Disorder is an inherent component of real materials, with significant implications for their application in functional devices. Despite this, the theoretical modelling of disorder remains restricted, primarily due to the large simulation cells required to adequately represent disordered systems, and the associated computational costs. This has been remedied in part by the increased availability of resources for high performance computing. In this thesis, using a combination of computational techniques, primarily density functional theory and ab initio as well as classical molecular dynamics, we investigate disorder in two broad categories – physical and chemical disorder, in three distinct classes of materials: palladium nanoparticles, the negative thermal expansion cuprite Ag2O and the complex quaternary chalcogenide Cu2ZnSnS4, known commonly as kesterite. The ‘physical’ disorder discussed in the thesis constitutes shape- and adsorption-induced mechanical softening on the surface of Palladium nanocrystals used for nanocatalysis. This includes one study on the the adsorption of organic capping agents, and another on the adsorption of oxygen molecules and the subsequent oxidation of Pd. In the former, it was observed that the strain effect due to adsorption-induced surface disorder is significantly greater than that due to variations in surface termination, i.e. nanoparticle shape. Moreover in the latter case, different crystallographic facets with different degrees of disorder were found to affect the spin-flip induced activation of oxygen atoms, relevant to the catalytic oxygen reduction reaction in hydrogen fuel cells. In each case, the computational results were combined with a sophisticated, phenomenological whole powder pattern modeling of X-ray diffraction data primarily from synchrotron radiation, leading to an accurate characterization of the Debye-Waller coefficient, which was established as a reliable metric for disorder in crystalline systems. In the case of Ag2O instead, we demonstrated that the large experimental Debye-Waller coefficient was due to thermal diffuse scattering arising from the strong distortion of the Ag4O coordination tetrahedra. The second form of disorder which was investigated is ‘chemical’ disorder, which refers to cation disorder in the quaternary chalcogenide Cu2ZnSnS4 studied for its performance as a thermoelectric material. Similar to the studies on palladium, the disorder was quantified through the Debye-Waller coefficient using molecular dynamics simulations, this time from ab initio methods, and compared with X-ray diffraction data from a synchrotron source. The ordered phase of CZTS is known to crystallize in a tetragonal phase, with alternating Cu-Zn and Cu-Sn cation layers sandwiched between sulfur layers. Two forms of cation disorder were studied: disorder only in the Cu-Zn layer, leading to a disordered tetragonal phase, and full cation site randomization, leading to a disordered cubic polymorph. In the former case, it was found that the higher symmetry of the disordered tetragonal structure led to an average symmetrization of the nearest neighborhood of each individual cation, as a result of which there was a convergence of bands at the valence band maximum, leading to an experimentally observed increase in p-type carrier concentration. In the case of CZTS with full cation disorder, inhomogenous bond led to favorable modifications of the electronic and phonon properties, allowing for a simultaneous improvement of the experimentally measured electrical and thermal conductivities as well as the Seebeck coefficients. Finally, by studying the atypical electronic band structure of this cubic polymorph, we were able to identify topologically non-trivial behavior evidence of bulk band inversion, robust surface states, and an adiabatically continuous connection to a known TI phase. As such, we were able predict disordered cubic CZTS to be the first disorder-induced topological Anderson insulator in a real material system.

This manuscript investigates the use of microring resonators to create all-optical reservoir-computing networks implemented in silicon photonics. Artificial neural networks and reservoir-computing are promising applications for integrated photonics, as they could make use of the bandwidth and the intrinsic parallelism of optical signals. This work mainly illustrates two aspects: the modelling of photonic integrated circuits and the experimental results obtained with all-optical devices. The modelling of photonic integrated circuits is examined in detail, both concerning fundamental theory and from the point of view of numerical simulations. In particular, the simulations focus on the nonlinear effects present in integrated optical cavities, which increase the inherent complexity of their optical response. Toward this objective, I developed a new numerical tool, precise, which can simulate arbitrary circuits, taking into account both linear propagation and nonlinear effects. The experimental results concentrate on the use of SCISSORs and a single microring resonator as reservoirs and the complex perceptron scheme. The devices have been extensively tested with logical operations, achieving bit error rates of less than 10^−5 at 16 Gbps in the case of the complex perceptron. Additionally, an in-depth explanation of the experimental setup and the description of the manufactured designs are provided. The achievements reported in this work mark an encouraging first step in the direction of the development of novel networks that employ the full potential of all-optical devices.

Artificial intelligence is a new paradigm of information processing where machines emulate human intelligence and perform tasks that cannot be done with standard computers. Neuromorphic computing is in particular inspired by how the brain computes. Large network of interconnected neurons whose synapses are varied during a learning phase, and where the information flows in parallel throughout different connections. Photonics platforms represent an interesting possibility where to implement neuromorphic processing schemes, exploiting light and its advantages in terms of speed, low energy consumption and inherent parallelism via wavelength division multiplexing. In particular, a candidate playing a diversity of key roles in integrated networks is the microring resonator. In silicon photonics, the microring resonator can implement the strength of a synapse, the spiking emission of a biological neuron, and it can exhibit a fading memory based on its multiple linear and nonlinear dynamical timescales. This manuscript presents an overview of the main applications of silicon microring resonators in neuromorphic silicon photonics, and then focuses on its implementation in a processing scheme, named time delay reservoir computing (RC). Time delay RC is a hardwarefriendly approach by which implement a large neural network, where this is folded in the nonlinear dynamical response of only one physical node, such as a dynamical system with delay feedback. The manuscript illustrates, both numerically and experimentally, how to make time delay RC exploiting the linear and nonlinear dynamical response of a silicon microring resonator. The microring is coupled to an external optical feedback and the results on a diversity of time series prediction tasks and delayed-boolean tasks are presented. Numerically, it is shown that the microring nonlinearities can be exploited to improve the performance on prediction tasks, such as the Santa Fe and Mackey Glass ones. Experimentally, it is shown how the network can be set to solve delayed boolean tasks with error-free operation, at 12 MHz operational speed, together with possible upgrades and alternative implementations that can boost its performances. / La inteligencia artificial es un nuevo paradigma de procesamiento de información en el que las máquinas emulan la inteligencia humana y realizan tareas que no pueden ser realizadas con ordenadores estándar. La computación neuromórfica está particularmente inspirada en cómo el cerebro realiza cálculos. Consiste en una gran red de neuronas interconectadas cuyas sinapsis varían durante una fase de aprendizaje, y donde la información fluye en paralelo a través de diferentes conexiones. Las plataformas fotónicas representan una interesante posibilidad para implementar esquemas de procesamiento neuromórfico, aprovechando las ventajas de la luz en términos de velocidad, bajo consumo de energía e inherente paralelismo a través de la multiplexación por división de longitud de onda. En particular, un candidato que desempeña una diversidad de roles clave en redes integradas es el micro-anillo resonador. En la fotónica de silicio, el micro-anillo resonador puede implementar la intensidad sináptica, la emisión de pulsos de una neurona biológica, y puede exhibir una memoria que decae con el tiempo basada en sus múltiples escalas temporales dinámicas lineales y no lineales. Esta tesis presenta una visión general de las principales aplicaciones de los resonadores de anillo microscópicos de silicio en la fotónica neuromórfica de silicio y se centra en su implementación en un esquema de procesamiento llamado time delay reservoir computing (RC). Time delay RC es un enfoque favorable para el hardware mediante el cual se implementa una gran red neural, a través de la respuesta dinámica no lineal de solo un nodo físico, como un sistema dinámico sujeto a retroalimentación. Este trabajo ilustra, tanto numérica como experimentalmente, cómo realizar la computación en time delay RC utilizando la respuesta dinámica lineal y no lineal de un resonador de anillo microscópico de silicio. El microanillo resonador está acoplado a una retroalimentación óptica externa y se presentan los resultados de una diversidad de tareas de predicción de series temporales y tareas booleanas retrasadas. Numéricamente, se muestra que las no-linealidades del micro-anillo resonador se pueden aprovechar para mejorar el rendimiento en tareas de predicción, como las de Santa Fe y Mackey Glass. Experimentalmente, se muestra cómo la red se puede configurar para resolver tareas booleanas retrasadas sin errores, a una velocidad operativa de 12 MHz, junto con posibles mejoras e implementaciones alternativas que pueden aumentar su rendimiento.

In this thesis we aim to study the structural evolution under x-ray illumination in a selection of oxide glass formers as a function of the radiation dose. The irradiated specimens are investigated by means of XPCS experiments and also by means of “exsitu” experiments as luminescence, Raman and x-ray diffraction that have been carried out after the irradiation protocol. As far as we know, the “ex-situ” investigations here reported are the first in the field and have allowed us to evaluate the nature of the point defects, the structural modifications and the nature of the vibrational excitations of the samples exposed to the x-ray dose. The work carried out in the thesis has helped us to elucidate the microscopic evolution of the glass structure under irradiation and to verify whether the transformation can be envisioned as an evolution in the energy landscape, either as an “annealing” or as a “rejuvenation” process.

The atomic mean square displacement (MSD,  ̄(σ_i^2 ) ) is often used in computational materials science studies to calculate measurable properties from the atomic trajectories of simulations; for example, the diffusion coefficient, which according to Einstein relations (Einstein 1905) on the random walk is 1/6 of the slope of the trend of  ̄(σ_i^2 ) vs. time (Chandler 1987). Equally relevant is the mean square relative displacement (MSRD,  ̄(σ_ij^2 )), used in X-ray Spectroscopies, mainly EXAFS, to describe the atomic disorder in solids (Calvin 2013) (Fornasini 2014). Less known is the relevance of the MSRD in X-ray scattering from nanoparticles. In particular, in Total Scattering methods (Pair Distribution Function and Debye Scattering Equation), which rely on an atomistic description of the nanoparticles, the MSRD is the key to distinguish dynamic (thermal) and static disorder (Krivoglaz 1969) (Kuhs 2006). Interestingly, the trend of the MRSD with the distance is characteristic of the nanoparticle shape, an aspect investigated in some detail in this Thesis work. More generally it can be shown that beyond the expected effect of nanocrystal size, the shape alters the contribution of the surface, which is quite relevant for the MSRD. The importance of the shape and of the surface region holds also in case of clusters of nanoparticles, not only in isolated particles. Besides the MSRD, the atomic configurations simulated by molecular dynamics (MD) can also be used to calculate the so-called Warren plot (or diagram), originally introduced in the seminal work of Warren & Averbach of the †̃50s to describe the effects of plastic deformation in metals (Warren B.E. 1950). Recent work has shown how to obtain Warren plots from the analysis of the diffraction line profiles according to the Whole Powder Pattern Modelling (WPPM) (L. M. Scardi P. 2002) (Scardi P 2017) (P. E.-W. Scardi 2018), in particular from the analysis of the strain component of the diffraction peak profile broadening. As proposed in this work, If the Warren plot can be calculated directly from MD simulations, then it is possible to proceed backwards, and construct more reliable strain functions from an atomistic knowledge of the local atomic displacement caused by static and dynamic disorder components. This thesis is divided in two main parts, discussing two different but complementary topics: atomistic modelling and calculations of displacement quantities, application of the above results to experimental case studies, based on the modelling of diffraction data from nanocrystalline systems. We start by describing the atomistic simulations and vibrational properties calculated for several atomic configurations. The main case study concerns Palladium nanoparticles of different sizes and shapes, for which we show that vibrational properties and correlation properties between atoms pairs are greatly influenced by the geometric shape of the nanoparticle and to a lesser extent by their size. The interest is on truncated cubes, i.e. cubes whose edges and corners are progressively removed, as in the series of so-called Wulff solids, ranging from the cubic to the octahedral shape (Wulff G. 1901). As shown in (ii), these are the object of several experimental studies. The developed methodologies are nevertheless applicable to other cases, like the clusters of nanocrystals observed in powders produced by high-energy ball milling, which is also a topic discussed in (ii). The work aims to show a general approach to atomistic modelling, both for isolated nanoparticles with definite shapes, and grains of unspecified shape in plastically deformed polycrystalline materials. We then use the values for displacement quantities (e.g., MSD, MSRD) calculated for the simulated systems to compare them to the experimental results. An underlying fact that seems to hold in all the different cases is that the surface behaviour of nanomaterials has the largest influence on the displacement quantities. For isolated particles we observe strong correlation between displacement quantities and the shape; whereas in the case of a nanocrystalline grain clusters (Figure 1 1) we see that no matter the defects inside the grain, the main contribution to MSRD is given by the grain boundary.

The Alpine region is a sensitive area to air pollution, as it presents specific characteristics, which expose it to a greater environmental burden with respect to flat areas. During the last decades, the scientific community has developed many different modeling tools to tackle the problem of air pollution. This issue demands at least three distinct procedures: the modeling of the meteorological fields, the modeling of the transport and dispersion of the pollutants and the modeling of the emitting sources. Each of these procedures performs differently across different space and time scales and carries its own strengths and weaknesses, which affect results in terms of pollutant dispersion patterns. The present work focuses on testing and improving different modeling tools at a local scale, over very complex topography, where most of them are brought to work at the limit of their applicability, but they are still the best available tools to face the problem. Different case studies are used in this research in order to evaluate strengths and deficiencies of the models and, where possible, to improve their performance. The experimental datasets used for this purpose come from both previously performed field campaigns and specifically designed campaigns, including meteorological and air quality observations. The performance of Land Surface Models within the Weather Research and Forecasting Model is evaluated and improved, focusing on their ability in reproducing near-ground variables, with specific attention to the frequent ground thermal inversion occurring in the mountainous areas. The performance of dispersion models recommended for applications over complex terrain is also tested and their results are compared with unique measurements (PM10 vertical profiles and tracer gas ground concentration), under challenging wintertime conditions. Atmospheric turbulence parameterizations are also analyzed, in order to understand their role and effects in a modeling chain for dispersion assessment purposes.

This thesis explored fundamental concepts of linear optics focusing on the modal interaction within waveguide/microresonator systems. In addition, it investigated a nonlinear process of stimulated degenerate four-wave mixing in a channel waveguide exploiting the analogy between photons and cold boson atoms. The backscattering phenomenon due to the surface wall roughness of a microresonator is addressed by adding to the usual conservative (Hermitian) coupling coefficient, a dissipative (non-Hermitian) term. This allows explaining the experimental measurements of a multimodal microresonator, which exhibits an asymmetrical resonance splitting characterized by a difference in the peak depths of the transmission spectra. It is shown theoretically, numerically and experimentally that the stochastic nature of the roughness along with the inter-modal dissipative coupling could give rise to a different exchange of energy between the co-propagating and the counter-propagating mode. The unbalanced exchange of energy between the two modes with opposite angular momenta can generate a different reflection by swapping the injection of the light between the input and the output ports. This effect lies at the heart of the realization of an unidirectional reflection device and it finds an explanation in the physics of the exceptional points. The realization of an optical setup based on a Mach-Zehnder interferometer, which exploits some particular techniques of data acquisition, allows obtaining a full knowledge of the complex electric field of a propagating mode. In this way, the spectrum of a wedge microresonator vertically coupled to a bus waveguide is explained using analysis methods based on parametric phasors and inverse complex representations. In addition, the energy exchange between the co-propagating and counter-propagating modes is studied from a temporal point of view by extrapolating a simple model based on the Green function. In particular, it is discussed the analytical temporal response of a microring resonator excited through a bus waveguide by an optical rectangular pulse. Here, it is shown theoretically and experimentally, how the temporal response leads to the characterization of the coupling regime simply from the knowledge of the electric field intensity. In this thesis, the isomorphism between the Schroedinger’s equation and the Helmholtz wave equation is analyzed in the nonlinear case. Considering a bulk nonlinear medium of the Kerr type, the complex amplitude of the optical field is a slowly varying function of space and time, which satisfies a nonlinear Schroedinger equation. The well-known nonlinear optical phenomenon of stimulated degenerate four wave mixing is reformulated in the language of the Bogoliubov theory. This parallelism between photons and cold atoms allows showing that the phase of the signal assumes a peculiar sound-like dispersion under proper assumptions.

Today, the application of particle beams in cancer therapy is a well-established strategy and its combination with surgery and chemotherapy is becoming an increasingly reliable approach for some several clinical cases (e.g. skull base tumors). Currently, protons and 12C ions are used for patients’ treatment, due to their characteristic depth-dose deposition profile featuring a pronounced peak (the Bragg Peak) at the end of range. Clinical energies typically span between 60 and 250 MeV for protons and up to 400 MeV/u for 12C ions, in order to deliver treatments to various disease sites. Interactions between the primary beam and the patient’s body always occur during treatment, changing the primary radiation composition, energy and direction and thus affecting its depth dose and lateral profile. In carbon therapy, both projectile and target fragments can be generated during a treatment: the former are characterized by a kinetic energy spectrum peaked at the same energy of the primary beam and are mostly emitted in the forward direction; the latter are emitted with a much lower energy because they are produced from the target, which is at rest before the collision, and they are generated isotropically in the target frame. Moreover, the interaction of carbon ions with the patient's body is currently modeled in the treatment planning on the basis of experimental data measured in water. For all other biological materials, the contribution of nuclear interactions is taken into account by rescaling the values measured in water with a density factor. This approximation neglects the influence of the elemental composition, which might become relevant in cases where the material encountered by the beam significantly differs from water (e.g. bone or lung tissues) and result in a non-uniform and incorrect dose profile. Thus, experimental data with target different from water are clearly needed in order to correctly evaluate the contribution of all biological elements inside the human body. Treatments with protons can only generate target fragments, leading to the production of low-energy and therefore short-range fragments. Heavy secondary fragments will have a higher biological effectiveness than to protons, thus affecting the proton Relative Biological Effectiveness (RBE, i.e. the ratio of photon to charged particles dose necessary to achieve the same biological effect), nowadays assumed as a constant value (RBE=1.1) in clinical practice. Another aspect related to nuclear interactions is the overlap between radiotherapy and space radiation protection. The group of particle species either currently available in radiotherapy or considered promising alternative candidates (i.e. Helium, Lithium and Oxygen) are among the most abundant in the space radiation environment. Moreover, the proton energy range used in radiotherapy is similar to that of Solar Particle Events (SPEs) and Van Allen trapped protons. The radiation environment in space can lead to serious health risks for astronauts, especially in long duration and far from Earth space missions (like human explorations to Mars). Protection against space radiation are of paramount importance for preserving the astronauts’ life. Today, the only possible countermeasure is passive shielding. Nuclear fragmentation processes can occur inside the spaceship hull, causing the production of lighter and highly penetrating radiation that must be considered when a shielding is designed. Therefore, experimental data for beam and targets combinations relevant in space radiation applications must be collected for characterizing the interaction of mixed generated radiation field and assess the radiation-induced health risk. Despite the many fundamental open issues in particle therapy and space radiation protection fields, such the ones mentioned above, the current lack of experimental fragmentation cross section data in their energy range of interest is undeniable. Thus, accurate measurements for different ions species with energies up to 1000 MeV/u would be of great importance in order to further optimize particles treatments and improve the shielding design of spaceship. Moreover, additional experimental data would be of great importance for benchmarking Monte Carlo codes, which are extensively used by the scientific communities in both research fields. In fact, the available transport codes suffer from many uncertainties and they need to be verified with reliable experimental data. Due to high energy and long range of projectile fragments, the standard approach for their identification is collect data from several detector types, usually two plastic scintillators coupled with a Barium Fluoride or LYSO crystal, placed both upstream or downstream the target, providing information about the charge, energy loss, the residual kinetic energy and the time of flight of the emitted fragments. This experimental setup allows the identification of particle species in terms of charge, isotope, emission angle and kinetic energy and it has been widely exploited to perform several fragmentation measurements, both in particle therapy and space application fields. An example is the ROSSINI (RadiatiOn Shielding by ISRU and/or INnovative materIals for EVA, Vehicle and Habitat) project financed by the European Space Agency (ESA) to select innovative shielding materials and provide recommendations on space radioprotection for different mission scenarios. However, such standard approach is not useful for the characterization of target fragments. In fact, because of their low energy and short range, a much more complex setup and finer experimental strategies are required for their detection. The FOOT (FragmentatiOn Of Target) experiment has been designed to measure fragment production cross sections with ~5% uncertainty. Target fragmentation induced by 50-250 MeV proton beams will be studied taking advantage of an inverse kinematic approach. Specifically, O, C and He beams impinging on different targets (e.g., C, C2H4) will be employed, thus boosting the fragments energy and making their detection possible. Fragmentation cross section of hydrogen will be then obtained by subtraction. The same configuration provides also a measurement of projectile fragments with the direct kinematics approach. FOOT experimental setup consists of two different apparatus: a dedicated “table-top” electronic setup, based on a magnetic spectrometer, were conceived for the detection of heavier fragments (Z≥3). Alternatively, an emulsion spectrometer was designed in order to measure the production of low Z fragments (Z≤3) that would not cross the whole magnetic spectrometer. The purpose of the work presented in this doctoral thesis is the experimental characterization of particles originated in nuclear fragmentation processes for targets and beams of interest for particle therapy and space radiation protection, providing inputs to improve the accuracy of Monte Carlo transport codes presently used. Data collected in experimental campaigns using the standard setup to study the interaction of 400 MeV/u 12C ions beam with bone-like materials and 1000 MeV/u 58Ni ions beam with targets relevant for space applications have been analyzed. The presented fragments characterization comprehends the fraction of primary particles surviving the target and the yield and kinetic energy spectra of charged particles emitted at several angles with respect to the primary beam direction. The )*Ni beam data were collected in the frame of the ROSSINI experiment and focused on characterizing secondary neutrons production. Moreover, the analysis of the performances and fragments reconstruction capabilities of the FOOT electronic setup has been accomplished with Monte Carlo simulations. A dedicated analysis software has been developed in order to reconstruct fragments charge and mass, energy yields and production cross sections. A preliminary analysis of experimental data collected by a partial FOOT electronic setup is presented as well.

This diploma thesis is divided into a theoretical part and a practical one. The theoretical part deals with the characteristics of sport events and presents the International Ski Federation FIS, including its structure, and marketing and technical concept of the races in the FIS Tour de Ski. The introduction to the practical part focuses on the history of cross-country skiing, the structure of the most important cross-country races in the world, and presents the town of Nove Mesto na Morave and the race Golden Ski. Then follows the description of the allocation process of the World Cup race and possibilities of its abolition. The main part of the diploma thesis is a detailed analysis of the Golden Ski race in the Tour de Ski. The analysis deals with the topics related to the organizing committee, technical support, scoring, invitations, accreditation, accommodation, marketing, transportation, and accompanying program of Golden Ski. The thesis concludes with financial background and short mention of Slavic Cup.

Questa tesi di dottorato studia l’interazione della cooperatività con il rumore in sistemi realistici, focalizzandosi principalmente sulla superradianza. Gli effetti cooperativi emergono dall’interazione collettiva di un insieme di elementi con un campo esterno. Esempi degni di nota sono la superconduttività, dove le coppie di Cooper elettroniche interagiscono con le vibrazioni reticolari, le eccitazioni di plasma, che sorgono dall'interazione collettiva degli elettroni in un metallo con il campo coulombiano, e la superradianza, ovvero quel processo di emissione spontanea cooperativa che sorge da un aggregato di emettitori identici. Gli effetti cooperativi sono tipicamente robusti al disordine e al rumore, cosa che li rende interessanti per delle applicazioni a dispositivi quantistici che possano operare a temperatura ambiente. In questo lavoro, inizialmente, introduciamo un formalismo di “master equations” che descrive l’accoppiamento collettivo di un aggregato di emettitori/assorbitori con il campo elettromagnetico, valido quando le dimensioni dell'aggregato sono sia maggiori che minori della lunghezza d’onda emessa/assorbita. Inoltre, il formalismo è valido per accoppiamento sia debole che forte con il campo elettromagnetico e, cosa più importante, permette di descrivere correttamente la superradianza in diversi regimi. In tale formalismo, studiamo l’interazione tra superradianza e rumore termico sia per nanotubi molecolari (di dimensioni minori della lunghezza d’onda associata alla transizione) che sono presenti nei complessi antenna fotosintetici dei Green Sulfur Bacteria, sia pure per superreticoli di quantum dots di nuova generazione, aventi dimensioni maggiori della lunghezza d’onda emessa. In entrambi i casi si dimostra che la coerenza può permanere in presenza di rumore termico alle temperature a cui questi sistemi sono stati analizzati sperimentalmente (temperatura ambiente per i nanotubi molecolari, e 6 K per i superreticoli di quantum dots). Nello specifico, nei nanotubi molecolari mostriamo che la macroscopica delocalizzazione coerente delle eccitazioni a temperatura ambiente, che copre centinaia di molecole, può essere considerata un effetto emergente che origina dall’effetto combinato della specifica disposizione geometrica delle molecole e della presenza di accoppiamenti tra subunità del cilindro, incrementati dagli effetti cooperativi. Questi risultati aprono la strada a nuovi modi per ingegnerizzare dei “quantum wires” robusti al rumore grazie alla cooperatività. Inoltre, la presente analisi di sistemi allo stato solido basati su superreticoli di “quantum dots” di perovskite (CsPbBr3) fornisce una base teorica in grado di comprendere recenti osservazioni di emissione superradiante. Sulla base della nostra teoria, suggeriamo che futuri esperimenti dove si utilizzino quantum dots più piccoli, potrebbe aumentare significativamente la robustezza del sistema al rumore termico, aprendo la strada verso la superradianza a temperatura ambiente in sistemi allo stato solido. Si considerano anche i complessi antenna dei Purple Bacteria, dove è ben risaputo che gli effetti cooperativi incrementano il trasferimento e l’accumulo di eccitazioni generate dalla luce assorbita. Mostriamo come queste proprietà possono essere sfruttate per creare un laser ispirato a sistemi biologici e basato su aggregati molecolari, dove la luce solare, benché debole, sarebbe utilizzata come sorgente di pompaggio. Il trasferimento efficiente di energia dentro questo sistema, all’atto pratico, focalizzerebbe l’eccitazione assorbita in direzione di un dimero molecolare, composto da una coppia di molecole interagenti, opportunamente scelte. L’orientazione dei momenti di dipolo di transizione in ciascun dimero è tale da concentrare tutta l’intensità del dipolo nel livello a più alta energia, lasciando lo stato eccitonico inferiore otticamente inattivo. Un dimero molecolare in tale configurazione, che è ideale per ottenere inversione di popolazione, è chiamato “H-dimer”. Tale H-dimer, nell’archittettura qui proposta per un laser ispirato a sistemi biologici, è posto al centro di un aggregato molecolare ispirato a sistemi biologici. Gli H-dimers, eccitati dagli aggregati molecolari circostanti, raggiungono inversione di popolazione e, dunque, possono emettere luce laser quando tali aggregati sono posti in una cavità ottica. Convertire l’energia incoerente fornita dal Sole in un fascio laser coerente supererebbe diverse limitazioni pratiche inerenti all’utilzzo della luce solare come sorgente di energia pulita. Per esempio, i fasci laser sono molto efficienti nell’avviare reazioni chimiche che convertono la luce solare in energia chimica. Inoltre, dal momento che i complessi fotosintetici batterici tendono ad operare nella regione spettrale del vicino infrarosso, la nostra proposta si presta in modo naturale a realizzare laser a infrarossi a corta lunghezza d’onda, i cui fasci viaggerebbero per lunghe distanze senza quasi perdere energia, quindi distribuendo in modo efficiente l’energia solare raccolta. Nella ricerca di un meccanismo comune alla cooperatività e alla sua robustezza, abbiamo confrontato il modello delle coppie di Cooper della superconduttività con la superradianza in singola eccitazione, mostrando molte somiglianze tra i due fenomeni: in particolare, i sistemi superradianti presentano una “gap” immaginaria nel piano complesso (ovvero, una segregazione tra i tempi di vita degli autostati del sistema) che, in modo simile alla gap superconduttiva, rende questi sistemi robusti al rumore statico. Più in generale, mostriamo che ogni interazione a lungo raggio tra i costituenti di un sistema induce effetti collettivi, manifestati da delle gap nello spettro eccitonico. Perciò, la nostra analisi successiva considera l’effetto delle interazioni a lungo raggio sul trasporto eccitonico lungo catene disordinate. Dimostriamo che la presenza di uno stato collettivo ben separato dagli altri stati influenza tutto lo spettro del sistema, generando dei regimi molto controintuitivi dove il trasporto è incrementato dal disordine o è indipendente da esso, e tali regimi si estendono su molti ordini di grandezza nell’intensità del disordine. Dimostriamo anche che una catena fortemente accoppiata a un modo del campo elettromagnetico in una cavità ottica è equivalente a una catena con interazione a lungo raggio, mostrandosi dunque molto promettente per esperimenti e applicazioni future. Nello specifico, mostriamo che catene molecolari realistiche, ioni intrappolati realizzati allo stato dell’arte e atomi di Rydberg sono tutti in grado di raggiungere l’intensità di interazione a lungo raggio tale per cui il trasporto sarebbe incrementato dal disordine o indipendente da esso, puntando alla realizzazione di un trasporto di energia senza dissipazione in “quantum wires” disordinati. / This Ph.D. thesis studies the interplay of cooperativity and noise in realistic systems, largely focusing on superradiance. Cooperative effects emerge from the collective interaction of an ensemble of elements to an external field. Notable examples are superconductivity, where the electron Cooper pairs interact with the lattice vibrations, plasmon excitations, arising from the collective interaction of electrons in a metal with the Coulomb field, and superradiance, that is a cooperative spontaneous emission process stemming from an aggregate of identical emitters. Cooperative effects are typically robust to disorder and noise, making them interesting for applications to quantum devices operating at room temperature. In this work, we first present a general master equation formalism that describes the collective coupling of an aggregate of emitters/absorbers to the electromagnetic field, valid both when the size of the aggregate is larger or smaller than the emitted/absorbed wavelength. Also, the formalism is valid both for weak and strong coupling of the emitters to the electromagnetic field and, most importantly, it allows to correctly describe superradiance in different regimes. Within such formalism, the interplay of superradiance and thermal noise is studied both for molecular nanotubes (of size smaller than the transition wavelength) that are present in the antenna complexes of photosynthetic Green Sulfur Bacteria, and also for novel solid state quantum dot superlattices, having size larger than the emitted wavelength. In both cases it is shown that coherence can persist in presence of thermal noise at the temperatures where these systems have been experimentally analyzed (room temperature for molecular nanotubes, and 6 K for quantum dot superlattices). Specifically, in natural molecular nanotubes we show that the macroscopic coherent delocalization of the excitation at room temperature, covering hundreds of molecules, can be considered an emergent effect originating from the combined effect of the specific geometric disposition of the molecules and the presence of cooperatively enhanced couplings between cylinder subunits. These results open the path to new ways of engineering quantum wires robust to noise thanks to cooperativity. Moreover, our analysis of solid state systems based on perovskite (CsPbBr3) quantum dot superlattices provides a theoretical framework able to explain recent observations of superradiant emission. Based on our theory, we suggest that further experiments, using smaller quantum dots, could significantly increase the robustness of the system to thermal noise, paving the way towards room-temperature superradiance in solid-state systems. We also considered the antenna complexes of Purple Bacteria, where cooperative effects are well known to boost the transfer and storage of photo-absorbed excitations. We show how these properties can be exploited to create a bio-inspired molecular aggregate laser medium, where natural sunlight, although weak, would be used as a pumping source. The efficient energy transfer within this system would effectively focus the absorbed excitation on a suitably chosen molecular dimer, composed by a pair of interacting molecules. The orientation of the molecule transition dipole moment in each dimer is such to concentrate all the dipole strength in the highest energy level, leaving the lower excitonic state dark. A molecular dimer in such configuration, which is ideal to achieve population inversion, is called H-dimer. Such an H-dimer in our proposed architecture for a bio-inspired laser medium, is placed at the center of the bio-inspired molecular aggregates. The H-dimers, pumped by the surrounding molecular aggregates, reach population inversion and, therefore, can lase when such aggregates are placed in an optical cavity. Turning the incoherent energy supply provided by the Sun into a coherent laser beam would overcome several of the practical limitations inherent in using sunlight as a source of clean energy. For example, laser beams are highly effective at driving chemical reactions which convert sunlight into chemical energy. Further, since bacterial photosynthetic complexes tend to operate in the near-infrared spectral region, our proposal naturally lends itself for realising short-wavelength infrared lasers which would allow their beams to travel nearly losslessly over large distances, thus efficiently distributing the collected sunlight energy. In search of a common mechanism to cooperativity and its robustness, we have compared the Cooper pair model of superconductivity and single-excitation superradiance, showing many similarities between the two: in particular, superradiant systems present an imaginary gap in the complex plane (that is, a segregation between the lifetimes of the system eigenstates) that, similarly to the superconducting gap, makes these systems robust to static disorder. More in general, we show that any long-range interaction between the constituents of a system generates collective behaviours, manifested by gaps in the excitonic spectrum. Therefore, our further analysis considers the effect of long-range interactions on excitation transport along disordered chains. We show that the presence of a gapped, collective state affects the whole spectrum of the system, generating quite counter-intuitive disorder-enhanced and disorder-independent transport regimes, that extend over many orders of magnitude of the disorder strength. We also prove that a chain strongly coupled to a cavity mode is equivalent to a long-range interacting chain, thus being very promising for future experiments and applications. Specifically, we show that realistic molecular chains, state-of-the-art trapped ions and Rydberg atoms are all able to reach the needed long-range interaction strength that would show disorder-enhanced or disorder-independent transport, aiming to the realization of dissipationless transport of energy in disordered quantum wires.