by new
versions. The paper is now significantly reduced in size: 11 pages, 7 figure
Genotyping-by-Sequencing Derived High-Density Linkage Map and its Application to QTL Mapping of Flag Leaf Traits in Bread Wheat
Winter wheat parents ‘Harry’ (drought tolerant) and ‘Wesley’ (drought susceptible) were used to develop a recombinant inbred population with future goals of identifying genomic regions associated with drought tolerance. To precisely map genomic regions, high-density linkage maps are a prerequisite. In this study genotyping-by-sequencing (GBS) was used to construct the high-density linkage map. The map contained 3,641 markers distributed on 21 chromosomes and spanned 1,959 cM with an average distance of 1.8 cM between markers. The constructed linkage map revealed strong collinearity in marker order across 21 chromosomes with POPSEQ-v2.0, which was based on a high-density linkage map. The reliability of the linkage map for QTL mapping was demonstrated by co-localizing the genes to previously mapped genomic regions for two highly heritable traits, chaff color, and leaf cuticular wax. Applicability of linkage map for QTL mapping of three quantitative traits, flag leaf length, width, and area, identified 21 QTLs in four environments, and QTL expression varied across the environments. Two major stable QTLs, one each for flag leaf length (Qfll.hww-7A) and flag leaf width (Qflw.hww-5A) were identified. The map constructed will facilitate QTL and fine mapping of quantitative traits, map-based cloning, comparative mapping, and in marker-assisted wheat breeding endeavors
Matching fields of a long superconducting film
We obtain the vortex configurations, the matching fields and the
magnetization of a superconducting film with a finite cross section. The
applied magnetic field is normal to this cross section, and we use London
theory to calculate many of its properties, such as the local magnetic field,
the free energy and the induction for the mixed state. Thus previous similar
theoretical works, done for an infinitely long superconducting film, are
recovered here, in the special limit of a very long cross section.Comment: Contains a REVTeX file and 4 figure
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