16,186 research outputs found
On a class of generating vector fields for the extremum seeking problem: Lie bracket approximation and stability properties
In this paper, we describe a broad class of control functions for extremum
seeking problems. We show that it unifies and generalizes existing extremum
seeking strategies which are based on Lie bracket approximations, and allows to
design new controls with favorable properties in extremum seeking and
vibrational stabilization tasks. The second result of this paper is a novel
approach for studying the asymptotic behavior of extremum seeking systems. It
provides a constructive procedure for defining frequencies of control functions
to ensure the practical asymptotic and exponential stability. In contrast to
many known results, we also prove asymptotic and exponential stability in the
sense of Lyapunov for the proposed class of extremum seeking systems under
appropriate assumptions on the vector fields
Time-domain response of nabla discrete fractional order systems
This paper investigates the time--domain response of nabla discrete
fractional order systems by exploring several useful properties of the nabla
discrete Laplace transform and the discrete Mittag--Leffler function. In
particular, we establish two fundamental properties of a nabla discrete
fractional order system with nonzero initial instant: i) the existence and
uniqueness of the system time--domain response; and ii) the dynamic behavior of
the zero input response. Finally, one numerical example is provided to show the
validity of the theoretical results.Comment: 13 pages, 6 figure
Catastrophe optics of caustics in single and bilayer graphene: fine structure of caustics
We theoretically study the scattering of a plane wave of a ballistic electron
on a circular n-p junction in single and bilayer graphene. We compare the exact
wave function inside the junction to that obtained from a semiclassical formula
developed in catastrophe optics. In the semiclassical picture short-wavelength
electrons are treated as rays of particles that can get reflected and refracted
at the n-p junction according to Snell's law with negative refraction index. We
show that for short wavelength and close to caustics this semiclassical
approximation gives good agreement with the exact results in the case of
single-layer graphene. We also verify the universal scaling laws that govern
the shrinking rate and intensity divergence of caustics in the semiclassical
limit. It is straightforward to generalize our semiclassical method to more
complex geometries, offering a way to efficiently design and model
graphene-based electron-optical systems.Comment: 4 pages, 5 figures This manuscript should be published in the
proceedings volume of the conference IWEPNM 2010, in the Physica Status
Solidi
A Stochastic Liouville Equation Approach for the Effect of Noise in Quantum Computations
We propose a model based on a generalized effective Hamiltonian for studying
the effect of noise in quantum computations. The system-environment
interactions are taken into account by including stochastic fluctuating terms
in the system Hamiltonian. Treating these fluctuations as Gaussian Markov
processes with zero mean and delta function correlation times, we derive an
exact equation of motion describing the dissipative dynamics for a system of n
qubits. We then apply this model to study the effect of noise on the quantum
teleportation and a generic quantum controlled-NOT (CNOT) gate. For the quantum
CNOT gate, we study the effect of noise on a set of one- and two-qubit quantum
gates, and show that the results can be assembled together to investigate the
quality of a quantum CNOT gate operation. We compute the averaged gate fidelity
and gate purity for the quantum CNOT gate, and investigate phase, bit-flip, and
flip-flop errors during the CNOT gate operation. The effects of direct
inter-qubit coupling and fluctuations on the control fields are also studied.
We discuss the limitations and possible extensions of this model. In sum, we
demonstrate a simple model that enables us to investigate the effect of noise
in arbitrary quantum circuits under realistic device conditions.Comment: 36 pages, 6 figures; to be submitted to Phys. Rev.
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