Molecules in a very cold gas are like tiny magnets. If they get too close, they stick together and drop out of the game. That makes it hard to cool a gas to a state where all particles behave as one. Physicists found a way to put an invisible barrier between the molecules: they turned on an electric field that repels molecules at short distances.
The field was created with eight electrodes around a cloud of sodium-rubidium molecules. They tuned it so that molecules start repelling each other more strongly as they approach. It's as if a hill grew between them that they can't jump over. As a result, molecules stopped sticking together, and the gas survived.
After cooling, 7200 molecules remained in the trap, and all of them entered a special quantum state. At the end, a droplet of 230 molecules assembled by itself from the gas. It didn't fly apart even though nothing held it. It's like a water droplet in zero gravity, but instead of surface tension, it's held by forces between molecules. The droplet stretched out along the field like a thread.
This simple electric-field method can be used in other labs to create quantum gases and study how droplets and other unusual forms of matter emerge from molecules.
A droplet of 230 molecules has a density of about 6.3×10¹³ cm⁻³, thousands of times denser than a typical ultracold molecular gas, and lives for just 260 milliseconds.
Self-bound filamentary structures of ultracold molecules evoke exotic forms of matter from science fiction, such as the 'filamentary crystals' in Greg Egan's novel 'Schild's Ladder'.