1,720,979 research outputs found
Misure Generalizzate e Strumenti Avanzati per i Protocolli di Informazione Quantistica
No full textL'informazione quantistica sta dando vita a numerose promettenti tecnologie e permette di raggiungere una più profonda comprensione di tutta la fisica quantistica. Ciononostante, il confine fra ciò che può e non può fare non è ancora stato ben definito. Questa tesi di dottorato spiega come la rinuncia ad alcune comuni assunzioni e idealizzazioni apre nuove e interessanti possibilità.
Per prima cosa si sofferma sulle misure quantistiche generalizzate, le quali possono superare le limitazioni delle più tipiche misure proiettive e possono sfruttare una risorsa quantistica più volte. Nella tesi, esse sono applicate ai random access codes, alla certificazione dell'entanglement e alla produzione di numeri casuali. Questi protocolli vengono studiati nei loro punti di forza, di debolezza, nelle applicazioni, nelle limitazioni e, soprattutto, nella fattibilità sperimentale.
Dopodiché, la tesi presenta due nuovi strumenti avanzati per la comunicazione quantistica. Il primo è un modello analitico generale che stima le performance di un sistema di distribuzione di chiave quantistica (QKD, dall'inglese quantum key distribution) a partire da una descrizione comprensiva di come esso si compone. Il modello viene usato come supporto per risultati sperimentali, per aiutare il design di apparati di QKD e per studiare meglio alcuni dettagli della QKD stessa. Il secondo è un'estensione del metodo dei decoy states che richiede meno assunzioni sulla distribuzione statistica dei fotoni. Così facendo, esso facilita l'uso di nuove sorgenti di fotoni in QKD.
Questa tesi cerca di trasmettere gran parte di ciò che imparato durante i tre anni della mia formazione da dottorando. Spero che possa allargare le vedute del lettore sull'informazione quantistica e dare un piccolo contributo allo sviluppo di nuove tecnologie quantistiche.Quantum information stands at the foundations of several promising technologies and is a window to a better understanding of quantum physics as a whole. However, because it is a fairly new research area, the boundaries of what it can and cannot do are still to be defined. This doctoral thesis explains the new and interesting possibilities that are opened up when some common idealizations and assumptions are removed.
First, it focuses on general quantum measurements, which, overcoming the restrictions of the usual projective ones, can harvest a quantum resource repeatedly. This is applied to random access codes, certification of entanglement and nonlocality, and production of random numbers. These protocols are studied in their strengths, weaknesses, applications, limitations, and, most importantly, in their feasibility, which is evaluated experimentally.
Then, the thesis introduces two advanced tools for quantum communications. One is a general analytical model that starts from a comprehensive description of quantum key distribution (QKD) systems and estimates their performance indicators. This is used to support experimental evidence, guide optimal design choices, and answer questions about the hidden facets of QKD. The other is a variation of the decoy-state method which requires weaker assumptions on the photon-number distribution in use. This enables the application of a wider set of photon sources to QKD.
This thesis transmits most of what I learned during the three years of my Ph.D. studies. I hope that it can broaden its readers' view on quantum information and give a small contribution to the development of quantum technologies
Experimental test of sequential weak measurements for certified quantum randomness extraction
Get PDFQuantum nonlocality offers a secure way to produce random numbers: their unpredictability is intrinsic and can be certified just by observing the statistic of the measurement outcomes, without assumptions on how they are produced. To do this, entangled pairs are generated and measured to violate a Bell inequality with the outcome statistics. However, after a projective quantum measurement, entanglement is entirely destroyed and cannot be used again. This fact poses an upper bound to the number of random numbers that can be produced from each quantum state when projective measurements are employed. Instead, by using weak measurements, some entanglement can be maintained and reutilized, and a sequence of weak measurements can extract an unbounded amount of randomness from a single state as predicted in Phys. Rev. A 95, 020102(R) (2017). We study the feasibility of these weak measurements, analyze the robustness to imperfections in the quantum state they are applied to, and then test them using an optical setup based on polarization-entangled photon pairs. We show that the weak measurements are realizable, but can improve the performance of randomness generation only in close-to-ideal conditions
Time-bin Quantum Key Distribution exploiting the iPOGNAC polarization moulator and Qubit4Sync temporal synchronization
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Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Simple quantum key distribution with qubit-based synchronization and a self-compensating polarization encoder
Get PDFQuantum key distribution (QKD) relies on quantum communication to allow distant parties to share a secure cryptographic key. Widespread adoption of QKD in current telecommunication networks will require the development of simple, low-cost, and stable systems. However, current QKD implementations usually include additional hardware that perform auxiliary tasks such as temporal synchronization and polarization basis tracking. Here we present a polarization-based QKD system operating at 1550 nm that performs synchronization and polarization compensation by exploiting only the hardware already needed for the quantum communication task. Polarization encoding is performed by a self-compensating Sagnac loop modulator that exhibits high temporal stability and the lowest intrinsic quantum bit error rate reported so far. The QKD system was tested over a fiber-optic link, demonstrating tolerance up to about 40 dB of channel losses. Due to its reduced hardware requirements and the quality of the s..
A Versatile and Low-Error Polarization Encoder for Quantum Communications
No full textIn addition to stable, low-error and calibration-free polarization modulation, the iPOGNAC encoder can be promptly reconfigured for time-bin encoding, intensity modulation, and operations at different wavelengths. An overview of these applications and results is given
Variations on the Author
Get PDF“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
Get PDFWe 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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