Physics students tend to learn about collisions early in their education. They study billiard balls flying away from each other or lumps of clay sticking and moving together after colliding. Scientists can study collisions of objects that are much smaller and much colder as well. 

In a recent experiment, physicists observed two calcium fluoride, or CaF, molecules colliding at a shocking low temperature — only a few millionths of a degree above absolute zero. Molecular collision measurements are important. Scientists can use them to better design future experiments involving ultracold CaF molecules. Since each collision helps molecules lose some energy and therefore become colder, understanding collisions is important for reaching very low temperatures. 

Making molecules ultracold is hard because they can find any of a bunch of different ways to warm up collecting energy, such as by rotating or vibrating. However, researchers learned how to use lasers and magnets to cool molecules and are learning how to control them with incredible precision. 

In this experiment, they used highly focused laser beams, called optical tweezers, to hold two CaF molecules apart and then bring them into an optical trap. Here, it is crucial that light and matter interact. If a laser beam doesn’t have constant intensity, then its electric field would yank the molecules in undesirable ways with electrical force. By manipulating light very precisely, optical tweezers use these forces to pluck each molecule and move it around. 

In optical traps, electric forces due to light-matter interactions keep molecules stuck in place. By holding two CaF molecules with two separate tweezers then bringing them together, physicists were able to study their collisions in detail. 

The optical tweezer method these researchers developed is now another tool in the toolbox of scientists studying ultracold systems. Other scientists could use this tool in the future to investigate collisions of more complicated molecules or to engineer collisions, and therefore reactions, between other ultracold particles.