Committee:
     PRESIDENT: AVEZOV, EDWARD
     SECRETARI: KRIEG, MICHAEL
     VOCAL: GERSHLICK, DAVID
Thesis abstract: Intracellular trafficking, particularly protein secretion, faces numerous unresolved challenges. This thesis aims to provide and evaluate tools for quantitative investigation of these processes using fluorescent microscopy. Quantitative analysis offers two main benefits: detailed characterization of molecular dynamics for mechanistic understanding and objective measurements for accurate comparisons across experiments. In Chapter 1, we introduce the secretory pathway, a cellular pathway responsible for the synthesis, processing, sorting and delivery of secretory proteins to the extracellular environment. In Chapter 2, we provide a thorough description of the methodologies used in this thesis. They include various fluorescence microscopy techniques, automated image analysis, and biological methods tailored to the secretory pathway. The tools were selected to achieve high spatial and temporal resolution, enable quantitative analysis, and allow live-cell characterization. In Chapter 3, we used fluorescence imaging to objectively evaluate results in four projects addressing protein secretion and intracellular trafficking. These included quantifying colocalization and proximity of structures, measuring fluorescent intensity differences, and characterizing dynamics of particles like ERGIC-derived nanotubules. Consistent sample preparation and image acquisition, coupled with computational analysis, are crucial for accurate, unbiased results.Chapter 4 focuses on single-particle tracking (SPT) in the secretory pathway, proposing control experiments and parameter descriptors to maximize data quality. We emphasized labeling strategies, imaging, and data analysis considerations for reliable results.Chapter 5 applied these methodologies to study protein sorting at the TGN, examining the role of ER-Golgi membrane contact sites (MCS) in TGN-derived carrier biogenesis. Using super-resolution fluorescence microscopy, we identified cargo accumulation regions and conducted SPT experiments, revealing confined, slow motion of cargo proteins near MCS. This effect was inhibited by the lipid transfer blocker 25-HC, indicating upstream regulation of cargo localization preferences by MCS.

Committee:
     PRESIDENT: BUSQUETS FACIABÉN, ALBERT
     SECRETARI: ARTIGAS GARCIA, DAVID
     VOCAL: ARMENDOLA, CATERINA
Thesis abstract: Hybrid diffuse optical devices offer a non-invasive and continuous and cost-effective method for monitoring cerebral blood flow and metabolism on the bedside use and realistic simulation applications. The incorporation of diffuse correlation spectroscopy (DCS) and near-infrared spectroscopy (NIRS) in these devices extends their versatility. This PhD project focused on utilizing diffuse optics to assess brain activity during functional and stress tests in older populations.Ageing is the primary risk factor for various brain conditions such as stroke, cognitive disorders, and mobility issues. As the population becomes increasingly older, these age-related pathologies are becoming a significant social and economic burden. The underlying assumption is that microvascular damage and changes in brain blood flow regulation contribute significantly to an increased risk of cerebrovascular diseases, cognitive and mobility disorders. This underscores the importance of creating a widely accessible monitoring system and associated protocols able to detect these changes early on, ultimately leading to personalised interventions. Two multi-disciplinary studies were performed during my doctorate studies to identify alterations in the haemodynamic parameters of older adults in response to existing pathologies.Microvascular cerebral blood flow (CBF) in a cohort of younger and older adults (>65 y.o.) with and without mild cognitive impairment (MCI) in overall good health was monitored during functional and stress tests. It was observed that CBF of older adults with MCI could not recover to baseline conditions compared to younger participants indicating possible autoregulation and vasoreactivity problems similar to those previously observed in chronic sleep apnea and chronic carotid stenosis patients. CBF measurements during functional cognitive tasks revealed gender differences. For a given test MCI participants presented a statistically higher response than normocognitive (NC) subjects. The combination of these results favour the "inefficiency hypothesis" that suggests that older adults activate the brain networks as NC individuals to cope with behavioural demands but with increased activity. A new hybrid diffuse optics device was developed combining a custom-made fast-DCS with a commercial NIRS device along with external devices for physiological signal recordings in the second study. The project aimed to measure changes in cerebral haemodynamics in older adults with Motoric Risk Syndrome (MCR) during functional cognitive and motor tasks protocols to evaluate the pre-post impact at 3 and 6 months of physical exercise alone or combined with transcranial direct current stimulation (tDC). Results revealed higher CBF but not oxy-haemoglobin (HbO2) responses in dual tasks (DT) compared to single (ST). There were no differences between groups at baseline and 3 months but statistically different responses in CBF were observed at 6 months for both intervention groups compared to the control group but not in HbO2 response, indicating that intervention affects CBF response possibly due to improvements of vascular health, highlighting the importance of physical activity and transcranial stimulation on the maintenance of vascular health. A big part of my research focused on the development of new algorithms for de-contaminating the measured data from extracerebral signal to develop an optimal model to minimise the effect for both studies. In summary this study proves the capability of hybrid optics to capture the evoked haemodynamic responses in the pre-frontal cortex and offers insights into the use of techniques to assess cognitive function in older adults, specifically those with MCI and MCR. The findings highlight the complex relationship between blood flow responses and cognitive activities suggesting that compensatory mechanisms may play a role in individuals facing cognitive challenges. Future research in these areas holds promise, for enhanc

Committee:
     PRESIDENT: TURA I BRUGUÉS, JORDI
     SECRETARI: LEWENSTEIN, MACIEJ
     VOCAL: MANCINSKA, LAURA
Thesis abstract: In the beginning of the last century, we witnessed a change of paradigm in how physics described the world with the formulation of quantum mechanics. This new theory shook the pillars of science by setting fundamental limits on our ability to describe nature. It was able to explain the laws that govern physics at the microscopic level, which could not be explained by means of the existing laws. The behavior at such small scales differs significantly from our daily experience. For instance, exotic phenomena such as entanglement or nonlocality are exclusively observed at the microscale. Entanglement and nonlocal correlations represent two essential resources in quantum information processing, enabling novel tasks that are unattainable within a classical framework.The end of the twentieth century has seen a wave of studies on the fundamental properties of quantum theory. Nowadays, as a consequence of these advances in quantum theory and experiments, various companies are selling devices claimed to perform a quantum information task with no classical analog, such as quantum random number generators, prototypes of quantum computers, or quantum key distribution devices. Since quantum devices cannot be simulated classically, it is hard to verify them using only classical resources, which are the ones available to the average user. Hence, a natural question to ask ourselves is how we can verify the properties and functioning of quantum devices in an efficient way.In this context, device-independent protocols have been developed in quantum information theory over the past decade. The main advantage of such protocols is that users do not have to make any assumption about the inner workings of their devices, considering them as black boxes. The security and success of a device-independent protocol relies on the observation of nonlocal correlations in a Bell experiment. This thesis is dedicated to provide tools to achieve the certification of quantum information devices or tasks in a device-independent way.In the first part of this thesis, we focus on certifying the security of device-independent quantum key distribution. To this end, we first study whether Bell nonlocality is a sufficient condition for security in the most used protocols, proving that there exist nonlocal correlations that are not useful for secure device-independent quantum key distribution using these protocols. Moreover, we study noisy scenarios, that is when experimental imperfections are present, and derive upper bounds on the two-way and one-way key rates for this kind of protocols.In the second part, we study self-testing, which is one of the simplest device-independent protocols. Its goal is to recover quantum states solely from the observed measurement correlations. In the majority of quantum information processing tasks one needs to consider a particular quantum state, making the certification of quantum states of great importance in the device-independent paradigm. We prove that all multipartite states of qubits can be self-tested. Moreover, we study self-testing in higher-dimensional systems.Finally, in the third part of this thesis, we tackle the problem of certification of entanglement. It is well known that certifying the presence of entanglement in a system is a hard task. The key methods for entanglement detection, entanglement witnesses and positive maps, rely on our understanding of the mathematical features of multilinear algebra. By using the fact that any separable state is one to one related to a matrix inequality, we port previously known results on the entanglement of states with positive partial transpose into the domain of matrix inequalities, which also allow us to translate multilinear positive maps back into entanglement witnesses. This approach leads to a unified treatment of a large class of matrix inequalities, allowing us to find new inequalities on the basis of advances in entanglement theory.

Committee:
     PRESIDENT: ECKARDT, ANDRÉ
     SECRETARI: ACÍN DAL MASCHIO, ANTONIO
     VOCAL: BAÑULS POLO, MARIA CARMEN
Thesis abstract: When matter is cooled to temperatures near absolute zero, its quantum nature begins to emerge. The interactions between its microscopic constituents can then lead to the emergence of fascinating physical properties. While the framework of spontaneous symmetry breaking has been incredibly successful in describing how a macroscopic number of particles cooperate to give a system its properties, there are manysituations where this is not sufficient to describe quantum systems. This is especially true for strongly interacting many-body systems.In recent years, multiple techniques have been developed to address this problem. On the one hand, the incredible advances in classical computing hardware and algorithms, have made it possible to study systems with a number of elementary components that were unimaginable just a few decades ago. In particular, the development of techniques such as tensor networks has unified the framework of quantum information with condensed matter physics, making it possible to optimize the computational complexity of simulating a system, based on its entanglement content.On the other hand, the development of platforms to directly perform simulations on quantum systems is a highly sought objective. While a hypothetical universal quantum computer could dramatically increase our understanding of the quantum nature of matter, its difficult development makes it essential to study analog platforms where specific many-body models can be studied directly in a controlled environment. In these quantum simulators, novel quantum phenomena can be studied in an environment free of disorder, with excellent control over parameters and measurement capabilities.In this thesis, we aim to explore these two paths to study some of the most relevant active topics in physics beyond the symmetry breaking paradigm. In the first part, devoted to topology, we propose and analyze new techniques for the detection of topological excitations. We start by proposing a protocol to detect anyons, quasiparticles that do not behave either as bosons or fermions, in Fractional QuantumHall Effect systems through measuring the angular momentum of impurities binding to the anyons. We then show how similar excitations can be identified in topological superconductors through an interaction between the electromagnetic field of a strong laser pulse and the system in a process called High Harmonic Generation In the second part, we move to the study of quantum frustration. This phenomenon,which describes a situation in which various constraints of the system cannot be satisfied simultaneously, can lead to the emergence of unexpected phases of matter. In particular, we study how frustrated phases and a particular class of quantum critical points, called deconfined can emerge in one-dimensional frustrated systems, potentially realizable in quantum simulators. We then study how frustrationcould explain the onset of superconductivity mixed with charge density modulations in two-dimensional strongly-correlated systems.

Committee:
     PRESIDENT: LEVITT, MALCOLM HARRIS
     SECRETARI: GARCÍA DE ARQUER, FRANCISCO PELAYO
     VOCAL: SAUER, KAREN LOUISE
Thesis abstract: This thesis describes theoretical background, simulations, experimental apparatus and measurements of nuclear spin dynamics via optically pumped magnetometers in unconventional magnetic field regimes. It is divided into four parts: Magnetometry, Nuclear Magnetic Resonance Spectroscopy, Nuclear Relaxation Dispersion, and Nuclear Spin Control, each looking at different aspects of this topic.The magnetometry section describes how through integration of techniques from DC spin-exchange relaxation-free and rf magnetometers, a widely tunable magnetometer is developed that offers a nearly flat response from DC up to few kHz with a sensitivity of less than 20 fT √Hz. Within this range, it surpasses the capabilities of inductive detection methods and eliminates the necessity for cryogenic temperatures that are required for superconducting quantum interference devices (SQUIDs).The subsequent part employs the magnetometer for conducting nuclear magnetic resonance spectroscopy experiments involving coupled nuclear spin systems. A comprehensive analysis is undertaken to ascertain the optimal magnetic field that yields the most precise determination of the J-coupling constant. It is shown that for some systems the ultra-low field regime offers advantages compared to the zero- and high-field regime.A key factor in choosing the optimal field is the nuclear spin relaxation’s strong field dependency, explored in the thesis’s third part. This section thoroughly examines this subject in the unconventional ultra-low field range, discussing long-lived coherences and the impact of long correlations in molecular dynamics. The thesis experimentally investigates this by adapting the established fast-field cycling method to ultra-low fields and combining it with optical detection.The thesis’ s final part focuses on enhancing nuclear spin dynamics manipulation through advanced methods that ensure selective, efficient, accurate, and fault-tolerant spin control. Ultra-low fields possess unique attributes, making even basic techniques like spin-selective resonant pulses challenging to implement. To address this, novel concepts were devised, enabling effective spin control in the ultra-low field range, rivaling or surpassing high-field counterparts. The efficiency of theseimproved pulse sequences is demonstrated in dynamical decoupling, polarimetry, and spectral filtering experiments.

Committee:
     PRESIDENT: TAVAKOLI, ARMIN
     SECRETARI: LEWENSTEIN, MACIEJ
     VOCAL: VENKATESH, VILASINI
Thesis abstract: Understanding the cause and effect relationships behind observed correlations is central to how we reason and interact with the world. Causal relationships help us make sense of the patterns we observe and predict what interventions in nature might lead to a desired outcome. These patterns can be mathematically framed as the joint probability distribution of a set of classical random variables which capture information gathered from the environment. This information may range from abstract data, like survey response statistics, to physical events, such as the probability of triggering a photon detector. A fundamental question is that of causal compatibility: Are the observed correlations compatible with a given causal explanation? A causal explanation can be expressed in terms of causal models, which can be systematically studied with the tools provided by the field of causal inference. Causal models consist of observable random variables with known probability distributions and latent variables with unknown distributions which, together, explain observed correlations through causal influences, that is, functional relationships between the values of these variables. Quantum theory---one of the most accurate theories at a fundamental level---is inherently probabilistic. Measurement results are, therefore, represented as random variables. This naturally leads to causal analysis: Which cause and effect relationships can explain observed measurement statistics in a quantum experiment? One of the simplest quantum experiments is that of two distant parties performing space-like separated, independently chosen measurements on a shared quantum state. In 1964, John Bell showed that in this experiment quantum theory predicts correlations that defy any classical common-cause explanation through a result known as Bell's Theorem. This phenomenon is known as Bell nonlocality. This thesis aims to operationally characterize the fundamental differences between classical and quantum theories within causal scenarios beyond Bell's common-cause scenario. Such an understanding may eventually help integrate quantum phenomena into a coherent, conceptually clear framework of causality. Towards this goal, we explore how classical and quantum causal models diverge in operational tasks in specific causal scenarios. We focus on simple scenarios that go beyond Bell's, while seeking to discover new forms of quantum advantage that are fundamentally different from traditional Bell nonlocality. Our goal is to link these new forms of quantum advantage to different nonclassical features of quantum theory and study their potential applications. A critical component of this research is testing for the causal compatibility of specific correlations with a given causal model. As such, an important part of this thesis is dedicated to expanding and refining the scope of current methods for testing causal compatibility.

Committee:
     PRESIDENT: COELHO MESQUITA, RICKSON
     SECRETARI: CAMPELO AUBARELL, FELIX
     VOCAL: GOROSTIZA LANGA, PAU
Thesis abstract: Brain is a crucial organ that controls all body functions. Its health is influenced by multiple factors, and any abnormality in them can negatively impact its smooth and seamless functioning, causing pathological conditions. So, tools and techniques have also been developed in order to explore its structure and functions. Due to the complexity of the brain, its study comprises of a highly diverse field of research. This doctoral work is based on a particular category of this field that focuses on development and usage of neuromonitoring tools to visualize its activity and status. Optical imaging is an ever growing and robust neuromonitoring methodology that includes a range of techniques, which monitor brain activity by tracking cerebral hemodynamics, which is known to form a close relationship with neural activity due to 'neurovascular coupling’. The primary focus of this doctoral study is to develop new multi-modal neuroimaging modalities to explore the complex activities that occur within the brain. Undoubtedly, 'neurovascular coupling’ is a fundamental concept that depicts the link relating neural activity and hemodynamics, and this link needs to be fully understood in order to interpret on brain status or function during health or pathology. This coupling has always been a crucial aspect for exploration by the neuroscientists. In this doctoral study, investigation of the neurovascular coupling was carried out during a specific brain state when slow wave activity prevails in the cerebral cortex, which is observed during non-repetitive eye movement sleep or deep anesthesia. This cortical activity is related to vital functions of the brain to maintain its health, and its disruption leads to pathologies related to sleep and cognition.In this doctoral study, cortical slow wave activity was investigated by simultaneously monitoring neural activity and hemodynamics, by building a platform that consists of synchronized electrophysiology and optical imaging systems for experiments on rodents. Spatial and temporal assessment of neurovascular relationship was carried out during both spontaneous unperturbed cortical state during slow wave activity and during externally perturbed state. It was observed that the neuronal firing periods of this cortical activity lead to a hike in the cerebral blood flow during both spontaneous and evoked states, and this response was quantified in detail. In addition, hemodynamic response function was plotted for further understanding. Note that both electrophysiology and optical imaging were able to continuously and simultaneously monitor brain activity at multiple cortical locations over a large region of the rodent brain, and so, helped in carrying out spatial comparisons.In addition to this project, this doctoral study also involved development of a hybrid diffuse optics-based tomographic system that can simultaneously monitor both cerebral blood flow and cerebral blood oxygenation. It can execute tomographic monitoring as both high density-speckle contrast optical tomography (HD-SCOT) and high density- diffuse optical tomography (HD-DOT) systems in parallel. This The high density-speckle contrast optical tomography and diffuse optical tomography (HD-SCOT/DOT) device was developed to explore brain activity and address complex brain functions related to the neurovascular unit during preclinical experimental studies in small animal models. It is a novel hybrid diffuse optics-based device, and its development during this doctoral study included all related tasks ranging from building the device instrumentation to developing its control system and user-device interface. The functionality of this device was also tested through experiments on tunable tissue-mimicking liquid phantoms and through in-vivo experiments on rodents. This device has the potential for usage in studies focused on investigation of oxygen metabolism in the brain in future.

Committee:
     PRESIDENT: MAZZANTI CASTRILLEJO, FERNANDO PABLO
     SECRETARI: PRUNERI, VALERIO
     VOCAL: DELLANTONIO, LUCA
Thesis abstract: With applicability on almost every aspect of our lives, optimization problems are ubiquitous to a broad range of fields within both scientific research and industrial environments. As such, these are growing in size and complexity at a fast pace, and are only expected to continue to do so. Accordingly, the urgency for better methods that can yield more optimal solutions in shorter times is increasing and, while the development of quantum computing technologies that are capable of tackling these problems evolves steadily, it does so too slowly for the challenges that nowadays society's demands represent. Consequently, a lot of effort is being invested to further develop classical methods and machines that are specially designed to solve optimization problems of relevant enough sizes. The present thesis is framed within this paradigm: classical optimization techniques are studied from various different perspectives, with the goal of improving their efficiency. To this end, we first dive into basic concerns related to the physical properties of the systems that allow for the convenient formulation of industrially-relevant optimization problems, namely spin glasses with quenched disorders. The understanding of such properties is of utmost importance for the correct designing of the annealing schedules used by thermally-based optimization methods. We then study the impact that the hidden correlations of the pseudo random number streams used in their simulations have in the results by comparing simulations using PRNGs of various qualities and perfectly random QRNGs. To conclude, we investigate novel ways, inspired by quantum-mechanical systems, to efficiently navigate the energy landscapes of spin glasses in classical algorithms, which has the potential of preventing the simulations getting stuck into local energy minima and thus reaching more optimal solutions.

Committee:
     PRESIDENT: PICAUD, SERGE
     SECRETARI: KRIEG, MICHAEL
     VOCAL: ACCANTO, NICOLÒ
Thesis abstract: Studying vision restoration is paramount for addressing degenerative blinding diseases, which significantly impact quality of life and public health. With more than 230M people worldwide affected by moderate to severe vision loss, and an estimated increase in blindness from 38M to 115M by 2050, the urgency for effective treatments is clear. Vision, being the most complex and crucial human sense, relies on an intricate network of structures. Light is captured by photoreceptors in the retina and translated into neural signals processed by the brain, enabling sight. Degenerative diseases often involve the progressive deterioration of photoreceptors, leading to blindness. Currently, there is no cure but various approaches are being researched to restore sight. These include gene and cell therapies targeting diseased tissue, as well as methods like optogenetics and neuroprosthetics to modulate neuron activity and bypass dysfunction. By understanding and manipulating neural activity, scientists aim to restore vision or slow down the degenerative processes.The results of this PhD thesis highlight progress across four areas for vision restoration research. Chapter 2a explores a new retinal implant using biocompatible reduced graphene oxide microelectrodes, demonstrating its ability to record neural signals from retinal tissue and capture light-induced firing patterns in retinal ganglion cells. It also details a protocol for retinal calcium imaging, facilitating future studies combining electrical stimulation with optical techniques. Chapter 2b focuses on adapting an ophthalmoscope for in vivo, single-cell resolution imaging of the retina in rodents, aiming to set the basis for future implementations to monitor the functionality of implanted retinal prostheses. Chapter 3 delves into the potential of human-induced pluripotent stem cell-derived retinal organoids for vision research. Using advanced imaging techniques, researchers successfully observed the formation of distinct retinal cell types within the organoids and identified differences in calcium dynamics between healthy and diseased models. Future work aims to refine MEA recordings and investigate the link between retinal organoid structure and function. Chapter 4 introduces a novel three-photon excitation technique offering deeper brain penetration and higher precision compared to traditional methods. This approach successfully regulates neuronal activity in zebrafish with minimal light exposure, showcasing its potential for revolutionizing the study of neural circuits and development of neuromodulation therapies.Taken together, these advancements across retinal implant design, in vivo monitoring of the retina, studying the potential of patient-derived organoid models, and development of non-invasive three-photon brain stimulation techniques, pave the way for future development of more effective vision restoration therapies.

Committee:
     PRESIDENT: WEITZ, THOMAS
     SECRETARI: RUBIO VERDÚ, CARMEN
     VOCAL: RIBEIRO PALAU, REBECA LISSETTE
Thesis abstract: In 2018, following a theoretical prediction from 2011, it was found that stacking two layers of graphene with a relative twist angle of 1.1° between them leads to multiple new properties. At this so-called magic angle, the electronic band structure of the material reconstructs, creating a narrow flat band at the Fermi level. The formation of a flat band enhances electron-electron interactions, resulting in the emergence of states of matter not present in the original graphene layers, including correlated insulators, superconductivity, ferromagnetism and non-trivial topological states. The understanding of the origin of these correlated states could help unravel the physics of highly correlated flat band systems which could potentially provide key technological developments. The main objective of this thesis is to study magic-angle twisted bilayer graphene (MATBG) by creating monolithic gate-defined Josephson junctions. By exploiting the rich phase space of the material, we can create a Josephson junction by independently tuning the superconductor and the weak link state. Studying the Josephson effect is a first step towards understanding fundamental properties of a superconductor, such as its order parameter. First, we have optimized the fabrication of these gate-defined junctions made of all van der Waals materials. We have made double-graphite-gated hBN encapsulated MATBG devices where the top gate is split into two parts via nanolithography techniques. This configuration allows to independently control the three regions of the Josephson junction (superconductor, weak-link and superconductor). Then, we have studied the gate-defined Josephson junctions via low-temperature transport measurements. After demonstrating the Josephson effect in the fabricated devices, we focus on the behavior of one of these junctions in great detail. In particular, we have observed an unconventional behavior when the weak link of the junction is set close to the correlated insulator at half-filling of the hole-side flatband. We have observed a phase shifted Fraunhofer pattern with a pronounced magnetic hysteresis, characteristic of magnetic Josephson junctions. To understand the origin of the signals, we have performed a critical current distribution Fourier analysis as well as a tight binding calculation of a MATBG Josephson junction. Our theoretical calculations with a valley polarized state as the weak link can explain the key signatures observed in the experiment. Lastly, the combination of magnetization and its current-induced magnetization switching has allowed us to realize a programmable zero-field superconducting diode.Finally, we have shown the flexibility of these devices by studying a MATBG p-n junction under light illumination. We have studied the relaxation dynamics of hot electrons using time and frequency-resolved photovoltage measurements. The measurements have revealed an ultrafast cooling in MATBG compared to Bernal-bilayer from room temperature down to 5 K. The enhanced cooling in MATBG can be explained by the presence of the moiré pattern and corresponding mini-Brillouin zone. In summary, we have demonstrated that by integrating various MATBG states within a single device, we can gain a deeper insight into the system's properties and can engineer innovative, complex hybrid structures, such as magnetic Josephson junctions and superconducting diodes.

Committee:
     PRESIDENT: SANTOS SANCHEZ, LUIS
     SECRETARI: CHANG, DARRICK
     VOCAL: HAUKE, PHILIPP HANS-JÜRGEN
Thesis abstract: In recent years, the evolution of quantum technologies has resulted in an unprecedented control over individual quantum particles and many-body systems. This remarkable progress has given rise to a new era, marked by the convergence of classical and quantum methodologies to investigate fundamental questions concerning the nature of quantum matter, improving our understanding of the role of entanglement in solid-state systems or the mechanisms behind high-energy physics. From analog quantum simulators to digital quantum computers, these advancements hold potential across diverse domains.This thesis explores the interplay between classical and quantum methods in understanding topological phases of matter. We concentrate on three distinct directions: non-conventional topological superconductors, interaction-induced topologicalphases in ultracold atom quantum simulators, and applications of variational quantum algorithms. Each trajectory relies on the combination of different techniques with the aim of understanding and characterizing topological phenomena indifferent settings.Exploring non-conventional topological superconductors involves extending the paradigmatic Kitaev chain model by incorporating additional terms in the Hamiltonian such as long-range interactions and quasi-periodic potentials. This investigation is relevant to better understand the impact of real-world conditions on the both the topological and localization properties of systems hosting non-local Majorana modes, which are promising candidates for topological quantum computation.In the realm of interacting systems, we explore the realization of interaction-induced topological phases in systems of ultracold atoms in optical lattices, both in one and two dimensions. The remarkable control and versatility of such platforms enable the simulation of both theoretical topological models and strongly correlated physics.Notably, the interplay between interactions and topology can give rise to intriguing phenomena, such as delocalized fractional charges and gapless topological phases,challenging existing intuition. We employ advanced numerical methods based on tensor networks to benchmark the experimental proposals that open the door to the realization and detection of novel many-body phases of matter, including topological quantum critical points and a higher-order topological Peierls insulator in Bose-Hubbard models with long-range interactions.Variational quantum algorithms, conversely, have the potential to efficiently tackle a wide range of problems, including ground state search, phase classification or accessing topological invariants. Despite current limitations in trainability and scalability, these hybrid classical-quantum algorithms provide practical insights into current quantum hardware capabilities and can inspire future architectures. We explore the application of variational quantum algorithms to shed light on topological phenomena, raising questions about their ability to discern topological phase transitions and compute topological invariants in situations where classical approaches fail.This thesis presents a comprehensive exploration of distinct approaches to topological quantum matter by leveraging quantum technologies and quantum-inspired classical algorithms. Our results not only advance our understanding of quantumsystems but also pave the way for the realization and discovery of novel physics extending to quantum information processing, materials science, and beyond.

Committee:
     PRESIDENT: MANNELLI, ILARIA
     SECRETARI: GARCÍA DE ARQUER, FRANCISCO PELAYO
     VOCAL: ST CLAIR, TODD PARRISH
Thesis abstract: The properties of ultrathin materials present exciting opportunities to develop multifunctional surfaces. In addition, the use of plastic and thin glass as transparent substrates has the potential to extend the use of ultrathin materials beyond conventional substrates and provide vital advancements to existing and emerging technologies across a wide range of sectors. One of the main challenges facing next-generation transparent substrates is the substantially reduced temperature processing window which is not compatible with materials requiring high fabrication temperatures. This thesis describes the development of fabrication techniques to obtain ultrathin materials on low thermal budget transparent substrates to create surfaces with advanced optical and biological functionalities. More specifically, this thesis describes: • A novel, low temperature transfer technique onto flexible substrates for ultrathin materials such as graphene, MoS2 and nanostructured metals that were previously grown at a much higher temperature. The universality of the method extends the use of these ultrathin materials to a wide range of technologically relevant substrates such as cover glass for display modules and polymeric substrates for next generation foldable and bendable electronics. • A novel approach to naturally increase electrical conductivity of transparent surfaces based on graphene, without the need of post-treatment, electrical gating or high temperatures. Notably, the method achieves a conductivity of comparable magnitude or greater than what is reported in previous studies. Furthermore, the increase in electrical conductivity is realised simply by utilizing an ion-exchanged substrate, a technologically relevant transparent glass substrate that is widely used in touch screen displays (e.g. smart phones). • A low temperature metal dewetting technique to obtain transparent antimicrobial nanostructured coatings on a cover glass substrate for display modules. The durability of the coatings was evaluated under conditions designed to simulate real-world use cases such as capacitive touch displays. The results show that the coatings were capable of substantially retaining optical properties of the underlying substrate, such as haze, neutral colour, and visible light transmission, as well as retaining antimicrobial properties after repeated contact with external objects such as, for example, when wiping with a towel or cloth, or touching with human fingers.The results of this thesis demonstrate the implementation of ultrathin and nanostructured materials, such as graphene and nanostructured metals onto a wide range of technologically relevant transparent substrates, by methods that are industrially scalable and compatible with low temperature processing. At the same time, surfaces are engineered with advanced optical and biological functionalities that are relevant for applications such as transparent electrodes and antimicrobial coatings.

Committee:
     PRESIDENT: TORRONTEGUI MUÑOZ, ERIK
     SECRETARI: ACÍN DAL MASCHIO, ANTONIO
     VOCAL: TOMZA, MICHAL
Thesis abstract: In this thesis, I focused on introducing tools to quantify the performance of quantum computing algorithms and their applications. The main focus is on two of the most popular application areas of quantum computing, quantum machine learning and quantum chemistry. To this end, I analyze the properties of quantum machine learning models by following statistical method techniques, which can help us build our understanding of the capabilities of such quantum models. Moreover, I introduce the teacher-student scheme as a computational tool to benchmark the performance of different quantum models and their training capabilities. Until large-scale benchmarking is available, these tools can help us understand the potential of quantum machine learning and guide the research in the right direction. Next, in recent years substantial effort have been devoted to the development of quantum algorithms for quantum chemistry applications. I introduce tools to assess the utility of various combinations of quantum chemistry algorithms. I perform extensive numerical simulations on computationally affordable systems of intermediate size to explore how quantum methods can accelerate tasks of quantum chemistry. These works set a foundation from which to further explore the requirements to achieve quantum advantage in quantum chemistry. Finally, I discuss how research in quantum computing has tended to fall into one of two camps: near-term intermediate scale quantum (NISQ) and fault-tolerant quantum computing (FTQC). Through a quantum chemistry application, I explore how to use quantum computers in transition between these two eras, namely the early fault-tolerant quantum computing (EFTQC) regime.

Committee:
     PRESIDENT: FELIX PEREIRA, SILVANIA
     SECRETARI: ARTIGAS GARCIA, DAVID
     VOCAL: VOGL, ULRICH
Thesis abstract: As the world moves towards increasingly miniaturized and complex technologies and devices, the need for imaging and metrology tools for precise material characterization and fabrication process control is rising accordingly. For highly transparent and ultra-thin structures and samples (e.g., optical coatings, lithographic structures or biological cells), intensity-based imaging techniques fall short due to insufficient contrast, as well as failing to provide quantitative information.To overcome these limitations, the field of phase imaging, based on superposition and interference of light, has emerged. In order to create image contrast, phase imaging does not leverage changes in intensity, but rather, as the name implies, changes in the phase of the electro-magnetic wave. With a long-standing history, and Nobel prizes awarded in 1953 to Zernike’s “phase contrast microscope” and 1971 to Gabor’s holographic methods, the field has evolved to “quantitative phase imaging” (QPI), using sophisticated methods and setups to control and manipulate the state of light in order to recover the phase information quantitatively. Herein, the category of “common-path” techniques promises adaptable, compact, robust, and cost-efficient imaging devices, enabling use in industrial applications outside of a well-controlled lab environment.In this thesis, we will describe the development and technological innovations of a “common-path” phase imaging platform based on the “lateral-shearing interferometric microscopy” (LIM) technology. We will implement and adapt the platform to various optical setups, e.g., for large-area lens-free imaging and for high-resolution microscopic imaging. We will also demonstrate the performance and versatility of the platform by exploring a range of applications, with a focus given to material science and manufacturing. Specifically, we will perform volumetric imaging of the tiniest femtosecond laser-written refractive index (RI) changes inside glass. This is followed by the characterization of semi-transparent ultra-thin gold films using multispectral intensity and phase imaging, enabling us to determine the complex RIs of the films of varying thickness. Lastly, we will apply the platform to the imaging of curing grades and RI changes in photopolymers, such as those used in resin-based 3D printing. Further applications of the platform could include surface metrology, imaging of 2D materials, as well as quantitative phase imaging for bio- and cell-imaging applications, with the possibility of integrating the whole platform into a compact add-on which could be added to any commercial microscope. In summary, this thesis will make evident the significant potential of phase imaging in both research and industrial settings, enabled by the proposed compact phase imaging platform. The work builds the foundation for future innovations and developments with a potentially lasting impact on the photonics industry.

Committee:
     PRESIDENT: ACUNA, GUILLERMO
     SECRETARI: VAN HULST, NIEK
     VOCAL: ZIJLSTRA, PETER
Thesis abstract: The ability to study the dynamics of individual biomolecules is crucial to understanding the complex organization of biological systems beyond what can be learned from ensemble averages. These single-molecule dynamics often occur at high micro- to millimolar concentrations, where conventional optical techniques cannot isolate single molecules anymore due to fundamental physical laws. This thesis explores the design, fabrication, and application of advanced nanoantenna platforms to detect individual fluorescent molecules at such high concentrations with increased sensitivity.Here, the theoretical groundwork is provided to understand the interactions between fluorescent molecules and nanoantennas. It is discussed how the single-molecule detection sensitivity of nanoantenna platforms can be quantitatively assessed through analytical models and numerical simulations. Based on these quantitative models, antenna-in-box platforms are identified to provide superior sensing performance and suitable lithography processes for their fabrication are established.Both computational and experimental evidence are presented that cleverly combining materials in hybrid antenna-in-box platforms enhances single-molecule detection sensitivity at micromolar concentrations. This improvement is attributed to decreased background signals and the use of previously unexplored coupling mechanisms inherent in the antenna-in-box architecture. Furthermore, hexagonal close-packed antenna-inbox platforms are introduced to enable highly parallelized single-molecule detection at micromolar concentrations. Notably, these hexagonally ordered platforms constitute the first demonstration of antenna-in-box platforms capable of single-molecule detection across the visible spectral range.Lastly, a correlative approach is presented that combines nonlinear fluorescence and vibrational spectroscopy to study the organization of receptor proteins in the cell membrane of living cells using nanoantennas. Measures to protect both the nanoantennas and the living cells are discussed and their effectiveness is validated.Overall, this thesis presents novel approaches for studying single-molecule dynamics at high concentrations with enhanced sensitivity. The development of these approaches was enabled through analytical and numerical modeling, the creation of new fabrication processes, and the use of appropriate experimental methods. These advancements promise to offer previously inaccessible insights into dynamics within biological systems.

Committee:
     PRESIDENT: SCHRECK, FLORIAN EBERHARD
     SECRETARI: DE RIEDMATTEN, HUGUES
     VOCAL: WEITENBERG, CHRISTOF
Thesis abstract: The development of quantum-gas microscopes has revolutionized the field of quantum simulation with ultracold atoms. More specifically, their ability of direct observation and manipulation of degenerate quantum gases in optical lattices on a single particle level has brought novel ways of probing and engineering quantum degenerate many-body systems. So far, most of these setups have focused on alkali atoms. Combining quantum-gas microscopy with the properties of alkaline-earth atoms such as strontium gives rise to exciting research directions. In this thesis, we report on the design and construction of a strontium quantum-gas microscope. The findings in this thesis can be divided into three parts.In the first part, we focus on the accumulation of atoms in the science cell and develop a scheme to enhance the atom number in magneto-optical traps of strontium atoms operating on the 461-nm transition. This scheme resonantly populates a short-lived reservoir state, partially shielding the atomic cloud from losses in the cooling cycle. We demonstrate a factor of 2 enhancement in the atom number for the bosonic isotopes Sr-88 and Sr-84, and the fermionic isotope Sr-87, showing the efficient capture of these isotopes in our experiment. Our scheme can be readily implemented in the majority of strontium experiments, given that the shielding transition at 689 nm is commonly used for further cooling. In our case, the shielding scheme facilitates the generation of Bose-Einstein condensates.The second part of the thesis reports on the generation of degenerate quantum gases of Sr-84 with up to 200000 atoms. After summarizing the required cooling steps, we study the formation of Bose-Einstein condensates during evaporative cooling in our experiment. Analyzing the evolution of the horizontal and vertical size of our quantum-degenerate clouds in free fall leads to the characteristic asymmetric expansion, which we compare to theory for our experimental parameters. We also show the generation of smaller Bose-Einstein condensates of less than 20000 atoms with the help of a light-sheet potential. With this highly-anisotropic confinement we can consider our Bose-Einstein condensates two-dimensional for atom numbers of the order of 1000.In the third part we demonstrate site-resolved imaging of a Sr-84 bosonic quantum gas in a Hubbard-regime optical lattice potential. We confine the quantum gas by a two-dimensional optical lattice and the aforementioned light-sheet potential, both operating at strontium's clock-magic wavelength. A high-NA imaging objective enables single-atom and single-site resolved fluorescence imaging by scattering photons on strontium's broad 461-nm transition, while performing efficient attractive Sisyphus cooling of the atoms on a narrower transition at 689 nm. We reconstruct the atomic occupation of the lattice sites from the fluorescence images, obtaining imaging fidelities above 94%. Finally, we realize a Sr-84 superfluid in the Bose-Hubbard regime and observe its characteristic interference pattern after free expansion in the light sheet with single-atom resolution. Our strontium quantum-gas microscope provides a new platform to study dissipative Hubbard models and cooperative effects in atom-light interaction at the microscopic level. Moreover, the ability to capture also the fermionic isotope Sr-87 paves the way to generate degenerate Fermi gases with SU(N) symmetry and study SU(N) quantum magnetism.

Committee:
     PRESIDENT: MIHI, AGUSTIN
     SECRETARI: GARCÍA DE ARQUER, FRANCISCO PELAYO
     VOCAL: DI STASIO, FRANCESCO
Thesis abstract: Bolometer technology, crucial for uncooled thermal detection in thermography, industrial inspection, monitoring, and surveillance applications, relies on thermistors primarily made of VOx, a-Si, and Si/SiGe QWs materials. Sustained growth demands the exploration of new material platforms and continuous improvements in device development strategies. Recently, the scientific community has recognized colloidal quantum dots (CQDs) as a disruptive technology, highlighted by the Nobel Prize in Chemistry. Their tunable properties have led to the creation of cutting-edge devices such as photodetectors, solar cells, and lasers. Furthermore, the COVID-19 outbreak emphasized the urgent need for affordable and compact thermal sensing devices, underscoring the importance of uncooled thermal detectors for the welfare of humankind. CQDs have been investigated here as a new material platform for infrared (IR) bolometer devices. Following this, the CQD films with varying sizes of the QDs were first examined for their temperature-dependent behaviour, which revealed a size-dependent resistivity and temperature-coefficient of resistance (TCR) and exhibited a sharp rise in these parameters for QDs smaller than ~4.5 nm. The lateral geometry devices thus prepared showed TCR values > 4 %/K but with high resistivity (resistance) values > MΩ (< GΩ). To modulate the activation energy (Ea) and TCR effectively, a potential barrier structure was developed with QDs of different sizes stacked alternatively on top of each other and referred to as a QPBT structure. This configuration allowed precise modulation of the potential energy landscape for the charge carriers. Various geometrical parameters such as barrier height and width, and no. of barrier layers were found to affect its performance. This structure also helped in mitigating the problem of high-pixel resistance with lateral geometry. For IR absorption, a metal-insulator-metal-based plasmonic metamaterial absorber (MIM-PMA) has been developed by utilizing QDs for the dielectric spacer of the structure. Two types of absorber geometries have been studied and the results from FDTD-based simulations and fabricated structures have been compared. The first type of geometry utilizes one type of QDs as a dielectric layer and is designed to fit on top of the QPBT structure to perform photothermal conversion. The second geometry utilizes the QPBT structure itself as the dielectric layer and thus integrates the absorber and thermistor structures to simplify the fabrication process in addition to other distinct advantages. Further, the integration of the QPBT and MA structures has been demonstrated. The ease of integration of CQDs-based components allowed the fabrication of fully functional devices. The completed bolometer devices displayed IR sensing properties through thermal detection and the device current exhibited a peak at a wavelength corresponding to the resonant wavelength of the MA structure implying a successful integration. The devices showed fast time response with ~4 ms time response and peak detectivity around ~10^4 Jones. Although the devices showed lower detectivity than commercial bolometer devices, an improvement in the device performance is expected with further optimization of the device geometry. In summary, this thesis explores the suitability of QDs for bolometer technology, laying the groundwork for expanding QD technology horizons. The work presented here is anticipated to contribute to the continuous advancement and improvement of uncooled IR sensing devices and pave the way for low-cost development and wider dissemination of IR bolometer technology.

Committee:
     PRESIDENT: MARTÍN BLANCO, ENRIQUE
     SECRETARI: DURDURAN, TURGUT
     VOCAL: PEREZ BROWNE, MARCOS FRANCISCO
Thesis abstract: The locomotion of Caenorhabditis elegans (C. elegans) offers a unique platform for studying complex postures and motor behaviors. In this study, I investigated locomotor patterns across different ages and genetic backgrounds of C. elegans, utilizing customized tracking systems and advanced analysis techniques. A comprehensive examination of locomotion behaviors was conducted using the eigenworm approach. Eigenworms are the principal components of the animals’ posture space. I identified specific eigenworms associated with forward movement, turning, and exaggerated bends. Notably, spectrin-mutant animals showed a strong correlation between their bending movements and a specific eigenworm for turning in wild-type animals. These findings suggest that eigenworms offer a universal framework to compare different types of worm movement and assess the effects of mutations. This paves the way for a more informative analysis of worm behavior, especially when combined with studies of neuronal networks.Additionally, I explored the role of proprioception in coordinating motor activities within C. elegans, employing genetic and modeling approaches. The focus of my research was to elucidate the mechanisms underlying proprioceptive feedback, including mechanical stress and neuronal signaling, with a focus on age-related deficits. My findings elucidate that the spectral network associated with a singular proprioceptive DVA interneuron, which modulates tension and compression states, serves as a critical determinant of body posture. Intriguingly, a striking resemblance was observed between animals of early ageing and the mutant animals for β-spectrin, where both animals crawled with exaggerated body bends. Moreover, I show that proprioceptive neurons are found to encode body posture and exhibit age-dependent structural and functional alterations, including protein aggregation and decreased mechanical tension. Notably, spectrin, a cytoskeletal component, emerges as a key player in maintaining proprioceptive integrity during ageing.Furthermore, I investigated the molecular pathways underlying age-associated proprioceptive defects, more specifically, the role CLP-1 protease in the cleavage of UNC-70/β-spectrin in ageing animals. Conditional knockout of clp-1 in DVA interneuron revealed altered locomotor behaviors, along with the pan-neuronal knockout of clp-1. Given the role of spectrin in proprioception through DVA interneuron suggests that clp-1 regulates spectrin in age-related neurodegeneration. Lastly, I explored the effect of ectopic expression of human αβcrystalline on ageing. We hypothesized that αβ-crystallin (HSPB5), a small heat shock protein (sHsp), will stabilize β-spectrin and shield it from clp-1 proteolytic degradation during ageing. I ectopically expressed the constitutively active 3E mutant of αβ-crystallin pan-neuronally or specifically in DVA. Through locomotion analysis of animals from young adult to adult day 6, I observed a modest rescue in the locomotion behavioral pattern in both DVA specific and pan-neuronally expressed αβ-crystallin animals. We speculate that constitutively active αβ-crystallin may bind to proteolytically vulnerable domains/residues of the UNC-70 protein, providing protection against proteases such as clp-1. Collectively, these findings contribute to our understanding of proprioceptive mechanisms in ageing and offer insights into potential therapeutic targets for age-related neurodegenerative diseases.

Committee:
     PRESIDENT: DÜRR, STEPHAN CLAUS
     SECRETARI: CHANG, DARRICK
     VOCAL: ADAMS, CHARLES STUART
Thesis abstract: Photons have emerged as the main candidates for carrying quantum information due to their weak interaction with the environment. Unfortunately, their limited interaction with one another poses challenges for photonic quantum information processing. One of the possible solutions lies in the unique behavior of interacting Rydberg excitations in cold atomic ensembles, where strong nonlinearities enable engineering interactions among individual photons. This phenomenon makes Rydberg ensembles a promising platform for quantum information applications, notably in long-distance quantum communication. This thesis presents a series of experiments that explore and exploit Rydberg-mediated interactions, all with the long-term objective of building an efficient quantum repeater. The thesis begins with a concise theory overview of Rydberg and ensemble physics. This is followed by an explanation of the experimental setup. I discuss how, building upon a previously existing setup, we improved the stability and spectral properties of our laser system, along with enhancing the quality of the atomic ensemble. The introductory section of the thesis concludes with a description of two different single-photon generation methods and an in-depth review of various decoherence mechanisms impacting Rydberg ensemble excitations. The single-photon generation performance has been improved by the modifications implemented in the setup, resulting in higher generation rates and better single-photon purity. Supported by experimental data and a careful analysis of experimental parameters, we identify the most probable sources of significant decoherence and suggest potential strategies for mitigation. In our initial experiment, we achieve the storage and subsequent retrieval of an on-demand single photon. This photon is generated through the collective excitation of Rydberg states in one cold atomic ensemble, and it is stored in a low-noise Raman quantum memory situated in another cold atomic ensemble. Our results show the capability to store and retrieve these single photons while maintaining a high signal-to-noise ratio of up to 26 and preserving strong antibunching characteristics. We also explore the built-in temporal beam splitting capabilities of the Raman memory and successfully use the memory to control the single photon waveshape. In the second experiment, we demonstrate for the first time an interaction and storage of single photons in a highly non-linear medium based on cold Rydberg atoms. We employ the DLCZ protocol in a cold atomic ensemble to create single photons, guiding them to another ensemble for storage in a highly excited Rydberg state under conditions of electromagnetically induced transparency. By studying the statistics of the light retrieved from the Rydberg atoms, we show for the first time single-photon filtering with non-classical input light. Moreover, through Monte Carlo simulation, we get an intuitive understanding of the effect of the (partial) Rydberg blockade upon the Fock state distribution of arbitrary input light pulses. This allows us to conclude that the response of the medium is determined by the input Fock state distribution, what confirms the established understanding of Rydberg ensemble nonlinearity. This demonstration can be seen as a step towards realization of deterministic photon-photon gates based on Rydberg ensembles with single photon inputs. The results presented in this thesis affirm the potential of Rydberg ensembles to become central elements of future quantum networks, both as single photon sources and processing nodes. Furthermore, auxiliary outcomes provide an additional understanding of the Rydberg ensemble physics and offer insight into limitations that we need to overcome to improve further our setup.

Committee:
     PRESIDENT: POCCIA, NICOLA
     SECRETARI: KOPPENS, FRANK
     VOCAL: TÖRMÄ, PÄIVI
Thesis abstract: The study of strongly-correlated matter in two-dimensional materials has emerged as a exciting prospect for the exploration of condensed matter physics, as well as the design of novel device platforms. Moiré engineering , where the 20 layers feature an interlayer twist angle, has proven to be a powerful tool to engineer electronic correlations . In magic angle twisted bilayer graphene, a twist angle of 1.1° between the graphene layers generates a moiré superlattice potential. A flat electronic band appears at the Fermi level, in which a variety of interaction-driven , many-body quantum phases can emerge . Another avenue to study strong electronic correlations in two dimensions is the exfoliation of intrinsically correlated bulk crystals into the atomic limit.The optoelectronic study of strongly-correlated systems in 20 heterostructures stands out as a powerful probe, as it can provide insight into both the electronic transport properties and the fundamental light-matter interaction in these systems . In this thesis , we study two strongly correlated 20 materials: MATBG and the cuprate superconductor Bi2Sr2CaCu208-delta (BSCC0-2212). We leverage different optoelectronic techniques to study the fundamental properties of the correlated electrons in the MATBG flat bands and the potential of two-dimensional BSCC0-2212 layers for applications in quantum sensing .First, we investigate the electronic spectrum of the MATBG flat bands through the study of their thermoelectric transport . We use an optical excitation to induce a thermal gradient, which in turn generales a charge curren!. We report anomalous thermoelectricity which provides strong evidence for the coexistence of localized and de-localized electronic states in the strongly-interacting flat bands.Next, we study the dynamics of hot carrier cooling in the MATBG flat bands using a frequency-resolved photomixing technique . Strikingly, we find that hot carriers can efficiently relax their energy down to cryogenic temperatures ; in contras! to the case of bilayer graphene samples . We propose a novel Umklapp electron-phonon scattering mechanism for hot carriers in MATBG, enabled by the moiré superlattice potential.Lastly, we explore the development of superconducting photodetectors with high-T_c based on ultrathin BSCC0-2212 flakes . We fabricate high quality samples that exhibit remarkable performance at telecom wavelengths . We observe fast and sensitive bolometric response at T = 77 K in free-space and waveguide-coupled devices , as well as single-photon sensitivity at T = 20 K through a non-bolometric , avalanche detection mechanism .

Committee:
     PRESIDENT: VOLPE, GIOVANNI
     SECRETARI: ACÍN DAL MASCHIO, ANTONIO
     VOCAL: VAN NIEUWENBURG, EVERT
Thesis abstract: The integration of artificial intelligence into research is propelling progress and discoveries across the entire scientific landscape. Artificial intelligence tools boost the development of novel scientific insights and theories by processing extensive data sets, guiding exploration and hypothesis formation, enhancing experimental setups, and even enabling autonomous discovery. In this thesis, we harness the power of machine learning, a sub-field of artificial intelligence, to study non-deterministic systems, which are amongst the hardest to characterize.On one hand, we address problems inherent to the study of quantum systems and the development of quantum technologies. Quantum physics presents formidable challenges due to the associated exponential complexity with the size of the system at hand, as well as its intrinsic stochastic nature and the presence of intricate correlations between its components. We employ reinforcement learning, a machine learning technique that excels at dealing with vast hypothesis spaces, to address some of these challenges. Notably, reinforcement learning has demonstrated super-human performance in multiple complex games like Go, which present similar characteristics to the problems encountered in the study of quantum physics. We use it to systematically simplify complex common problems in condensed matter and quantum information processing tasks, as well as to implement robust calibration schemes for quantum computers.On the other hand, we focus on the characterization of complex stochastic processes, such as diffusion. Understanding diffusion processes is crucial to unravel the complex underlying physical and biological mechanisms governing them. This involves extracting meaningful parameters from the analysis of stochastic trajectories described by tracked particles. However, accurately capturing and analyzing the trajectories presents multiple challenges, stemming from the combination of their random nature, complex dynamics, and experimental drawbacks, such as noise. We develop machine learning algorithms to accurately extract such parameters, even when they vary with time, and demonstrate their applicability in experimental scenarios. Furthermore, we apply similar techniques to study the diffusion of internet users browsing an e-commerce website, predicting their likelihood to make a purchase before closing the session.

Committee:
     PRESIDENT: KAMINER, IDO
     SECRETARI: DE RIEDMATTEN, HUGUES
     VOCAL: AHUFINGER BRETO, VERÓNICA
Thesis abstract: The dawn of the last century marked the onset of the first quantum revolution, a period characterized by groundbreaking discoveries culminating in the establishment of quantum mechanics. Over time, the abstract concepts introduced by this new branch of Physics, evolved into indispensable practical devices shaping our daily lives. This technological evolution spurred our actual era, centered around information exchange and acquisition, laying the foundation for what is now termed the second quantum revolution. This phase aims to leverage quantum information science, which harnesses quantum mechanics' properties to propel advancements in information processing, communication, and computation, leading to revolutionary quantum technologies.At the heart of advancing quantum technologies lies the exploration of what are known as non-classical states --physical manifestations exhibiting behaviors diverging from classical physics, necessitating the framework of quantum mechanics for explanation. Manipulating and generating these states delineates the frontier of progress in quantum technology. Therefore, it is crucial to devise methodologies for generating and controlling non-classical states. Photonics emerges as a promising platform within this context due to its robustness and exceptional manageability of this kind of states.For the above reasons, this Thesis adopts a dual focus. Firstly, we delve into the generation of non-classical states of light through strong-field processes. These processes entail interactions between light and matter, where light intensities contend with the binding forces that keep electrons bound to their respective nuclei. Our exploration demonstrates the utility of strong-field phenomena in generating non-classical states of light, exhibiting intriguing features dependent on specific process dynamics and the materials involved in excitation. Secondly, we investigate the constraints and prerequisites of non-classical states of light sources --beyond those derived from the aforementioned strong-field processes-- for the advancement of quantum communication. In particular, we analyze quantum key distribution, aiming to create a secret key exclusively known by the communicating parties for encrypting and decrypting messages.Therefore, this Thesis can be understood as a zeroth step towards leveraging strong-field physics as a prospective tool for quantum information science applications, as well as an exploration about the advances and limitations of photonic-based setups for quantum key distribution.

Committee:
     PRESIDENT: RUBIO VERDÚ, CARMEN
     SECRETARI: GONZALEZ TUDELA, ALEJANDRO
     VOCAL: WEITENBERG, CHRISTOF
Thesis abstract: Over the past three decades, optically trapped ultra-cold atoms have served as a versatile platform for controlled exploration of numerous condensed matter phenomena. The successful fabrication of magic angle twisted bi-layer graphene (MATBG) has introduced a for condensed matter physicists, while concurrently posing a novel challenge for the quantum simulation community. This thesis is devoted to addressing this problem, focusing mainly on the simulation of MATBG structures using ultra-cold atoms within its initial three chapters.To overcome the issue of unit cell expansion resulting from rotation misalignment, in the first chapter we propose the concept of "twist-less twistronics” (twistronics, a term coined from twist and electronics). This innovative notion involves replacing the physical rotation of one layer with a light-modulated hopping amplitude between the layers. Enabled by the architecture of ultra-cold atoms, this approach yields quasi-flat bands, a pivotal ingredient for collective phenomena observed in Magic-Angle Twisted Bilayer Graphene (MATBG), achieved at significantly reduced unit cell sizes.The opening chapter also presents a suitable experimental set-up. Moreover, it provides a comprehensive theoretical framework, including tight-binding calculations and effective models derived from perturbative analysis. The second chapter delves into the topological properties of an analogous system, emphasizing the energy separation between the quasi-flat bands and the resulting spectrum. We demonstrate Quantum Anomalous Hall Effect across diverse parameter regimes, accompanied by an exhaustive phase diagram with respect to tunable parameters.In the third chapter, we extend our investigation to encompass onsite, density-density attractive interactions between lattice atoms. Employing the Hartree-Fock-Bogoliubov mean-field approximation, we consider all feasible interaction channels within/between layers and spins. This chapter aims to elucidate the relationship between band flattening, a fully controlled parameter in our system, and the emergence/size of a superconductive gap. Notably, we uncover a substantial enhancement in the critical (Kosterlitz-Thouless) temperature within the quasi-flat band regime at quarter filling, along with a comprehensive diagram illustrating superconducting order parameters corresponding to each interaction channel.The fourth chapter marks a departure from condensed matter simulations, delving into "special purpose quantum computing" within the context of quantum batteries. These devices, analogous to their classical counterparts, store and release energy on demand, a process inherently governed by the battery Hamiltonian. Our work establishes a novel framework for assessing quantum battery performance and setting fundamental bounds on two key attributes: power and capacity. We investigate the essential Hamiltonian terms of a for achieving quantum speed-up in battery charging.The last, fifth chapter describes the theoretical tools, that have been used to support the first experimental realisation of the Extended Bose Hubbard model with dipolar excitons. We discuss the parameters of interests and important observables, such as a structure factor and discuss both the exact diagonalization and mean-field methods, which were necessary to verify the observation of strongly correlated phases at half and unit filling.

Committee:
     PRESIDENT: WEGMANN, SUSANNE
     SECRETARI: CAMPELO AUBARELL, FELIX
     VOCAL: OREN, MEITAL
Thesis abstract: In recent years, our understanding of the ion channels and receptors that orchestrate the conversion of physical stimuli into physiological signals has deepened significantly. Nevertheless, there remains some ambiguity regarding the precise mechanism by which mechanical stresses reach the molecular mechanosensors. While it is well-established that many mechanosensitive ion channels respond to increased plasma membrane tension, emerging evidence underscores the crucial role played by the cytoskeleton within sensory cells. In this thesis, we asked how animals perceive mechanical stress, with specific focus on touch sensation. Our primary objective was to unravel the molecular and the mechanical pathways responsible for transmitting force within the touch receptor neurons of the nematode Caenorhabditis elegans. For over a decade, it has been postulated that the pore-forming subunit of the mechanoelectrical transduction channel forms a connection with the cytoskeleton through a highly conserved and widespread protein known as MEC-2, which bears structural similarities to Stomatin. Our study presents compelling evidence that MEC-2 assembles in liquid condensates that experience a shift in rigidity, transitioning from a fluid-like pool that allows transport along neurons to solid-like states that are mechanoelectrically active. We provide a new physiologically relevant context in which biomolecular condensates tune their function upon maturation and facilitate neuronal mechanotransduction in response to tactile stimuli. In order to contextualize the function of MEC-2 in force transmission, we developed a genetically encoded tension sensor module, which revealed that only stiffened condensates, as opposed to fluid-like ones, are capable of transmitting force within living organisms. Notably, this stiffening process does not occur autonomously. Within this study, we showed that this transition is instigated by a specific SH3 motif of UNC-89, a protein homologous to Titin and Obscurin, through a direct interaction with a proline-rich domain located in the C-terminal region of MEC-2. We propose that this change in rigidity serves a vital physiological function by contributing to the transmission of forces that vary in frequency during touch to the animal's body wall. Together, our data introduces a novel perspective on the significance of the MEC-2 liquid-to-solid phase transition in the realm of mechanotransduction. It also presents a new conceptual framework for understanding how animals, in a broader sense, perceive and respond to mechanical stresses.

Committee:
     PRESIDENT: BOGGILD, PETER
     SECRETARI: PRUNERI, VALERIO
     VOCAL: COLETTI, CAMILLA
Thesis abstract: Moore's law, a longstanding guide for the semiconductor industry, successfully predicted the exponential growth in computing power by doubling transistor counts every two years. However, recent challenges in maintaining this pace, attributed to physical limitations, energy consumption, and escalating costs, have prompted a shift in focus towards two-dimensional (2D) materials in semiconductor technology. This thesis aims to bridge the gap in understanding the complexities of incorporating 2D materials, such as Graphene (Gr), Transition Metal Dichalcogenides (TMD), and hexagonal boron nitride (hBN), into Complementary Metal-Oxide-Semiconductor CMOS platforms, paving the way for innovative optoelectronic devices with improved functionality to overcome these challenges. High-quality heterostructuresThis thesis investigates the crucial role of encapsulants and substrates in Gr-based heterostructures, highlighting their impact on electronic transport characteristics, such as hysteresis (∆n), carrier mobility (µ), and residual charge carrier concentration (n*). Owing to the quality and integration complexity of scalable large-area thick hBN, this thesis explores the utilization of TMD-like tungsten diselenide (WSe2) and tungsten disulfide (WS2) as substrates and encapsulants, respectively, for Gr. The hybrid heterostructures fabricated with WSe2/Gr/hBN and WS2/Gr/hBN exhibited a high µ of ~170,000 and ~140,000 cm2V-1s-1 with a n^*of ~7 and 8 x 1010 cm-2 respectively. This study underscores the significance of substrate engineering, particularly for WS2. A successful demonstration of the effectiveness of TFSI-treated WS2 in encapsulating Gr and its role as a gate dielectric has been established. The treated devices exhibited remarkable stability and resilience, leading to a low ∆n of ~2 x 109 cm-2 with a µ of ~62,000 cm2V-1s-1 and a n^* of ~1.7 x 1011 cm-2. Waveguide-integrated photodetectorsThe exponential growth of internet users and data traffic necessitates higher bandwidth capabilities in communication systems. Optical transceivers play a pivotal role in meeting this demand, particularly in data centers and broadband access networks. This thesis focuses on the crucial components of optical transceivers, specifically photodetectors (PD), optimized for a wavelength of 1550 nm, a standard for long-distance communication in optical fibers. This thesis explores a photothermoelectric (PTE) WSe2 encapsulated Gr photodetector on a waveguide to address this challenge. Up on a comprehensive analysis of the device's design, the fabricated PD with different widths exhibited a responsivity up to ~12 V/W (long) or 0.1 A/W and ~32 mA/W or 18 V/W (short) with a setup limited bandwidth of 110 GHz. PDs also demonstrated direct detection of NRZ and PAM-4 optical signals up to 120 and 160 Gbps, respectively.Wireless receiversMeanwhile, in wireless telecommunications, efforts must be directed towards boosting data rates to accommodate growing data traffic, as indicated by Edholm's law. The proposed 6G wireless devices are expected to achieve peak data rates of up to 1Tbps. To overcome speed bottlenecks, this thesis proposes exploring the terahertz (THz) range, with a focus on the sub-THz (~200GHz-300GHz) window, exhibiting low-attenuation demands for short-range (<200m) wireless applications. We performed an extensive investigation and optimization of the performance of a PTE-based Gr photodetector using various encapsulants. Among these, the hBN-encapsulated Gr PDs exhibited superior performance compared to their counterparts (PD with CVD Gr), with an elevated responsivity of ~240 (~30) V/W and low noise equivalent power (NEP) of ~1 (~9) x 10-11 W/√Hz. The fabricated PDs exhibited a bandwidth of approximately 1.9 GHz, enabling data rates of 2 Gbps. Finally, we developed a Gr-based receiver, establishing a sub-terahertz wireless communication link that achieved data rates of up to 3 Gbps and efficiently operated over a distance of 2.5 meters.