A cloud of 41K87Rb molecules held in an optical trap expands like fog at dawn. Each molecule is a weakly bound pair of potassium and rubidium atoms, teetering on the brink of dissociation. Cooling such a cloud to near absolute zero without shattering the molecules has long been a challenge. Theorists have now simulated a process where a short trap pulse acts as a lens, focusing the molecules' motion. The result: the expansion energy drops to tens of picokelvins, while the molecules' internal structure remains untouched.

The method, known as delta-kick collimation, has long been used for atoms: a short trap pulse compresses the cloud, which then expands with lower energy. For molecules, it's trickier: the kick can excite vibrations of the atoms within the molecule and tear it apart. The authors built a classical model of the coupled dynamics of the center of mass and the relative motion of the atoms, averaging over fast vibrations. They found that the coupling between these motions is three orders of magnitude weaker than the center-of-mass energy. During the 150-microsecond kick, only about 100 picokelvins is transferred to vibrations, which is 60,000 times less than the molecule's binding energy. The molecules survive.

Simulations across different regimes, from a Bose-Einstein condensate to a hydrodynamic and a thermal cloud, showed that the gain in expansion energy exceeds 550 for the condensate and drops to 90 for a purely thermal cloud. This means the method works even at relatively high temperatures, though with reduced efficiency. The practical value is immense: cooled molecular clouds are needed for interferometry, where the phase of a molecule's wavefunction is sensitive to gravity. Such interferometers could test the universality of free fall with unprecedented precision, a question that has intrigued physicists since Galileo's time.

The method's potential extends beyond a single molecule. Delta-kick collimation can be implemented on existing setups, including BECCAL and the Cold Atom Laboratory on the ISS. Simultaneous collimation of two atomic species will simplify differential interferometry, where the trajectories of different particles are compared. Ultimately, this paves the way to creating degenerate molecular gases, states where the quantum nature of molecules manifests macroscopically. The lens for a molecular cloud is already polished; all that remains is to point it at the sky.