Current Work to Improve Precision in Measurements of Helium Fine Structure

Measurements on the fine structure of the 2P state of the helium atom show good agreement, 0.22(30) kHz, between the most recent theory (complete m$\alpha ^{7}$ evaluation with 0.02 kHz numerical uncertainty) and experiment. Among other things, this result could be used to give a value for the fine structure constant alpha with a 5 ppb uncertainty. However, some of the uncalculated m$\alpha ^{8}$ terms (those with large Z scaling), might contribute as much as 1.2 kHz, limiting the precision and thus calling for further theoretical work. For application to a precision alpha determination, an order of magnitude experimental improvement is desirable, given the electron g factor (0.4 ppb) and photon recoil (0.7 ppb) uncertainties. To this end we are currently addressing a major source of experimental uncertainty in our previous measurements by incorporating a convenient and reliable tunable laser frequency selector.\footnote{Marc Smiciklas and David Shiner, Phys. Rev. Lett. 105, 123001 (2010) } An approach using a fiber grating and fiber circulator will be discussed.