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Tunable liquid-crystal source of photon pairs

Vitaliy Sultanov, Aljaž Kavčič, Manolis Kokkinakis, Matjaž Humar, Maria V. Chekhova

Published Jan 1, 2024• 0 citations
Publisher

Abstract

We demonstrate a liquid-crystal broadband source of entangled photons, whose flux and polarization are tunable through the electric field or molecular twist. This opens a path to a new generation of tunable quantum light sources.
DOI
10.1364/cleo_si.2024.sth1f.6

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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.