Over the past two decades, rapid progress in the development of quantum light sources has positioned quantum optics as a cornerstone of emerging quantum technologies. This thesis presents two entirely new and distinct advances in quantum optics, each introducing quantum light sources into unconventional environments. In the first part, nanoparticles of hexagonal boron nitride (hBN) with optically active color centers were successfully introduced into living cells, where they emitted light in the form of temporally isolated single photons. Unlike conventional light sources, each color center emits exactly one photon at a time — a fundamentally quantum mechanical phenomenon with no classical analogue. The emitters showed excellent single-photon purity, outstanding emission stability with no photobleaching or spectral drift over several hours, and negligible impact on cell health. Crucially, because different color centers emit at different wavelengths and with distinct spectral profiles, they can serve as unique optical fingerprints. Combining just a few per cell allows virtually unlimited numbers of cells to be uniquely tagged. Beyond labeling, the single-photon nature of these emitters opens a path toward quantum-limited sensing and imaging inside living tissue. In addition to single-photon emitters, we also investigated sources of entangled photon pairs. These have traditionally relied on solid-state nonlinear crystals, which are rigid, difficult to tune, and offer little flexibility once fabricated. To overcome these limitations, we turn to soft matter: we developed and investigated sources based on ferroelectric nematic liquid crystals (FNLCs), a newly discovered class of organic materials with exceptionally strong intrinsic electric polarization and optical nonlinearity, as a platform for generating entangled photon pairs through spontaneous parametric down-conversion (SPDC). The results presented here mark the first demonstration of photon-pair generation in organic matter via SPDC. By reorienting the liquid crystal molecules through sample geometry or applied electric fields, the intensity, polarization state, and degree of entanglement of the generated photon pairs can all be controlled — a level of tunability not available in conventional solid-state sources. Additionally, chiral FNLCs with a self-assembled helical structure were shown to produce photon-pair generation rates comparable to or exceeding those of the best solid-state platforms. By introducing scalable, efficient, and reconfigurable quantum light sources with functionalities beyond those of conventional solid-state platforms, this work opens new directions in quantum optics and related technologies.
Over the past two decades, rapid progress in the development of quantum light sources has positioned quantum optics as a cornerstone of emerging quantum technologies. This thesis presents two entirely new and distinct advances in quantum optics, each introducing quantum light sources into unconventional environments. In the first part, nanoparticles of hexagonal boron nitride (hBN) with optically active color centers were successfully introduced into living cells, where they emitted light in the form of temporally isolated single photons. Unlike conventional light sources, each color center emits exactly one photon at a time — a fundamentally quantum mechanical phenomenon with no classical analogue. The emitters showed excellent single-photon purity, outstanding emission stability with no photobleaching or spectral drift over several hours, and negligible impact on cell health. Crucially, because different color centers emit at different wavelengths and with distinct spectral profiles, they can serve as unique optical fingerprints. Combining just a few per cell allows virtually unlimited numbers of cells to be uniquely tagged. Beyond labeling, the single-photon nature of these emitters opens a path toward quantum-limited sensing and imaging inside living tissue. In addition to single-photon emitters, we also investigated sources of entangled photon pairs. These have traditionally relied on solid-state nonlinear crystals, which are rigid, difficult to tune, and offer little flexibility once fabricated. To overcome these limitations, we turn to soft matter: we developed and investigated sources based on ferroelectric nematic liquid crystals (FNLCs), a newly discovered class of organic materials with exceptionally strong intrinsic electric polarization and optical nonlinearity, as a platform for generating entangled photon pairs through spontaneous parametric down-conversion (SPDC). The results presented here mark the first demonstration of photon-pair generation in organic matter via SPDC. By reorienting the liquid crystal molecules through sample geometry or applied electric fields, the intensity, polarization state, and degree of entanglement of the generated photon pairs can all be controlled — a level of tunability not available in conventional solid-state sources. Additionally, chiral FNLCs with a self-assembled helical structure were shown to produce photon-pair generation rates comparable to or exceeding those of the best solid-state platforms. By introducing scalable, efficient, and reconfigurable quantum light sources with functionalities beyond those of conventional solid-state platforms, this work opens new directions in quantum optics and related technologies.
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.
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.
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 Quantum nonreciprocity—a fundamental phenomenon enabling directional control of quantum states and photon correlations—has long been recognized as pivotal for quantum technologies. However, the experimental realization of nonreciprocal quantum photon-pair generation, as a critical prerequisite for advancing quantum systems, continues to be an outstanding challenge that remains unaddressed in practice. Here, we experimentally implement a highly-efficient nonreciprocal quantum photon source in a micro/nano-scale helical structured nonlinear optical fluid. Intriguing helical quasi-phase matching is achieved by deliberately engineering the pitch of the chiral ferroelectric structure, thus enabling spontaneous parametric down-conversion with record-high brightness (5,801.6 Hz∙mW −1 , 10,071% enhancement over phase-mismatched systems) and high coincidence-to-accidental ratio, rivaling state-of-the-art centimeter-scale nonlinear crystals. In particular, by tailoring the ferroelectric helix structure with orthogonally aligned head and tail polarization vectors, we demonstrate up to 22.6 dB isolation in biphoton generation coupled with nonreciprocal quantum polarization states, while maintaining classical optical reciprocity . This quantum liquid-crystal-based platform, combining flexible tunability and superior performance of purely quantum nonreciprocity at micro/nano scales, builds a bridge between a wide range of soft-matter systems, nonreciprocal physics, and emerging quantum photonic technologies.
Quantum nonreciprocity-a fundamental phenomenon enabling directional control of quantum states and photon correlations-has long been recognized as pivotal for quantum technologies. However, the experimental realization of nonreciprocal quantum photon-pair generation, as a critical prerequisite for advancing quantum systems, continues to be an outstanding challenge that remains unaddressed in practice. Here, we experimentally implement a highly-efficient nonreciprocal quantum photon source in a micro/nano-scale helical structured nonlinear optical fluid. Intriguing helical quasi-phase matching is achieved by deliberately engineering the pitch of the chiral ferroelectric structure, thus enabling spontaneous parametric down-conversion with record-high brightness (5,801.6 Hz*mW-1, 10,071% enhancement over phase-mismatched systems) and high coincidence-to-accidental ratio, rivaling state-of-the-art centimeter-scale nonlinear crystals. In particular, by tailoring the ferroelectric helix structure with orthogonally aligned head and tail polarization vectors, we demonstrate up to 22.6 dB isolation in biphoton generation coupled with nonreciprocal quantum polarization states, while maintaining classical optical reciprocity. This quantum liquid-crystal-based platform, combining flexible tunability and superior performance of purely quantum nonreciprocity at micro/nano scales, builds a bridge between a wide range of soft-matter systems, nonreciprocal physics, and emerging quantum photonic technologies.
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.