Liquid crystals could improve quantum communication devices

Creating quantum light just became easier thanks to liquid crystals like the ones found in television screens.

Light with quantum properties is crucial for many future technologies. Entangled particles within such light could help produce quantum communications networks to support an unhackable internet or quantum imaging techniques for biomedicine. Matjaž Humar at Jožef Stefan Institute in Slovenia says that despite these advanced applications, the methods for making quantum light have barely changed for six decades – until now. He and his colleagues devised a way to create it with liquid crystals.

Team member Vitaliy Sultanov at the Max Planck Institute in Germany says traditionally, researchers hit special crystals with lasers to make them emit quantum light. With this technique, the structure of a crystal determines the properties of the light it emits, which then determines how it can be used. The only way to change these properties is to re-do the experiment with a new crystal, which is costly, time-consuming and impractical.

To circumvent this, the researchers used a liquid crystal, a substance made of rod-shaped molecules that can both slosh around like a liquid and assume special arrangements like a more conventional crystal. By exposing a liquid crystal to electric fields, they could adjust its structure – and therefore the properties of quantum light it emitted when shot with lasers.

“In this regard, liquid crystal is the perfect material,” says Sultanov.

Across several experiments, his team found the liquid crystal was much more tuneable than a solid one, and nearly as efficient at producing light full of entangled particles.

“Although the photons produced could perhaps have been generated using traditional crystals, the tuneability of the entanglement could not,” says Miles Padgett at the University of Glasgow in the UK. “These advances could be transformational in terms of applications in [quantum] imaging, communication and sensing.”

Maria Chekhova, also at the Max Planck Institute, says if used in quantum communication devices, liquid crystals could make it easier to transmit information through multiple channels at once. This is because the crystal could be tuned to produce light with quantum states capable of encoding lots of information in many of its properties.