There is a growing belief that we are currently in the midst of a Second Quantum Revolution. More and more quantum technologies, such as communication, computing, and imaging, are moving from academic exploration to practical industrial applications. These technologies often depend on entangled photons, and their increasing use requires the development of novel quantum light sources that are compact, versatile, and integrable into modern photonic systems. Additionally, the trend toward miniaturization and nano-optics is accelerating, driven by rapid advances in nanofabrication. Therefore, there is an urgent need to create new classes of miniaturized and tunable two-photon sources. This thesis is dedicated to the theoretical investigation and experimental demonstration of miniaturized sources of entangled photons based on spontaneous parametric down-conversion in thin nonlinear films. It focuses not only on traditional crystalline materials but also on the emerging platform of liquid crystals, marking the first-ever demonstration of photon-pair generation in a liquid crystal within this thesis. A significant achievement is the ability to relax phase-matching constraints in ultra-thin nonlinear layers, allowing the full utilization of the nonlinear tensor and enhancing the variety of possible entangled two-photon states. Several aspects of two-photon state engineering with relaxed phase-matching are discussed in the thesis. First, this relaxation facilitates the switching between different polarization two-photon states, with a varying degree of polarization entanglement and the ability to generate non-maximally entangled states. In terms of spectral properties, miniaturized two-photon sources generate exceptionally broad two-photon frequency spectra, leading to a high degree of frequency entanglement and an unprecedented number of frequency modes, which also leads to ultra-narrow time correlations. By considering the effects of Fabry-Perot resonances, it becomes possible to engineer the two-photon spectra and even achieve backward emission of photon pairs. Remarkably, the Fabry-Perot effect can be accurately described by a simple modification of the rigorous quantum theory, providing a powerful predictive tool for photon pair generation in advanced nanostructures, metasurfaces, and nonlinear nano-antennas when properly applied. Another central contribution of this work is the introduction of liquid crystals as tunable nonlinear media for photon pair generation. Controlling the molecular orientation through a static twist or applying an external electric field makes it possible to tune the generated two-photon states dynamically. A theoretical model of this process, verified through direct experimental measurements, predicts quasi-phase-matching-like behavior and enhanced conversion efficiency, which can be utilized to develop exceptionally tunable and integrable sources of photon pairs and squeezed light. Finally, a fundamental challenge addressed in this thesis is mitigating a parasitic photoluminescent background. This study demonstrates a two-order-of-magnitude reduction in photoluminescence-induced contamination of photon pairs generated from thin nonlinear films by switching from the commonly used continuous-wave regime to the pulsed regime and performing time-resolved distillation. It also shows a developed theory that relates measured experimental parameters to the intrinsic purity of the two-photon state. Overall, this thesis advances the quantum micro- and nano-photonics field by providing a deeper understanding and enhanced control over photon pair generation in thin nonlinear films. It bridges theoretical insights with experimental realizations and paves the way toward practical, tunable, and highly integrated quantum light sources.
There is a growing belief that we are currently in the midst of a Second Quantum Revolution. More and more quantum technologies, such as communication, computing, and imaging, are moving from academic exploration to practical industrial applications. These technologies often depend on entangled photons, and their increasing use requires the development of novel quantum light sources that are compact, versatile, and integrable into modern photonic systems. Additionally, the trend toward miniaturization and nano-optics is accelerating, driven by rapid advances in nanofabrication. Therefore, there is an urgent need to create new classes of miniaturized and tunable two-photon sources. This thesis is dedicated to the theoretical investigation and experimental demonstration of miniaturized sources of entangled photons based on spontaneous parametric down-conversion in thin nonlinear films. It focuses not only on traditional crystalline materials but also on the emerging platform of liquid crystals, marking the first-ever demonstration of photon-pair generation in a liquid crystal within this thesis. A significant achievement is the ability to relax phase-matching constraints in ultra-thin nonlinear layers, allowing the full utilization of the nonlinear tensor and enhancing the variety of possible entangled two-photon states. Several aspects of two-photon state engineering with relaxed phase-matching are discussed in the thesis. First, this relaxation facilitates the switching between different polarization two-photon states, with a varying degree of polarization entanglement and the ability to generate non-maximally entangled states. In terms of spectral properties, miniaturized two-photon sources generate exceptionally broad two-photon frequency spectra, leading to a high degree of frequency entanglement and an unprecedented number of frequency modes, which also leads to ultra-narrow time correlations. By considering the effects of Fabry-Perot resonances, it becomes possible to engineer the two-photon spectra and even achieve backward emission of photon pairs. Remarkably, the Fabry-Perot effect can be accurately described by a simple modification of the rigorous quantum theory, providing a powerful predictive tool for photon pair generation in advanced nanostructures, metasurfaces, and nonlinear nano-antennas when properly applied. Another central contribution of this work is the introduction of liquid crystals as tunable nonlinear media for photon pair generation. Controlling the molecular orientation through a static twist or applying an external electric field makes it possible to tune the generated two-photon states dynamically. A theoretical model of this process, verified through direct experimental measurements, predicts quasi-phase-matching-like behavior and enhanced conversion efficiency, which can be utilized to develop exceptionally tunable and integrable sources of photon pairs and squeezed light. Finally, a fundamental challenge addressed in this thesis is mitigating a parasitic photoluminescent background. This study demonstrates a two-order-of-magnitude reduction in photoluminescence-induced contamination of photon pairs generated from thin nonlinear films by switching from the commonly used continuous-wave regime to the pulsed regime and performing time-resolved distillation. It also shows a developed theory that relates measured experimental parameters to the intrinsic purity of the two-photon state. Overall, this thesis advances the quantum micro- and nano-photonics field by providing a deeper understanding and enhanced control over photon pair generation in thin nonlinear films. It bridges theoretical insights with experimental realizations and paves the way toward practical, tunable, and highly integrated quantum light sources.
Abstract Liquid crystals, with their ability to self-assemble, strong response to an electric field and integrability into complex systems, are key materials in light-beam manipulation 1 . The recently discovered ferroelectric nematic liquid crystals 2,3 also have considerable second-order optical nonlinearity, making them a potential material for nonlinear optics 4,5 . Their use as sources of quantum light could considerably extend the boundaries of photonic quantum technologies 6 . However, spontaneous parametric down-conversion, the basic source of entangled photons 7 , heralded single photons 8 and squeezed light 9 , has so far not been observed in liquid crystals—or in any liquids or organic materials. Here we implement spontaneous parametric down-conversion in a ferroelectric nematic liquid crystal and demonstrate electric-field tunable broadband generation of entangled photons, with an efficiency comparable to that of the best nonlinear crystals. The emission rate and polarization state of photon pairs is markedly varied by applying a few volts or twisting the molecular orientation along the sample. A liquid-crystal source enables a special type of quasi-phase matching 10 , which is based on the molecular twist structure and is therefore reconfigurable for the desired spectral and polarization properties of photon pairs. Such sources promise to outperform standard nonlinear optical materials in terms of functionality, brightness and the tunability of the generated quantum state. The concepts developed here can be extended to complex topological structures, macroscopic devices and multi-pixel tunable quantum light sources.
The aim of this thesis is to explore nonlinear nanostructures – photonic systems with one or more dimension with nanoscale feature size – as new platforms for parametric photon-pair generation. This is motivated by the need for photon-pair sources in many technological applications of quantum optics, such as quantum communication, imaging and computing. The initial hypothesis is that nonlinear nanostructures are a promising platform to complement established photon-pair source schemes based on bulk crystals or photonic integrated circuits in terms of quantum state-engineering, footprint, functional density etc. However, nonlinear pair generation, e.g. via spontaneous parametric down-conversion (SPDC), is a new subfield in nanophotonics with a very limited number of studies existing. This constitutes the knowledge-gap addressed in this thesis. Therefore, the objective is to provide first experimental demonstrations of SPDC in subwavelength-scale and nonlinear nanostructures. We investigate the ability of such systems to control the properties of two-photon quantum states such as frequency and angular spectrum or polarization entanglement. To this end we analyze nanostructures with different degrees of complexity, starting with an unstructured thin crystal with one nanoscale dimension, leading up to a 3D nano-scale resonator with subwavelength-sized features in all dimensions. Furthermore, we test different types of nonlinear materials such as lithium niobate, group III-V semiconductors and emerging, highly nonlinear materials such as molybdenum disulfide. Additionally, we also advance the theoretical understanding by developing a comprehensive model of the quantum process of nonlinear parametric pair-generation in photonic nanostructures.
Entangled photon pair sources relying on spontaneous parametric down-conversion (SPDC) are an essential ingredient for optical quantum technologies. Currently, the vast majority of SPDC sources is based on solid-state crystals. However, their rigid nature implies a careful design of the source as well as it limits the two-photon state that can be generated, without much possibility of tuning it. Here, we demonstrate a new generation of efficient, highly tunable sources of entangled photons based on recently discovered liquid crystal materials called ferroelectric nematic liquid crystals (FNLC). In addition to other highly desirable properties that liquid crystals possess, such as the ability to self-assemble into complex structures, strong response to the electric field and integrability into complex optical systems, FNLCs also possess considerable optical nonlinearity. This, as we demonstrate, enables also efficient SPDC generation and opens the door for promising new functionalities in quantum technologies. This work demonstrates the first-ever realization of SPDC in soft, organic matter, with the possibility of controlling the molecular order and thus tuning the two-photon polarization state. We show that almost any polarization state can be generated by simply adjusting the twist of the molecular orientation along the sample. Additionally, by applying only a few volts, we can drastically alter both the emission rate and the generated state of photon pairs, which enables real-time tunability. Developed concepts could lead to complex multi-pixel devices generating quantum light with real time tunability and therefore hold the potential to have a huge impact in the field of quantum technologies.
Entangled photons have acted as a workhorse in many quantum technologies, including cryptography, imaging, sensing, and computation. With the growing demands of quantum technology, the entangled photon pair sources are required to be compact and integrable. For the last few years, the field of quantum optics in the nanoscale has demonstrated photon pair generation in ’flat’ optics such as thin layers, Van der Waals crystals, micro-, nano-antennas, and metasurfaces. The word ’flat’ is used because such optical platforms have sub-wavelength thicknesses. Due to the short travel length inside the sources, they can possess multiple functions that their bulky counterparts cannot have. This doctoral thesis addresses photon pair generation in ’flat’ optics via second- and third-order nonlinear processes, which are spontaneous parametric down-conversion (SPDC) and spontaneous four-wave mixing (SFWM). The main aims of the thesis are to understand how the photon pair generation is realized via SPDC and SFWM on subwavelength-scale platforms, and to make a bridge between photon pair generation via two nonlinear processes under the presence of optical resonances. The thesis covers two experimental parts related to SPDC and SFWM, and one future expectation of the SFWM in metamaterials. The first part of the thesis addresses one of the multi-functionalities that has recently been reported: the emission direction control of the photon pairs. A 150 nm-thick gallium phosphide (GaP) metasurface is used as a source. The metasurface supports a quasi-bound state in the continuum (quasi-BIC) resonance, enhancing the electric field inside the material. I generate the entangled photons through the SPDC process. The generated photons are in two different wavelengths under the given pump wavelength: signal photon at the resonance, and idler photon out of the resonance. The photon pair generation rate is enhanced by 66 times compared to the unpatterned GaP slab since the vacuum field in the signal mode is enhanced by the resonance. The generated photons obey the transmission spectrum of the metasurface. As a result, the signal photons at resonance are mostly reflected, and the idler photons are transmitted after generation. This result is the first work of photon demultiplexing by a metasurface. The second part of the thesis covers the photon pair generation through SFWM in thin nonlinear films. Silicon-rich silicon nitride (SiN) layers are selected as the sources. The layers have an intriguing characteristic that the third-order nonlinear susceptibility (chi-3) differs with the nitrogen doping rate. I generate the photon pairs through chi-3 variant SiN layers. The thicknesses of the samples are all sub-wavelength level, having a few hundred nanometers. Time-energy correlation measurements are conducted for all samples, and the second-order correlation functions (g(2)(0)) are calculated. The maximum g(2)(0) of the samples exceeds 2. Interestingly, I observed two-photon interference effects between photon pairs generated in the layers and the fused-silica substrate, which allows me to estimate the chi-3 values of the samples by knowing the reference: chi-3 value of the substrate. The last part of the thesis introduces designs of metasurfaces and possible experimental schemes for photon pair generation through SFWM. Yet, due to its weak light-matter interaction in third-order nonlinear processes, the photon pair generation through SFWM has not been observed in metasurfaces. However, it is fairly possible to enhance the vacuum fields at the output photon modes and generate photon pairs because preliminary work in unpatterned slabs has already been observed in this thesis. Finally, I introduce a fabricated metasurface. It needs improvements, but it is quite promising for the ongoing project.
Entangled photons are a cornerstone of quantum technologies, enabling applications from secure communication to quantum computing. A longstanding challenge is to develop a compact source that would generate polarization-entangled photons with tunable quantum state on demand. The promising materials for such sources are ferroelectric nematic liquid crystals (FNLCs), due to their nonlinear optical properties and easily controllable configuration. In this work, it is demonstrated that the polarization state and the degree of entanglement of photon pairs generated within FNLCs can be changed in a controllable and reversible manner. First, tuning of the entanglement is demonstrated via sample geometry with twisted FNLC configurations in a sample of varying thickness. Secondly, by applying an electric field, the degree of entanglement can be tuned in real time. In both scenarios, the degree of entanglement can be adjusted from nearly entirely separate photons to fully entangled. These findings represent a significant step toward tunable quantum sources that can produce any desired polarization state on demand. In the future, by adding more electrodes, different parts of the sample could be controlled individually, allowing for the creation of pixelated quantum light sources.
The efficient and controllable generation of entangled photon pairs is a cornerstone for advancing quantum technologies, including quantum computing, communication, and sensing. Traditional methods, primarily relying on spontaneous parametric down-conversion (SPDC) or spontaneous four-wave mixing (SFWM) in bulk nonlinear crystals or waveguides, often suffer from low efficiency, bulky form factors, and limited control over the spectral and spatial properties of the generated photons. This paper explores a transformative approach leveraging resonant Bound States in the Continuum (BICs) within nanophotonic structures to overcome these limitations. BICs are extraordinary optical resonances characterized by theoretically infinite quality (Q) factors, enabling extreme light localization and immense field enhancement within subwavelength volumes. By integrating nonlinear optical materials into BIC-supporting structures, such as photonic crystal slabs or metasurfaces, the local electric field strength experienced by the nonlinear medium can be dramatically increased, boosting the efficiency of entangled photon generation. Furthermore, the sharp, tailor-able spectral response of BICs offers unprecedented control over the joint spectral amplitude of the photon pairs, promising high degrees of spectral purity and entanglement fidelity. This work outlines the theoretical underpinnings, potential device architectures, and anticipated advantages of BIC-enhanced entangled photon sources, paving the way for compact, high-brightness, and spectrally engineered quantum light sources critical for next-generation quantum technologies.