Record broken for the coldest temperature reached by large molecules

Molecules containing four atoms are the largest yet to be cooled down to only a hundred billionths of a degree above absolute zero. 

The techniques researchers use for cooling individual atoms, such as hitting them with lasers and magnetic forces, don’t work as well for molecules. This is especially true for molecules made of many atoms, because to be very cold they must be very still – the more moving parts a molecule has, the more opportunities it has to move and warm up.  

“We have a joke that we study molecules not because it is easy, but because it is hard,” says Xin-Yu Luo at the Max Planck Institute of Quantum Optics in Germany. He and his colleagues have now made four-atom molecules colder than ever before. 

They started with several thousand molecules composed of one sodium and one potassium atom, which they confined in an airless chamber and cooled – that is, made very still – with magnetic forces and bursts of light. The coldest possible temperature is 0 kelvin, or absolute zero; these molecules were just 97 billionths of a kelvin warmer.  

To turn these two-atom molecules into four-atom molecules, the researchers had to combine them in pairs without allowing them to warm up. They used microwave fields to “glue” molecules together based on theoretical calculations by Tao Shi and Su Yi the Chinese Academy of Sciences. “We really didn’t know if we could assemble these molecules, but Tao’s team did a calculation and he said to me, ‘this is possible, just try it’,” says Luo.  

Their trials were successful. The researchers created about 1100 molecules, each with two potassium and two sodium atoms, at a temperature of 134 billionths of a kelvin – the largest molecules to reach this ultracold temperature yet.  

“One of the reasons you make molecules ultracold in the first place is to have more control over them, and this is a big step forward in that sense,” says John Bohn at the University of Colorado Boulder. The new experiment is important not only because of the molecules’ unprecedented temperature, but also because at their coldest, they enter a known quantum state and could be pushed into another state or a process with precision, he says.  

Luo says the atoms in these molecules are not “glued” to each other as strongly as those in room-temperature molecules. But making them is a necessary step towards studying complicated chemical reactions, which are easier to observe when they are extremely cold and slow. 

The next question is what other, possibly even bigger molecules could be built at ultracold temperatures from similarly frigid ingredients with a similar microwave technique, says Sebastian Will at Columbia University in New York. “I think we are looking at exciting new opportunities for quantum chemistry!” he says.