1,720,965 research outputs found
Pseudo-classical theory for directed transport at quantum resonance
No full textRecent studies have demonstrated that a directed current arises in kicked atom systems at quantum resonance (so-called 'resonance ratchets'). Here, we demonstrate that this effect can be explained using a pseudo-classical model by taking classical initial conditions analogous to the initial quantum state. A corollary of our result is that a current is also expected to arise in the actual standard classical limit of the kicked atoms, demonstrating that the phenomenon can arise even in the absence of quantum interference. We show that in the standard classical limit, the momentum current undergoes far less saturation due to quasi-momentum spread than for the quantum resonance case. Additionally, we demonstrate that a phase-independent analytical scaling law exists for the mean momentum as a function of a single combined parameter and show that it predicts an unexpected current inversion regime
The role of quasi-momentum in the resonant dynamics of the atom-optics kicked rotor
No full textWe examine the effect of the initial atomic momentum distribution on the dynamics of the atom-optical realization of the quantum kicked rotor. The atoms are kicked by a pulsed optical lattice, the periodicity of which implies that quasi-momentum is conserved in the transport problem. We study and compare experimentally and theoretically two resonant limits of the kicked rotor: in the vicinity of the quantum resonances and in the semiclassical limit of the vanishing kicking period. It is found that for the same experimental distribution of quasi-momenta, significant deviations from the kicked rotor model are induced close to quantum resonance, while close to the classical resonance (i.e. for a small kicking period) the effect of the quasi-momentum vanishes
A Pseudoclassical Method for the Atom-Optics Kicked Rotor. from Theory to Experiment and Back
No full textWe review the concept and applications of a semiclassical (epsilon-classical or pseudoclassical) approximation to the resonant dynamics of an atom "kicked" by a pulsed, periodic potential. This powerful method allows us to derive analytical results in the deep quantum limit of the kicked rotor. Additionally, classical phase space portraits may be used to represent the dynamics even though the system is fundamentally quantum mechanical. The technique has been successfully adapted for systems including noise and decoherence, as well as systems for which the initial state is a trivial quantum super-position (leading to directed transport at quantum resonance). For almost a decade, theoretical investigations and experimental investigations have been proceeding hand-in-hand in this field, which has been stimulated regularly by experimental progress in controlling driven dynamical systems. Here, we review both theoretical and experimental advances, which in turn may inspire future applications of the presented pseudoclassical method
Phase-selected momentum transport in ultra-cold atoms
No full textWe propose a phase selected transport scenario for ultra-cold matter waves subject to pulsed optical lattices in which equally-populated momentum eigenstates with a flipped phase are localized while all other states are transported ballistically. The phenomenon relies on the concepts of quantum resonance and anti-resonance in two optical lattice potentials - one with half the wavelength of the other. We demonstrate the effectiveness of the technique in the presence of likely experimental noise sources, including the non-ideal situation where the momentum components are not all of equal amplitude. We also discuss possible uses for atom interferometric experiments and detail how to observe the phenomenon experimentally by creating an ideal initial state using standard atom-optical procedures
Experimental verification of a one-parameter scaling law for the quantum and "classical" resonances of the atom-optics kicked rotor
No full textWe present experimental measurements of the mean energy in the vicinity of the first and second quantum resonances of the atom-optics kicked rotor for a number of different experimental parameters. Our data are rescaled and compared with the one-parameter (ε) classical scaling function developed to describe the quantum resonance peaks. Additionally, experimental data are presented for the “classical” resonance which occurs in the limit as the kicking period goes to zero. This resonance is found to be analogous to the quantum resonances, and a similar one-parameter classical scaling function is derived, and found to match our experimental results. The widths of the quantum and classical resonance peaks are compared, and their sub-Fourier nature examined
Scaling law and stability for a noisy quantum system
No full textWe show that a scaling law exists for the near-resonant dynamics of cold kicked atoms in the presence of a randomly fluctuating pulse amplitude. Analysis of a quasiclassical phase-space representation of the quantum system with noise allows a new scaling law to be deduced. The scaling law and associated stability are confirmed by comparison with quantum simulations and experimental data
Engineering transport by concatenated maps
No full textWe present a generalized kick rotor model in which the phase of the kick can vary from kick to kick. This additional freedom allows one to control the transport in phase space. For a specific choice of kick-to-kick phases, we predict novel forms of accelerator modes which are potentially of high relevance for future experimental studies
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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