1,744 research outputs found
Counteracting systems of diabaticities using DRAG controls: The status after 10 years
The task of controlling a quantum system under time and bandwidth limitations
is made difficult by unwanted excitations of spectrally neighboring energy
levels. In this article we review the Derivative Removal by Adiabatic Gate
(DRAG) framework. DRAG is a multi-transition variant of counterdiabatic
driving, where multiple low-lying gapped states in an adiabatic evolution can
be avoided simultaneously, greatly reducing operation times compared to the
adiabatic limit. In its essence, the method corresponds to a convergent version
of the superadiabatic expansion where multiple counterdiabaticity conditions
can be met simultaneously. When transitions are strongly crowded, the system of
equations can instead be favorably solved by an average Hamiltonian (Magnus)
expansion, suggesting the use of additional sideband control. We give some
examples of common systems where DRAG and variants thereof can be applied to
improve performance.Comment: 7 pages, 2 figure
The structure of borders in a small world
Geographic borders are not only essential for the effective functioning of
government, the distribution of administrative responsibilities and the
allocation of public resources, they also influence the interregional flow of
information, cross-border trade operations, the diffusion of innovation and
technology, and the spatial spread of infectious diseases. However, as growing
interactions and mobility across long distances, cultural, and political
borders continue to amplify the small world effect and effectively decrease the
relative importance of local interactions, it is difficult to assess the
location and structure of effective borders that may play the most significant
role in mobility-driven processes. The paradigm of spatially coherent
communities may no longer be a plausible one, and it is unclear what structures
emerge from the interplay of interactions and activities across spatial scales.
Here we analyse a multi-scale proxy network for human mobility that
incorporates travel across a few to a few thousand kilometres. We determine an
effective system of geographically continuous borders implicitly encoded in
multi-scale mobility patterns. We find that effective large scale boundaries
define spatially coherent subdivisions and only partially coincide with
administrative borders. We find that spatial coherence is partially lost if
only long range traffic is taken into account and show that prevalent models
for multi-scale mobility networks cannot account for the observed patterns.
These results will allow for new types of quantitative, comparative analyses of
multi-scale interaction networks in general and may provide insight into a
multitude of spatiotemporal phenomena generated by human activity.Comment: 9 page
Engineering adiabaticity at an avoided crossing with optimal control
We investigate ways to optimize adiabaticity and diabaticity in the
Landau-Zener model with non-uniform sweeps. We show how diabaticity can be
engineered with a pulse consisting of a linear sweep augmented by an
oscillating term. We show that the oscillation leads to jumps in populations
whose value can be accurately modeled using a model of multiple,
photon-assisted Landau-Zener transitions, which generalizes work by Wubs et al.
[New J. Phys. 7, 218 (2005)]. We extend the study on diabaticity using methods
derived from optimal control. We also show how to preserve adiabaticity with
optimal pulses at limited time, finding a non-uniform quantum speed limit
Solar cycle variations of electron density and temperature in the Venusian nightside ionosphere
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95478/1/grl7104.pd
Molecular mode-coupling theory for supercooled liquids: Application to water
We present mode-coupling equations for the description of the slow dynamics
observed in supercooled molecular liquids close to the glass transition. The
mode-coupling theory (MCT) originally formulated to study the slow relaxation
in simple atomic liquids, and then extended to the analysis of liquids composed
by linear molecules, is here generalized to systems of arbitrarily shaped,
rigid molecules. We compare the predictions of the theory for the -vector
dependence of the molecular nonergodicity parameters, calculated by solving
numerically the molecular MCT equations in two different approximation schemes,
with ``exact'' results calculated from a molecular dynamics simulation of
supercooled water. The agreement between theory and simulation data supports
the view that MCT succeeds in describing the dynamics of supercooled molecular
liquids, even for network forming ones.Comment: 22 pages 4 figures Late
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