  Liquid crystals, with their ability of self-assembly, strong response to the
electric field, and integrability into complex systems, are key materials in
light beam manipulation. Recently discovered ferroelectric nematic liquid
crystals also possess a considerable second-order optical nonlinearity, making
them a perspective material for nonlinear optics. Their use as sources of
quantum light could drastically extend the boundaries of photonic quantum
technologies. However, spontaneous parametric down-conversion, the basic source
of entangled photons, heralded single photons, and squeezed light, has been so
far not 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 the efficiency comparable to that of the
best nonlinear crystals. The emission rate and polarization state of photon
pairs is dramatically varied by applying a few volts or twisting the molecular
orientation along the sample. A liquid crystal source enables a new type of
quasi-phasematching, which is based on 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.
