The widespread implementation of many quantum technologies are impeded by challenges associated with the messiness of the real world. Optical microcavities are proven to enhance and control light–matter interactions on the nanoscale, allowing for the utilisation of quantum effects in thin film devices and at room temperature. This thesis experimentally demonstrates how these cavity-mediated effects can be harnessed to overcome fundamental limitations in energy storage and quantum sensing, demonstrating extended storage times and superlinear scaling of charging and discharging in model quantum batteries, and cavity-enhanced quantum sensing in a scalable design. Understanding these challenges and probing the limits of their potential will contribute significantly to the emerging field of applied quantum technologies.
The widespread implementation of many quantum technologies are impeded by challenges associated with the messiness of the real world. Optical microcavities are proven to enhance and control light–matter interactions on the nanoscale, allowing for the utilisation of quantum effects in thin film devices and at room temperature. This thesis experimentally demonstrates how these cavity-mediated effects can be harnessed to overcome fundamental limitations in energy storage and quantum sensing, demonstrating extended storage times and superlinear scaling of charging and discharging in model quantum batteries, and cavity-enhanced quantum sensing in a scalable design. Understanding these challenges and probing the limits of their potential will contribute significantly to the emerging field of applied quantum technologies.
While the fundamental principles of light-matter interaction are well-understood and drive countless technologies, the world of multiphoton processes remains a fascinating puzzle, holding the potential to drastically alter our understanding of how light interacts with matter at its most basic level. This rich interplay of light and matter unveils novel phenomena that can be harnessed for sensing with exceptional precision, as exemplified by multiphoton quantum sensing. This thesis delves into the applications of multiphoton quantum protocols, particularly in imaging, communication, and plasmonic sensing, to surpass classical limitations and achieve enhanced sensitivity. We explore the potential of multiphoton quantum processes, particularly in the nanoscale regime and within subsystems of macroscopic systems, where novel and ultra-sensitive sensing methodologies emerge. Subsequent chapters of this thesis demonstrate the transformative potential of multiphoton quantum sensing, elucidating the design, implementation, and experimental results of specific sensing protocols tailored to diverse applications. Our analysis combines experimental observations and theoretical predictions to assess the sensitivity and performance of these protocols. Additionally, the thesis discusses potential future directions and advancements in the field, envisioning applications in biomolecule detection, environmental monitoring, and fundamental studies of light-matter interactions at the nanoscale. Concluding reflections highlight the implications of multiphoton quantum sensing across scientific disciplines and lay the groundwork for future research endeavors.
While the fundamental principles of light-matter interaction are well-understood and drive countless technologies, the world of multiphoton processes remains a fascinating puzzle, holding the potential to drastically alter our understanding of how light interacts with matter at its most basic level [1–4]. This rich interplay of light and matter unveils novel phenomena that can be harnessed for sensing with exceptional precision, as exemplified by multiphoton quantum sensing. This thesis delves into the applications of multiphoton quantum protocols, particularly in imaging, communication, and plasmonic sensing, to surpass classical limitations and achieve enhanced sensitivity. We explore the potential of multiphoton quantum processes, particularly in the nanoscale regime and within subsystems of macroscopic systems, where novel and ultra-sensitive sensing methodologies emerge. Subsequent chapters of this thesis demonstrate the transformative potential of multiphoton quantum sensing, elucidating the design, implementation, and experimental results of specific sensing protocols tailored to diverse applications. Our analysis combines experimental observations and theoretical predictions to assess the sensitivity and performance of these protocols. Additionally, the thesis discusses potential future directions and advancements in the field, envisioning applications in biomolecule detection, environmental monitoring, and fundamental studies of light-matter interactions at the nanoscale. Concluding reflections highlight the implications of multiphoton quantum sensing across scientific disciplines and lay the groundwork for future research endeavors.
The interaction of light with matter is at the heart of quantum optics, which itself enables insight into the fundamental aspects of quantum mechanics. First experimental access to this research field has been realized by coupling atoms to light. Here, the transition between discrete energy states of the atom is associated with the absorption and emission of a photon, a single quantum of the electromagnetic field. As a central aspect of quantum optics, the light-emitter interaction can be significantly enhanced by placing the emitter in optical cavity that is on resonance with the emitter. In recent years, this has led to a rapidly evolving research field known as cavity quantum electrodynamics (CQED). In CQED two different regimes are distinguished: the strong and the weak coupling regimes. In the strong coupling regime, the emitted photon is reflected from the cavity mirrors and eventually reabsorbed by the emitter. In contrast, the weak coupling regime describes the irreversible emission, where the photon leaks out of the cavity before it can be reabsorbed. Both the weak and strong coupling regimes enabled fundamental experiments for a better understanding of quantum optical phenomena. \n \n \n \nCQED grants access to the quantum world and hence offers potentially revolutionizing applications, particularly in the field of quantum information processing. A central aspect for the successful implementation of quantum applications is the system's scalability. Unfortunately, placing atoms deterministically inside a cavity remains technologically elaborate and hence minimizes the prospect of scaling a atom-CQED system. \n \nA possibility to address this issue is to implement CQED in the solid state, where sophisticated fabrication strategies allow miniaturization and scalability of the system. Particularly the development of self-assembled quantum dots (QD) in semiconductors represent a promising route. QDs can be considered as artificial atoms that mimic the atomic two-level system. These structures interact strongly with light and therefore have the potential for replacing atoms in CQED. As a central advantage, QDs are naturally trapped, which greatly simplifies the deterministic incorporation into the cavity. \n \nIn recent years, many efforts have been made to couple self-assembled QDs to microcavities. Generally, the successful implementation of CQED requires a cavity with a high quality factor Q and a low mode volume. In a majority of the approaches, the high Q/small mode volume cavities were monolithically defined around the QD, embedding the QD at a fixed position inside the cavity. Both the weak and strong coupling regimes have been reached with these systems. However, for future applications they suffer from several disadvantages. The fixed position of the QD inside the cavity minimizes the prospects for spectral tunability and spatial positioning the QD inside the cavity. Furthermore, prospects for further increasing of the cavity Q-factor and minimization of the mode volume remain limited in these systems. \n \nIn this thesis the mentioned disadvantages are addressed by developing a fully tunable miniaturized Fabry-Pérot microcavity with low mode volume. The design enables both spatial positioning of the emitter inside the cavity and spectral tunability. Successful coupling of a single QD to the microcavity is demonstrated including the strong coupling regime. Further a new approach to decrease the cavity mode volume is presented, where we demonstrate weak coupling is achieved
The rate at which matter emits or absorbs light can be modified by its environment, as markedly exemplified by the widely studied phenomenon of superradiance. The reverse process, superabsorption, is harder to demonstrate because of the challenges of probing ultrafast processes and has only been seen for small numbers of atoms. Its central idea—superextensive scaling of absorption, meaning larger systems absorb faster—is also the key idea underpinning quantum batteries. Here, we implement experimentally a paradigmatic model of a quantum battery, constructed of a microcavity enclosing a molecular dye. Ultrafast optical spectroscopy allows us to observe charging dynamics at femtosecond resolution to demonstrate superextensive charging rates and storage capacity, in agreement with our theoretical modeling. We find that decoherence plays an important role in stabilizing energy storage. Our work opens future opportunities for harnessing collective effects in light-matter coupling for nanoscale energy capture, storage, and transport technologies.
The work in this thesis is embedded in the framework of quantum metrology and explores quantum effects in solid state emitters and optical sensing. Specifically, the thesis comprises studies on silicon vacancy centres in nanodiamonds, phase measurements and cavity optomechanics utilising optical squeezed states, and a theoretical study on quantum amplifiers. Due to its similarity to single atoms, colour centres in diamond are ideal objects for exploring and exploiting quantum effects, because they are comparably easy to produce, probe and maintain. While nitrogen vacancy centres are the most renowned colour centres, we studied the silicon vacancy (SiV−) centre. In bulk diamond it features strong zero-phonon-line emission and, at cryogenic temperatures, a linewidth of hundreds of MHz, but it displays a weak spin coherence in the order of ns. To suppress the relaxation process which limits the coherence time, we utilised SiV− centres in nanodiamond. By means of confocal microscopy and resonant excitation at cryogenic temperatures, we measured linewidths in recently developed nanodiamond which were an order of magnitude smaller compared to previous studies on SiV− nanodiamonds. Furthermore, we identified spectral diffusion as the main hindrance in extending spin coherence times. Overcoming this issue will provide a promising candidate as an emitter for quantum information. Next, the question of how squeezed states of light can improve optical sensing was addressed. For this purpose, a squeezed light source was designed and built from scratch, which achieved a noise suppression of −8 dB at an optical pump power of 40mW. The generated squeezed light was first used to demonstrate how Gaussian states and detection can beat the shot noise limit and Rayleigh criterion in phase measurements simultaneously. Compared to quantum phase measurements based on single photon states, this approach is inherently deterministic. In addition, the applied homodyne detection enables close-to-unity detection efficiencies and thereby outperforms single photon state strategies which rely on comparably inefficient or demanding detection techniques. A second experiment combined squeezed light and feedback control to cool an optomechanical system. This proof-of-principle study is the first reported squeezing enhanced optomechanical cooling experiment. Despite losses of more than 50% (a resulting noise suppression of −2 dB), the mechanical resonator was cooled from room temperature to 130K. This represents a 12% improvement compared to the use of a coherent state protocol. Finally, we theoretically investigated the fundamental properties of quantum amplifiers. Such devices can be used in information and sensing technology to amplify signals to overcome e.g. technical detection limitations. Amplified communication channels were characterised by applying the measure of mutual information I, as it offers strict bounds on the maximum achievable performance, which enabled a fair comparison between different applications scenarios. As a result, we identified two peculiar configurations: A configuration where amplification does not affect I, and a configuration where quantum correlations do not always lead to an enhanced I.
Quantum properties like coherence and entanglement can lead to enhanced performance characteristics in a wide range of applications including quantum computation, quantum memory storage, optical sensing, and energy harvesting. Entanglement is very sensitive to static and dynamical disorder. Similarly, the generation of highly-entangled states requires strong coupling or strong driving fields. Satisfying all of these requirements is generally quite difficult. In the first part of this thesis, we present an approach to overcome these limitations through the use of exotic light-matter states in hyperbolic media which provide a new approach to control quantum correlations and interatomic interactions. We reveal a class of excited-state, long-range interactions, referred to as Super-Coulombic interactions that are singular along a material-dependent resonance angle. In practical systems, the Super-Coulombic interaction achieves dipole-dipole coupling that is orders of magnitude larger than conventional approaches, while also occurring across a large frequency bandwidth making it robust to static energy-level disorder. This unique hyperbolic response is not only naturally occurring, found in materials like h-BN, BiTe2, BiSe2, and mono-layered black phosphorus, but can also be designed with artificial nanostructured materials (metamaterials) to create the desired hyperbolic dispersion across different parts of the electromagnetic spectrum. Our theoretical prediction motivated an intense search for the effect and was confirmed by an experimental demonstration at room temperature. To obtain agreement with experimental results, we present a rigorous theoretical framework that takes into account ensemble effects, finite-sized effects, and dimensional effects that arise from confined geometries ultimately modifying the Super-Coulombic spatial scaling law. In the second part of this thesis, we solve an outstanding theoretical problem dealing with the control of resonance energy transfer in nanophotonic environments in both the incoherent and coherent coupling limits. Resonance energy transfer is a fundamental process that is the subject of intense research across all sciences. For example, in chemistry for drug delivery and chemical monitoring, in engineering for photovoltatic and up-conversion devices, and in biology for exciton transport within photosynthetic complexes. First, we consider the disordered and weak coupling limit of resonance energy transfer often encountered in chemistry. We propose new design principles for enhancing and suppressing the energy transfer rate and efficiency quantitatively captured by a simple image dipole model. Our theory explains a wide range of experimental results which have been the subject of an ongoing debate for the past 15 years. Second, we present our recent result aimed at understanding the fundamental role of entanglement and quantum coherence in resonance energy transfer. To uncover the role of these effects, we develop a unified theory of energy transfer valid from the incoherent to quantum coherent coupling regimes. Ultimately, our theory reveals a fundamental bound ηmax = γa for energy transfer efficiency arising from γd+γa the spontaneous emission rates γd and γa of the donor and acceptor. This bound provides an upper limit to the efficiency of energy transfer regardless of quantum coherence or entanglement, suggesting new design principles for achieving near-unity energy transfer efficiency in coherent systems. The result has important implications for the two-chromophore model found in photosynthetic complexes and paves the way for nanophotonic analogues of efficiency-enhancing environments mimicking biological photosynthetic systems.