9,430 research outputs found
Enhancement of Coherent X ray Diffraction from Nanocrystals by Introduction of X ray Optics
Coherent X-ray Diffraction is applied to investigate the structure of individual nanocrystalline silver particles in the 100nm size range. In order to enhance the available signal, Kirkpatrick-Baez focusing optics have been introduced in the 34-ID-C beamline at APS. Concerns about the preservation of coherence under these circumstances are addressed through experiment and by calculations
Quantum Memristors
Technology based on memristors, resistors with memory whose resistance
depends on the history of the crossing charges, has lately enhanced the
classical paradigm of computation with neuromorphic architectures. However, in
contrast to the known quantized models of passive circuit elements, such as
inductors, capacitors or resistors, the design and realization of a quantum
memristor is still missing. Here, we introduce the concept of a quantum
memristor as a quantum dissipative device, whose decoherence mechanism is
controlled by a continuous-measurement feedback scheme, which accounts for the
memory. Indeed, we provide numerical simulations showing that memory effects
actually persist in the quantum regime. Our quantization method, specifically
designed for superconducting circuits, may be extended to other quantum
platforms, allowing for memristor-type constructions in different quantum
technologies. The proposed quantum memristor is then a building block for
neuromorphic quantum computation and quantum simulations of non-Markovian
systems
On characteristic equations, trace identities and Casimir operators of simple Lie algebras
Two approaches are developed to exploit, for simple complex or compact real
Lie algebras g, the information that stems from the characteristic equations of
representation matrices and Casimir operators. These approaches are selected so
as to be viable not only for `small' Lie algebras and suitable for treatment by
computer algebra. A very large body of new results emerges in the forms, a) of
identities of a tensorial nature, involving structure constants etc. of g, b)
of trace identities for powers of matrices of the adjoint and defining
representations of g, c) of expressions of non-primitive Casimir operators of g
in terms of primitive ones. The methods are sufficiently tractable to allow not
only explicit proof by hand of the non-primitive nature of the quartic Casimir
of g2, f4, e6, but also e.g. of that of the tenth order Casimir of f4.Comment: 39 pages, 8 tables, late
Higher gauge theory -- differential versus integral formulation
The term higher gauge theory refers to the generalization of gauge theory to
a theory of connections at two levels, essentially given by 1- and 2-forms. So
far, there have been two approaches to this subject. The differential picture
uses non-Abelian 1- and 2-forms in order to generalize the connection 1-form of
a conventional gauge theory to the next level. The integral picture makes use
of curves and surfaces labeled with elements of non-Abelian groups and
generalizes the formulation of gauge theory in terms of parallel transports. We
recall how to circumvent the classic no-go theorems in order to define
non-Abelian surface ordered products in the integral picture. We then derive
the differential picture from the integral formulation under the assumption
that the curve and surface labels depend smoothly on the position of the curves
and surfaces. We show that some aspects of the no-go theorems are still present
in the differential (but not in the integral) picture. This implies a
substantial structural difference between non-perturbative and perturbative
approaches to higher gauge theory. We finally demonstrate that higher gauge
theory provides a geometrical explanation for the extended topological symmetry
of BF-theory in both pictures.Comment: 26 pages, LaTeX with XYPic diagrams; v2: typos corrected and
presentation improve
Quantum and classical surface acoustic wave induced magnetoresistance oscillations in a 2D electron gas
We study theoretically the geometrical and temporal commensurability
oscillations induced in the resistivity of 2D electrons in a perpendicular
magnetic field by surface acoustic waves (SAWs). We show that there is a
positive anisotropic dynamical classical contribution and an isotropic
non-equilibrium quantum contribution to the resistivity. We describe how the
commensurability oscillations modulate the resonances in the SAW-induced
resistivity at multiples of the cyclotron frequency. We study the effects of
both short-range and long-range disorder on the resistivity corrections for
both the classical and quantum non-equilibrium cases. We predict that the
quantum correction will give rise to zero-resistance states with associated
geometrical commensurability oscillations at large SAW amplitude for
sufficiently large inelastic scattering times. These zero resistance states are
qualitatively similar to those observed under microwave illumination, and their
nature depends crucially on whether the disorder is short- or long-range.
Finally, we discuss the implications of our results for current and future
experiments on two dimensional electron gases.Comment: 16 pages, 8 figure
Giant microwave photoresistivity in a high-mobility quantum Hall system
We report the observation of a remarkably strong microwave photoresistivity
effect in a high-mobility two-dimensional electron system subject to a weak
magnetic field and low temperature. The effect manifests itself as a giant
microwave-induced resistivity peak which, in contrast to microwave-induced
resistance oscillations, appears only near the second harmonic of the cyclotron
resonance and only at sufficiently high microwave frequencies. Appearing in the
regime linear in microwave intensity, the peak can be more than an order of
magnitude stronger than the microwave-induced resistance oscillations and
cannot be explained by existing theories.Comment: 4 pages, 4 figure
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