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Experimental study of anomalous diffusion and quantum correlations in an all optical quantum walk
Full text linkIn recent years, quantum walks (QWs), the quantum counterpart of classical random walks (CRWs), have become increasingly attractive for quantum information fields, ranging from quantum computation [1], to quantum simulation [2], and quantum cryptography [3]. Furthermore, QWs have been found to be a useful resource for the study of energy propagation and transport phenomena [4]. Indeed, QWs are featured by a variance of the position probability distribution of the walker which grows quadratically with the number of the step, namely σ^2 = α*n^2. This behavior is usually referred to as ballistic. Several natural propagation phenomena follow an anomalous diffusion regime, for which it holds σ^2 = α*n^β with 1 < β < 2 (β < 1) in the superdiffusive (subdiffusive) case. Being it an intermediate regime between the ballistic and the diffusive one, which is typical of CRWs, it is natural to ask whether QWs allow to reproduce such a behavior.
The aim of my thesis work consists in studying this possibility by experimentally investigating anomalous diffusion within a QW framework, exploiting both single and two photons evolution. The superdiffusive dynamics has been investigated by adopting a novel bulk optics scheme [5], whose basic idea has been used in the study of non-Markovian dynamics [6]. By adopting the so-called p-diluted disorder, we could investigate the average dynamics of a quantum particle moving in a random environment [7]. We found that, in both cases of single and two walkers, the evolution is characterized by a superdiffusive behavior, with the value of the parameter β depending on the disorder level experienced by the walkers.
Subdiffusivity has been studied by mean of a time-split QW. A static disorder, namely a disorder fixed in time but varying in space, can reproduce the so-called Anderson localization, consisting of an exponential localization of the walker around its starting position. By perturbing it through the p-diluted disordering technique, it is possible to break the static condition giving rise to Anderson localization. We found that in this case a clear subdiffusive behavior can be obtained, with a value of β increasing with the disorder level and reaching β = 1 when the disorder is maximum [8].
These results demonstrate that QWs provided with the p-diluted disorder are feasible resources to study anomalous diffusion processes, for both single and two photons cases, and that it can be helpful in shedding light on the physical mechanisms underlying anomalous diffusion phenomena.
Secondarily, by the same disordering technique, we studied how to quantify non-classical correlations between two walkers, by checking the violation of a certain inequality [9], as a function of the disorder experienced by the photons during their evolution. Preliminary results show that the disorder can control the non-classical correlation between the photons travelling along different pairs of modes, suggesting that it could be adopted as a useful instrument in a quantum protocol scenario, such as for instance metrology.
References
[1] A. M. Childs, “Universal computation by quantum walk," Physical review letters, vol. 102, no. 18, p. 180501, 2009.
[2] S. E. Venegas-Andraca, “Quantum walks: a comprehensive review," Quantum Information Processing, vol. 11, no. 5, pp. 1015-1106, 2012.
[3] C. Vlachou, W. Krawec, P. Mateus, N. Paunkovic, and A. Souto, “Quantum key distribution with quantum walks," Quantum Information Processing, vol. 17, no. 11, p. 288, 2018.
[4] M. Mohseni, P. Rebentrost, S. Lloyd, and A. Aspuru-Guzik, “Environment-assisted quantum walks in photosynthetic energy transfer," The Journal of chemical physics, vol. 129, no. 17, p. 11B603, 2008.
[5] A. Geraldi, L. D. Bonavena, C. Liorni, P. Mataloni, and A. Cuevas, “A novel bulk-optics scheme for quantum walk with high phase stability," Condensed Matter, vol. 4, no. 1, p. 14, 2019.
[6] A. Cuevas, A. Geraldi, C. Liorni, L. D. Bonavena, A. De Pasquale, F. Sciarrino, V. Giovannetti, and P. Mataloni, “All-optical implementation of collision-based evolutions of open quantum systems," Scientific reports, vol. 9, no. 1, pp. 1-8, 2019.
[7] A. Geraldi, A. Laneve, L. D. Bonavena, L. Sansoni, J. Ferraz, A. Fratalocchi, F. Sciarrino, A. Cuevas, and P. Mataloni, “Experimental investigation of superdiffusion via coherent disordered quantum walks," Physical review letters, vol. 123, no. 14, p. 140501, 2019.
[8] A. Geraldi, S. De, A. Laneve, S. Barkhofen, J. Sperling, P. Mataloni, and C. Silberhorn, “Paper in preparation,"
[9] Y. Bromberg, Y. Lahini, R. Morandotti, and Y. Silberberg, “Quantum and classical correlations in waveguide lattices," Physical review letters, vol. 102, no. 25, p. 253904, 2009
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
Transient subdiffusion via disordered quantum walks
Full text linkTransport phenomena play a crucial role in modern physics and applied sciences. Examples include thedissipation of energy across a large system, the distribution of quantum information in optical networks, andthe timely modeling of spreading diseases. In this work we experimentally prove the feasibility of disorderedquantum walks to realize a quantum simulator that is able to model general transient subdiffusive phenomena,exhibiting a sublinear spreading in space over time. Our experiment simulates such phenomena by means ofa finely controlled insertion of various levels of disorder during the evolution of the walker, enabled by theunique flexibility of our setup. This allows us to explore the full range of subdiffusive behaviors, ranging fromanomalous Anderson-like localization to normal diffusion for all experimentally accessible step numbers
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
A Novel Bulk-Optics Scheme for Quantum Walk with High Phase Stability
Full text linkA novel bulk optics scheme for quantum walks is presented. It consists of a one-dimensional lattice built on two concatenated displaced Sagnac interferometers that make it possible to reproduce all the possible trajectories of an optical quantum walk. Because of the closed loop configuration, the interferometric structure is intrinsically stable in phase. Moreover, the lattice structure is highly configurable, as any phase component perceived by the walker is accessible, and finally, all output modes can be measured at any step of the quantum walk evolution. We report here on the experimental implementation of ordered and disordered quantum walks
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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