A polar molecule is like a tiny dumbbell with opposite charges at its ends. Normally, such particles quickly stick together or fly apart when they meet — electric forces are too strong. But the authors of the work found a way to turn this aggressiveness into a controlled game.
They took the two lowest vibrational energy levels of the molecule (v=0 and v=1) and declared them states of a pseudospin — a quantum analog of a top that can point 'up' or 'down.' Then they dressed the molecules in a dual microwave 'shield': radiation at two frequencies envelops both states, quenches dangerous approaches, and forbids triple collisions. Particle losses drop by hundreds of times.
In an ordinary gas, such a system would allow observing the wandering of magnetic excitations and the birth of quantum droplets with vortex-like spin patterns. If molecules are arranged in the sites of an optical lattice (a kind of light crystal lattice), one can realize extended Hubbard and t–Jz models. These are workhorses of solid-state physics, describing electron behavior in materials. Now they can be studied directly, moving molecules with optical tweezers.
Polar molecules can interact at distances a hundred times their own size — roughly like two people tossing a ball across a football field.