4,876 research outputs found
Dressing the chopped-random-basis optimization: a bandwidth-limited access to the trap-free landscape
In quantum optimal control theory the success of an optimization algorithm is
highly influenced by how the figure of merit to be optimized behaves as a
function of the control field, i.e. by the control landscape. Constraints on
the control field introduce local minima in the landscape --false traps-- which
might prevent an efficient solution of the optimal control problem. Rabitz et
al. [Science 303, 1998 (2004)] showed that local minima occur only rarely for
unconstrained optimization. Here, we extend this result to the case of
bandwidth-limited control pulses showing that in this case one can eliminate
the false traps arising from the constraint. Based on this theoretical
understanding, we modify the Chopped Random Basis (CRAB) optimal control
algorithm and show that this development exploits the advantages of both
(unconstrained) gradient algorithms and of truncated basis methods, allowing to
always follow the gradient of the unconstrained landscape by bandwidth-limited
control functions. We study the effects of additional constraints and show that
for reasonable constraints the convergence properties are still maintained.
Finally, we numerically show that this approach saturates the theoretical bound
on the minimal bandwidth of the control needed to optimally drive the system.Comment: 8 pages, 6 figure
Charge fluctuations in nonlinear heat transport
We show that charge fluctuation processes are crucial for the nonlinear heat
conductance through an interacting nanostructure, even far from a resonance. We
illustrate this for an Anderson quantum dot accounting for the first two
leading orders of the tunneling in a master equation. The often made assumption
that off-resonant transport proceeds entirely by virtual occupation of charge
states, underlying exchange-scattering models, can fail dramatically for heat
transport. The identified energy-transport resonances in the Coulomb blockade
regime provide new qualitative information about relaxation processes, for
instance by strong negative differential heat conductance relative to the heat
current. These can go unnoticed in the charge current, making nonlinear
heat-transport spectroscopy with energy-level control a promising experimental
tool
Significant Subgraph Mining with Multiple Testing Correction
The problem of finding itemsets that are statistically significantly enriched
in a class of transactions is complicated by the need to correct for multiple
hypothesis testing. Pruning untestable hypotheses was recently proposed as a
strategy for this task of significant itemset mining. It was shown to lead to
greater statistical power, the discovery of more truly significant itemsets,
than the standard Bonferroni correction on real-world datasets. An open
question, however, is whether this strategy of excluding untestable hypotheses
also leads to greater statistical power in subgraph mining, in which the number
of hypotheses is much larger than in itemset mining. Here we answer this
question by an empirical investigation on eight popular graph benchmark
datasets. We propose a new efficient search strategy, which always returns the
same solution as the state-of-the-art approach and is approximately two orders
of magnitude faster. Moreover, we exploit the dependence between subgraphs by
considering the effective number of tests and thereby further increase the
statistical power.Comment: 18 pages, 5 figure, accepted to the 2015 SIAM International
Conference on Data Mining (SDM15
Supercontinuum generation in media with sign-alternated dispersion
When an ultrafast optical pulse with high intensity is propagating through
transparent material a supercontinuum can be coherently generated by self-phase
modulation, which is essential to many photonic applications in fibers and
integrated waveguides. However, the presence of dispersion causes stagnation of
spectral broadening past a certain propagation length, requiring an increased
input peak power for further broadening. We present a concept to drive
supercontinuum generation with significantly lower input power by counteracting
spectral stagnation via alternating the sign of group velocity dispersion along
the propagation. We demonstrate the effect experimentally in dispersion
alternating fiber in excellent agreement with modeling, revealing almost an
order of magnitude reduced peak power compared to uniform dispersion.
Calculations reveal a similar power reduction also with integrated optical
waveguides, simultaneously with a significant increase of flat bandwidth, which
is important for on-chip broadband photonics.Comment: Main text and supplementary informatio
Holograms of Conformal Chern-Simons Gravity
We show that conformal Chern-Simons gravity in three dimensions has various
holographic descriptions. They depend on the boundary conditions on the
conformal equivalence class and the Weyl factor, even when the former is
restricted to asymptotic Anti-deSitter behavior. For constant or fixed Weyl
factor our results agree with a suitable scaling limit of topologically massive
gravity results. For varying Weyl factor we find an enhancement of the
asymptotic symmetry group, the details of which depend on certain choices. We
focus on a particular example where an affine u(1) algebra related to
holomorphic Weyl rescalings shifts one of the central charges by 1. The Weyl
factor then behaves as a free chiral boson in the dual conformal field theory.Comment: 5
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