Spontaneous parametric down-conversion (SPDC) is the workhorse of quantum optics and photonic quantum technologies. It is used as a source of entangled photons, single photons, and squeezed light. SPDC is a second-order nonlinear process and requires materials without centre of symmetry, like crystals. SPDC in liquids or gases has never been observed up to now.

Recently synthesized ferroelectric nematic liquid crystals change this situation drastically. They combine considerable second-order nonlinear susceptibility with the strong response to electric field, typical for usual liquid crystals, and with fluidity, typical for all liquids. In my talk I will show experiments where we generate entangled photons via SPDC in a ferroelectric nematic liquid crystal, switch the process on and off with external electric field, reconfigure the molecular orientations, and tune the polarization state of the photon pairs. The efficiency of photon pair generation is comparable to the one for typical nonlinear crystals of the same length.  Moreover, by creating a constant twist of the molecules orientation along the sample one can implement a new type of phase matching, more efficient and more flexible than periodic poling.