1,720,958 research outputs found
Temporal properties of counter propagating twin photons
Full text linkTwin photon pairs generated through parametric downconversion (PDC) in a χ 2 medium is one of the most widely used source of entanglement. We focus here on a non-conventional geometry in which one of the twin photons propagates in the opposite direction with respect to the pump beam, exploiting quasi-phase-matching in a periodically poled crystal. Through predicted almost 50 years ago, this new PDC configuration has been realized experimentally only recently [1] thanks to new fabrication techniques achieving the required sub-micrometer poling period. Because of the presence of distributed feedback, the optical system has been shown to behave as a Mirrorless Optical Parametric Oscillator (MOPO) and exhibits peculiar spectral properties which strongly differ from those found in more common geometries involving co-propagating beams. In this work we provide a detailed analysis of the correlation and coherence properties of counter-propagating twin beams both in the purely spontaneous regime and in the neighborhood of the MOPO threshold. We consider on the on side the regime of spontaneous pair production where the characteristic narrow band of the counter-propagating twin beams offers the unique opportunity of generating heralded single photon states with a high degree of purity, a relevant property for applications in quantum communication. In this context, we investigate how the degree of separability of the twin photon state varies with the pump pulse duration τp. We find that two well separated time scales characterize the system dynamics: a short time scale τgvm, in the picoseconds range, corresponding to the typical temporal delay of co-propagating waves due to group-velocity mismatch, and a much longer time scale τgvs associated with the temporal separation of counter-propagating waves. Such a difference of time scales occurs naturally in the counterpropagating configuration, for basically any kind of material and tuning condition. Because of this same feature, counter-propagating twin photons in a pure state can in principle be heralded at any wavelength by choosing the appropriate poling period. We show that a high degree of separability can be achieved when the pump pulse duration satisfy the condition τgvm ≪ τp ≪ τgvs, as put in evidence from the evaluation of Schmidt number as a function of the pump pulse duration which reaches a minimum close to unity in this region. The separability is lost in the nearly monochromatic limit ( τp ≫ τgvs ) as well as for ultra-short pulses ( τp ≪ τgvm ), where the entanglement between the signal and idler frequencies can be inferred by the non factorable shape of the spectral biphoton amplitude. We offer a physical interpretation of such a behaviour, and a detailed analysis of the Schmidt number characterizing the entanglement of the state. We also considered a completely different regime of operation, close to the MOPO threshold, where the combined effect of stimulated PDC and distributed feedback affects dramatically the property of coherence of the field. Our analysis put in evidence a progressive narrowing of both the spectral twin beam correlation and the intensity spectra as the pump field intensity approaches its threshold value. This translates into a drastic increase of the correlation and coherence times in the temporal domain, a feature which can be attributed to the critical slowing down of the fluctuation dynamics characterizing the transition toward coherent emission occurring at the MOPO threshold. Furthermore, we investigate the potentiality of the source to generate squeezing and EPR type correlations in the threshold vicinity. In this regards, the obtained results shows that the system displays a behaviour which is very similar to that found in standard optical parametric oscillators enclosed in a resonant cavity. In the last part of the work, we present some preliminary results from numerical simulations illustrating the transition above the MOPO thresholds. We also take into account non collinear PDC emission, showing explicitely that the spatial and the temporal degrees of freedoms of the emitted twin photons are almost uncoupled. This feature strongly distinguish the counter-propagating configuration from standard co-propagating geometries where the phase-matching mechanism usually leads to strong angular dispersion
Temporal properties of counter-propagating twin beams close to the threshold for mirrorless optical parametric oscillation
Full text linkWe investigate the coherence and correlation properties of counter-propagating twin beams generated in a Mirrorless Optical Parametric Oscillator (MOPO) with distributed feedback in the neighborhood of the device emission threshold
Squeezing and Einstein-Podolsky-Rosen correlation in the mirrorless optical parametric oscillator
No full textThis work analyses the quantum properties of counterpropagating twin beams generated by a mirrorless optical parametric oscillator in the continuous-variable regime. Despite the lack of the filtering effect of a cavity, we show that in the vicinity of its threshold it may generate high levels of narrowband squeezing and Einstein-Podolsky-Rosen correlation, completely comparable to what can be obtained in standard optical parametric oscillators
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
Temporal coherence and correlation of counterpropagating twin photons
No full textThis work analyzes the temporal coherence and correlation of counterpropagating twin photons generated in a
quasiphase matched nonlinear crystal by spontaneous parametric down-conversion.We find out different pictures
depending on the pump pulse duration relative to two characteristic temporal scales, determined, respectively, by
the temporal separation between the counterpropagating and the co-propagating wave packets. When the pump
duration is intermediate between the two scales, we show a transition from a highly entangled state to an almost
separable state, with strongly asymmetric spectral properties of the photons
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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