Ordinarily, two neutral atoms separated by much more than a few angstroms take no notice of each other. But when the valence electrons of two alkali atoms are excited to Rydberg states—states with very high principal quantum numbers n—the atoms can interact strongly at separations exceeding 104 Å (1 µ m). When their nuclei are held a few microns apart, the strength of the interaction between two atoms raised to the same Rydberg state increases like n 11.
As reported in back-to-back papers appearing this month, two groups of experimenters have exploited that long-range interaction to demonstrate significant steps toward the goal of quantum computing with neutral atoms. A group led by Mark Saffman and Thad Walker at the University of Wisconsin–Madison reported the first observation of “blockade” interaction between a pair of Rydberg atoms. 1 (When the quantum state of one part of a system...
