We have measured the forbidden $2{}^3{S}_1\to 2{}^1{P}_1$ transition at 887 nm in a quantum degenerate gas of metastable ${}^4\mathrm{He}$ atoms confined in an optical dipole trap. The determined transition frequency is 338 133 594.4 (0.5) MHz, from which we obtain an ionization energy of the $2{}^1{P}_1$ state of 814 709 148.6 (0.5) MHz. This ionization energy is in disagreement by $>3\sigma$ with the most accurate quantum electrodynamics calculations available. Our measurements also provide a new determination of the lifetime of the $2{}^1{P}_1$ state of $0.551(0.004{)}_{\text{stat}}{(}_{-0.000}^{+0.013}{)}_{\text{syst}}\mathrm{ns}$, which is the most accurate determination to date and in excellent agreement with theory.

• Received 27 March 2014

DOI:https://doi.org/10.1103/PhysRevLett.112.253002