Prospects for precision measurements of atomic helium using direct frequency comb spectroscopy

We analyze several possibilities for precisely measuring electronic transitions in atomic helium by the direct use of phase-stabilized femtosecond frequency combs. Because the comb is self-calibrating and can be shifted into the ultraviolet spectral region via harmonic generation, it offers the prospect of greatly improved accuracy for UV and far-UV transitions. To take advantage of this accuracy an ultracold helium sample is needed. For measurements of the triplet spectrum a magneto-optical trap (MOT) can be used to cool and trap metastable 23S state atoms. We analyze schemes for measuring the two-photon 23S →43S interval, and for resonant two-photon excitation to high Rydberg states, 23S →33P →n3S, D. We also analyze experiments on the singlet-state spectrum. To accomplish this we propose schemes for producing and trapping ultracold helium in the 11S or 21S state via intercombination transitions. A particularly intriguing scenario is the possibility of measuring the 11S →21S transition with extremely high accuracy by use of two-photon excitation in a magic wavelength trap that operates identically for both states. We predict a “triple magic wavelength” at 412 nm that could facilitate numerous experiments on trapped helium atoms, because here the polarizabilities of the 11S, 21S and 23S states are all similar, small, and positive.