Light is fully described by its three degrees of freedom (DoFs), wavelength/time, space/momentum, and polarization. A deep understanding of the DoFs is central in both classical and quantum optical research. By exploring the unique possibilities that correlations of different DoFs or the correlation of one DoF with other systems offer, new technological opportunities arise. This thesis offers new experimental and theoretical perspectives on optical correlations of laser pulses and photon pairs generated in spontaneous parametric down-conversion (SPDC). This thesis introduces spectral vector beams - light pulses in which wavelength and polarization are correlated. These beams can be used for high-speed spectro-scopic measurements, where the lights spectrum can be obtained from polarization measurements alone. I extend this concept by the third DoF, correlating wavelength, space, and polarization within a single light pulse. I investigate the similarity of the loss of their degree of polarization in imprecise measurements to the loss of coherence of nonseparable quantum systems. Additionally, the thesis explores the conservation of orbital angular momentum (OAM) in photon pairs produced with SPDC. I show, that the OAM of a pump light field is transferred to the photon pair in agreement with the conservation of OAM. For the first time, I demonstrate that this conservation law is also valid for a single-photon pump. This work not only explores fundamental descriptions but also provides the ex- perimental framework to generate light fields with complex hybrid correlations. The research demonstrates the potential for a deeper understanding of light’s versatile DoFs.
Light is fully described by its three degrees of freedom (DoFs), wavelength/time, space/momentum, and polarization. A deep understanding of the DoFs is central in both classical and quantum optical research. By exploring the unique possibilities that correlations of different DoFs or the correlation of one DoF with other systems offer, new technological opportunities arise. This thesis offers new experimental and theoretical perspectives on optical correlations of laser pulses and photon pairs generated in spontaneous parametric down-conversion (SPDC). This thesis introduces spectral vector beams - light pulses in which wavelength and polarization are correlated. These beams can be used for high-speed spectro-scopic measurements, where the lights spectrum can be obtained from polarization measurements alone. I extend this concept by the third DoF, correlating wavelength, space, and polarization within a single light pulse. I investigate the similarity of the loss of their degree of polarization in imprecise measurements to the loss of coherence of nonseparable quantum systems. Additionally, the thesis explores the conservation of orbital angular momentum (OAM) in photon pairs produced with SPDC. I show, that the OAM of a pump light field is transferred to the photon pair in agreement with the conservation of OAM. For the first time, I demonstrate that this conservation law is also valid for a single-photon pump. This work not only explores fundamental descriptions but also provides the ex- perimental framework to generate light fields with complex hybrid correlations. The research demonstrates the potential for a deeper understanding of light’s versatile DoFs.
Summary form only given. Spontaneous parametric down-conversion (SPDC), one of the most important nonlinear processes, is of paramount interest especially in the field of quantum optics due to its intrinsic capability in generating entangled photon pairs in different degree of freedoms (DoFs). The spatial distribution of the paired photons generated through SPDC process are highly influenced by different crystal parameters including birefringence and length, and the spatial structure of the pump beam [1]. Recent studies have shown the transfer of pump properties such as phase profile and doughnut intensity distribution [2] into the transverse amplitude of heralded single photon. Here we report, first time to best of our knowledge, the direct transfer of the non-separable state in OAM and polarization DoFs of classical pump beam to generate two photons state with hybrid entanglement in both OAM and polarization. Using same scheme one can produce macroscopic entanglement with very high quanta of OAM.
Spontaneous parametric down-conversion (SPDC) can produce entangled photons from a nonlinear medium driven by a pump beam, and has thus been a key resource in photonic quantum technologies. Historically, the low efficiency of SPDC prompted the use of lasers, which are high-intensity light sources, as pump sources. However, the practical implementation of laser-driven systems faces an imminent energy bottleneck. Lasers typically have suboptimal energy efficiencies due to lasing thresholds and the technical demands of thermal management. As a result, laser-pumped entangled photon sources suffer from significant power overhead, rendering quantum devices less sustainable. Over the past few decades, light-emitting diodes (LEDs) have emerged as inexpensive and energy-efficient alternatives for high-intensity light sources. Since 2010, studies have shown that LEDs can drive SPDC and generate photon pairs. These results suggests the potential to replace lasers with ubiquitous and natural light sources to build environmentally friendly quantum devices. However, replacing lasers with other light sources entails a lack of coherence in all degrees of freedom (DoFs) of the pump beam, including spatial, temporal-spectral, and polarization, which may influence the resulting entanglement generated from SPDC. While earlier studies established that pump coherence in a specific DoF sets an upper bound for entanglement in that same DoF, the cross-influence between different DoFs has not been fully investigated. In this thesis, we study the influence of the pump's spatiotemporal coherence on the polarization entanglement generated from SPDC. First, we experimentally demonstrate that polarization-entangled two-photon states can be generated from SPDC pumped by spatiotemporally incoherent light emitted from an LED. Although the resulting entanglement appears lower than that of laser-pumped SPDC, we theoretically show that spatiotemporal incoherence does not fundamentally constrain polarization entanglement; rather, the reduction arises from a coupling between spatiotemporal and polarization DoFs introduced by the experimental setup. We determined experimentally that this cross-influence acts as a deterministic mapping from the spatial components of the pump to the two-photon polarization state. Because this mapping depends on technical aspects of the setup, we envisioned that it could be either (i) engineered to structure entanglement in higher dimensions, or (ii) compensated to reduce spatial distinguishability. Building on the latter, we show that by placing the nonlinear medium in a phase-compensated interferometric setup, SPDC pumped by spatially incoherent light can produce high polarization entanglement that violates local realism, even without postselecting the SPDC field for a single spatial mode. Finally, as a proof-of-principle demonstration of a truly environmentally friendly quantum light source, we demonstrate the generation of polarization-entangled two-photon states from SPDC pumped by a natural source - sunlight.
Spontaneous parametric down-conversion (SPDC) is one of the most important sources of entangled photons in quantum optics and underpins a wide range of applications in quantum communication, quantum imaging, quantum sensing, and photonic quantum information processing. In this nonlinear optical process, a pump photon is converted into a pair of lower-energy photons that exhibit strong correlations in multiple degrees of freedom. While the generation of photon pairs is governed by energy and momentum conservation, the resulting biphoton state possesses a rich spatial structure whose properties are determined by the interplay among the pump field, the nonlinear medium, and subsequent propagation. This thesis investigates the spatial properties of entangled photon pairs generated through SPDC, focusing on the evolution of their correlations in the near field and on the behavior of orbital-angular-momentum (OAM) entangled states under tight focusing conditions. A common theme throughout this work is the examination of physical effects that emerge when the full biphoton wavefunction is retained rather than reduced through the simplifying approximations often employed in theoretical treatments. The principal contribution of this thesis concerns the near-field propagation of SPDC photon pairs. The spatial correlations of SPDC-generated photons are commonly analyzed within the thin-crystal approximation, where the biphoton wavefunction is assumed to factorize into independent functions of the sum and difference coordinates. We demonstrate that birefringence-induced transverse walk-off breaks this factorization and introduces an intrinsic coupling between these coordinates. More importantly, this coupling survives free-space propagation and remains observable even for crystals that nominally satisfy the thin-crystal condition. As a result, the joint spatial correlations develop a characteristic tapering near the crystal image plane that cannot be captured by conventional factorized models. Numerical simulations and experimental measurements are found to be in excellent agreement, revealing a previously overlooked feature of SPDC and providing a more complete description of photon-pair generation and propagation in birefringent nonlinear media. The second part of the thesis explores the propagation of OAM-entangled photon pairs beyond the paraxial regime. While OAM correlations are typically studied under weak-focusing conditions, many emerging quantum technologies require structured quantum states to be manipulated in highly confined optical systems. We show that tight focusing modifies the OAM correlation spectrum, leading to a broadening of the accessible mode distribution. This broadening exhibits distinct transverse and longitudinal characteristics and arises from the emergence of nonparaxial field components absent in conventional scalar descriptions. These results provide new insight into the behavior of structured quantum light in strongly focused optical fields and establish a framework for understanding OAM entanglement beyond the paraxial approximation. Together, the results presented in this thesis reveal previously unexplored aspects of the structure and propagation of entangled photon pairs. By moving beyond widely used approximations, this work uncovers physical effects that influence the generation, evolution, and measurement of spatially entangled states, with implications for quantum imaging, high-dimensional quantum communication, and the broader development of structured-light-based quantum technologies.
Conservation laws are at the heart of our natural scientific understanding, as they dictate why certain phenomena do or do not exist in nature. In the realm of nonlinear optics — where light-matter interactions enable the generation of photon pairs — these principles are also essential. In the common interpretation of spontaneous parametric down-conversion (SPDC), the conservation of momenta is attributed to the transfer of the well-defined linear or orbital angular momenta (OAM) of the single pump photon to the generated photon pair [1], [2]. However, due to the low conversion efficiencies, this conservation rule has only been tested with a strong coherent laser pumping the process. Here, we show for the first time that the OAM of a single pump photon is indeed conserved and that an OAM quanta of up to 2 is transferred to the generated photon pair.
Polarization-entangled photon pairs can be efficiently prepared into pure Bell states with a high fidelity via type-II spontaneous parametric down-conversion (SPDC) of narrow-band pump light. However, the use of femtosecond pump pulses to generate multiphoton states with precise timing often requires spectral filtering to maintain a high quality of polarization entanglement. This typically reduces the efficiency of photon pair collection. We experimentally map the polarization correlations of photon pairs from such a source over a range of down-converted wavelengths with a high spectral resolution and find strong polarization correlations everywhere. A spectrally dependent imbalance between contributions from the two possible decay paths of SPDC is identified as the reason for a reduction in entanglement quality observed with femtosecond pump pulses. Our spectral measurements allow us to predict the polarization correlations for arbitrary filter profiles when the frequency degree of freedom of the photon pairs is ignored.
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.