Giant atom hides its neighbors under a single-electron skirt

Enlarge (credit: Technische Universitit Wien)

You may be familiar with matryoshka dolls: nested sets of painted figurines that fit within painted figurines. In the case of wooden dolls, the concept is pretty straightforward: hollow out a large bit and fit smaller bits in. You might think that doing the same thing with atoms is kind of tough. Yes, atoms are mostly space, but they are kind of difficult to hollow out. And, convincing another atom to enter the empty space (and make itself smaller) seems an impossible task.

Yet, this is kind of what a group of researchers has done. They created a crowd of very cold atoms; within that crowd, they took a single atom and convinced one of the electrons to orbit at a very large distance. Pairs of atoms kind of fell into the gap between that electron and its nucleus. And there they sat, trapped between an outer shell of a fast-orbiting electron and a hard wall of the inner electrons and nucleus. It's not quite a matryoshka doll-all of the trapped atoms sit together in the gap, not inside each other-but it's an impressive approximation.

So, how did the researchers manage this feat of atomic woodwork? To get a grip on that, we need to introduce three concepts: the Bose Einstein Condensate, which forms the crowd of very cold atoms; a polaron, which is the mechanism that traps the atoms inside the outer shell of the doll; and finally, Rydberg atoms, a way to make an atom very large and very empty.

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