A phenomenon where excitation of one atom prevents excitation of its neighbors due to strong interactions.The Rydberg blockade is a collective effect in which atoms that have been excited into high-lying, or Rydberg, states prevent nearby atoms from being excited in the same way. A Rydberg atom is an atom with one electron promoted to a very large orbit, far from the nucleus. Such an atom is physically huge — its electron cloud can span hundreds of nanometres — and it is easily polarized, meaning its internal charge distribution shifts in response to nearby charges. When one atom in a small group is in this state, its shifted charge distribution changes the energy levels of its neighbours so strongly that the laser frequency used to excite the first atom no longer matches the transition for the others. The result is a hard limit: within a certain radius, called the blockade radius, only one atom can be excited at a time. This turns a loose cloud of atoms into a system with an all-or-nothing constraint, which is exactly the kind of constraint that makes a physical system useful for storing and processing information.
The mechanism is a shift in atomic energy levels caused by the interaction between two highly excited atoms. When two Rydberg atoms are close, the electric field from one atom's separated charges perturbs the other atom's energy levels. This interaction grows very rapidly as the distance between the atoms decreases — much faster than the usual inverse-square falloff of ordinary forces. Because the excitation laser is tuned to a precise frequency, any atom whose levels have been shifted by a neighbour simply does not absorb the light. The first atom to be excited therefore blocks the excitation of all atoms within its blockade radius. The radius itself depends on the strength of the interaction and on how precisely the laser is tuned: a narrower laser linewidth allows the blockade to extend further.
The blockade is used in neutral-atom quantum computers and simulators, where it allows a single laser pulse to create exactly one excitation in a group of atoms, and in experiments that generate single photons on demand.
The blockade radius can be larger than the wavelength of the light used to excite the atoms, so a single photon can be prevented from being absorbed by a region much wider than itself.
