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.
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.
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 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.
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.
For high-intensity photon pair generation via spontaneous parametric down-conversion, phase matching or quasi-phase-matching must be realized. The most common method is to use periodically poled crystals, where quasi-phase-matching is achieved through the periodic modulation of the nonlinear susceptibility. In contrast to solid-state materials typically employed, an alternative approach is demonstrated here by using chiral ferroelectric nematic liquid crystals (FNLCs). It involves the spontaneous formation of a twisted periodic arrangement of liquid crystal molecules that depends only on the concentration of chiral dopants. This enables the rapid and scalable fabrication of nonlinear optical media with a tunable periodicity of the structure. We show that within only 20 µ m , we can enhance the signal by 2 orders of magnitude compared to the maximal achievable rate for the non-phase-matched case. This process enables us to achieve a significantly enhanced photon pair generation rate of over 1200 Hz/mW in a very short interaction length while simultaneously maintaining a high degree of polarization entanglement. These results establish chiral FNLCs as a versatile, reconfigurable, and low-cost platform for nonlinear and quantum photonics.
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.