Towards chemistry at absolute zero

The prospect of cooling matter down to temperatures that are close to absolute zero raises intriguing questions about how chemical reactivity changes under these extreme conditions. Although some types of chemical reaction still occur at 1 mu K, they can no longer adhere to the conventional picture of reactants passing over an activation energy barrier to become products. Indeed, at ultracold temperatures, the system enters a fully quantum regime, and quantum mechanics replaces the classical picture of colliding particles. In this Review, we discuss recent experimental and theoretical developments that allow us to explore chemical reactions at temperatures that range from 100 K to 500 nK. Although the field is still in its infancy, exceptional control has already been demonstrated over reactivity at low temperatures. Chemical reactions can still occur at temperatures as low as 1 mu K. Under such conditions, however, quantum effects are increasingly important, challenging the common understanding of chemical reactivity. The current developments of experimental and theoretical approaches are starting to provide us with a clearer picture of chemistry close to absolute zero.