Thomas, KieranRoss, JacobHenson, BryceShin, DongkiBaldwin, KennethHodgman, SeanTruscott, Andrew2023-01-132023-01-130031-9007http://hdl.handle.net/1885/282757We present the detection of the highly forbidden 2 3S1→ 3 3S1 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 develop two methods using ultracold metastable (2 3S1) 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 ultraweak transitions and precision metrology.This work was supported through Australian Research Council (ARC) Discovery Project Grants No. DP160102337 and DP180101093, as well as Linkage Project No. LE180100142. K. F. T. and D. K. S. are supported by Australian Government Research Training Program (RTP) scholarships. S. S. H. is supported by ARC Discovery Early Career Researcher Award No. DE150100315.application/pdfen-AU© 2020 American Physical Society2020-07-0110.1103/PhysRevLett.125.0130022021-11-28