TechCrunch summarizes how the technique stores what amounts to a 0 or a 1 in a single Holmium atom:
It works like this: A single Holmium atom (a large one with many unpaired electrons) is set on a bed of magnesium oxide. In this configuration, the atom has what’s called magnetic bistability: It has two stable magnetic states with different spins (just go with it).
The researchers use a scanning tunneling microscope (also invented at IBM, in the 1980s) to apply about 150 millivolts at 10 microamps to the atom — it doesn’t sound like a lot, but at that scale, it’s like a lightning strike. This huge influx of electrons causes the Holmium atom to switch its magnetic spin state. Because the two states have different conductivity profiles, the STM tip can detect which state the atom is in by applying a lower voltage (about 75 millivolts) and sensing its resistance.