Books in the Springer Theses series

Time, space and entanglement are the main characters in this book. Their nature is still a great mystery in physics and we study here the possibility that these three phenomena are closely connected, showing how entanglement can be at the basis of the emergence of time and space within closed quantum systems. We revisit and extend the Page and Wootters theory that was originally introduced in order to describe the emergence of time through entanglement between subsystems in a globally static, quantum Universe. In the book, after providing a complete review of the salient aspects of the theory, we establish a connection with recent research on the foundations of statistical mechanics and we provide a new understanding of the thermalization process. Furthermore, we generalize the framework in order describe the spatial degree of freedom and we provide a model of 3+1 dimensional, quantum spacetime emerging from entanglement among different subsystems in a globally "timeless" and "positionless" Universe. Finally, via the Page and Wootters theory, the evolution of quantum clocks within a gravitational field is treated and a time dilation effect is obtained in agreement with the Schwarzschild solution.

In recent decades, twistor theory has grown into an irreplaceable tool for the study of scattering amplitudes in gauge theory and gravity. This book introduces the reader to cutting-edge advances in twistor theory and its applications to general relativity. The problem of graviton scattering in four dimensions is shown to be dual to dramatically simpler computations in a two-dimensional CFT known as a twistor sigma model. Twistor sigma models are the first step toward a holographic description of gravity in asymptotically flat space-times. They underpin the infinitely many asymptotic symmetries of flat space physics discovered in celestial holography, and extend them to exciting new arenas like curved space-times. They also yield intrinsically mathematical results in the field of hyperkähler manifolds. This volume will be of broad interest to students and researchers looking for an accessible entry point into twistor geometry, scattering amplitudes, and celestial holography. It will also provide an invaluable reference for specialists by bringing together results from a host of different disciplines.

This thesis describes how the rich internal degrees of freedom of molecules can be exploited to construct the first ¿clock¿ based on ultracold molecules, rather than atoms. By holding the molecules in an optical lattice trap, the vibrational clock is engineered to have a high oscillation quality factor, facilitating the full characterization of frequency shifts affecting the clock at the hertz level. The prototypical vibrational molecular clock is shown to have a systematic fractional uncertainty at the 14th decimal place, matching the performance of the earliest optical atomic lattice clocks. As part of this effort, deeply bound strontium dimers are coherently created, and ultracold collisions of these Van der Waals molecules are studied for the first time, revealing inelastic losses at the universal rate. The thesis reports one of the most accurate measurements of a molecule¿s vibrational transition frequency to date. The molecular clock lays the groundwork for explorations into terahertz metrology, quantum chemistry, and fundamental interactions at atomic length scales.

This thesis outlines the principles, device physics, and technological applications of electronics based on the ultra-wide bandgap semiconductor aluminum nitride. It discusses the basic principles of electrostatics and transport properties of polarization-induced two-dimensional electron and hole channels in semiconductor heterostructures based on aluminum nitride. It explains the discovery of high-density two-dimensional hole gases in undoped heterojunctions, and shows how these high conductivity n- and p-type channels are used for high performance nFETs and pFETs, along with wide bandgap RF, mm-wave, and CMOS applications. The thesis goes on to discuss how the several material advantages of aluminum nitride, such as its high thermal conductivity and piezoelectric coefficient, enable not just high performance of transistors, but also monolithic integration of passive elements such as high frequency filters, enabling a new form factor for integrated RF electronics.

This thesis makes significant advances in the use of microspheres in optical traps as highly precise sensing platforms. While optically trapped microspheres have recently proven their dominance in aqueous and vacuum environments, achieving state-of-the-art measurements of miniscule forces and torques, their sensitivity to perturbations in air has remained relatively unexplored. This thesis shows that, by uniquely operating in air and measuring its thermally-fluctuating instantaneous velocity, an optically trapped microsphere is an ultra-sensitive probe of both mass and sound. The mass of the microsphere is determined with similar accuracy to competitive methods but in a fraction of the measurement time and all while maintaining thermal equilibrium, unlike alternative methods. As an acoustic transducer, the air-based microsphere is uniquely sensitive to the velocity of sound, as opposed to the pressure measured by a traditional microphone. By comparison to state-of-the-art commercially-available velocity and pressure sensors, including the world¿s smallest measurement microphone, the microsphere sensing modality is shown to be both accurate and to have superior sensitivity at high frequencies. Applications for such high-frequency acoustic sensing include dosage monitoring in proton therapy for cancer and event discrimination in bubble chamber searches for dark matter. In addition to reporting these scientific results, the thesis is pedagogically organized to present the relevant history, theory, and technology in a straightforward way.

This book focuses on proposing a tsunami early warning system using data assimilation of offshore data. First, Green's Function-based Tsunami Data Assimilation (GFTDA) is proposed to reduce the computation time for assimilation. It can forecast the waveform at Points of Interest (PoIs) by superposing Green's functions between observational stations and PoIs. GFTDA achieves an equivalently high accuracy of tsunami forecasting to the previous approaches, while saving sufficient time to achieve an early warning. Second, a modified tsunami data assimilation method is explored for regions with a sparse observation network. The method uses interpolated waveforms at virtual stations to construct the complete wavefront for tsunami propagation. Its application to the 2009 Dusky Sound, New Zealand earthquake, and the 2015 Illapel earthquake revealed that adopting virtual stations greatly improved the tsunami forecasting accuracy for regions without a dense observation network. Finally, a real-time tsunami detection algorithm using Ensemble Empirical Mode Decomposition (EEMD) is presented. The tsunami signals of the offshore bottom pressure gauge can be automatically separated from the tidal components, seismic waves, and background noise. The algorithm could detect tsunami arrival with a short detection delay and accurately characterize the tsunami amplitude. Furthermore, the tsunami data assimilation approach is combined with the real-time tsunami detection algorithm, which is applied to the tsunami of the 2016 Fukushima earthquake. The proposed tsunami data assimilation approach can be put into practice with the help of the real-time tsunami detection algorithm.

This thesis investigates the detection efficiency of field-resolved measurements of ultrashort mid-infrared waves via electro-optic sampling for the first time. Employing high-power gate pulses and phase-matched upconversion in thick nonlinear crystals, unprecedented efficiencies are achieved for octave-spanning fields in this wavelength range. In combination with state-of-the art, high-power, ultrashort mid-infrared sources, this allows to demonstrate a new regime of linear detection dynamic range for field strengths from mV/cm to MV/cm-levels. These results crucially contribute to the development of field-resolved spectrometers for early disease detection, as fundamental vibrational modes of (bio-)molecules lie in the investigated spectral range.The results are discussed and compared with previous sensitivity records for electric-field measurements and reference is made to related implementations of the described characterization technique. Including a detailed theoretical description and simulation results, the work elucidates crucial scaling laws, characteristics and limitations. The thesis will thus serve as an educational introduction to the topic of field-resolved measurements using electro-optic sampling, giving detailed instructions on simulations and experimental implementations. At the same time, it showcases the state-of-the-art in terms of detection sensitivity for characterizing mid-infrared waves.

The thesis presents a systematic study of the Mpemba effect in a colloidal system with a micron-sized particle diffusing in a water bath. While the Mpemba effect, where a system's thermal relaxation time is a non-monotonic function of the initial temperature, has been observed in water since Aristotle's era, the underlying mechanism of the effect is still unknown. Recent studies indicate that the effect is not limited to water and has been studied both experimentally and numerically in a wide variety of systems. By carefully designing a double-well potential using feedback-based optical tweezers, the author demonstrates that an initially hot system can sometimes cool faster than an initially warm system. The author also presents the first observation in any system of another counterintuitive effect-the inverse Mpemba effect-where the colder of the two samples reaches the thermal equilibrium at a hot temperature first. The results for both the observations agree with theoretical predictions based on the Fokker-Planck equation. The experiments reveal that, for carefully chosen conditions, a strong version of both of the effects are observed where a system can relax to the bath temperature exponentially faster than under typical conditions.

This thesis presents the first realization of non-reciprocal energy transfer between two cantilevers by quantum vacuum fluctuations. According to quantum mechanics, vacuum is not empty but full of fluctuations due to zero-point energy. Such quantum vacuum fluctuations can lead to an attractive force between two neutral plates in vacuum ¿ the so-called Casimir effect ¿ which has attracted great attention as macroscopic evidence of quantum electromagnetic fluctuations, and can dominate the interaction between neutral surfaces at small separations. The first experimental demonstration of diode-like energy transport in vacuum reported in this thesis is a breakthrough in Casimir-based devices. It represents an efficient and robust way of regulating phonon transport along one preferable direction in vacuum. In addition, the three-body Casimir effects investigated in this thesis were used to realize a transistor-like three-terminal device with quantum vacuum fluctuations. These two breakthroughs pave the way for exploring and developing advanced Casimir-based devices with potential applications in quantum information science. This thesis also includes a study of the non-contact Casimir friction, which will enrich the understanding of quantum vacuum fluctuations.

Line intensity mapping (LIM) is an observational technique that probes the large-scale structure of the Universe by collecting light from a wide field of the sky. This book demonstrates a novel analysis method for LIM using machine learning (ML) technologies. The author develops a conditional generative adversarial network that separates designated emission signals from sources at different epochs. It thus provides, for the first time, an efficient way to extract signals from LIM data with foreground noise. The method is complementary to conventional statistical methods such as cross-correlation analysis. When applied to three-dimensional LIM data with wavelength information, high reproducibility is achieved under realistic conditions. The book further investigates how the trained machine extracts the signals, and discusses the limitation of the ML methods. Lastly an application of the LIM data to a study of cosmic reionization is presented. This book benefits students and researchers who are interested in using machine learning to multi-dimensional data not only in astronomy but also in general applications.

This thesis describes in-depth studies of the remarkable electronic transport within the ultrahigh conductivity delafossite metals PtCoO_2 and PdCoO_2 using the tool of focused ion beam (FIB) microstucturing. Despite being first synthesised over 50 years ago, important questions remain regarding both the origin of the unusually high conductivity of these compounds and the consequences of their unique properties for unconventional electronic transport, such as that within the ballistic regime. The thesis explores both these areas.High-energy electron irradiation is used to examine the effects of deliberately introducing point defects into PdCoO_2 and PtCoO_2, demonstrating that the extremely low resistivity of these materials stems from an extreme purity as high as 1 defect in 120,000 atoms, rather than a novel scattering suppression mechanism. In addition, studies of the electronic transport in micron-scale squares of these metals show that their broadly hexagonal Fermi surfaces lead not only to long range ballistic behaviour but novel ballistic regime phenomena which cannot be observed in materials with a higher-symmetry Fermi surface.

This work systematically investigates the use of high-quality (high-Q) resonators as coding particles of chipless cooperative radar targets to overcome clutter. Due to their high-Q, the backscattered signature can outlast clutter and permit reliable readouts in dynamic environments as well as its integration in other types of cooperative radar targets for joint identification, sensing, and ranging capabilities.This is first demonstrated with temperature and pressure sensors in the microwave frequency range, which include the characterization of a novel temperature sensor for machine tool monitoring up to 400 (deg)C, as well as inside the machine. Afterwards, the thesis proposes and demonstrates the use of metallic as well as dielectric Electromagnetic BandGap (EBG) structures to enable the realization and to enhance the capabilities at mm-Wave and THz frequencies compared to microwave frequencies with compact monolithic multi-resonator cooperative radar targets. Furthermore, this work studies the integration of resonators as coding particles inside larger retroreflective configurations such as Luneburg lenses to achieve long-range and high accuracy for localization and, at the same time, frequency coding robust against clutter for identification. Finally, the successful readout of these cooperative radar targets is demonstrated in cluttered dynamic environments, as well as with readers based on Frequency-Modulated Continuous-Wave (FMCW) radars.

This book presents innovative laser desorption ionization (LDI)-active nanophotonic structures for addressing the challenges that matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) is currently facing and for enhancing LDI efficiency. It presents a variety of cutting-edge nanophotonic structures to satisfy the mass-analytical needs of sensitivity, reproducibility, and quantification. As opposed to the commercialized, conventional organic matrix used in MALDI-MS, these nanostructures are validated to be highly effective in detecting small metabolites and drugs, highlighting their considerable potential in the mass spectrometry field. It also systematically elucidates fundamental LDI processes in terms of the photo-thermal, electronic, and structural characteristics of nanophotonic structures, offering mechanistic knowledge of LDI-MS. Even though LDI-MS performance is heavily influenced by a number of nanostructure parameters, relatively little is known about the LDI processes associated with those characteristics. An in-depth understanding of nanostructure characteristics and LDI mechanisms thus paves the way for more effective, rational design and development of nanostructures with improved LDI capabilities. Further, with a focus on multiple cascades in nanostructure functions in response to laser pulse stimuli, this book provides detailed, step-by-step procedures to design and construct a nanophotonic, LDI-active platform, which may serve as a roadmap for graduate students in the field of mass spectrometry. Readers, including graduate students, researchers, and experts working in the related areas of mass spectrometry, nanophotonics, and material science and material engineering, will find a wealth of useful information in this book.

This book introduces readers to the preparation of two-dimensional metal sulfide/oxide for CO2 photoreduction. Based on two-dimensional metal sulfide/oxide materials, this book establishes the structure-to-property relationships of photocatalyst for CO2 photoreduction, and reveals the intrinsic mechanism of the CO2 photoreduction by virtue of the in situ characterization techniques and the density functional theory calculations. It is anticipated that this book will help to identify empirical guidelines for designing and fabricating high-performance catalysts of solar-driven CO2 reduction.

This thesis makes significant advances towards an understanding of superconductivity in the cuprate family of unconventional, high-temperature superconductors. Even though the high-temperature superconductors were discovered over 35 years ago, there is not yet a general consensus on an acceptable theory of superconductivity in these materials. One of the early proposals suggested that collective magnetic excitations of the conduction electrons could lead them to form pairs, which in turn condense to form the superconducting state at a critical temperature Tc. Quantitative calculations of Tc using experimental data were, however, not available to verify the applicability of this magnetic mechanism. In this thesis, the author constructed an angle-resolved photoemission apparatus that could provide sufficiently accurate data of the electronic excitation spectra of samples in the normal state, data which was furthermore unusually devoid of any surface contamination. The author also applied the Bethe-Salpeter method to his uncommonly pristine and precise normal state data, and was able to predict the approximate superconducting transition temperatures of different samples. This rare combination of experiment with sophisticated theoretical calculations leads to the conclusion that antiferromagnetic correlations are a viable candidate for the pairing interaction in the cuprate superconductors.

This thesis highlights research explorations in quantum contextuality with photons.Quantum contextuality is one of the most intriguing and peculiar predictions of quantum mechanics. It is also a cornerstone in modern quantum information science. It is the origin of the famous quantum nonlocality and various nonclassical paradoxes. It is also a resource for many quantum information processing tasks and even universal quantum computing. Therefore, the study of quantum contextuality not only advances the comprehension of the foundations of quantum physics, but also facilitates the practical applications of quantum information technology.In the last fifteen years, the study of quantum contextuality has developed from a purely theoretical level to a stage where direct experimental tests become amenable. However, the experimental research on contextuality at the current stage largely focuses on direct validations of some most famous predictions of contextuality, while other forms of contextuality and its practical applications in quantum information science are rarely involved. The research in this thesis is committed to bridge this gap from two directions: (1) to construct and test stronger forms of contextuality and relieve the requirements of contextuality experiments on experimental platforms, and (2) to explore the connections between contextuality and the other concepts in quantum information science and directly demonstrate the application of contextuality in broader scenarios. Specifically, the thesis have discussed the research topics about the relationship between quantum contextuality and nonlocality, the ¿all-versus-nothing¿ paradoxes from quantum contextuality, the ore- and post-selection paradoxes from quantum contextuality, and the topological protection and braiding dynamics of quantum contextuality in quasiparticle systems.

The structure of quantum theory permits interference of indistinguishable paths. At the same time, however, it also limits such interference to certain orders and any higher-order interference is prohibited. This thesis develops and studies concepts to test quantum theory with higher-order interference using many-particle correlations, the latter being generally richer and typically more subtle than single-particle correlations. It is demonstrated that quantum theory in general allows for interference up to order 2M in M-particle correlations. Depending on the mutual coherence of the particles, however, the related interference hierarchy can terminate earlier. In this thesis, we show that mutually coherent particles can exhibit interference of the highest orders allowed. We further demonstrate that interference of mutually incoherent particles truncates already at order M+1, although interference of the latter is principally more multifaceted than their coherent counterpart. We introduce two families of many-particle Sorkin parameters, whose members are expected to be all zero when quantum mechanics holds. As proof of concept, we demonstrate the disparate vanishing of such higher-order interference terms as a function of coherence in experiments with mutually coherent and incoherent sources. Finally, we investigate the influence of exotic kinked or looped quantum paths, which are permitted by Feynman's path integral approach, in such setups.