Superfluid liquid Helium is shot under very high pressure out of a tiny nozzle and into vacuum. Outside in front of the nozzle, the excess pressure bursts the liquid apart violently into a myriad of fragments. A cloud of ultra small droplets comes into existence. The liquid beam is almost completely atomized.

If you take a randomly drawn droplet from the explosion, the number of atoms inside of it is mostly just one, a single atom. Finding two atoms is less likely, three atoms even less, and so on.