Selected Student Papers

In this article, Emmanouil Kokkinakis, a Master student in the Department of Physics, in collaboration with researchers from Max Planck Institute for the Science of Light (Erlangen, Germany) and Jožef Stefan Institute (Ljubljana, Slovenia), under the leadership of Professors Maria Chekhova and Matjaž Humar, have observed, for the first time, the generation of entangled photon pairs through a liquid crystal. The results of their work, recently published in Nature, pave the way for a new generation of quantum sources, efficient and tunable by applying an electric field or engineering the molecular orientation of the sample.

The splitting of a photon in two via a nonlinear optical process known as Spontaneous Parametric Down-Conversion (SPDC) is one of the most useful tools in quantum photonics as it can create entangled photon pairs, heralded photons, squeezed light as well as more complex light states. SPDC is feasible in materials without central symmetry where the existence of nonlinear second order susceptibility is allowed. It is therefore not observed in ordinary liquids/gasses due to their isotropy, but only in certain solid nonlinear crystals.

Recently, however, the so called ferroelectric nematic liquid crystals (FNLCs) have been discovered, which consist of elongated asymmetric molecules and do not exhibit central symmetry, although they are fluidic.  The important property of their molecules to reorient under the influence of an electric field allowed the researchers in this work to change in real time the rate and polarization state of the photon pairs generated. In fact, the efficiency of entangled photon pair generation is as high as the most efficient non-linear crystals of similar thickness, such as lithium niobate. Therefore, the integration of such a material as a flexible, tunable and efficient source of entangled photon pairs in more complex devices is expected to find a plethora of applications in modern optical quantum technologies. 

Image: a) The main concept: both the flux and the polarization state of the photon pairs can be altered by reconfiguring the molecular orientation, achieved by either engineering the sample geometry or applying an electric field. b) Typical peak of photon-pair detection (coincidences) at the zero delay between the counts of two detectors. c) The rate of two-photon detection increases linearly with the power of the pump laser. Inset: Coincidence-to-accidentals ratio versus the pump power. The inverse dependence indicates two-photon emission. d) The spectrum of the generated photon pairs is broadband (limited by the used filters) and relatively flat.

Research article: Sultanov, V., Kavčič, A., Kokkinakis, E., Sebastian N., Chekhova M.V., Humar M., Tunable entangled photon-pair generation in a liquid crystal. Nature (2024). https://doi.org/10.1038/s41586-024-07543-5