Rydberg atom ($n\gg 1$)

A Rydberg atom has an electron excited to a large principal quantum number $n$. In a hydrogen-like model, the binding energy scales as $1/n^2$ and the orbital size as $n^2$. The outer electron is therefore weakly bound and its motion extends far beyond the compact core.

These properties make Rydberg atoms sensitive to applied electric fields and useful for studying controllable interactions such as Rydberg blockade. A field ramp can ionize different excited states at different thresholds, enabling state-selective field ionization. Real multi-electron atoms depart from the simple hydrogen-like spectrum, especially when the outer electron penetrates the core.

Rydberg blockade suppresses simultaneous excitation of nearby chemical atoms into strongly interacting Rydberg atom states. If the interaction shifts the doubly excited level by much more than the excitation linewidth and coherent drive scale, a pulse resonant for one isolated chemical atom becomes off-resonant when a neighbour is already excited. The resulting conditional dynamics can implement an entangling two-qubit gate for neutral-atom qubits.