To create these droplets, the researchers cooled molecules to nanokelvin temperatures, just one billionth of a degree above absolute zero. Under these conditions, quantum effects dominate and the molecules behave collectively, forming a Bose-Einstein condensate in which many molecules share the same quantum state.
Controlling interactions
Until recently, researchers had only managed to create a Bose-Einstein condensate with atoms. With molecules, this proved impossible for a long time. The problem: when two molecules collide, they both disappear from the system, effectively undergoing a chemical reaction with each other.
'We have learned to switch off these losses', says Tijs Karman, researcher at Radboud University. Molecules have a dipole moment, which causes them to behave like tiny magnets. Using microwaves, these magnets can be controlled so that they all rotate synchronously. This can be done such that molecules that come too close repel each other instead of colliding. 'We essentially create a small shield around the molecules,' Karman explains.
In earlier research, the researchers kept that repulsion in balance so that the molecules influenced each other as little as possible. This produced a weakly interacting quantum gas: stable, but with little interaction between the molecules. Karman: 'In this new research, we deliberately break that balance. Using the same microwaves, we essentially turn a knob to increase the attractive force between the molecules.'
