Ultrafast laser-based micromanufacturing is essential for fabricating high-precision quantum devices such as quantum gates, photonic circuits, and waveguides. Besides the area of micromachining, recent applications of laser beams also include quantum imaging and sensing. The laser beams have been characterized for their wavelength and pulse durations. The techniques include homemade setups of intensity autocorrelation, frequency-resolved optical gating (FROG), and spectral phase interferometry for direct electric-field reconstruction (SPIDER) besides the commercial spectrographs. Beams obtained from non-collinear optical parametric amplifier (NOPA) capable of delivering tunable ultrafast pulses are also described. The dark core beams through Hermite Gaussian (HG) modes and thermal effects are also generated. All these beams are capable of generating polarization-entangled photons by the nonlinear-optical phenomenon of spontaneous parametric down-conversion (SPDC). After describing the construction of the entangled photon source (EPS), an interferometric setup for imaging based on extracting the phase and intensity of an object is discussed. The application areas of the EPS for quantum imaging, multi-photon applications and adaptive imaging are also highlighted. We propose that quantum imaging can overcome the existing challenges of real-time feedback during micromanufacturing and help prevent the wastage of resources.
Ultrafast laser-based micromanufacturing is essential for fabricating high-precision quantum devices such as quantum gates, photonic circuits, and waveguides. Besides the area of micromachining, recent applications of laser beams also include quantum imaging and sensing. The laser beams have been characterized for their wavelength and pulse durations. The techniques include homemade setups of intensity autocorrelation, frequency-resolved optical gating (FROG), and spectral phase interferometry for direct electric-field reconstruction (SPIDER) besides the commercial spectrographs. Beams obtained from non-collinear optical parametric amplifier (NOPA) capable of delivering tunable ultrafast pulses are also described. The dark core beams through Hermite Gaussian (HG) modes and thermal effects are also generated. All these beams are capable of generating polarization-entangled photons by the nonlinear-optical phenomenon of spontaneous parametric down-conversion (SPDC). After describing the construction of the entangled photon source (EPS), an interferometric setup for imaging based on extracting the phase and intensity of an object is discussed. The application areas of the EPS for quantum imaging, multi-photon applications and adaptive imaging are also highlighted. We propose that quantum imaging can overcome the existing challenges of real-time feedback during micromanufacturing and help prevent the wastage of resources.
The fields of quantum and nonlinear optics have given rise to a variety of nonclassical states of light that have been proven to surpass certain limitations set by classical physics. Namely, certain squeezed and entangled states have been shown to beat the shot-noise limit when making precision phase measurements in interferometry, as well as write lithographic patterns that are smaller than classically allowed by the Rayleigh diffraction limit. Additionally, single-photon sources and entangled photon pairs have given rise to provably secure quantum key distribution for cryptography. Producing these quantum states of light has proven a difficult task. Nonlinear crystals, when pumped by a laser, produce pairs of single photons via the process of spontaneous parametric down conversion (SPDC). This process is mediated by the second order nonlinear susceptibility of the material. When pumped in a high gain regime, these crystals give rise to optical parametric amplification, which is a viable source of squeezed light. The vast majority of research in this area has focused on crystals that are seeded by vacuum in their two modes. This dissertation concerns the field of quantum nonlinear optics. It is an investigation into the processes that occur when nonlinear materials interact with the electromagnetic field on the single photon level. I have focused on seeding nonlinear crystals with quantum states of light, including single photons and entangled states. This process results in various states directly applicable to interferometry, imaging, and cryptography. Another application investigated is an absolute radiance measurement via stimulated parametric down conversion resulting from non-vacuum seeding of a nonlinear crystal. Additionally, other nonlinear processes, including four-wave mixing, nonlinear magneto-optical effects and coherent population trapping in warm atomic vapor involving quantum states of light are investigated. The process of seeding third-order nonlinear interactions, such as in atomic vapors, gives rise to a variety of interesting, nonclassical phenomena such as entangled image transfer and nonlocal imaging. Strong analogies between SPDC and four-wave mixing are drawn. I also experimentally show an all optical pi-only phase shift of one light beam via another in warm Cesium vapor.
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.
Ultrafast quantum light pulses with durations of 1ps and below show great promise as information carriers in quantum communication and computation. In future they may also be used to probe physical processes at ultrashort timescales with a resolution beyond the limits of Heisenberg uncertainty. This thesis focuses on the creation and manipulation of quantum light pulses with second order nonlinear optical processes in optical waveguides. In chapter 1. Introduction, we lead the reader towards this work's topic by giving a brief qualitative overview over the EPR paradox, quantum entanglement, three-wave-mixing and quantum light pulses. Chapter 2. Basic concepts familiarizes the reader with the physical concepts and mathematical tools underlying this thesis. In chapter 3. Spectral engineering, we discuss the requirements to produce separable photon pair states with group velocity matching and examine several nonlinear materials widely used for optical waveguide inscription for their suitability to group velocity matching in the telecom wavelength regime. In chapter 4. A PP-KTP waveguide as parametric downconversion source, we give the basic spontaneous parametric downconversion source setup, as well as some initial measurements to characterize the single photon detectors, to demonstrate the production of correlated photons, and to determine the phasematching properties of the PP-KTP source. In the following chapter, 5. Fiber spectrometer, we present the single-photon fiber spectrometer. We discuss the experimental setup and calibration of the device, and measure the joint spectrum of photon pairs from our PP-KTP source. Finally, we investigate the behavior of a joint spectrum measurement of a high mean photon number source with binary detectors. In 6. Two-mode squeezed vacuum source, we characterize spectral correlations of an ultrafast SPDC source with the second order correlation function g². We present the g² measurement results and show that we can control the spectral correlations to produce a two-mode squeezed vacuum state of light. We then determine the mean photon number and gain of the source. 7. Quantum pulse gate: While the previous chapters focus on the creation and characterization of ultrafast quantum pulses of light, we now propose a way to manipulate the mode structure of a given quantum light state. We discuss the concept of an active optical filter sensitive to spectral/temporal pulse shape: The quantum pulse gate. In the final chapter 8. Conclusion and outlook, we recapitulate the main results of this thesis and provide a few pointers towards possibilities for future research building on it.
The quantum description of light offers a unique set of optical effects that has led to promising applications beyond those described by classical physics. Although well-defined quantum states of light do not persist in typical classical environments, phenomena such as entanglement often enhance optical approaches to communication, measurement, and sensing. With the emergence of new tasks in classical and quantum optical technology, new tools are required that must be specifically engineered including the generation of quantum states. This thesis is concerned with three principle tasks in engineering and implementing entangled photonic states. First, the use of frequency anti-correlated and polarization entangled two-photon states generated during spontaneous parametric down conversion (SPDC) to precisely evaluate optical delays with quantum interferometry is demonstrated in a realistic commercially available optical telecommunication device. Second, the study of correlated orbital angular momentum (OAM) states for efficient object identification is presented. Finally, experimental efforts towards the development of sources for entangled weak coherent states are discussed. The generation of broadband entangled states leading to well-defined second order interference patterns is a necessary step for the application of low coherence quantum interferometry as a metrological device. The flexibility of non-uniformly chirped periodically poled nonlinear crystals offers a rich set of tools for precise state engineering. The experimental evaluation of a broadband source of polarization entanglement is presented. In addition, design considerations for applications that require optimized quantum interference features are discussed along with a numerical investigation of the limits of quantum interferometry with even order dispersion cancellation. We present an experimental demonstration of correlated OAM sensing exploiting the two-dimensional and correlated nature of states produced during SPDC projected onto the OAM basis. Efficient object recognition through the identification of azimuthal symmetries of arbitrary objects is achieved by observing the full two-photon joint OAM spectrum and focusing on non-conserved OAM components not found in the natural OAM spectrum of SPDC. Finally, quantum key distribution (QKD) is currently the most successful quantum optical application; however, a limiting trade off between the achievable rates and distances confines the approach to niche applications. The generation of entangled coherent states has been proposed to transition QKD into a new regime that would set aside single photons and two-photon entangled states for higher intensity coherent pulses. The key technical limitation that has prohibited the demonstration of such states is a reliable source of single-photon cross phase modulation. The plausibility and experimental efforts towards creating such an environment in a solid state device is presented.
Single photons are a natural platform for quantum technologies as they support entanglement in many degrees of freedom and are inherently well protected from a detrimental type of noise called decoherence. In order to fully exploit photons for quantum technologies, it will be necessary to be able to shape, control, and measure their properties in all degrees of freedom: space, time, frequency, and polarization. \n \n The time-frequency degree of freedom of light is of particular interest for quantum information tasks as it supports various encodings, including frequency bins and time bins, and is intrinsically robust for propagation through long-distance fibre links. Applications which harness quantum correlations in this degree of freedom, referred to as energy-time entanglement, include dispersion cancellation and high-dimensional quantum key distribution. However, detection of this entanglement and observation of these effects requires time resolution beyond the capabilities of current photon detectors. Thus, for operations on ultrafast time scales, more powerful and complex methods are required. \n \n In this thesis, we use a nonlinear technique known as optical gating to surpass the limitations in current detectors and measure single photon pairs, improving the time resolution by two orders of magnitude. When a single photon enters a nonlinear medium at the same time as a strong laser pulse, it may upconvert to a higher energy photon. The strong laser pulse effectively acts as an ultrafast shutter or gate and this creates a high resolution snapshot of the photon in time. Optical gating in conjunction with single photon spectrometers then allow us to measure both the spectral and temporal features of photons on subpicosecond time scales. \n \n These high resolution measurements enable us to explore a host of quantum effects which remained previously inaccessible. First, we directly observe energy-time entanglement, analogous to Einstein-Rosen-Podolsky correlations but \\emph{in frequency and time}. Then, with full control over the dispersion of each photon, we observe nonlocal dispersion cancellation on femtosecond time scales. We also explore ultrafast interferometry in the quantum domain by temporally resolving two-photon interference from a Franson interferometer and observe a violation of the CHSH-Bell inequality. Finally, we show that, using these measurements, it is possible to reconstruct a two-photon energy-time entangled state. Such measurement capabilities will be essential to precisely control both the spectral and temporal shape of entangled photons and to develop new applications in quantum state engineering.
Non-classical correlations in optical beams offer the unprecedented opportunity of surpassing conventional limits of sensitivity and resolution in optical measurements and imaging, especially but not only, when a low photon flux down/single photons are measured. We review the principles quantum imaging and sensing techniques presenting some state-of-the-art achievements in the field. These quantum protocols have the potential to trigger major steps in many application ranging from radiometry to biophotonics.