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What is Rydberg physics?

Atoms are extremely small. You could string ten thousands of them together in the length of a single bacterium. And yet an atom can be "inflated" to almost the size of a microorganism.

How does that happen?

Crudely put, an atom consists of electrons orbiting a nucleus. With laser light of precisely measured energy, an electron can be lifted into a path with a diameter that is in relative terms gigantic. It then circles around the nucleus at a distance of a few micrometres. These colossuses of atomic physics are known as "Rydberg atoms", after the Swedish physicist Johannes Rydberg.

It is not only size that distinguishes Rydberg atoms from other atoms. The far-off nucleus has only a tenuous hold on the electron. Hence, it reacts extremely sensitively to external stimuli such as electric or magnetic fields. Nevertheless, Rydberg atoms are stable for a relatively long time. With its electrical charge, the outermost electron of a Rydberg atom also has an impact on its environment. It reaches out for a second atom and traps it. The result is a "Rydberg molecule" which is held together by a completely different type of interaction than a conventional molecule. Rydberg physics also comprises many other exotic states of matter, such as the so-called Rydberg polaron, in which about one hundred atoms are held in a cage within a Rydberg atom.

All this makes Rydberg atoms highly interesting research objects for physicists. But there are also ground-breaking applications on the horizon.

Text: Christian Meier. Translation: Kern Group.