Phys. Rev. Lett. (in the press)

Rydberg atoms are atoms that have one or more electrons in a highly excited state. The outermost occupied electronic orbital of such an atom can be micrometres wide, producing a strong dipole moment that enables Rydberg atoms to interact over much longer distances than atoms in their ground state. Such interactions can cause the laser-driven excitation of one atom to shift the resonant frequency of neighbouring atoms, which prevents their excitation by the laser that excited the first — a process known as Rydberg blockade.

Andrew Schwarzkopf and colleagues have developed a technique to image the location of many Rydberg atoms in a cold rubidium gas. Autocorrelation functions obtained from these images enabled them to determine the radius and shape of the region over which Rydberg blockade occurs for different excitation states. For the highest measured state, the blockade radius was about 5 μm — 100,000 times larger than a rubidium atom's ground-state radius.

The authors also saw unexpected periodic structures at distances beyond this radius. The large distance of such interactions, and the fact that the atoms involved in Rydberg blockade are quantum entangled, could be useful in building the logic circuits of a quantum computer.