Committee:
     PRESIDENT: RUPRECHT, VERENA
     SECRETARI: LOZA ALVAREZ, PABLO
     VOCAL: FLAMES BONILLA, NURIA
Thesis abstract: In Caenorhabditis elegans, the embryo develops within the protective confines of an eggshell, shielded from direct interactions with the outside world. In this isolated environment, neuronal mechanosensory circuits, responsible for translating physical forces into biochemical signals, are among the first to emerge during development. While their primary function is to mediate interactions with the external mechanical world, they also play a significant role in broader physiological and behavioral processes, including synaptic plasticity and social bonding. C. elegans neurons responsible for the sense of touch arise very early during embryogenesis, long before any physical interaction, raising the question: when does the nervous system first awaken to the mechanical world it has yet to experience?Unraveling the onsets of mechanosensation has critical implications for neuroscience and human health, as its dysregulation is linked to various diseases and neurodevelopmental disorders. For example, studies on mouse models of autism spectrum disorder (ASD) have demonstrated that the timing of mechanosensory disruptions during embryogenesis plays a pivotal role in determining the severity of the condition. Despite its importance, our understanding of when mechanosensitivity first emerges remains limited, also due to the practical and ethical challenges of studying the application of mechanical forces to developing neural systems.To address these challenges, the goal of this project was to develop a multifunctional experimental platform that combines precision mechanical stimulation with live volumetric imaging, enabling the investigation of induced calcium signals in the developing neural system of C. elegans embryos. Central to this platform is a custom open-top light sheet fluorescence microscope, optimized for fast, 3D imaging of calcium dynamics. The imaging unit is integrated with a fiber-optic-based nanoindenter, which provides precise force application and quantitative characterization of the mechanical properties of the sample. This setup allows for controlled mechanical stimulation while capturing real-time neuronal activity, facilitating the analysis of how and when external forces influence mechanosensory circuits during critical developmental stages.Using this platform, we conducted proof-of-concept experiments to explore mechanosensory responses in C. elegans embryos. These studies validated the system's ability to trigger and record precise neuronal activity, demonstrating its experimental effectiveness. Preliminary findings suggest that mechanosensory functionality might begin to emerge during the late stages of embryogenesis of C. elegans embryos, offering a glimpse into the elusive timing of sensory circuits development. Overall, this work sets the stage for future investigations into how these circuits awaken and their vital role in early neural development.

Committee:
     PRESIDENT: FAUSTI, DANIELE
     SECRETARI: VAN HULST, NIEK
     VOCAL: SCHICK, DANIEL
Thesis abstract: Strongly-correlated materials have emerged as one of the most active areas of research in Condensed Matter Physics. Interests in these materials arise mainly from the pliability of their properties, offering the possibility of tailoring these materials for specific applications. This is, in turn, due to the rich interplay of interactions between electronic, orbital and lattice degrees of freedom. This complex coupling of the different degrees of freedom, on the other hand, makes strongly-correlated materials difficult to understand.Ultrafast spectroscopy offers the possibility of resolving this bottleneck and provides insight into aspects of correlated materials crucial for enhancing our understanding of these materials. One such aspect is photoinduced phase transitions, where light drives a symmetry change in a material. To date, research has focused on using light to force materials to cross a single structural transition. In this work, we investigate the possibility of making multiple phase jumps with a single pulse of light. A suitablesystem for such study is the manganite, Pr0.5Ca1.5Mn04, which despite its prospects remains less explored. This layered manganite exhibits multiple phase transitions of electronic, orbital and structural origins, as a function of temperature. The presence of more than one phase transition in Pr0.5Ca1.5Mn04 allows us to examine the possibility and mechanism of multi-phase transition, an aspect of photoinduced phase transition that has hitherto not received much attention. The physics of the manganites is strongly dictated by the dynamics of Jahn-Teller phonons, which occur at a very high frequency (>15 THz). Studies involving these phonons thus call for setups with a very high time resolution.This thesis first discusses the construction of a novel setup that makes use of few-cycle pulses from the visible to the near infrared wavelength regions. Then, leveraging on the capabilities of this setup, we undertake ultrafast measurements on Pr0.5Ca1.5Mn04 in two parts: the linear and nonlinear pumping regimes. In the linear regime, we perform broadband, low-fluence measurements to characterize the sample. From this, we identify key structural and electronic changes that occur during the thermal transition pathway, allowing us to map out the sample into different symmetry regions, in agreement with literature. In the nonlinear pumping regime, we study the fluence dependence of the changes identified from the linear regime. By analyzing the coherent lattice response, we find indications of both single and double phase transitions occurring.

Committee:
     PRESIDENT: HAUKE, PHILIPP HANS-JÜRGEN
     SECRETARI: ACÍN DAL MASCHIO, ANTONIO
     VOCAL: CERVERA LIERTA, ALBA
Thesis abstract: This is a thesis in theoretical physics about analog quantum simulations, digital quantum simulations (quantum computing), and quantum state preparation using different quantum platforms (neutral atoms, trapped ions, and superconducting circuits). We live in a quantum era with such a wide variety of platforms available, however performing experiments on existing quantum devices remains challenging due to limitations in control, scale, and connectivity. Therefore, innovative strategies must be developed to achieve quantum advantage using current quantum technology. We are primarily interested in applications to high-energy physics, as quantum computing provides a natural framework for simulating the real-time evolution of gauge theories. While the field of quantum simulations and quantum computing is still in its infancy and may be far from uncovering relevant insights about the Standard Model in regimes inaccessible to analytical methods, classical simulations, or direct experiments, interesting discoveries are emerging. Significant developments include the observation of quantum many-body scar states and the reformulation of quantum field theories as quantum link models.Most part of the thesis is dedicated to the quantum simulations of lattice gauge theories, which we explore under different lenses. First, we propose a scheme to effectively generate three body interactions in trapped-ion platforms which consists of a generalization of the Mølmer-Sørensen scheme for three spins. In this project, we envision the quantum simulation of the spin 1/2 quantum link model description of the massless Schwinger model, which features a three-body interaction. Such interaction requires at least 12 two-qubit gates to be performed, which in principle accumulates more errors than a single three-qubit gate. This is what makes analog quantum simulations so powerful: We can tailor the platform to generate interactions of a specific target model, potentially reducing quantum errors.Next, assuming the existence of a perfect three-body gate, we study quantum many-body scar states in the Schwinger model. We use a mapping from the spin 1/2 Schwinger quantum link model to the PXP model to identify the relevant physical configurations. Then, we compare the evolution of thermal and non-thermal states under sequential Trotterized quantum circuits to their evolution under randomized quantum circuits. Our results indicate that the non-thermal sector of the Hilbert space, which includes the quantum many-body scars, are more sensitive to randomization.Then, on a more realistic note, we use real quantum devices from IBMQ to perform digital quantum simulations of the Schwinger model. These quantum computers are based on superconducting circuits, and we currently have access to up to 156-qubits together with a basis of single and two-qubit gates. The devices impose strong limitations on connectivity and depth of the quantum circuits, hence we propose using gauge invariance, in the form of the Gauss' law, for quantum error detection.In the last part of this thesis, we shift focus to study an interesting many-body behavior that emerges from the presence of a static gauge field. Specifically, we propose a protocol for the quasi-adiabatic preparation of the 1/2-Laughlin state, a fractional quantum Hall state, using rotating ultracold atoms to create artificial gauge fields. From the condensate phase to the Laughlin state there are three points of closed gaps, and we make trap largely anisotropic to cross these regions without losing fidelity. We improved the preparation times by a factor of ten compared to previous studies.

Committee:
     PRESIDENT: SONG, JUSTIN CHIEN WEN
     SECRETARI: RUBIO VERDÚ, CARMEN
     VOCAL: BASCONES FERNANDEZ DE VELASCO, MARÍA ELENA
Thesis abstract: Since the discovery of graphene, two-dimensional (2D) materials have garnered significant attention from the condensed matter physics community owing to their potential to engineer new physical, optical, and mechanical properties. The 2D material class now includes insulators (hexagonal boron nitride, hBN), semiconductors (transition metal dichalcogenides, TMDs), superconductors (NbSe2), topological insulators (Bi2Te3), and ferromagnets (CrI3). Beyond their inherent properties, layered materials allow for new characteristics through vertical stacking. Recent developments have led to the discovery of moiré materials, in which electronic properties are significantly altered by twisting adjacent 2D layers.The discovery of superconductivity in magic-angle twisted bilayer graphene (MATBG) marked a milestone in moiré physics, initiating a rapidly growing field. The resulting phase diagrams of other high-Tc superconductors, MATBG, serve as a platform for exploring highly tunable strongly correlated states. At a twist angle of approximately 1.1°, the "magic angle,” MATBG shows significant band flattening near the Dirac points, reducing the Fermi velocity and making the kinetic energy smaller than the repulsive Coulomb interactions. This results in superconductivity and various emergent phases dominated by many-body physics, including correlated insulators, orbital magnetism, nematic orders, and topological states.Moiré materials with large superlattice unit cells facilitate the exploration of strongly correlated phenomena at low charge carrier densities. Local back-gate electrodes enable capacitive tuning between strongly correlated states in-situ, a unique feature not available in other high-Tc superconductors. Advances in scanning probe techniques have allowed researchers to determine local properties at the sub-nanometer scale. Scattering-type scanning near-field optical microscopy (s-SNOM) is particularly suited for exploring MATBG because it can measure scattering and photovoltage signals at the nanometer scale while simultaneously probing mesoscopic electron transport. Utilizing a groundbreaking cryo-near-field nanoscopy method, we will conduct s-SNOM measurements at cryogenic temperatures (as low as 8 K) to assess the optical and photovoltage near-field responses. This approach employs energies in the mid-infrared (MIR) and terahertz (THz) ranges, which align with the anticipated optical transition energies in the band structures of these materials. The primary objectives of this thesis are to ascertain the pertinent optical and thermoelectric coefficients in twisted moiré materials, evaluate the impact of inhomogeneities through gate-tuned near-field photovoltage and optical measurements, visualize correlated phenomena and broken symmetry states, and comprehend the nature of dephased signals in various measurements. This dissertation seeks to highlight crucial advancements in quantum phases, quantum nano-optoelectronics, and thermoelectricity, while supporting interest in unresolved questions, such as the characteristics of low-temperature correlated states. Additionally, it outlines future objectives for near- and far-field photovoltage experiments.

Committee:
     PRESIDENT: MAZZERA, MARGHERITA
     SECRETARI: CHANG, DARRICK
     VOCAL: WARBURTON, RICHARD
Thesis abstract: Despite decades of research, practical quantum computing and long-distance quantum communication remain elusive, hindered by significant challenges in current platforms. Single rare earth ions (SREI) in the solid state offer a promising alternative, with potential to form quantum computing nodes containing around 100 highly connected qubits capable of photonic networking. Nanoparticles are ideal for this system, as they enable high doping concentrations for strong interactions while maintaining the required spectral distinguishability.SREI experiments benefit from optical cavities that enhance emission via the Purcell effect. The open-access Fabry–Perot fiber cavity, formed by a fiber-tip micromirror and a planar or fiber mirror, is particularly versatile: a wide range of emitters can be integrated on the mirror surface, optical access is easy via the fiber, and three-dimensional tunability is possible. This flexibility has enabled studies across various quantum emitters and 2D materials.This thesis presents our work developing the SREI platform using nanoparticles in fiber cavities. It begins with an introduction to quantum computing, quantum communication with quantum repeaters, and rare earth ions as a basis for quantum computers, along with an overview of our experimental design. A review of background knowledge follows, covering optical cavities, the Purcell effect, the optical Bloch equations, and single-photon light statistics.The absence of commercial nanopositioners suitable for controlling our fiber cavity led us to design our own. This positioner enabled the first detection of single ions in nanoparticles. We studied the ⁴I15/2 → ⁴I13/2 transition at 1535 nm in 20 ppm erbium-doped 150 nm Y₂O₃ nanoparticles, and identified an ion with excellent spectral stability, a linewidth of 3.8(3) MHz, and a g(2)(0) compatible with a perfect single emitter.We then developed a significantly improved second positioner with 2.5 pm RMS stability, 130 µm × 130 µm XY scan range, and MHz-rate cavity modulation, all at 1.65 K in a closed-cycle cryostat. The broad potential of fiber cavities enhances this device's impact, marking it as one of the thesis's main contributions.Equipped with this improved positioner, we proceeded with a new experiment to detect interactions between single ions. We studied the ³H₄ → ¹1D₂ transition at 619 nm in two sets of praseodymium-doped Y₂O₃ nanoparticles, but were so far unable to observe any praseodymium emission in the cavity. To diagnose why this was happening, we performed additional experiments with a confocal microscope, which confirmed the presence of praseodymium in a majority of objects and found the absorption resonance near where we expected.The thesis ends with conclusions and future directions, including emission shaping and a novel microscopy technique. A closing reflection on this work and recent breakthroughs in the field paints a promising future for quantum information technologies.

Committee:
     PRESIDENT: SALA LLONCH, ROSER
     SECRETARI: KRIEG, MICHAEL
     VOCAL: RE, REBECCA
Thesis abstract: The generation of energy in the human body relies on oxygen metabolism, determined by oxygen delivery through blood flow and extraction at the tissue level. Reliable assessment of these parameters is crucial for understanding physiological function and tissue adaptations under various stimuli. Conventional monitoring tools for blood flow and oxygen saturation face trade-offs between cost, portability, and technical limitations (depth, resolution, dynamics), restricting their real-time deep-tissue use.This thesis advances diffuse optics, a non-invasive, safe, scalable approach exploiting light diffusion in scattering media, and introduces methodological and instrumental innovations for monitoring blood flow and oxygenation in adult skeletal muscle and brain—two of the most oxygen-demanding organs.Part I investigated long-term physiological adaptations in forearm muscles of advanced rock climbers versus healthy controls. Rock climbing requires exceptional grip endurance, making it an ideal model for localized neuromuscular and hemodynamic adaptations to chronic training. Two protocols were applied: (1) a resting vascular occlusion test (VOT) combining near-infrared spectroscopy (NIRS, oxygenation) and diffuse correlation spectroscopy (DCS, blood flow), and (2) an intermittent grip endurance test measuring force, NIRS, and electromyography (EMG). Results showed climbers had faster blood flow recovery and higher hemoglobin concentrations after occlusion, indicating enhanced vascular response. During exercise, they maintained force longer and used oxygen more efficiently. However, steady-state measures revealed no significant inter-group differences, suggesting adaptations are demand-driven rather than evident at rest. This study is novel in (1) applying DCS to climbing physiology and (2) integrating mechanical, neuromuscular, and hemodynamic measures in one framework.Part II focused on high-density (HD) cerebral blood flow (CBF) mapping, a key marker of brain metabolism. Current systems are bulky, costly, and clinical-only. We developed a new diffuse optics platform using speckle contrast optical spectroscopy (SCOS) and its tomographic extension (SCOT), leveraging cost-effective CMOS technology to improve signal-to-noise ratio (SNR) and scalability while retaining cortical sensitivity. A fiber-based prototype validated signal quality and flow sensitivity in forearm and forehead tests. Building on this, we designed a full-scale HD-SCOT system, nearing completion, intended for real-time, non-invasive mapping of CBF over large cortical areas (e.g., visual cortex).Final contribution: a proof-of-concept SCOS extension enabling simultaneous blood flow and oxygenation measurement. Using multiple wavelengths, source-detector separations, and exposure times, it offers a simplified alternative to dual NIRS-DCS systems. Preliminary forearm tests confirmed feasibility, suggesting applications in muscle and brain monitoring.In summary, this thesis advances diffuse optical monitoring by developing new instruments and methodologies for deep-tissue hemodynamics. Applications in sport physiology and neuroimaging highlight the potential of multi-modal, high-density optical systems to deepen understanding of oxygen metabolism in naturalistic, real-time contexts, paving the way for broader physiological and clinical applications.

Committee:
     PRESIDENT: NOVOTNY, LUKAS
     SECRETARI: EBRAHIM-ZADEH, MAJID
     VOCAL: ODOM, TERI
Thesis abstract: This thesis investigates light-matter interactions within plasmonic nanocavities, using ultra- confined optical environments to observe and control molecular dynamics at the nanoscale. Plasmonic cavities, formed by metallic nanoparticles in close proximity to metal films, generate highly localized optical fields within nanometer gaps. Such intense confinement allows molecules in these gaps to interact strongly with light at room temperature, producing hybrid states that blend characteristics of both photons and molecules- a phenomenon known as strong coupling. By focusing light into such small volumes, these cavities enable a platform to probe both strong and weak coupling regimes.To explore these interactions, a range of experimental setups were developed that enabled simultaneous Rayleigh and Raman scattering measurements from individual nanocavities and introduced new methods for tracking polariton dynamics with femtosecond pulses. In conditions of strong coupling, plasmonic nanoparticle on mirror cavities paired with Methylene Blue molecules demonstrated hybrid light-matter states. Through extensive quantitative measurements, this study identified key parameters that drive strong coupling and distinguished it from the mechanisms underlying Surface-Enhanced Raman Scattering, offering a new understanding of light-molecule interactions. The study also delves into dynamic phenomena like spectral diffusion, observed in host-guest molecular systems within plasmonic nanocavities, revealing time-dependent spectral shifts that shed light on molecular behavior in confined fields.Control over these nanoscale interactions was achieved by tuning the cavity resonance through both refractive index means and femtosecond optical pulses, enabling targeted shifts in the plasmonic resonance. Additionally, the use of femtosecond pulses allowed investigation of decay dynamics in strongly coupled systems, advancing optical control methods for polaritons and capturing previously unobserved spectral evolutions in hybrid light-matter states.Finally, a broadband white-light interferometric technique was employed to measure the spectral phase of polaritons directly, revealing phase shifts across hybrid light-matter states and adding to the understanding of phase dynamics in strongly coupled plasmonic systems.By bridging the themes of understanding and control this thesis contributes a holistic view of light-matter interactions at the nanoscale, setting the stage for future advancements in plasmonic nanocavities and their potential applications in areas such as chemical sensing and nanoscale spectroscopy.

Committee:
     PRESIDENT: D\'ANGELO, MILENA
     SECRETARI: LOZA ALVAREZ, PABLO
     VOCAL: JULIA DIAZ, BRUNO
Thesis abstract: Two-photon quantum correlations are a resource for quantum communication, sensing, and imaging. Coincidence-based quantum imaging leverages spatiotemporal quantum correlations to form images from two-photon coincidences rather than intensity alone. To be useful beyond free-space laboratory setups, such as in access-constrained, realistic settings, quantum correlations must be distributed through compact waveguides while remaining usable after propagation across many spatial modes and at practical rates.This thesis develops methods for waveguide-based quantum imaging by integrating and advancing three main technology components: engineered quantum light sources based on spontaneous parametric down-conversion (SPDC), optical waveguides (including disordered and multicore optical fibre), and detection with single-photon avalanche diode (SPAD) array cameras, through both time-tagging and rolling-delay compensation. We demonstrate and quantify transport of quantum spatiotemporal correlations, and implement quantum imaging in a waveguided geometry using optical fibre. In addition, we show real-time operation with continuous coincidence image frames. The main specific achievements are:- Foundations for correlation transport. We model the propagation of quantum correlated SPDC two-photon states in media (continuous and discrete pictures), identifying measurable signatures of correlation conservation after transport. We develop a protocol that measures the waveguide point-spread function via coincidence imaging with a time-tagging SPAD array and timestamp post-processing. Using these tools, we define metrics to certify and quantify correlation transport through waveguides, and map its dependence on waveguide parameters. - Waveguide-based transport of SPDC correlations We validate transmission of SPDC spatiotemporal correlations through a custom-fabricated transverse Anderson localization optical fibre and a commercially available multicore fibre (MCF), showing high-dimensional, mode-parallel delivery of correlations necessary for coincidence imaging protocols. We inject position anticorrelated photon pairs into the fibres, extract coincidences from SPAD array timestamps, characterize the waveguide point-spread function through the coincidence-imaging protocol and quantify preservation of correlations. - Waveguided quantum ghost imaging. Building on the established quantum correlation distribution, we implement waveguided quantum ghost imaging (QGI) through a MCF using a non-degenerate signal-idler photon pair SPDC source. The idler illuminates the sample through the waveguide and is detected with a single SPAD, while the signal, which has not interacted with the sample, is imaged with spatial resolution on a SPAD array camera. We report image quality and imaging rates, and identify limits of optical resolution. - Real-time quantum ghost imaging. We realize real-time, low-latency QGI (free-space and waveguided) by leveraging in-pixel asynchronous coincidence extraction in the SPAD array camera, eliminating external post-processing latency. Coincidence image frames are produced continuously, supporting live alignment and use.Together, these results provide methods for waveguided, mode-parallel delivery and measurement of quantum correlations using optical fibre transport and SPAD array camera coincidence imaging, and thus extend quantum imaging technology towards practical waveguided settings. This may outline routes to applications in constrained environments, low photon flux regimes, and imaging at unusual wavelengths, potentially enabling new use-cases for quantum imaging, for example in the life sciences.

Committee:
     PRESIDENT: ALONSO GONZÁLEZ, PABLO
     SECRETARI: PRUNERI, VALERIO
     VOCAL: CALDWELL, JOSHUA
Thesis abstract: The mid-infrared spectral region holds significant potential for applications in energy harvesting and waste-heat recovery, radiative cooling, spectroscopy, sensing, thermal camouflage and night vision, among others. Conventional approaches to controlling mid-infrared (mid-IR) radiation with metamaterials and metasurfaces often rely on intricate fabrication methods. Commercial components for mid-IR photonics rely on materials that limit their scalability and accessibility. In this thesis, we explore how van der Waals (vdW) heterostructures, with their intrinsically anisotropic optical properties and deeply subwavelength thicknesses, enable unprecedented manipulation of thermal emission in terms of directionality, polarization, and chirality.We first introduce a straightforward far-field method to extract the complex dielectric function of microscopic exfoliated flakes, facilitating accurate characterization of highly dispersive polar materials without sophisticated near-field instrumentation. We demonstrate how ultrathin flakes of α-molybdenum trioxide (α-MoO₃) can serve as deeply subwavelength phase retarders in the mid-IR, enabling efficient polarization control at spectral regions inaccessible to conventional bulk optical components. Moreover, we show that by simply twisting two anisotropic flakes, intrinsic mid-IR chirality can be engineered, resulting in circular dichroism in both absorption and thermal emission, effectively transforming inherently incoherent blackbody radiation into circularly polarized emission.Finally, we develop structures based on anisotropic dielectric spacers within Salisbury screen configurations, enabling simultaneous control over the azimuthal and zenithal angles of emitted thermal radiation. Through analytical and numerical analysis, clear design principles are derived and validated using realistic materials. The results presented here establish vdW materials and their heterostructures as versatile platforms for advanced mid-infrared photonic applications, significantly enhancing our capability to precisely tailor thermal radiation across a broad range of practical applications.

Committee:
     PRESIDENT: SBALZARINI, IVO
     SECRETARI: GARCÍA PARAJO, MARÍA
     VOCAL: HÜMBELI, PATRICK
Thesis abstract: Understanding how a complex system works from its components, such as a virus invading a cell or particles aggregating in a liquid, is a fundamental question in the study of nature that provides great biological benefits. To solve this question, it is interesting to observe the path taken by the components of a system, as this contains valuable information that helps us to characterize them and understand how they interact with each other. Advances in the last decade in the field of machine learning offer a promising numerical tool, as they allow the automatic extraction of relevant features and relationships, while also predicting the system's behavior.In this thesis, we focus on the analysis of particle trajectories observed in complex systems, addressing two fundamental aspects:the random and therefore difficult-to-characterize individual behavior, as occurs in the lungs, where we inhale air and oxygen diffuses into the capillaries of the alveoli; and behavior due to multiple ways of interacting, in some cases unknown, such as that of a large flock of birds migrating together.In particular, we consider three problems:1) the accurate estimation of parameters that characterize the anomalous diffusion observed in biological processes,2) the identification of significant parameters to describe stochastic processes,and 3) the extraction of the functional form of the multiple forces present in particle systems.To tackle each of the problems, we developed a specific machine learning model designed to extract meaningful information from trajectories and rigorously evaluated it on a series of simulated systems with known dynamics.The first method, KISTEP, predicts anomalous diffusion properties at each time step, for trajectory segments, and for a set of trajectories, allowing for detailed analysis at each level, based on individual trajectories. With this method, we participated in the AnDi Challenge 2, a scientific competition comparing computational methods dedicated to characterizing fractional Brownian motion trajectories that resemble biological phenomena observed in experiments such as cell endocytosis or protein immobilization.The second method, SPIVAE, helps to identify the minimal representation of stochastic processes thanks to its unsupervised, interpretable, and generative features. Furthermore, it is capable of generating new trajectories that reproduce the learned characteristics of the process. The analysis performed with SPIVAE revealed the expected parameters of BM, fractional BM, and confined BM, while it learned a nonlinear combination in the case of the scaled BM.The third method, FISGAE, employs a graph neural network to infer in an unsupervised manner the functional form of the forces acting between particles. FISGAE successfully learned the forces between 21 interacting particles with non-reciprocal linear forces, while in the more complex scenario of a Lennard-Jones gas, it learned well the force at short distances.In conclusion, this research provides methods to facilitate the analysis of particle systems directly from their trajectories, unlocking insights otherwise unavailable. The proposed methods have the potential to benefit experimental and theoretical researchers, and even artificial intelligence developers, by enabling a more comprehensive understanding of complex systems. Furthermore, the developed frameworks are ready for future improvements, which could be achieved through the integration of more sophisticated architectures, thereby paving the way for even more advanced applications and discoveries.

Committee:
     PRESIDENT: STIER, ANDREAS
     SECRETARI: VAN HULST, NIEK
     VOCAL: GERARDOT, BRIAN
Thesis abstract: We investigate the physics of quantum-confined excitons in an electrostatically defined PN junction. Such a PN junction can be generated in an encapsulated monolayer \MoSe{} along the patterned edge of a top-gate electrode. Applying a voltage gradient between the top and bottom gates results in the formation of an in-plane electric-field gradient in the depletion region and strong doping gradients in the P- and N-doped regions of the PN junction. The exciton experiences an attractive force within the electric field gradient and repulsion from exciton-charge carrier interactions. The combined effects are sufficiently strong to quantize the number of available excitonic states within the potential. We show measurements of these quantum-confined excitons in reflection contrast and photoluminescence spectroscopy. We demonstrate how the confinement potential results in fine structure splitting with linearly polarized excitonic states that are aligned either along or perpendicular to the top gate edge. The states can be gradually tuned between linear and circular polarization using an out-of-plane magnetic field. Our particular sample geometry, where the formation of the PN junction electrically isolates the sample from ground, allows us to investigate the electrostatic model of a photo-biased PN junction, with the bias voltage as an additional tuning parameter of the confinement potential. The bias voltage is modified by the measurement itself and varies with the location of the measurement. We demonstrate how the combined impact of exciton dissociation and Auger-assisted hot-hole tunneling modifies the bias voltage over a timescale up to $\qty{100}{\s}$. The bias voltage can be further controlled using a second laser, which enables the tuning of the energy of the quantized states over the range of $\qty{15}{\meV}$. Ultimately, these findings allow us to simulate the exact shape of the confinement potential and to investigate how the in-plane electric field modifies the internal exciton structure, impacting the exciton oscillator strength, lifetime, and dissociation. The results of this thesis illustrate the possibilities to pattern tailored exciton potential shapes for photonic and quantum technologies.

Committee:
     PRESIDENT: ZEIHER, JOHANNES
     SECRETARI: DE RIEDMATTEN, HUGUES
     VOCAL: MALZ, DANIEL
Thesis abstract: Cold atom platforms have become central to quantum technologies such as information processing, simulation, and metrology. Their versatility and high degree of control make them especially powerful. A key breakthrough in the field was the ability to trap individual atoms using light. Far-off-resonant optical dipole traps—like optical tweezers and lattices—enable precise positioning of atoms in diverse geometries, from simple 2D arrays to complex 3D structures. Another major advantage of cold atoms is their ability to mediate interactions between photons, which do not naturally interact in free space. Cold atomic ensembles act as a nonlinear medium, enabling strong interactions even at the two-photon level. Using collective atom-photon coupling and Rydberg-state excitations, they allow for photon-photon gates and the creation of non-classical states of light. These two features—strong optical nonlinearities and precise atomic positioning—make cold atoms a leading platform for quantum networks, quantum simulations, and studies of light-matter interaction.A phenomenon common to both platforms is photon scattering, which can either be of an intended or unintended nature. Up until recently, simple theories of scattering were sufficient for the community. However, the advance of atomic platforms now requires more nuanced and sophisticated theories to understand scattering and their consequences on applications. This constitutes the main theme of the thesis. In the application of atom trapping, in many practical situations atoms may experience state-dependent potentials. The potential mismatch can lead to excess heating and reduced elastic scattering of light, as compared to well-known limits like an atom in “magic-wavelength” traps or a trapped ion. In the first part of the thesis, we develop a model to analyze these effects, which can have important consequences in quantum optics or in atom imaging.In the second part of the thesis, we investigate how Rydberg spin waves decohere in the presence of light scattering, within the context of Rydberg Electromagnetically Induced Transparency (EIT). Within Rydberg EIT, an initial photon is stored as a coherent, extended superposition across atoms. This initial photon can strongly modify the propagation of subsequent photons, leading to large nonlinearities, but the scattering of subsequent photons can reveal information about where the first photon was stored, leading to decoherence of the initial superposition state. This in turn can lead to decreased utility or ability to retrieve the first photon. Here, we elucidate the nature of decoherence, and in particular for the first time we take fully into account the three-dimensional nature of the ensemble and its multiple scattering of light. We find regimes in which multiple scattering might offer additional protection from decoherence, as compared to previous simplified theories. Overall, this thesis makes new advances in understanding the nature of microscopic atom-light interactions and scattering, and connects this fundamental physics to key consequences in real-life applications.

Committee:
     PRESIDENT: SCALARI, GIACOMO
     SECRETARI: VAN HULST, NIEK
     VOCAL: RESERBAT-PLANTEY, ANTOINE
Thesis abstract: The goal of this thesis is to probe the infrared optical response of twisted bilayer graphene (TBG) using Fourier transform infrared spectroscopy (FTIR). First, I used a commercial FTIR to measure the TBG in the mid-infrared range at room temperature. I improved the device fabrication technique and fabricated the TBG devices with a large area and simultaneously a low inhomogeneity. I observe that the TBG has abundant optical absorption features originating from the interband transitions that are uniquely determined by the twist angle. Then, I want to probe the interband transition of the TBG that lies in the terahertz range, which evolves the flat band of the TBG that hosts strongly correlated effects. I built a homemade FTIR that works in both the mid-infrared and terahertz range. I wired the cryostat carefully and achieved an electrical noise level approaching the Johnson noise limit. By guiding the light from the FITR into the cryostat, I successfully measured the exciton states in the Bernal bilayer graphene device over a broad spectral range, demonstrating that the system is ready for future experimental study of TBG.

Committee:
     PRESIDENT: CERULLO, GIULIO
     SECRETARI: LIGUORI, NICOLETTA
     VOCAL: PRINS, FERRY
Thesis abstract: The growing global energy demand, coupled with the need to reduce CO2 emissions, highlights the urgency of developing sustainable energy solutions. Despite its vast potential, solar energy remains underutilized due to technological challenges. Most solar technologies— including photovoltaics (PV), concentrated solar power, and artificial photosynthesis— depend on three core processes: light absorption, energy conversion, and energy transport. Understanding how light is converted and transmitted— primarily via exciton diffusion— in materials ranging from semiconductors to biomimetic and biological systems is essential for improving the performance of both optoelectronic devices and natural photosynthesis.This thesis examines how dimensionality, defects, and molecular geometry affect exciton diffusion, aiming to uncover structure-function relationships that govern energy transport in energy-related materials. Using advanced spatiotemporal microscopy — including Time-Correlated Single-Photon Counting Microscopy, Transient Reflection Microscopy, and a novel technique developed by my group, Structured Excitation Energy Transfer (StrEET) — this research investigates exciton diffusion in organic semiconductors, 2D perovskites, transition metal dichalcogenides (TMDCs), and bio-inspired systems.Key findings show that dimensionality critically influences exciton mobility. In Y6 organic films — a leading non-fullerene acceptor for organic photovoltaics — I performed the first direct measurements of exciton diffusion, revealing that confinement enhances mobility. Combined with morphological tuning via additives, diffusion coefficients increase by over 50%. In 2D perovskites, increasing thickness boosts both diffusion and anisotropy, yielding diffusion lengths well beyond those of conventional organic systems.TMDC studies reveal that, beyond dimensionality, defects and substrate interactions significantly affect exciton mobility. In suspended monolayers, multiple transport regimes — rapid, negative, and slow diffusion — are observed, each constrained by trap states and sensitive to structural and environmental changes.Lastly, this work explores how molecular packing and geometry influence exciton transport in bio-inspired systems like porphyrin films and bacterial LH2 networks. Using the novel highly sensitive StrEET technique, I conducted the first direct measurements of exciton transport in photosynthetic systems. Denser molecular packing enhances diffusion while reducing exciton lifetimes; the optimal diffusion length, comparable to that of top organic semiconductors, arises from a balance between these competing effects, offering insights into the design of artificial light-harvesting systems.Overall, this research demonstrates that excitonic transport can be engineered by tuning material properties such as dimensionality, defect density, and molecular organization. These findings provide guiding principles for developing more efficient optoelectronic and bio-inspired energy technologies, supporting the transition to sustainable energy solutions.

Committee:
     PRESIDENT: DOS SANTOS DUARTE VIEIRA HENRIQUES, RICARDO JOSÉ
     SECRETARI: DURDURAN, TURGUT
     VOCAL: ACCANTO, NICOLÒ
Thesis abstract: Bioluminescence microscopy presents a powerful alternative to fluorescence imaging by eliminating the need for external illumination, thereby avoiding issues such as phototoxicity, photobleaching, and background autofluorescence. However, the inherently low photon output of luciferase-based reporters significantly restricts the signal-to-noise ratio (SNR), as well as the achievable spatial and temporal resolution—challenges that are especially pronounced in dynamic or volumetric biological imaging. This thesis addresses these limitations by introducing a deep learning-driven imaging pipeline designed to enhance bioluminescence microscopy at both the data acquisition and image reconstruction stages.Our strategy integrates optical system design with advanced neural networks to enable rapid, high-resolution 3D imaging under extremely low-light conditions. We engineered a custom microscope featuring a highly compact optical axis and paired it with a single-photon sensitive camera, significantly boosting the SNR of bioluminescent images. To achieve fast volumetric imaging, we incorporated light field microscopy (LFM) and Fourier light field microscopy (FLFM), enabling single-shot 3D acquisition while improving axial and lateral resolution via Fourier-domain filtering. The primary objective of this work is to demonstrate how deep learning can substantially enhance bioluminescence microscopy, pushing the technique beyond its traditional limits in both 2D and 3D imaging.At the core of our approach is a suite of convolutional neural networks specifically trained on bioluminescent data. Using both synthetic and experimental datasets, we designed and trained models capable of extracting meaningful information from low-SNR raw data, recovering otherwise lost details and offering deeper insight into the biological sample. The models developed in this thesis cover key tasks such as denoising and reconstruction of wide-field, light field, and Fourier light field bioluminescent images. Together, they form a modular, learnable pipeline that significantly elevates the performance of bioluminescence microscopy in terms of both quality and speed.We validate our system using live biological samples, including Caenorhabditis elegans, mouse stem cells, and zebrafish embryos, capturing neuronal activity and intracellular dynamics at subsecond timescales. By placing deep learning at the heart of the imaging process, this work establishes a new paradigm for bioluminescence microscopy, transforming a traditionally low-SNR modality into a robust tool for fast, high-resolution, and label-specific imaging in living organisms.

Committee:
     PRESIDENT: NAVASCUES COBO, MIGUEL
     SECRETARI: CHANG, DARRICK
     VOCAL: WRIGHT, VICTORIA JANE
Thesis abstract: The thesis considers a number of optimisation techniques applied in the context of quantum information theory. After a pedagogical introduction of both quantum information theory and optimisation, it considers three main avenues of research. The first is the well-known foundational open problem of mutually unbiased bases, which consists of finding sets of orthonormal bases that are each unbiased with one another. More specifically, it remains unknown whether one can find a set of 4 mutually unbiased bases in dimension 6. A variety of optimisation techniques are applied, including non-linear semidefinite programming, see-saw optimisation, semidefinite programming relaxations, branch-and-cut, gradient descent methods and the method of Lagrange multipliers, each providing further insights into the problem. The second avenue is that of Bell nonlocality, more specifically attempting to simplify the hierarchy of semidefinite programs known as the NPA (Navascués-Pironio-Acín) hierarchy used to find bounds on the maximum quantum violation of Bell inequalities. For the case in which one has a large number of inputs per party, advantage in both memory and time versus state-of-the-art solvers is demonstrated using a combination several optimisation techniques. The third avenue is that of many-body quantum physics, which encompasses a wide range of topics. The thesis considers the problems of bounding expectation values of observables over the steady-states of open quantum systems, finding improved Fermion-to-qubit mappings and solving the graph colouring problem with a novel qudit-inspired optimisation algorithm. In each case, advantage versus comparable methods is demonstrated.

Committee:
     PRESIDENT: GRANGIER, PHILIPPE
     SECRETARI: ACÍN DAL MASCHIO, ANTONIO
     VOCAL: BELEN SAINZ, ANA
Thesis abstract: This thesis primarily aims at developing reliable theoretical tools to certify the preparation of entangled states and other quantum correlations in many-body systems from accessible observables. In doing so, we reconcile various information-theoretic measures to the laboratory byconstructing witnesses that can be readily tested in current experiments. In the course of this work, we address the certification of a number of resources related to quantum entanglement ranging from coherence to Bell nonlocality. A common aspect among these resources is their convexity, namely, the fact that the resource content cannot be produced nor amplified by mere statistical mixing of different states. This observation is also a key technical property for almost all of our contributions. Here, we focus on those many-body systems that are most easily probed by permutation-invariant or collective observables, such as spin ensembles or spinor Bose Einstein condensates. In this respect, the symmetries of the observables can be leveraged to construct entanglement criteria with a more favorable scaling. The resource content of a physical system is certified from the statisticsit produces. Within the quantum formalism, such statistics are encoded in the density matrix, which is reconstructed based on finite information from experimentally available probes. We start the thesis by outlining a practical machine-learning assisted protocol to improve and denoise the inference of such statistics in realistic scenarios. Subsequently, we discuss the certification of metrologically useful entanglement by introducing a simple algorithm to evaluate the minimal quantum Fisher information compatible with a set of arbitrary mean values. Our approach enables to systematically tighten well-known spin-squeezing parameters and reveal the sensing power of many-body states with minimal experimental effort. Next, we address the detection of entanglement from averages and uncertainties of collective observables by formulating a single condition testing a number of witnesses, including those proposed in the past such as the generalized spin squeezing inequalities. We apply our approach to unveil new entanglement witnesses tailored to Bose-Einstein condensates based on Zeeman sublevels populations. We also discuss, to some extent, the witnessing of the Schmidt number, the central bipartite entanglement measure, using similar observables. Then, we tackle the converse problem of detecting separable states from mathematical techniques based on invertible positive maps. The last part of the thesis is devoted to Bell nonlocality, one of the strongest forms of nonclassicality beyond quantum entanglement. We first scale Bell dimension witness, i.e. criteria whose violation signals the impossibility of explaining the inferred statistics with a Hilbert space of a given local dimension, to the many-body regime. In particular, we propose that the violation depth of a specific three-outcome Bell inequality can be used to robustly certify the number of qutrits in an ensemble. We close the thesis by presenting a data-driven approach to detect Bellnonlocality from one- and two-body spin correlations averaged over all permutation of parties. This methodology allows us to discover tighter Bell inequalities tailored to spin squeezed states and many-body spin singlets of arbitrary spin.

Committee:
     PRESIDENT: DIAMANTI, ELENI
     SECRETARI: PRUNERI, VALERIO
     VOCAL: BROWN, PETER
Thesis abstract: With this Thesis, we will focus on diverse technologies as well as techniques relevant for the development of secure communication systems based on Quantum Mechanics. This theoretical analysis involves the application of diverse methodologies, which represent formidable challenges from a purely analytical perspective, as well as from a numerical stance. The contents of this manuscript are divided according to the nature of the particular technology under scrutiny.As a first step, we will provide a study of Quantum Key Distribution based on continuous variable states and coarse-grained measurements. This inquiry is split according to two chapters. In the first one, a thorough description of our approach will be provided, as well as diverse outcomes regarding the addition of postselection techniques for an improved perfomance; a study of optimization in infinite dimensions, as well as results in the finite regime under collective attacks. In the second part, we will observe how performing a full discretization on the measurements grants not only full security against any adversary allowed by Quantum Mechanics, but also a new formalism via entropy accumulation approaches that permits the analysis of the statistical fluctuations that emerge when only finite statistics are accessible.This discussion will be followed by a study of Quantum Random Number Generation according to a semi Device Independent perspective where the assumptions on the experimental setups are limited. Our discussion will be based on using a trusted measurement and imposing a bound on the corresponding expectation value -- which requires an adaption from the well-known asymptotic limit to the the finite case in order to prevent emerging security pitfalls. This examination is completed with a thorough security proof against general attacks and diverse results in the finite regime. To conclude, this Thesis finishes with an assessment of Quantum Key Distribution for large networks based on classical Internet models, which provides insights on the scalability and optimality conditions of Quantum Communications according to the density of users and their connections. In particular, this indagation is mainly based on continuous variable systems, albeit another approach grounded on discrete variables is also added to the model in order to enhance its performance.

Committee:
     PRESIDENT: ANTHOPOULOS, THOMAS
     SECRETARI: BURGUES CEBALLOS, IGNASI
     VOCAL: CHRISTODOULOU, SOTIRIOS
Thesis abstract: Shortwave infrared (SWIR) light sources are highly significant due to their ability to interact with molecular bonds and penetrate materials with reduced scattering and absorption. These properties make SWIR light exceptionally valuable across diverse applications, including spectroscopic analysis, non-invasive biomedical imaging, food and agriculture, and environmental monitoring. However, traditional SWIR sources tend to be bulky, inefficient, and characterized by high bulb temperatures, prolonged warm-up times, limited dimming capabilities, relatively short lifespans, and high costs. This has resulted in an increasing demand for more efficient, compact, and cost-effective alternatives.Solution-processed colloidal quantum dots (CQDs) are promising candidates for advanced SWIR light sources due to their wavelength tunability, high photoluminescence quantum yield (PLQY), and cost-effective synthesis. While CQD-based light sources are well-established in the visible range, there is a need for further development of SWIR emitters. This thesis addresses this gap by utilizing CQDs to create efficient, flexible SWIR light emitters through a simplified and cost-effective fabrication method.We developed SWIR light emitters with an emission wavelength of around 1350 nm using the down-conversion (DC) technique, where lead sulfide (PbS) quantum dots (QDs) absorb high-energy photons and emit lower-energy SWIR photons. Down-conversion using QDs addresses certain drawbacks of conventional phosphor-converted LEDs based on lanthanides or transition metal ions, such as the need for complex fabrication processes involving high-temperature sintering or annealing, limited emission band tunability, and challenges in supporting pulsed operations. We used binary blends of large-bandgap matrix QDs and small-bandgap emitter QDs, as they are reported to improve the PLQY.Initially, flexible DC films were fabricated on PET substrates via solid-state ligand exchange (SSLE) and spin coating, with various ligands, including 3-Mercaptopropionic Acid (MPA), combinations of Zinc Iodide (ZnI2) and MPA, and combinations of 1-Ethyl-3-methylimidazolium Iodide (EMII) and MPA, studied. The best performance was achieved using MPA as the ligand, and selectively exciting the emitter QDs proved more efficient than exciting both matrix and emitter QDs. The DC film treated with MPA, when excited by a 980 nm LED produced a SWIR power density of 0.19 mW mm⁻². Despite these promising results, spin coating was found to be inefficient and labor-intensive, necessitating a more scalable method.To address this, we developed an alternative fabrication method using ethyl cellulose (EC) polymer, where oleic acid-capped QDs are mixed with EC to form flexible QD-EC composites. This approach is industrially adaptable, reduces QD usage by a factor of 20, eliminates wastage, and requires less manual effort than the SSLE process. It also allows for scalable fabrication of DC films in any size or shape. The maximum SWIR power density achieved for a DC film with OA-capped PbS QDs, without ligand modification, was 0.18 mW mm⁻² .To further enhance efficiency, we applied solution-phase ligand exchange (SPLE) using 1-dodecanethiol (DDthiol) to improve surface passivation and reduce non-radiative recombination. DC films made with DDthiol-treated PbS QDs demonstrated a three-fold increase in SWIR power output and reduced efficiency roll-off by 37% at higher excitation power, compared to films with oleic acid (OA)-capped QDs. The best-performing film, composed of DDthiol-treated matrix QDs and OA-capped emitter QDs, achieved a maximum SWIR power density of 0.54 mW mm⁻² . This methodology was further extended to develop SWIR light sources emitting at 1470 nm. In summary, we developed efficient and flexible SWIR light sources using solution-processed CQDs through a cost-effective and scalable fabrication method by overcoming the limitations of conventional sources.

Committee:
     PRESIDENT: MORTENSEN, NIELS ASGER
     SECRETARI: BACHTOLD, ADRIAN
     VOCAL: MANJAVACAS ARÉVALO, ALEJANDRO
Thesis abstract: In this thesis, we provided a theoretical description of how focused electron beams interact with low-energy excitations and generate nanoscale light, which is of great interest in the field of nanophotonics. In addition, we described light generation by tunneling electrons and applied this phenomenon to the detection of analytes through the changes that they produce in the far-field emitted intensity.Specifically, through detailed modeling, we have investigated the plasmonic behavior in nanostructured materials, the phononic responses in polar crystals, and light generation through inelastic electron tunneling. We also calculated the spectral distribution of energy losses experienced by fast electrons interacting with these excitations and obtained spatially resolved electron energy-loss spectra for diverse systems. Here, we summarize our key findings:In Chapter2, we explored that the infrared plasmon response of fluorine-doped indium oxide nanocube dimers is controlled by how the cubes touch: point and edge contacts produce a singular low-energy dipolar mode that vanishes when the gap opens, whereas face contacts give a smooth spectral shift. Electron energy loss spectroscopy measurements and simulations confirm this geometry-dependent behavior, challenging the assumption that all dimers are identical. These results provide new insights into gap plasmons for sensing and nonlinear optics applications.In Chapter3, our study of hexagonal boron nitride nano-ellipsoids shows that nonlocal effects rule the low-energy phonon polaritons when the particle size drops below a few tens of nanometers. An atomistic model that includes long-range dipole–dipole interactions explains both the surface-confined and bulk-like modes seen in monochromated scanning transmission electron microscopy-electron energy loss spectroscopy, while a simple local dielectric model misses the surface feature entirely. We also find that finite size and surface polarization reshape the vibrational spectrum. These results underline that any realistic design of mid-infrared hexagonal boron nitride devices must account for spatial dispersion at the atomic scale. By integrating advanced microscopy, atomistic theory, and computational modeling, this research provides a blueprint for studying vibrational excitations in low-dimensional polar materials. In Chapter5, it introduced a self-illuminating plasmonic biosensor driven by electron tunneling. The device utilizes an metal-insulator-metal tunnel junction with a gold nanowire metasurface on top, enhancing electron-to-light conversion via plasmonic resonance. The developed sensor demonstrates spatially uniform emission over large areas and exhibits high sensitivity for detecting thin polymer films and biomolecular layers. This work offers a practical platform suited for integrated biosensing without relying on external illumination sources. Hence, angle-resolved photodetection could provide deeper insights into plasmonic mode structures, leading to enhanced spectral selectivity and sensitivity. The interplay between electron probes, excitations, and nanoscale light generation offers numerous exciting opportunities for future research. We believe that the insights and methods presented in this thesis can contribute significantly to exploring these promising new directions.

Committee:
     PRESIDENT: VON KLITZING, WOLF DIETRICH CARL
     SECRETARI: DE RIEDMATTEN, HUGUES
     VOCAL: HUGBART, MATHILDE
Thesis abstract: This thesis studies the use of a single trapped neutral 87Rb atom as a photon counter. Detection of quantum jumps (QJs), i.e., abrupt changes between atomic states observable by a change in atomic fluorescence, is used to infer the arrival of single photons. This is referred to as the quantum jump photodetection (QJPD) technique.The thesis first situates QJPD in the context of photodetection. Compared to traditional detectors, QJPD technique has lower speed and quantum efficiency (QE), but has exceptional performance in other figures of merit: QJPD is intrinsically narrowband, has strong out-of-band rejection, and very low dark counts (DCs). These features make the QJPD interesting for applications that detect weak optical signals in the presence of a strong broadband background.Experimental methods to study QJPD are described. A 87Rb atom is loaded from a magneto-optical trap (MOT) into a far-off resonance trap (FORT) at the center of four orthogonal, co-focal, high numerical aperture lenses. These lenses create the FORT, couple probe light onto the atom and collect the atomic fluorescence, which is used to identify the atomic state. A typical QJPD sequence is presented, which consists of trapping and cooling an atom in the FORT, optically pumping it into the dark state, illuminating with probe light, illuminating with readout light and collecting fluorescence photons, and checking that the atom has not left the FORT during the sequence.Statistical methods for measurement of QE and DC contributions are introduced. These compare the observed fluorescence count distribution against measured hyperfine-state fluorescence distributions. A QE of (2.4±0.1)×e−3 is demonstrated, a record for single photon absorption by a single atom in free space.Dark count contributions are measured. To produce low DC, the QJPD technique is implemented in two time windows: an exposure time for the single photon absorption followed by a short fluorescence time to read out the atomic state. This implies distinct acquisition and readout DC contributions, similarly to CCD and CMOS detectors. A dark jump rate (analogous to CCD/CMOS dark current) is measured of (5 ± 10) × e−3 jumps/s, consistent with zero and limited by measurement statistics. The measured readout contribution is (4.0 ± 0.4) × e−3 jumps per ms of fluorescence readout. For a 1 Hz readout rate, with 1 ms readout pulses, a net dark count rate of (15 ± 10) × e−3 counts/acquisition is demonstrated, which is already competitive with any non-cryogenic detector.The background rejection capabilities of the system are tested by measuring quantum jump rates when the atom is illuminated with direct sunlight, and with light scattered by the atmosphere (skylight). A rate equation model is developed to describe QJ probabilities in the presence of both intense broadband background and weak resonant probe light. This model is used to interpret experiments in which a weak signal beam competes with strong broadband background and validated using direct sunlight. Measurements where the atom is illuminated with skylight show no observable background-induced QJs. Finally, measurements of sky brightness and its fluctuations are presented, showing large fluctuations even on mostly clear days, a factor that further increases the need for background rejection.A number of contemporary applications of extreme photodetection, including free-space quantum communication in daylight, classical optical communications in space, and fundamental physics experiments, are discussed as possible applications of the QJPD technique. A realistic scenario where the demonstrated QJPD capabilities surpass the current performance of commercial single photodetectors is presented.Finally, potential improvements are discussed. It is shown that existing atomic and optical technologies could be applied to reach different wavelength ranges, narrower bandwidths, higher quantum efficiency, and lower dark counts.