11,508 research outputs found
Particle simulations of magnetic field reconnection and applications to flux transfer events
Thesis (Ph.D.) University of Alaska Fairbanks, 1990Basic plasma processes associated with driven collisionless magnetic reconnection at the Earth's dayside magnetopause are studied on the basis of particle simulations. A two-and-one-half-dimensional (2-D) electromagnetic particle simulation model with a driven inflow boundary and an open outflow boundary is developed for the present study. The driven inflow boundary is featured with a driving electric field for the vector potential, while the open outflow boundary is characterized by a vacuum force free condition for the electrostatic potential. The major findings are as follows. (1) The simulations exhibit both quasi-steady single X-line reconnection (SXR) and intermittent multiple X line reconnection (MXR). The MXR process is characterized by repeated formation and convection of magnetic islands (flux tubes or plasmoids). (2) Particle acceleration in the MXR process occurs mainly in O line regions as particles are trapped within magnetic islands, not in X line regions. The MXR process results in a power law particle energy spectrum of f(E)\sim E\sp{-4}. (3) Field-aligned particle heat fluxes and intense plasma waves associated with the collisionless magnetic reconnection process are also observed. (4) When applied to the dayside magnetopause, simulation results show that the MXR process tends to generate a simultaneous magnetic field perturbation on both sides of the dayside magnetopause, resembling the observed features of two-regime flux transfer events (FTEs). (5) An intrusion of magnetosheath plasma bulge into the magnetosphere due to the formation of magnetic islands may lead to the layered structures observed in magnetospheric FTEs. (6) In the current sheet, the enhanced tearing mode instability caused by the driving force applied at the driven inflow boundary creates an energy source at a specific wavenumber range with k\sb{z}L\sim 0.3 in the modal spectrum of the magnetic field B\sb{x} component. An inverse cascade of the modal spectrum of B\sb{x} leads to the formation of the large-scale ordered magnetic island structures observed in the simulations. (7) In addition, the results of a theoretical study show that the tearing mode instability, and hence the magnetic reconnection at the dayside magnetopause, do not exhibit strong dependence on the magnetosheath values
Experimental demonstration of the DPTS QKD protocol over a 170 km fiber link
Quantum key distribution (QKD) is a promising technology aiming at solving
the security problem arising from the advent of quantum computers. While the
main theoretical aspects are well developed today, limited performances, in
terms of achievable link distance and secret key rate, are preventing the
deployment of this technology on a large scale. More recent QKD protocols,
which use multiple degrees of freedom for the encoding of the quantum states,
allow an enhancement of the system performances. Here, we present the
experimental demonstration of the differential phase-time shifting protocol
(DPTS) up to 170 km of fiber link. We compare its performance with the
well-known coherent one-way (COW) and the differential phase shifting (DPS)
protocols, demonstrating a higher secret key rate up to 100 km. Moreover, we
propagate a classical signal in the same fiber, proving the compatibility of
quantum and classical light.Comment: 5 pages, 3 figures, journal pape
Spatial frequency based video stream analysis for object classification and recognition in clouds
The recent rise in multimedia technology has made it easier to perform a number of tasks. One of these tasks is monitoring where cheap cameras are producing large amount of video data. This video data is then processed for object classification to extract useful information. However, the video data obtained by these cheap cameras is often of low quality and results in blur video content. Moreover, various illumination effects caused by lightning conditions also degrade the video quality. These effects present severe challenges for object classification. We present a cloud-based blur and illumination invariant approach for object classification from images and video data. The bi-dimensional empirical mode decomposition (BEMD) has been adopted to decompose a video frame into intrinsic mode functions (IMFs). These IMFs further undergo to first order Reisz transform to generate monogenic video frames. The analysis of each IMF has been carried out by observing its local properties (amplitude, phase and orientation) generated from each monogenic video frame. We propose a stack based hierarchy of local pattern features generated from the amplitudes of each IMF which results in blur and illumination invariant object classification. The extensive experimentation on video streams as well as publically available image datasets reveals that our system achieves high accuracy from 0.97 to 0.91 for increasing Gaussian blur ranging from 0.5 to 5 and outperforms state of the art techniques under uncontrolled conditions. The system also proved to be scalable with high throughput when tested on a number of video streams using cloud infrastructure
Orbital angular momentum states enabling fiber-based high-dimensional quantum communication
Quantum networks are the ultimate target in quantum communication, where many
connected users can share information carried by quantum systems. The keystones
of such structures are the reliable generation, transmission and manipulation
of quantum states. Two-dimensional quantum states, qubits, are steadily adopted
as information units. However, high-dimensional quantum states, qudits,
constitute a richer resource for future quantum networks, exceeding the
limitations imposed by the ubiquitous qubits. The generation and manipulation
of such -level systems have been improved over the last ten years, but their
reliable transmission between remote locations remains the main challenge.
Here, we show how a recent air-core fiber supporting orbital angular momentum
(OAM) modes can be exploited to faithfully transmit -dimensional states.
Four OAM quantum states and their superpositions are created, propagated in a
1.2 km long fiber and detected with high fidelities. In addition, three quantum
key distribution (QKD) protocols are implemented as concrete applications to
assert the practicality of our results. This experiment enhances the
distribution of high-dimensional quantum states, attesting the orbital angular
momentum as vessel for the future quantum network
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