be enclosed within a vacuum chamber) or multi-photon transitions (relying on absorption of multiple photons almost simultaneously), both of which are rather inefficient and result in systems with high internal energy. Features The bond length in a heavy Rydberg system is 10,000 times larger than in a typical diatomic molecule. As well as producing the characteristic hydrogen-like behaviour, this also makes them extremely sensitive to perturbation by external electric and magnetic fields. Heavy Rydberg systems have a relatively large reduced mass, given by: This leads to a very slow time evolution, which makes them easy to manipulate both spatially and energetically, while their low binding energy makes them relatively simple to detect through field dissociation and detection of the resulting ions, in a process known as threshold ion-pair production spectroscopy. Kepler's third law states that the period of an orbit is proportional to the cube of the semi-major axis; this can be applied to the Coulomb force: where is the time-period, is the reduced mass, is the semi-major axis and . Classically we can say that a system with a large reduced mass has a long orbital period. Quantum mechanically, a large reduced mass in a system leads to narrow spacing of the energy levels and the rate of time-evolution of the wavefunction depends on this energy spacing. This slow time-evolution makes heavy Rydberg systems ideal for experimentally probing the dynamics of quantum systems. References Atomic, molecular, and optical physics