Useful applications for squeezed states of light have been theorized for decades, however, only recently have advancements in the technology allowed for experimental investigations into applications of squeezed light. Most experiments which involve squeezing could be benefited by having a squeezing source which produces stable, highly squeezed states. This thesis investigates the generation of squeezed states of light which exhibit these properties, stability and high levels of squeezing. The key investigations are; • Initially a model is produced which estimates the expected level of squeezing in a singly or doubly resonant cavity. The properties of the cavity are altered by varying the reflectivity of one of the cavity mirrors for either the pump or seed frequencies. The effects of increasing pump power and introducing additional losses into the cavity are investigated as well as the optimum resonance conditions for maximum squeezing. • Built 3 squeezers with 3 different nonlinear mediums. A maximum squeezing value was found for each of the crystals and any effects which degraded or improved the results were investigated. Using PPKTP 5.8±0.1dB of squeezing was directly observed. Using PPSLT 4.9±0.1dB of squeezing was directly observed. PPSLT was found to have an unusually high threshold power, indicating that the crystal may have had a nonlinear coefficient much lower than expected. Using 1.7% MgO:PPSLN 1.1±0.2dB of squeezing was observed. The poor squeezing results were due to an anomaly in the material which affected its reflectivity and is investigated in the thesis. • The intra-cavity loss introduced to the system through the addition of each of the crystals was inferred from measurements of the cavity finesse, and the effect of this loss is explored. • Demonstrated the usefulness of a wedge in periodically poled materials to reach the optimum resonance condition in the squeezing setup. It is shown that the wedge is capable of cancelling the dispersion introduced in a round trip of the cavity, with no discernible negative effects. These investigations are used to comment on the suitability of each of these crystals for future experiments involving squeezing and possible modifications to improve the results.
Useful applications for squeezed states of light have been theorized for decades, however, only recently have advancements in the technology allowed for experimental investigations into applications of squeezed light. Most experiments which involve squeezing could be benefited by having a squeezing source which produces stable, highly squeezed states. This thesis investigates the generation of squeezed states of light which exhibit these properties, stability and high levels of squeezing. The key investigations are; • Initially a model is produced which estimates the expected level of squeezing in a singly or doubly resonant cavity. The properties of the cavity are altered by varying the reflectivity of one of the cavity mirrors for either the pump or seed frequencies. The effects of increasing pump power and introducing additional losses into the cavity are investigated as well as the optimum resonance conditions for maximum squeezing. • Built 3 squeezers with 3 different nonlinear mediums. A maximum squeezing value was found for each of the crystals and any effects which degraded or improved the results were investigated. Using PPKTP 5.8±0.1dB of squeezing was directly observed. Using PPSLT 4.9±0.1dB of squeezing was directly observed. PPSLT was found to have an unusually high threshold power, indicating that the crystal may have had a nonlinear coefficient much lower than expected. Using 1.7% MgO:PPSLN 1.1±0.2dB of squeezing was observed. The poor squeezing results were due to an anomaly in the material which affected its reflectivity and is investigated in the thesis. • The intra-cavity loss introduced to the system through the addition of each of the crystals was inferred from measurements of the cavity finesse, and the effect of this loss is explored. • Demonstrated the usefulness of a wedge in periodically poled materials to reach the optimum resonance condition in the squeezing setup. It is shown that the wedge is capable of cancelling the dispersion introduced in a round trip of the cavity, with no discernible negative effects. These investigations are used to comment on the suitability of each of these crystals for future experiments involving squeezing and possible modifications to improve the results.
A squeezed light source requires properties such as high squeezing amplitude, high bandwidth and stability over time, ideally using as few resources, such as laser power, as possible. We compare three nonlinear materials, two of which have not been well characterized for squeezed state production, and also investigate the viability of doubly-resonant optical parametric oscillator cavities in achieving these requirements. A model is produced that provides a new way of looking at the construction of an optical parametric oscillator/optical parametric amplifier setup where second harmonic power is treated as a limited resource. The well-characterized periodically poled potassium titanyl phosphate (PPKTP) is compared in an essentially identical setup to two relatively new materials, periodically poled stoichiometric lithium tantalate (PPSLT) and 1.7% magnesium oxide doped periodically poled stoichiometric lithium niobate (PPSLN). Although from the literature PPSLT and PPSLN present advantages such as a higher damage threshold and a higher nonlinearity, respectively, PPKTP was still found to have the most desirable properties. With PPKTP, 5.8 dB of squeezing below the shot noise limit was achieved. With PPSLT, 5.0 dB of squeezing was observed but the power required to see this squeezing was much higher than expected. A technical problem with the PPSLN limited the observed squeezing to around 1.0 dB. This problem is discussed.
We report on the generation of squeezed states with optical parametric oscillators. Firstly, we discuss experiments aimed toward producing stronger squeezing. We characterise three different nonlinear materials (PPKTP, PPSLN and PPSLT) in terms of their nonlinearity and optical losses to evaluate their squeezing efficacy. Secondly, we present results showing the development of squeezed states at audio sideband frequencies. Squeezing in this frequency range is required for applications in interferometric gravitational wave (GW) detectors. We discuss the significance of the results in terms of potential sensitivity improvements of current and future GW detectors and also potential upgrades to our experiment.
Squeezed states of light are quantum states that can be used in numerous protocols for quantum computation and quantum communication. Their generation in labora- tories has been investigated before, but they still lack compactness and practicality to easily integrate them into larger experiments. This thesis considers two experiments: one conducted in France, the miniOPO; and one conducted in Australia, the SquOPO. Both are new designs of compact sources of squeezed states of light towards an integrated system. The miniOPO is a linear cavity of 5mm length between the end of a fiber and a curved mirror with a PPKTP crystal of 1mm inside it. The squeezing generated in this cavity is coupled into the fiber to be able to be brought to a measurement device (homodyne) or to a larger experiment. The cavity is resonant for the squeezed light and the pump light, and locked in frequency using self-locking effects due to absorption of the pump in the crystal. The double resonance is achieved by changing the temperature of the crystal. Two different fibers have been tested in this experiment, a standard single-mode fiber and a photonic large core single-mode fiber. The squeezing obtained is still quite low (0.5dB with the standard fiber and 0.9dB for the photonic fiber) but a number of ameliorations are investigated to increase these levels in the future. The SqOPO is a monolithic square cavity made in a Lithium Niobate crystal using four total internal reflections on the four faces of the square to define an optical mode for the squeezed mode and the pump mode. The light is coupled in the resonator using frustrated internal reflection with prisms. The distance between the prisms and the resonator defined the coupling of the light, which allows us to control the finesse of the light in the resonator and by using birefringent prisms it is possible to tune independently the two frequencies in the resonator to achieve an optimal regime. The frequency of the light is locked using absorption of the pump light in the resonator to achieve self-locking, and double resonance is controlled by tuning the temperature of the crystal. We demonstrated 2.6dB of vacuum squeezing with this system. Once again, the amount of squeezing is low, but ameliorations that could be implemented in the future are discussed.
Squeezed light is expected to enable new quantum metrology and quantum computation applications, but the very high degree of experimental control required in practice places severe demands on the physical devices used to generate it. As ultrafast squeezed light sources are reaching an unprecedented degree of brightness and modal control, there is a strong interest in developing methods for their experimental characterization and theoretical understanding. In this thesis, we demonstrate both a new application and a new characterization technique for pulsed squeezed light. In the first part of the thesis, we present a new metrological application of pulsed squeezed light: quantum-enhanced label-free nonlinear microscopy. Stimulated emission microscopy (SEM) is a state-of-the-art pump-probe microscopy technique whose sensitivity is ultimately limited by shot noise in the probe beam. We propose to overcome this fundamental limitation by using a sub-Poissonian probe generated via optical parametric amplification. We demonstrate an intensity-squeezed sub-picosecond pulsed probe, and a simple stimulated emission microscope. Using this squeezed probe, sub-Poissonian statistics are observed in the SEM signal. After this proof-of-principle experiment, we aim to provide an improved design for a source of intensity-squeezed light. First, we develop a theoretical framework to simulate the photon statistics of a general sub-Poissonian source, considering pulsed and focused pump and seed fields in a unified spatio-temporal description. Next, we use this framework to suggest possible improvements to our previous squeezed probe. In the second part of this thesis, we provide new theoretical and experimental tools to analyze the high gain regime of pulsed spontaneous parametric down-conversion (SPDC). We study a waveguided source capable of generating twin-beams with tens of photons in a single spatio-temporal mode. We develop a framework to efficiently simulate SPDC for an arbitrary nonlinear gain, including the effect of parasitic nonlinearities such as self-phase modulation (SPM) of the pump and cross-phase modulation (XPM) between pump and twin-beams. We experimentally characterize our source using a new technique based on stimulated difference-frequency generation (DFG). In a type-II down-conversion process, we measure the frequency generation produced not only in the orthogonal polarization to the seeding beam, but also in its same polarization, which we interpret as cascaded difference-frequency generation. We argue that the additional information obtained in this measurement allows for a self-referenced estimation of the squeezing gain. Using these measurements, we accurately fit our theoretical model, revealing that, in the high gain regime, parasitic nonlinearities strongly affect the spectral structure of the twin-beams. We propose a simple strategy to reduce the impact of these nonlinearities on the modal structure of SPDC.
We demonstrate the methods for increasing the observed squeezing level in the generation of squeezed states of light at 1064 nm with periodically poled KTiOPO4 (PPKTP) crystal. We analyze the technical limits to the reduction of noise in the squeezed quadrature theoretically, including the intra-cavity loss of the optical parametric amplifier (OPA) cavity, the normalized amplitude, the detection frequency, and the mode matching on the balanced homodyning stage. To observe a high degree of squeezing, we improve the quality of mode matching into the OPA cavity and the mode cleaning (MC) cavity experimentally. By optimizing mode matching of the light into cavities, the finesse of the cavities would be higher in practice and the desirable spatial mode can be realized. As the intensity noise of the laser light reach the shot noise limit above the frequency of 1 MHz, which is sufficient for our experiment, so the MC cavity is incorporated to filter higher-order transverse modes of the local oscillator (LO) beam for the balanced homodyne detection. The experimental and theoretical results show that it is essential to optimize the mode matching efficiency to generate and detect high degree of squeezing otherwise an inefficiency mode matching will throw away the squeezing and transform the squeezed field into a vacuum field.
Squeezed states of light are used for precision metrology and quantum-enhanced measurements, with applications spanning communication and sensing. State-of-the-art squeezed-light sources typically rely on optical cavities to achieve high, usable levels of squeezing. Recently, waveguide-based squeezed-light sources have demonstrated significant improvements in achievable squeezing, with performance currently limited by fabrication-induced losses. In this work, we present a detailed analysis of waveguide-based squeezers, examining the effects of phase noise, multiple loss mechanisms, and fundamental light leakage seeding the squeezer. We further investigate a cascaded squeezer architecture, in which a second waveguide operates as a phase-sensitive amplifier to mitigate out-coupling and detection losses. Owing to their ease of integration, robustness to high pump powers, and low intrinsic phase noise, we propose waveguide-based squeezed-light sources as a promising alternative for quantum noise reduction in future gravitational wave detectors, such as the Einstein Telescope.