1,721,018 research outputs found
Novel techniques for the trapping and manipulation of ultracold atoms
No full textWe describe several novel techniques for the optical trapping of ultracold atoms and for the production of wavelength-stabilised, coherent light at the frequencies required for use in atomic physics experiments.The greater part of this thesis deals with work towards the creation of regular arrays of microscopic optical dipole traps formed at the foci of truncated spherical cavities in a metallic film, in which the inter-site spacing can be set anywhere between one and several hundred micrometers. Arrays of such cavities are synthesised and structurally characterised via optical and electron microscopy, and numerical simulations of the light intensity distribution near the foci of such cavities under normal illumination are used to confirm their suitability for dipole trap production. A method for the construction of arrays of magneto-optical traps based on such structures is proposed and theoretically examined, and some preliminary experimental work towards the synthesis of the required microstructures is also described. Possible approaches to the loading of such traps and the imaging of the atoms contained therein are discussed - experimental work towards ballistic atom transfer from a specialised form of magneto-optical trap that can be formed close to a microstructured surface is carried out, and the efficacy of wavelength-selective fluorescence imaging as a means of reducing the effects of background scatter from the surface is experimentally demonstrated.Further work described herein includes the proposal and experimental demonstration of two novel techniques for the removal of the carrier wave from a phase-modulated laser beam, one of which is based on a fiber-optic Mach-Zehnder interferometer that is shown to be an effective device for splitting or combining beams of nearly equal frequencies. A spontaneous-force based atom trapping mechanism that does not rely on the use of a magnetic field, but rather on spatially-dependent optical pumping between different metastable atomic states, is also proposed, and a proof of principle experiment is carried out to demonstrate the validity of the suggested mechanism. We find that this trapping scheme allows the spatial dependence of the trapping force to be tailored with a greater degree of flexibility than is usually possible with magneto-optical trapping, and that it is also capable of producing traps with stronger spring constants than are typically achievable with magneto-optical trapping under realistic experimental constraints
Trapping of 85Rb atoms by optical pumping between metastable hyperfine states
No full textWe describe an atom trapping mechanism based upon differential optical pumping between metastable hyperfine states by partially displaced laser beams in the absence of a magnetic field. With realistic laser powers, trap spring constants should match or exceed those typical of magneto-optical traps, and highly flexible tailored trap shapes should be achievable. In a proof-of-principle experiment, we have combined a 1D implementation with magneto-optical trapping in the orthogonal directions, capturing ~104 85Rb atoms
Stabilized fiber-optic Mach–Zehnder interferometer for carrier-frequency rejection
No full textWe have demonstrated stabilization of a fiber-optic Mach–Zehnder interferometer, with a centimeter-scale path difference, to the transmission minimum for the carrier wave of a frequency-modulated laser beam. A time-averaged extinction of 32 dB, limited by the bandwidth of the feedback, was maintained over several hours. The interferometer was used to remove the carrier wave from a 780 nm laser beam that had been phase modulated at 2.7 GHz
Actively stabilized wavelength-insensitive carrier elimination from an electro-optically modulated laser beam
No full textWe demonstrate a simple and robust technique for removal of the carrier wave from a phase-modulated laser beam, using a noninterferometric method that is insensitive to the modulation frequency and instead exploits the polarization dependence of electro-optic modulation. An actively stabilized system using feedback via a liquid crystal cell yields long-term carrier suppression in excess of 28 dB at the expense of a 6.5 dB reduction in sideband power
Visualisation of quantum evolution in the Stern–Gerlach and Rabi experiments
Full text linkThe Stern–Gerlach experiment is a seminal experiment in quantum physics, involving the interaction between a particle with spin and an applied magnetic field gradient. A recent article [Wennerström et al., Phys. Chem. Chem. Phys., 2012, 14, 1677–1684] claimed that a full understanding of the Stern–Gerlach experiment can only be attained if transverse spin relaxation is taken into account, generated by fluctuating magnetic fields originating in the magnetic materials which generate the field gradient. This interpretation is contrary to the standard quantum description of the Stern–Gerlach experiment, which requires no dissipative effects. We present simulations of conventional quantum dynamics in the Stern–Gerlach experiment, using extended Wigner functions to describe the propagation of the quantum state in space and time. No relaxation effects are required to reproduce the qualitative experimental behaviour. We also present simulations of quantum dynamics in the Rabi experiment, in which an applied radiofrequency field induces spin transitions in the particle wave
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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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