  We present the detection of the highly forbidden $2^{3\!}S_1 \rightarrow
3^{3\!}S_1$ atomic transition in helium, the weakest transition observed in any
neutral atom. Our measurements of the transition frequency, upper state
lifetime, and transition strength agree well with published theoretical values,
and can lead to tests of both QED contributions and different QED frameworks.
To measure such a weak transition, we developed two methods using ultracold
metastable ($2^{3\!}S_1$) helium atoms: low background direct detection of
excited then decayed atoms for sensitive measurement of the transition
frequency and lifetime; and a pulsed atom laser heating measurement for
determining the transition strength. These methods could possibly be applied to
other atoms, providing new tools in the search for ultra-weak transitions and
precision metrology.
