very fast (of order nanoseconds). However, quantum-mechanical selection rules can prevent
such rapid decay, in which case these “metastable” states can have lifetimes of order
seconds or longer. In this Letter, we determine experimentally the lifetime of the longest-
lived neutral atomic state—the first excited state of helium (the 2 S 1 3 metastable state)—to
the highest accuracy yet measured. We use laser cooling and magnetic trapping to isolate a …