Cooling atoms to ultracold temperatures is a routine task in atomic physics labs, but molecules are a trickier proposition. Researchers in the US have now used a widely-applicable combination of methods to make molecules colder than ever before – a feat that could pave the way for applications in areas as diverse as high-temperature superconductivity and quantum computing.

In everyday life, we do not see the bizarre effects of quantum mechanics because the quantum states of the particles around us are constantly collapsing, or decohering, as they interact. At temperatures near absolute zero, however, some identical particles will simultaneously occupy the lowest energy quantum state available. This phenomenon is known as quantum degeneracy, and it was experimentally demonstrated in 1995, when groups led by Eric Cornell and Carl Wieman (then at the University of Colorado, Boulder) and Wolfgang Ketterle of the Massachusetts Institute of Technology (MIT) created the first Bose-Einstein condensates (BECs) with rubidium and sodium atoms, respectively.

Other groups have subsequently made condensates using other atomic species, and various techniques have been developed to cool atoms to quantum degeneracy. In one of the simplest methods, a sample of atoms is confined in a magnetic or optical trap. Hotter atoms with more kinetic energy are more readily able to escape, or evaporate, from this trap, so the remaining atoms become cooler. In another method, known as sympathetic cooling, one type of atom is cooled directly and allowed to thermalize with atoms of another type, thereby cooling them by extracting their kinetic energy.