Observation of superradiant and subradiant spontaneous emission of two trapped ions

This paper reports a laboratory realization of the gedanken experiment that Dicke used to introduce superradiance. Two Ba138+ ions are laser-cooled and crystallized in a microscopic Paul trap so that they come to rest ≅ 1.5 microns from each other. This experiment probes different physics than earlier superfluorescence experiments, because propagation effects (nonlinear Maxwell-Bloch equations) are absent and precise theory is possible without approximations. With the development of stronger traps to make R ≤ λ, superradiance may be resolved with high precision to test such effects as Van der Waals dephasing, multiexponential decay, and crystal shape dependence. It also demonstrates that atom-by-atom studies of atomic interactions are practical, which makes possible new types of microscopically resolved, detailed, and precise tests of QED and other atomic interactions.