Spatial separation of atomic states in a laser-cooled ion crystal

We have produced a laser-cooled ion crystal in a linear Paul trap containing several hundred Ca+ ions. A portion of the ions can be excited by an additional laser to a long-lived metastable energy level that decouples them from the cooling laser radiation. The light pressure acting upon the remaining laser-cooled ions forces them into the direction of the laser beam, while the ions in the metastable state drift to the crystal side that points toward the cooling laser. Depending on the number of ions in the crystal and on the fraction of the excited ions, the spatial separation of the ions in the metastable state and the ions in the cooling cycle can be as large as several hundred micrometers.