Demonstration of a novel atomic beam splitter

We have demonstrated a novel beam splitter for atoms. The J=1 to J=0 system in metastable helium has two independent ‘‘dark’’ states which are radiatively stable. By preparing atoms in the two ‘‘dark’’ states of the $2^3$ ${\mathit{S}}_1$ level and passing them through a sequence of partially overlapping laser beams tuned to the $2^3$ ${\mathit{S}}_{1\mathrm{-}}$ $2^2$ ${\mathit{P}}_0$ transition, we cause the atoms in one ‘‘dark’’ state to be deflected by integral multiples of 2ħk, while the atoms in the other ‘‘dark’’ state are undeflected. The excited $2^3$ ${\mathit{P}}_0$ state is not populated, so the process is unaffected by spontaneous emission. We have observed up to 90% coherent momentum transfer with 4ħk deflection. We conclude by describing how one might construct an atom interferometer based on these ‘‘dark state’’ beam splitters.