From Rydberg atoms ...

Rydberg atoms are true giants in the atomic world with sizes reaching 10 micrometer for = 300 and constitute mesoscopic entities. They allow to probe classical-quantum correspondence and the transition from the quantum to the classical world. They also provide a useful testing ground for concepts of non-linear dynamics since even moderate external (magnetic or electric) fields provide strong non-separable perturbations. Rydberg atoms have recently attracted attention also in the quest for building blocks of quantum information systems. Rydberg atoms have been proposed as a quantum phase register and as quantum gates employing the Rydberg dipole blockade. The latter makes use of the strong long-range dipole-dipole interactions which conditionally block excitations of atoms in the vicinity of a Rydberg atom already created. More generally, an ensemble of ultracold Rydberg atoms constitutes a strongly interacting many-body system far from the ground state.

Image of a Rydberg atom in a cloud of small atoms

Due to the large distance between the electron and the atomic nucleus and the slow motion of the electron, the Rydberg atom creates a rotating dipole in whose potential other atoms of the gas can be bound.

... to Rydberg molecules

Rydberg molecules are another interesting topic which can, for example, be used as a sensitive probe for the spatial distribution of ultracold gases. When a Rydberg atom is excited in a dense gas of atoms, one or more ground-state atom(s) can be found within the Rydberg electron orbit. For an attractive interaction between the quasi-free Rydberg electron and a ground-state atom an ultralong-range Rydberg molecule can be formed. Typically the interaction and the resulting binding energy are very small and a low kinetic energy of a ground state atom is required and the formation of Rydberg molecules is observed in an ultracold gas of atoms (of the order of 1 μK or less). Within the Born-Oppenheimer approximation, the molecular potential experienced by a ground state atom is approximately proportional to the squared wave function of Rydberg electron. This is because the interaction is larger at higher probability density of Rydberg electron. The vibrational levels are formed by trapping a ground state atom within these potential wells.

For a Rydberg dimer the lowest energy vibrational state has a wavefunction well localized in space around Rn = 2n2 (a.u.). Thus, the probability of creating the dimer molecule will depend on the likelihood of initially finding a pair of ground-state atoms with the appropriate internuclear separation, Rn. By varying the principal quantum number of the Rydberg atom the wavefunction can be localized at different positions and a pair distribution at different distances can be probed. For example, the excitation of Rydberg dimers between = 30 and 50 leads to a pair correlation in the range of R = 90 nm and 250 nm. This technique has been applied to an ultracold gas of bosons as well as fermions and the exchange hole in the two-body correlation of a fermion gas has been observed.

The Rydberg molecule can also be extended to involve more than one ground-state atom. An excitation of Rydberg trimer in which two ground-state atoms are bound may open a possibility to probe a three-body correlation of ultracold gases. By increasing the density of a gas or the principal quantum number n, the number of ground state atoms within a Rydberg molecule can be increased. The current record Rydberg molecule involves up to about 10000 atoms.

For more information about this topic, please consult the following website:, opens an external URL in a new window


J. Burgdörfer, opens an external URL in a new window, S. Yoshida, opens an external URL in a new window