There is now compelling evidence that ordinary baryonic matter only represents 15% of the matter content of the Universe. Observational results suggest that the remaining 85% may be constituted of dark matter possibly in the form of weakly interacting massive particles (WIMPs). One of the potential ways to detect these WIMPS is to look for their scattering interactions with nuclei. This is the basis of direct detection experiments. In particular, galactic signature direct detection experiments look for the characteristic properties of the WIMP signal. The current landscape of galactic signature experiments is dominated by two main types of experiments. Firstly, NaI (Tl) detectors are searching for the annual modulation of the WIMP recoil rate induced by the revolution of the Earth. Secondly, directional time projection chambers (TPCs) can reconstruct the momentum of the incoming scattering particle and determinate whether its origin is compatible with the WIMP wind. In this thesis, both types of experiments are addressed. For these rare-event searches, the performance of the detector is dictated by two linked parameters, the mass of target materials and the rate of background events. A new generation of galactic signature experiments is currently being developed. This work addresses the issue of the background levels through the use of Monte-Carlo simulations to predict the event rate associated with the different backgrounds. In the context of the COSINE experiment, these simulations investigate the neutron background in the detector and compare the associated rate to a theoretical model which proposes that neutrons may be responsible for the positive signal seen by the DAMA experiment. Otherwise, for the proposed CYGNUS experiments, these simulations are done in a way to facilitate the design effort of the collaboration and orientate the blueprints towards detectors which could potentially achieve background event rates below 1 per year. These efforts may potentially lead to the creation of background-free experiments larger than the DRIFT-IId TPC. Background-free status was achieved in DRIFT with the discovery of minority carriers in 2013. This thesis presents the current world-leading directional limit on the spin-dependent WIMP-proton cross section achieved with the DRIFT-IId detector. The recent detection of fast neutrons from the rock at the Boulby underground laboratory is also discussed. This is the first ever measurement of the concentration of radioisotopes in an underground laboratory using a TPC. This thesis is considering the impact that this new technique may have on future dark matter searches and how it may provide a new tool for neutron metrology in nuclear physics.

Liquid argon (LAr) time projection chambers (TPC) have rapidly increased in importance as particle detectors throughout the past four decades. While much research has been completed, there are still many areas which require further development to build and operate the next generation LAr TPC experiment, such as the Deep Underground Neutrino Experiment (DUNE). These include high voltage breakdown, argon purification and purity monitoring, and vacuum ultraviolet (VUV) scintillation light measurement. Visual monitoring of high voltage breakdown is helpful in allowing assessment of the performance of high voltage component design. Thus, a system of cryogenic cameras, the first of its kind, was developed for use in a large LAr cryostat, without the need for additional electronics heating. The system functioned without problem for 50 days at cryogenic temperature, with some degradation of image quality, and provided a useful monitor for the DUNE 35-ton cryogenics systems. The system did not observe any high voltage breakdowns during the run. Further development of the concept is ongoing for future installation in other experiments. The monitoring of LAr purity using TPC data is a fundamental study for LAr TPC experiments. However, the study has not been performed for a large LAr TPC in the presence of high electronic noise. Custom software was developed and validated for the accurate reconstruction of signals in noisy TPC data. The results of the reconstruction were used to successfully measure the LAr electron lifetime with an uncertainty comparable to alternate methods of measurement. The electron lifetime of the 35-ton Phase II run is determined to be $4.12\pm0.17$~(stat.)~$\pm0.40$~(syst.)~ms. For general purpose research and development of high purity LAr as a particle detection medium, a dedicated test stand was designed, constructed, and commissioned. The system is used to test the gaseous photomultiplier (GPM) performance at cryogenic temperatures. The GPM functions with photoelectron multiplication at 77~K, at a reduced gain. Further study is required to show the detector's direct sensitivity to LAr VUV scintillation light.

The work in this thesis is motivated by the goal of creating scalable quantum computers, and equally by the physical understanding that develops alongside and follows from this. The fields of physics and technology are symbiotic, and quantum information processing is a prime example. The field has the potential to test quantum mechanics in new and profound ways. Here we approach the technological problem by building upon the foundations laid by the semiconductor chip manufacturing industry. This architecture is based on the III-V semiconductors Gallium Arsenide and Indium Arsenide. Combining the two we can create chip-embedded atom-like light sources -- quantum dots -- that can produce quantum photonic states in lithographically etched nanoscale waveguides and cavities. We demonstrate the integration of quantum light sources and single-mode beam splitters in the same on-chip device. These are the two primary ingredients that are needed to produce the entangled states that are the basis of this type of quantum computing. Next we look at the quantum light source in more detail, showing that with cavity-enhancement we can significantly mitigate the detrimental dephasing associated with nanostructures. The source can be used as a means to produce coherently scattered photons in the waveguides. More importantly, the on-demand photons obtained from pulsed excitation are more indistinguishable and thus more suitable for quantum information carrying and processing. Through experiments and simulations, we investigate some aspects of single-photon sources under pulsed excitation, including emission rate, emission number probabilities, and indistinguishability. A new technique to measure very short lifetimes is demonstrated and examined theoretically. Finally we look at preliminary steps to extend the platform further. The inclusion of photonic crystals and superconducting nanowires provides on-chip filters and detectors, and etched diode structures enable electrical excitation and tunability of the circuit components. These show some clear paths that the work can continue to evolve along.

There are couple types of semiconductor microcavities. The most general structure type is fabricated using two distributed Bragg reflector (DBR) mirrors attached by a layer of semiconductor, whereas in open microcavity it is separated by a gap of air or vacum. In this device periodically repeating pairs of low and high refractive index semiconductor layes allows to reflect light very efficiently. Lately, interest increased in the use of open microcavities as they allow to investigate light and matter interactions in an alternative and more freely tunnable way. This system allows creating and studying of polaritons (which are quasi-particles created, when photons and excitons couple strongly) in case QWs (quantum wells) are introduced into the microcavity or even making a single photon source, in case a SAW (sound acoustic wave) device will be manufactured on the bottom mirror, which has QWs, successfully. Single photon source is very important in order to develop secure communication and quantum computers. A very welcoming use case of the open cavities is allowing to very effectively investigate vey small amounts of fluids in absorption spectroscopy, as well as development of ultralow-threshold lasers (low threshold appears, because photons spend more time in the cavity, which depends on Q-factor), because of recirculation insmall volume mode, interaction for the same incident power increases as I = P in λ2πnQV, enhanced emitters, tunable microfilters and etc. ([1],[2], [3]). Polaritons also have long propagation distances, which allows to consider using them for optical circuits ([4]). The project aim is to use this optical semiconductor microcavity system, which allows easy cavity tuning into resonance, to investigate various effects in coupled mirrors, achieve polariton blockade and etc. Samples were made by University of Cambridge, University of Oxford and III-IV semiconductor center in Sheffield. In Chapter 2 experiments using planar DBR mirrors inside open cavity are present. For example, Rabi dependence on cavity length and optimum threshold dependence on detuning. Chapter 3 has data collected using top sample containing top concave mirror. Effective mass and coupled concave mirror coupling strength dependence on cavity length are good examples. Chapter 4 contains introduction of new setup design, which works in transmission and not reflectivity mode, and experiments carried on using it, like achievement of a very efficient polariton transfer from LG01 to LG00 mode. Chapter 5 will introduce to experiments which will be carried out in the future. This includes use of Surface Acoustic Waves (SAW), organic materials and etc.

Cataclysmic variables (CVs) are binary star systems comprising a white dwarf which is accreting material from a Roche-lobe filling companion, usually a late-main sequence or evolved star. The accreted material accumulates on the surface of the white dwarf and ultimately leads to a thermonuclear runaway explosion, called a nova eruption. There are ∼ 400 recorded novae and some have shown more than one eruption. These systems are the recurrent novae, and they are one of the leading progenitor candidates of Type Ia Supernovae. In this thesis, I describe how I used high speed spectroscopy to determine the component masses of the stars in the eclipsing recurrent nova CI Aquilae. The masses I determined suggest it is indeed a progenitor of a Type Ia supernova, and will explode in 10 Myr. The long term impact of nova events on the evolution of CVs is poorly understood, and may be the reason for the diversity of CV types observed at the same orbital period. One theory, known as hibernation theory, proposes that the nova event causes systems to cycle through the various classes of CVs, due to heating and bloating of the secondary. In the second part of this thesis, I undertook searches for nova shells around known CVs, in an effort to determine the frequency and life-cycle of novae. I examined over 150 systems and found one definite shell around V1315 Aquilae. I then used high–resolution spectroscopy to determine the mass and age of this shell. This is the first discovery of a nova shell around a novalike variable. By combining my search results with simulations and the results of other recent searches for nova shells, I find that the lifetime of the novalike state is broadly in line with the nova-induced cycle theory (hibernation theory).

We have designed and implemented a low noise evanescent wave sensor (EWS) based on lock-in amplifier and sensitized glass optical fibre for detection of waterborne and airborne analytes. We stripped the buffer and cladding from a 2 cm section in the middle of a 20 cm long 400 μm core multimode glass optical fibre and coated it with a film of a sensitive molecule. We then ran the coated optical fibre through a sample vessel such that the film was in contact with a sample that may contain analyte, and probed with modulated light, coupled into the fibre from an LED. We matched the LED peak emission wavelength to the absorption peak of the sensitizer-analyte complex formed and used the reference voltage output of a digital lock-in amplifier for modulation. To account for possible LED drift, we propagated the modulated light through a 50:50 beam splitter, where one beam was coupled to the coated fibre (sample beam) and the other acts as a reference beam. We then projected the two beams onto two photodiodes connected to a 'light balance' circuit that delivers a differential current ∆i, which is proportional to the absorbance along the sample beam and converts it into a voltage. Finally, this voltage was fed into the digital lock-in amplifier for low- noise measurement. As a proof of concept, we spray-coated the stripped section of the optical fibre with 5,10,15,20-tetrakis[3,4-bis(2-ethylhexyloxy)phenyl]-21H,23H-porphine (EHO), which is sensitive to acetic acid and ran the fibre through a gas exposure chamber. Then we exposed acetic acid vapour, which is being detected, into the chamber and found a limit of detection (LoD), (defined as the lowest concentration that a sensor can detect) equal to 1.491 ppt (61.48 µM). To compare the performance of our EWS in air and in aqueous media, we coated the same sensitizer (EHO) to the stripped section of the fibre and run through deionised water in a sample vessel. Titrating acetic acid solution into the vessel, we obtained a LoD equal to 18.43 µM, which is 3.3 times better than in air. We further tested the performance of our EWS by spray-coating the stripped section of the fibre with 1-(2-pyridylazo)-2-naphthol (PAN) to detect aqueous Zn2+ and obtained 31.07 nM as a LoD. To improve the analytical performance of our EWS, we roughened the stripped section of the sensing fibre with a home-made roughening tool based on a Dremel tool. Then we spray-coated optical fibres with a macrocyclic sensitizer, zinc 5-(4-carboxyphenyl),10,15,20-triphenyl porphyrin (Zn(P-CO2H-TPP)) and used them to detect waterborne octylamine realizing a LoD equal to 2.17 μM. In conclusion, we find EWS sensors are particularly useful for sensing in the aqueous medium, as the higher refractive index (1.333) of water, (compared to air (1)) allows deeper evanescence of the light propagating inside the fibre, and hence better coupling to the sensitizer coating, than in gas sensing.

This thesis documents a data-driven methodology for the estimation of the multijet background in fully hadronic final states with high missing transverse momentum within the ATLAS detector at the Large Hadron Collider. The implementation of this methodology within two analyses searching for evidence of supersymmetric particle production using 36.1 fb−1 of √s = 13 TeV proton-proton collision data is described. The two analyses described in this thesis investigate supersymmetric particle production via the strong interaction both inclusively via the superpartners of the first and second generation quarks and exclusively via the superpartners of the third generation t and b quarks. Both analyses consider the eventual decay of any produced supersymmetric particles to standard model particles and the lightest neutralino, which escapes without detection and is observed as missing momentum in the transverse plane of the detector. Additionally, an investigation into the level of radiation damage received by the ATLAS silicon Semiconductor Tracker is presented with predictions of the level of radiation damage faced over the course of the planned lifetime of the detector.

As the precision of neutrino experiments is steadily improving, new data is beginning to highlight problems with current models of neutrino interactions. There exist several unresolved tensions within the global neutrino scattering dataset, and faced with these problems, experiments are forced to turn to data-driven models with errors that can span the spread in experimental observations. The NUISANCE software package was developed to provide a flexible framework for the community to use in the building of these models. This thesis focuses on the extraction of empirical model corrections, to account for several observed tensions in the global scattering dataset, that could have an impact on the T2K oscillation analysis. Charged current neutrino interaction measurements from MiniBooNE are used to study systematic shifts when tuning the NEUT event generator under different sterile neutrino mixing hypotheses. The results are used to derive an error rescaling method for use in future T2K short baseline oscillation fits. Alternative models for the quasi-elastic axial form factor are developed, and implemented into NEUT. Data from neutrino-deuterium scattering is used to test each model's implementation, and extract systematic uncertainties for input to the T2K long baseline oscillation analysis. In addition, NEUT predictions are compared to CC-inclusive data from the MINERvA experiment, highlighting a clear deficiency in its CC0π model. An empirical correction to both the quasi-elastic and multi-nucleon contributions is developed to account for this. Exclusive data from MINERvA is also used to test the GENIE event generator's pion production model. The standard GENIE dials are found to provide insufficient coverage of the chosen datasets, and clear tensions are found between these results and earlier tunings to deuterium scattering data. An ad-hoc model correction is developed to account for disagreements in the muon scattering angle distributions, reducing the tensions in the joint fits.

The main scope of this thesis is the investigation of quantum properties of nonlinear, dissipative, optical and condensed matter systems. Compared with previous studies on this subjects, we reached several milestone of general interests for the theoretical and experimental community. The main experimental platform we refer to in this thesis is microcavity polaritons: polaritons arise from the coupling of light with excitons in a quantum well, i.e., a 2D semiconductor material embedded in the microcavity. They have attracted the interest of the scientific community for their rich physics. Polaritons can form out-of-equilibrium Bose-Einstein condensates, they can experience superfluid phase transition and, due to their interaction via the excitonic component, they show a variety of phenomena which are typical of nonlinear physics, such as bistability and parametric scattering. In particular, motivated by recent proposals about the generation of single photons from weakly nonlinear system, like the unconventional photon blockade, and by the production of squeezed states in polariton systems, we studied the possibility of obtaining antibunched light and other quantum states (like squeezed states and entangled states), when the nonlinear system shows some peculiar features: in particular we consider polariton soliton and cascade systems. Finally, when considering nonlinear incoherent processes, we investigate the effect of the PT-symmetry and the PT-symmetry breaking on the quantum state of the emitted light. Hence, this research work has two main objectives: first, from the fundamental physics point of view, to understand the interplay between nonlinear phenomena and quantum optics, second to provide useful tools for future technological applications. Specifically, for the latter case, we demonstrate a theorem that links the measurement of intensity correlations with the occurrence of certain phase transitions, and we propose a setup to maximize the non-classicality of the light emitted by weakly non-linear systems via a cascade configuration.

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