Atoms are not always small. When excited to a Rydberg state—in which the outermost electron occupies a diffuse orbital that’s barely bound to the nucleus—an atom can have macroscopic dimensions of a millimeter or more. Rydberg states’ large size and polarizability make them an important tool of atomic physics because they offer researchers a way of controllably switching interactions on and off between atoms in neighboring optical traps. (See the article by David Weiss and Mark Saffman, Physics Today, July 2017, page 44.)
But if an atom is to be stably excited into a Rydberg state, it needs to be held at least several millimeters away from any solid surface, including the surface of the lens that’s used to focus the trapping light. For a typical trap size of 1 µm, that’s like confining an object to a spot the size of a golf ball from the opposite...
