Liquid crystals as tunable sources of entangled photon pairs

Entangled photon pair sources are an essential ingredient for quantum technologies. However, currently existing sources struggle from the lack of tunability of the generated quantum state of light. Their linear and nonlinear optical properties strictly define the generated state and can barely be modified in real time in a reversible and controllable way, mainly due to their rigid structure. Here, we investigate a new approach of entangled photon-pair generation in liquid crystals that can overcome these limitations. Their use as sources of quantum states of light was so far impossible due to inversion symmetry and consequently insufficient nonlinear response. However, recently discovered ferroelectric nematic liquid crystals have polar order which leads to considerable nonlinear response comparable to state-of-the-art nonlinear crystals. This, as we demonstrate, enables efficient SPDC generation with the addition of broad tunability of the generated state. The emission rate, polarization state and degree of entanglement of the photon pairs strongly depend on the orientation of the molecules, which we can drastically alter by either applying electric field or twisting the molecules along the sample. This work holds the potential to have a huge impact in the field of quantum technologies as tunable quantum light sources in liquid crystals can outperform conventional sources of photon pairs in terms of tunability, efficiency and overall functionality.