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Particelle localizzate per informazione quantistica e quantum sensing in dispositivi-prototipo a semiconduttore
No full textLa teoria dell’informazione quantistica possiede potenziale a lungo termine in diverse aree, tra le quali calcolo e comunicazione. Tuttavia, nonostante il rapido progresso della comunità quantistica, le realizzazioni pratiche sono ancora limitate. I principali obiettivi ancora da raggiungere includono la realizzazione di potenti processori quantistici universali con un numero sufficiente di qubit, e la loro connessione tramite canali quantistici adatti. Prospettive promettenti in questo senso sono offerte dall'uso di nanostrutture a semiconduttore, grazie alla loro compatibilità con i circuiti classici e con le tecniche di produzione avanzate dei dispositivi moderni.
La prima parte di questa tesi studia gli interferometri Mach-Zehnder nel regime quantum Hall intero in grafene, come possibile piattaforma per l'implementazione di flying qubit. In electron quantum optics, la presenza di un'interazione Coulombiana forte e manipolabile consente la diretta implementazione di gate a uno e due qubit. Inoltre, la chiralità dei canali di edge impedisce il backscattering e consente un’alta coerenza dei portatori generati da sorgenti a singolo elettrone. Nello specifico, nel grafene, la coerenza è ulteriormente migliorata dalla dispersione lineare dei fermioni di Dirac. Nel Capitolo 1 analizziamo lo stato dell'arte sui flying qubit e la loro implementazione in grafene. Nel Capitolo 2 mostriamo come trattare analiticamente singole cariche in nanoribbon di grafene, tenendo conto sia sia degli pseudospin di valle che di reticolo. In più spieghiamo come simulare la loro evoluzione tempo-dipendente tramite il metodo split-step di Fourier. Nel Capitolo 3, caratterizziamo il moto di pacchetti d'onda elettronici Gaussiani all'interno di nanoribbon di grafene, concentrandoci sul comportamento degli interferometri a singola particella. Studiamo nello specifico implementazioni sia con quantum point contact che valley beam splitter, questi ultimi fondamentali per la valleytronics, e analizziamo l'effetto della struttura dei canali di edge sulla visibilità.
Nella seconda parte della tesi, studiamo i qubit di spin di lacuna integrati in dispositivi in silicio. Un grande vantaggio di questo approccio è la scalabilità dei dispositivi in silicio compatibili con le risorse in commercio. Infatti, questi offrono un alto grado di controllo sull'integrazione e la manipolazione dei qubit, fino anche a 4 K di temperatura. Inoltre, il rumore dovuto all'interazione iperfine è ridotto grazie all'abbondanza naturale di isotopi senza spin nel silicio. Le lacune mostrano specificamente un accoppiamento spin-orbita molto più grande degli elettroni, consentendo così una manipolazione dei qubit completamente elettrica. Lo stato dell'arte è illustrato nel Capitolo 4, mentre descriviamo il nostro quadro teorico, basato sull'approccio di Luttinger-Kohn, e altri modelli nel Capitolo 5. Nel Capitolo 6 studiamo il possibile utilizzo dei qubit di spin di lacuna per applicazioni di quantum sensing, e in particolare charge sensing. Sfruttiamo la sensibilità di un double quantum dot alla presenza e alla posizione di una carica remota, e come questa viene riflessa nello stato del qubit e nelle sue frequenze caratteristiche. Consideriamo sia un approccio statico che dinamico e generalizziamo i nostri risultati attraverso un modello di Hubbard a due siti. Inoltre, troviamo le espressioni generali delle Fisher information dopo uno schema di Rabi o Ramsey. Infine, nel Capitolo 7 esploriamo la possibilità di sfruttare per la manipolazione i primi due stati oltre il sottospazio del qubit. Questi sono molto più sensibili al grado di anisotropia del sistema e sono di conseguenza molto più manipolabili, consentendo un miglioramento di diversi ordini di grandezza sulle frequenze del qubit.Quantum information processing holds significant long-term potential in several areas, including computation and communication. However, despite the rapid progress of the quantum community, practical realizations are still limited. The main objectives yet to be achieved include the realization of powerful, universal quantum processors with a sufficient number of qubits, and their connection through suitable quantum channels. Promising perspectives in these respects are offered by the use of semiconductor nanostructures, due to their compatibility with classical circuitry and with the advanced manufacturing techniques of modern devices.
The first part of this Thesis is devoted to Mach-Zehnder interferometers in the integer quantum Hall regime in graphene, as a possible platform for the implementation of flying qubits. In electron quantum optics, the presence of a strong and tunable Coulomb interaction allows for a straightforward implementation of single- and two-qubit gates. Additionally, the chirality of edge channels forbids back-scattering and allows for long coherence lengths of carriers generated by single-electron sources. In graphene specifically, coherence is further enhanced by the linear dispersion of massless Dirac fermions. In Chapter 1 we analyze the state of the art on flying qubits and their implementation with graphene. In Chapter 2 we show how to treat single carriers in graphene nanoribbons analytically, by taking into account both the sublattice and the valley pseudospins. Then, we explain how to simulate their time-dependent evolution through the split-step Fourier method. In Chapter 3, we characterize the motion of Gaussian electron wavepackets inside graphene nanoribbons, focusing on the behavior of single-particle interferometers. We study implementations with either quantum point contacts or valley beam splitters, the latter being fundamental for valleytronics, and analyze the effect of the edge-channel structure on visibility.
In the second part of the Thesis, we study hole-spin qubits integrated into Si-based devices. A significant advantage of this approach is the scalability of foundry-compatible Si devices. Indeed, these offer a high degree of control on qubit integration and manipulation, even at temperatures as high as 4 K. Additionally, noise due to the hyperfine interaction is reduced thanks to the natural abundance of spinless isotopes in Si. Holes specifically show a much larger spin-orbit coupling than electrons, thus allowing for an all-electrical qubit manipulation. The state of the art is illustrated in Chapter 4, while we describe our theoretical framework, based on the Luttinger-Kohn approach, and other models in Chapter 5. In Chapter 6 we report on the possible use of hole-spin qubits for quantum sensing applications, specifically for charge sensing. We exploit the sensitivity of a double quantum dot to the presence and position of a remote charge, and how it is reflected into the qubit state and its characteristic frequencies. We consider either a static or dynamic approach, and generalize our results through a generic two-site Hubbard model. In addition, we find the general expressions of the Fisher information after both a Rabi and Ramsey scheme. Finally, in Chapter 7 we explore the possibility of exploiting the first two states beyond the qubit subspace for manipulation. These states are much more sensitive to the system’s degree of anisotropy and are consequently much more tunable, allowing for an enhancement of several orders of magnitude on the qubit’s frequencies
Time-dependent transport in Graphene Mach-Zender Interferometers
Get PDFGraphene nanoribbons provide an ideal platform for electronic interferometry
in the Integer Quantum Hall regime. Here, we solve the time-dependent
four-component Schroedinger equation for single carriers in graphene and expose
several dynamical effects of the carrier localization on their transport
characteristics in pn junctions. We simulate two kinds of Mach-Zender
Interferometers (MZI). The first is based on Quantum Point Contacts and is
similar to traditional GaAs/AlGaAs interferometers. As expected, we observe
Aharonov-Bohm oscillations and phase averaging. The second is based on Valley
Beam Splitters, where we observe unexpected phenomena due to the intersection
of the Edge Channels that constitute the MZI. Our results provide further
insights into the behavior of graphene interferometers. Additionally, they
highlight the operative regime of such nanodevices for feasible single-particle
implementations.Comment: 14 pages, 7 figure
Quantum estimation and remote charge sensing with a hole-spin qubit in silicon
Get PDFHole-spin qubits in semiconductors represent a mature platform for quantum
technological applications. Here we consider their use as quantum sensors, and
specifically for inferring the presence and estimating the distance from the
qubit of a remote charge. Different approaches are considered - based on the
use of single or double quantum dots, ground and out-of-equilibrium states,
Rabi and Ramsey measurements - and comparatively analyzed by means of the
discrimination probability, and of the classical and quantum Fisher
information. Detailed quantitative aspects result from the multiband character
of the hole states, which we account for by means of the Luttinger-Kohn
Hamiltonian. Furthermore, general conclusions can be drawn on the relative
efficiency of the above options, and analytical expressions are derived for the
Fisher information of a generic qubit within the Rabi and Ramsey schemes.Comment: 16 pages, 14 figure
Adhesion, Friction and Tribochemical Reactions at the Diamond-Silica Interface
Get PDFDiamond-based coatings are employed in several technological applications,
for their outstanding mechanical properties, biocompatibility, and chemical
stability. Of significant relevance is the interface with silicon oxide, where
phenomena of adhesion, friction, and wear can affect drastically the
performance of the coating. Here we monitor such phenomena in real-time by
performing massive ab initio molecular dynamics simulations in tribological
conditions. We take into account many relevant factors that can play a role,
i.e. the diamond surface orientation and reconstruction, silanol density, as
well as, the type and concentration of passivating species. The large systems
size and the long simulations time, put our work at the frontier of what can be
currently done with fully ab initio molecular dynamics. The results of our work
point to full hydrogenation as an effective way to reduce both friction and
wear for all diamond surfaces, while graphitization is competitive only on the
(111) surface. Overall we expect that our observations will be useful to
improve technological applications where the silica-diamond interface plays a
key role. Moreover, we demonstrate that realistic and accurate in silico
experiments are feasible nowadays exploiting HPC resources and HPC optimized
software, paving the way to a more general understanding of the relationship
between surface chemistry and nanoscale-tribology
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
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
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe 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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