9,748 research outputs found
Gaussian fields and Gaussian sheets with generalized Cauchy covariance structure
Two types of Gaussian processes, namely the Gaussian field with generalized
Cauchy covariance (GFGCC) and the Gaussian sheet with generalized Cauchy
covariance (GSGCC) are considered. Some of the basic properties and the
asymptotic properties of the spectral densities of these random fields are
studied. The associated self-similar random fields obtained by applying the
Lamperti transformation to GFGCC and GSGCC are studied.Comment: 32 pages, 6 figure
Optimized numerical gradient and Hessian estimation for variational quantum algorithms
Sampling noisy intermediate-scale quantum devices is a fundamental step that
converts coherent quantum-circuit outputs to measurement data for running
variational quantum algorithms that utilize gradient and Hessian methods in
cost-function optimization tasks. This step, however, introduces estimation
errors in the resulting gradient or Hessian computations. To minimize these
errors, we discuss tunable numerical estimators, which are the
finite-difference (including their generalized versions) and scaled
parameter-shift estimators [introduced in Phys. Rev. A 103, 012405 (2021)], and
propose operational circuit-averaged methods to optimize them. We show that
these optimized numerical estimators offer estimation errors that drop
exponentially with the number of circuit qubits for a given sampling-copy
number, revealing a direct compatibility with the barren-plateau phenomenon. In
particular, there exists a critical sampling-copy number below which an
optimized difference estimator gives a smaller average estimation error in
contrast to the standard (analytical) parameter-shift estimator, which exactly
computes gradient and Hessian components. Moreover, this critical number grows
exponentially with the circuit-qubit number. Finally, by forsaking analyticity,
we demonstrate that the scaled parameter-shift estimators beat the standard
unscaled ones in estimation accuracy under any situation, with comparable
performances to those of the difference estimators within significant
copy-number ranges, and are the best ones if larger copy numbers are
affordable.Comment: 24 pages, 7 figures (updated Fig. 4, new Fig. 6, new Secs. IV C, V C,
VII and Appendix C5 since last version
Effective response theory for zero energy Majorana bound states in three spatial dimensions
We propose a gravitational response theory for point defects (hedgehogs)
binding Majorana zero modes in (3+1)-dimensional superconductors. Starting in
4+1 dimensions, where the point defect is extended into a line, a coupling of
the bulk defect texture with the gravitational field is introduced.
Diffeomorphism invariance then leads to an Kac-Moody current running
along the defect line. The Kac-Moody algebra accounts for the
non-Abelian nature of the zero modes in 3+1 dimensions. It is then shown to
also encode the angular momentum density which permeates throughout the bulk
between hedgehog-anti-hedgehog pairs.Comment: 7 pages, 3 figure
Wnt-Frizzled Signaling Regulates Activity-Mediated Synapse Formation
The formation of synapses is a tightly regulated process that requires the coordinated assembly of the presynaptic and postsynaptic sides. Defects in synaptogenesis during development or in the adult can lead to neurodevelopmental disorders, neurological disorders, and neurodegenerative diseases. In order to develop therapeutic approaches for these neurological conditions, we must first understand the molecular mechanisms that regulate synapse formation. The Wnt family of secreted glycoproteins are key regulators of synapse formation in different model systems from invertebrates to mammals. In this review, we will discuss the role of Wnt signaling in the formation of excitatory synapses in the mammalian brain by focusing on Wnt7a and Wnt5a, two Wnt ligands that play an in vivo role in this process. We will also discuss how changes in neuronal activity modulate the expression and/or release of Wnts, resulting in changes in the localization of surface levels of Frizzled, key Wnt receptors, at the synapse. Thus, changes in neuronal activity influence the magnitude of Wnt signaling, which in turn contributes to activity-mediated synapse formation
Optimal integral force feedback and structured PI tracking control : application for objective lens positioner
Peer reviewedPostprin
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