A precise few-nucleon size difference by isotope shift measurements of helium

We perform high precision measurements of an isotope shift between the two stable isotopes of helium. We use laser excitation of the 23 S1 -- 23P0 transition at 1083 .... in a metastable beam of 3He and 4He atoms. A newly developed tunable laser frequency selector along with our previous electro-optic frequency modulation technique provides extremely reliable, adaptable, and precise frequency and intensity control. The intensity control contributes negligibly to overall experimental uncertainty by selecting (t selection < 50 ) and stabilizing the intensity of the required sideband and eliminating (∼10-5) the unwanted frequencies generated during the modulation of 1083 nm laser carrier frequency. The selection technique uses a MEMS based fiber switch (tswitch ≈ 10 ms) and several temperature stabilized narrow band (∼3 GHz) fiber gratings. A fiber based optical circulator and an inline fiber amplifier provide the desired isolation and the net gain for the selected frequency. Also rapid (∼2 sec.) alternating measurements of the 23 S1 -- 23P0 interval for both species of helium is achieved with a custom fiber laser for simultaneous optical pumping. A servo-controlled retro-reflected laser beam eliminates residual Doppler effects during the isotope shift measurement. An improved detection design and software control makes negligible subtle potential biases in the data collection. With these advances, combined with new internal and external consistency checks, we are able to obtain results consistent with the best previous measurements, but with substantially improved precision. Our measurement of the 23S 1 -- 23P0 isotope shift between 3He and 4He is 31 097 535.2 (5)kHz. The most recent theoretic calculation combined with this measuremen.;yields a new determination for nuclear size differences between 3He and 4He: Deltarc = 0.292 6 (1)exp (8)th(52)expfm, with a precision of less than a part in 104 coming from the experimental uncertainty (first parenthesis), and a part in 10 3 coming from theory. This value is consistent with electron scattering measurement, but a factor of 10 more precise. It is inconsistent (4 sigma) with a recent isotope shift measurement on another helium transition (2 1S0 -- 23 S1). Comparisons with ongoing muonic helium measurements may provide clues to the origin of what is currently called the proton puzzle: electronic and muonic measurements of the proton size do not agree. In the future, the experimental improvements described here can be used for higher precision tests of atomic theory and quantum electrodynamics, as well as an important atomic physics source of the fine structure constant.