Entangled photons are ideally suited to the transmission of photonic quantum information. Mitigating the effects of decoherence is fundamental to distributing photonic entanglement across large distances. One such proposal is entanglement distillation, in which operations on a large ensemble of weakly entangled states generate a smaller ensemble of more strongly entangled states. In this thesis, we experimentally and theoretically analyse various tools required for demonstrating continuous-variable (CV) entanglement distillation, following the proposal by Browne et al., [Phys. Rev. A <b>67</b>, 062320 (2003)]. Specifically, we propose figures of merit to account for the practical limitations of non-deterministic non-Gaussian operations, and analyse the experimental parameters necessary to optimise them. We develop a source of pulsed two-mode squeezed states, which are the initial states of our entanglement distillation protocol. We use weak-field homodyne detection as a phase-dependent photon counting detector, and demonstrate its utility in conditional state generation. Using these states, we demonstrate sub-binomial light as a tool for benchmarking quantum states. Finally, we applied two-mode weak-field homodyne detection to two entangled states and demonstrate correlations in the photon counting statistics which depend on a joint phase from two independent local oscillators. This setup is sufficient to apply an entanglement witness developed by Puentes et al. [New J. Phys. <b>12</b>, 033042 (2010)]. Despite encouraging simulations, we do not witness entanglement with this scheme, which we attribute to a noise source unaccounted for in the simulations. Although we do not demonstrate entanglement distillation outright, the tools we develop to do so represent a general, hybrid approach to CV quantum optics. Developing tools such as phase-resolved projective measurement on two-mode states allows us to probe both the wave and particle nature of entangled light at the single-photon level. Using and expanding these techniques to probe larger quantum systems may prove useful in studies of fundamental physics and quantum enhanced technologies.

The development of multimode absorption spectroscopy (MUMAS) for multi-species detec- tion and its potential for process control or environmental monitoring is reported. The simultaneous detection of CO and CO2 is demonstrated in a proof-of-principle experiment for applications in industrially relevant gas species monitoring. The technique of MUMAS is extended to the near infrared in order to detect these and other industrially relevant species. A laser was designed and constructed to emit a multimode spectrum in the region of 1.57um to take advantage of the spectral overlap of the second vibrational overtone of CO and the combination band 3ν1 + ν3 of CO2. The laser consisted of a semi-confocal cavity employing an Er:Yb glass chip as the gain medium. The laser was pumped by a 1W laser diode at 980nm and emitted up to 30mW in a bandwidth of 180GHz. The laser emitted between 6-10 modes depending upon the selective cavity length. Mode spacings varied between 18GHz to 33GHz with an individual mode linewidth of less than 8MHz. The laser modes were simultaneously scanned using a piezo-electric transducer (PZT) in order to modulate the cavity length at frequencies between 1Hz and 10Hz. A system for linearizing the MUMAS spectra with respect to frequency was devised based on a transmission spectra of a confocal Fabry-Perot etalon. Refinements to the MUMAS fitting code were developed to improve the computational efficiency. An initial demonstration of MUMAS on a known gas mixture of CO and CO2 was per- formed. The ratio of CO:CO2 concentrations in the gas mixture was measured with an accuracy of 0.4% which was within the supplier’s quoted uncertainty. MUMAS is then applied to the detection of CO and CO2 concentrations in exhaust gas produced by a 1.3 litre 4-cylinder turbo-charged spark ignition engine. Relative and absolute concentrations were derived from MUMAS signals and values compared to measurements using a 4-gas analyser. Concentrations of CO and CO2 were measured using MUMAS to a precision of 0.17% and 0.23% respectively compared to less than 0.1% for the 4-gas analyser. Ratios of CO and CO2 were determined with a precision of 0.28 using MUMAS compared to 0.11 with the 4-gas analyser. The detection limit of CO was found to be 1486ppm in these circumstances. Finally a discussion is presented of potential improvements arising from wavelength mod- ulation spectroscopy and cavity enhancement techniques.

In the present quantum optics and laser physics study, the non-linear interaction of electromagnetic fields with atomic, molecular and biomolecular media is analyzed. Particular emphasis is given to coherent phenomena, while propagation and dispersion effects are also extensively investigated. The fields involved vary from ultra short pulses to continuous waves; while their energies range from the very strong that are addressed classically, to the very weak which are described quantum mechanically. Applications and problems addressed span a wide range. A scheme for a real time detector of chemical and biological hazards, like anthrax spores, is presented; in it, a strong spectroscopic signature is obtained from complex molecules by using ultrashort, femtosecond, laser pulses and inducing vibrational coherence on them. Furthermore, a way of reversing the phase matching condition in coherent spectroscopy, based on dispersion, is developed; which allows for the use of such spectroscopic methods in remote detection. More fundamental questions addressed include a resolution of the centennial old paradox of Maxwell's demon via quantum thermodynamics, and the role of atomic coherence in enhancing the efficiency of a heat engine as well as in obtaining lasing without population inversion. Additionally, a quantum storage scheme is presented, in which the information contained in an optical pulse is stored and restored via photon echoes.

This thesis is about the experimental implementation of a high-speed and robust quantum memory for light. A novel far off-resonant Raman approach to ensemble-based quantum memories in a room-temperature environment is developed and demonstrated. Storage and retrieval of sub-nanosecond, weak coherent light pulses at the single-photon-level with total efficiencies exceeding 30% and storage times of up to 4 μs are achieved. The coherence of the memory is shown by directly interfering a copy of the incident signal with the retrieved signal from the memory. The unconditional noise floor of the memory is found to be low enough to operate the memory in the quantum regime at room temperature. Multiple readout of a single stored excitation is demonstrated, suggesting that 100% readout is possible in different temporal modes. Furthermore, first results regarding the storage and retrieval of polarisation encoded qubits are obtained. This and the memory’s ability to operate in the quantum regime at room temperature with a low unconditional noise floor illustrate its potential usefulness for real world applications.

The development of high average and peak power ultrashort pulsed fiber lasers is important for many critical research, industrial, and defense applications. However, the performance of mode-locked fiber oscillators still lags behind that of solid-state counterparts such as Kerr-lens mode-locked Ti:sapphire lasers. Despite the drawbacks in cost, size and required maintenance, Ti:sapphire remains the workhorse of ultrafast science. One of the remaining challenges for fiber lasers to overcome is their limited set of accessible wavelengths. Unfortunately, readily available ytterbium, erbium and thulium fiber lasers can produce coherent radiation only near 1, 1.55 and 2μm, respectively. There remain a significant number of wavelength regions that fiber lasers cannot address. In this thesis, novel fiber lasers producing ultrashort pulses at wavelengths not currently accessible with established active rare-earth-doped fibers are investigated. Our main approach is to use various nonlinear optical effects to generate new laser wavelengths. First, a watt-level synchronously pumped Raman fiber oscillator generating tens of nanojoules femtosecond pulses is demonstrated. Stimulated Raman scattering in a passive fiber within an oscillator cavity allows formation of Raman pulses that are spectrally redshifted with respect to the pump pulses. World-record output pulse energy and conversion efficiency have been achieved with our femtosecond Raman fiber laser design. We have also demonstrated a high power, widely tunable all-fiber optical parametric oscillator (FOPO) based on four-wave mixing in a passive fiber. The FOPO is synchronously pumped with an Yb³⁺-doped mode-locked fiber laser working at ~1040nm. The FOPO produces ultrashort pulses tunable from 760 to 1560nm. Record pulse energy is generated at the output of the femtosecond FOPO. Depending on the configuration of the FOPO, the duration of produced pulses varies between 170fs and 3ps. This new laser source has similar performance to standard Ti:sa femtosecond lasers so it can potentially replace the latter in many applications. Ultrashort optical pulses in the mid-IR and long-IR range (2-20 μm) have many important applications in gas sensing, counter-measures, etc. The realization of the ultrashort pulses in the mid-IR and long-IR wavelengths requires the use of free-space nonlinear crystals. An efficient mid-IR source based on difference frequency generation (DFG) in an AgGaS₂ crystal using femtosecond erbium/thulium pump fiber laser has been proposed and demonstrated. The photon conversion efficiency of the pump wave (1.55μm) to idler wave (9.2μm) has been measured to be 16%, which is today a record for conversion of near-IR light radiation from fiber lasers to 9μm spectral range. Potentially the photon conversion efficiency can be increased up to 60% by using pump pulses having higher peak power. Finally, generation of supercontinuum (SC) light in the mid-IR spectral range is also demonstrated. It is well known that SC produced in standard optical fibers is limited to ~6μm by material absorption. The liquid core optical fiber platform has been proposed to address this matter. Several highly nonlinear liquids have minimal absorption in the mid-IR wavelength range, which potentially allows us to create broadband SC light in this spectral region. SC generation up to 2.4μm in an integrated hollow core optical fiber filled with CS₂ has been demonstrated. Further development of the liquid core optical fiber platform should allow generation of the SC covering wavelengths beyond 6μm.

The prospect of a universal quantum computer is alluring, yet formidable. Smaller scale quantum information processing, however, has been demonstrated. Quantum networks, interlinking flying and stationary qubits, and linear optical quantum computing (LOQC) are both good candidates for scaling up such computations. A strongly coupled atom-cavity system is a promising approach for applications in these fields, both as a node in a quantum network, and as a source of photons for LOQC. This thesis demonstrates the versatile capabilities of an atom-cavity system comprising a single ⁸⁷Rb atom within a macroscopic high-finesse Fabry-P&eacute;rot cavity. It operates intermittently for periods of up to 100 &mu;s, with single-photon repetition rates of 1 MHz and an intra-cavity production efficiency of up to 85%. Exploiting the long coherence time of around 500 ns, the photons are subdivided into d time bins, with arbitrary amplitudes and phases, thus encoding arbitrary qudits. High fidelity quantum logic is shown, operating a controlled-NOT gate integrated into a photonic chip with a classical fidelity of 95.9^+1.4<sub style='position: relative; left: -1.6em;'>-1.7</sub> %. Additionally, the generation of entanglement is verified and non-classical correlations between events separated by periods exceeding the travel time across the chip by three orders of magnitude are observed. Photonic quantum simulation is performed, using temporally encoded qudits to mimic the correlation statistics of both fermions and anyons, in addition to bosons. Finally measurement-based quantum feedback is demonstrated and used to actively control the routing of temporal qubits.

This thesis investigates the coherent phase modulation of ultrashort pulses using vibrating hydrogen as a molecular medium. Self-phase modulation in a gas-filled hollow core capillary allows the generation of highpower few-cycle pulses in the NIR. Such pulses can be used to drive high harmonic generation (HHG) to deliver attosecond duration pulses in the extreme ultraviolet and soft X-ray spectral region. While reaching unrivalled pulse durations (down to 67 as), these sources have characteristically low efficiencies. The pump-probe spectroscopy community would greatly benefit from brighter short wavelength sources with sub-5 fs duration. In this work I apply Amplified RamaN Impulsive Excitation for Molecular Phase Modulation (ARNIEMPM), a multiple pulse scheme, to coherently prepare vibrating hydrogen molecules and exploit them for the phase modulation of ultrashort pulses. The preparation of the molecular motion is performed via impulsive stimulated Raman scattering and transient stimulated Raman scattering. The generated in-phase motion of molecules creates an oscillating optical polarizability in the medium which can be exploited by a probe pulse propagating through it, acting as a 125THz frequency phase modulator, the fastest among molecular media. This technique has the potential to provide bright, isolated subfemtosecond duration ultra-violet (UV) pulses via spectral broadening of broadband pulses. I experimentally investigate the preparation of the molecular motion against multiple experimental parameters. I then demonstrate the molecular phase modulation of ultrashort broadband probes in the near-infrared (NIR) and UV via a degenerate interferometric scheme. I used a waveguide to increase the interaction length of the process and reduce the energy requirements for the medium preparation. This allowed the use of a single laser system to generate all the required pulses, which are largely diverse in terms of wavelength, duration and power. Additionally, I present a novel technique named Attosecond Resolved Interferometric Electric-field Sampling (ARIES), which is capable of directly measuring the waveform of arbitrary pulses with attosecond resolution. This technique is based on high-harmonic generation (HHG) acting as a temporal gate for an applied secondary field, and tracking its electric field amplitude as a shift in the HHG cut-off frequency. I present experimental demonstration of a pulse waveform measurement by accurately retrieving a know inserted variation in dispersion and carrier-envelope-phase. A theoretical calculation of the technique applicability over a wide spectral range is also presented.

The advent of the 4th generation X-ray sources paves the way for a new phase of experimental investigation of Hot-Dense plasmas. At the Linac Coherent Light Source (LCLS), pulses of keV X-rays, shorter than 100 fs, and with intensities up to 10¹⁸ W·cm^-2, are routinely produced, allowing for the production of uniform samples of solid-density plasmas. The simple single-photon X-ray absorption mechanism can be easily modelled, so that the plasma conditions can be accurately retrieved, without relying on diagnostic techniques that are not benchmarked in this high density regime. The work presented here describes the results of the first experiment where the LCLS interacts with a solid Al target, isochorically heating it at temperatures up to 190 eV. The system is described by the SCFLY non-LTE model, where the density and temperature are computed self consistently, as a consequence of the detailed atomic processes, rather than imposed by the user. The approximations affecting the simulations are discussed in detail. The code is first validated, by modelling the charge state distribution measured in a previous experiment (L. Young et. al), where the LCLS interacts with a Ne gas, a simplified (collisionless) problem. Then it is used to model the K-alpha spectroscopic data obtained for Al. The plasma evolution, explained by SCFLY simulations, is found to be primarily determined by collisions, whose visible effects on the experimental spectra are discussed. By varying the wavelength of the laser and observing the change in the K-alpha fluorescence, the K-edges for different ions in the plasma were determined, leading to a charge resolved measurement of continuum lowering in the HDM system. The results disagree with the widely used Stewart-Pyatt model, with the disagreement increasing for higher charge states, but are consistent with the older Ecker-Kroell model. These results have profound implications for dense plasma modelling.

This thesis focuses on the development of high harmonic generation (HHG) by using polarisation controlled quasi-phase matching QPM as well as related topics. A new class of QPM techniques called polarisation-controlled QPM is introduced where linear or circlar birefringence enables the modulation of the driving field's polarisation state called polarisation-beating QPM (PBQPM) for linear birefringence and optical rotation QPM (ORQPM) for circular birefringence respectively. PBQPM uses a linear birefringence to modulate periodically the driving pulse between linear and circular/elliptical polarisation states. Because elliptical or circular polarisation of the driving pulse suppresses harmonic generation, by appropriately matching the beat length of the driving field's polarisation state to the coherence length of the harmonic generation, QPM can be achieved. In the second technique, ORQPM, propagation of the driving radiation in a system exhibiting circular birefringence causes its plane of polarisation to rotate; by appropriately matching the period of rotation to the coherence length, it is possible to avoid destructive interference of the generated radiation. Not only does ORQPM have similar enhancements as true-phase matching, it is also the first proposed QPM source for circularly polarised high harmonics. The importance of phase modulation in QPM, especially relating to modebeating in hollow-core waveguides where harmonics is being generated are also explored theoretically. Based on this, a novel technique for analyzing random phase matching using a continuous phase-diffusion treatment has been developed; theoretical analytical models are shown to produce excellent agreement with simulations. It is further shown that random phase matching may be responsible for additional broadening of the high harmonic spectrum, especially at higher harmonic orders. Because mode and polarisation control is central to polarisation-controlled QPM, four waveguide mode decomposition techniques from single shot CCD data have been developed. The extraction of phase and coupling coefficients are demonstrated experimentally. A novel analytical general solution for the phase introduced by a phase-only spatial light modulator to generate a given far-field phase and amplitude was developed. The solution was demonstrated experimentally and shown to enable excellent control of the far-field amplitude and phase. Finally, circular and linear birefringent waveguides were explored. Analytic solutions to rectangular birefringent hollow-core waveguides were developed and some initial demonstration experiments were performed.

The effect of uncertainty in gas composition on the accuracy of gas-phase thermometry using Laser-Induced Thermal Grating Spectroscopy, LITGS, is studied. Temperatures are obtained from measurements of the sound speed derived from the frequency of oscillations &fnof;_OSC imposed upon the LITGS signal arising from the transit of acoustic waves across the density modulation feature. The dependence of the sound speed, c_s on &radic;&gamma;/m, where &gamma; is the ratio of specific heats and m is the mean molecular mass leads to a dependence upon gas composition. LITGS signals were generated in acetone vapour in a variety of gas mixtures in a temperature controlled cell at 4 bar total pressure using pump pulses from a frequency quadrupled Q-switched Nd:YAG laser at 266 nm and a cw diode pumped solid state probe laser at 671 nm. Studies were undertaken of the variation in &fnof;_OSC with gas composition using gas mixtures of O₂ and N₂ with component concentrations in the range 0-100 &percnt;, and was found to agree with theoretical predictions. Measurement precision of the data (one standard deviation in 50 measurements) was found to be typically &plusmn; 1.7 &percnt; for measurements at 4 bar total pressure. The effect of varying concentrations in exhaust gas residuals (EGR) typical of pre-ignition gases in a spark ignition internal combustion engine were studied using synthetic air (N₂/ O₂ mixtures) containing variable amounts of simulated EGR components, CO₂ and H₂O. The effect of variation in CO₂ concentration in dry synthetic air was measured at 4 bar and 30&deg;C and found to agree with theoretical predictions. Experiments conducted at 30&deg;C, with the addition of a saturated vapour pressure of water indicate that the effect of a saturated vapour pressure of water on the oscillation frequency in synthetic EGR is on the borderline of resolution. The effect of variable amounts of typical hydrocarbon fuel vapour on &fnof;_OSC was studied using 2,2,4-trimethyl-pentane in gas mixtures composed of synthetic air and variable amounts of EGR and water vapour at 80&deg;C. Kinetic theory was used in order to model the dependence of the oscillation frequency &fnof;_OSC on various gas compositions containing fuel and EGR, in order to construct an error surface for comparison with experimental measurements. Experimental data were found to agree with the model predictions to within experimental error for a representative data set within the range of calculated values. The results indicate that uncertainties in temperature values derived from LITGS thermometry can be estimated with confidence within reasonable estimates of composition variations in an internal combustion engine, and should lead to absolute temperature accuracy of within 2-3 &percnt;.