Molecules in an ultracold gas are like tiny magnets: they attract when far apart, but when they get close they can stick together for good. Such sticking destroys the gas: particles drop out of the game, and you never reach the quantum regime where all of them behave as one. Physicists from Princeton and Vienna placed an invisible barrier between the molecules, preventing them from getting too close. The barrier was a static electric field tuned so that molecules repel at short distances.
The field was applied around a cloud of 23Na87Rb (sodium-rubidium molecules). Eight electrodes created a uniform field, and its strength was set near a Förster resonance. This is a condition where two rotational levels of the molecule nearly match in energy. When that happens, molecules feel each other more strongly, and a repulsive hill grows between them. To get closer, molecules would need to jump over the hill, but they lack the energy to do so.
The effect was powerful: collision losses dropped by a factor of ten thousand. Previously, molecules vanished so quickly that evaporative cooling couldn't bring the gas to the desired temperature. Now, by removing the hottest molecules, the physicists cooled the rest to 44 nanokelvin, almost absolute zero. The trap held 7200 molecules, all in a quantum-degenerate state where their wave functions overlap.
But the most intriguing part came at the end of evaporation: a droplet of 230 molecules condensed out of the gas. It didn't fly apart, even though nothing held it. This is a self-bound droplet, like a tiny water droplet in zero gravity that holds together via its own surface tension. Here, the tension comes from dipolar forces between molecules. The droplet was elongated along the electric field, like a thread.
The invisible barrier, which first saved the gas from destruction, ultimately allowed matter to organize itself into new forms. Now this technique can be used in other labs to more easily reach quantum states of molecules and study how droplets, crystals, and possibly superfluid phases emerge from them.
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'.