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Precision Laser Spectroscopy of Helium Testing QED Atomic Calculations
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Precision Laser Spectroscopy
of Helium Testing QED Atomic Calculations

Helium is the simplest multi-electron atom and has been the best testing ground for many-body QED atomic calculations. Unlike the hydrogen atom, for which analytical solutions exists, similar studies of helium requires extensive numerical calculations in order to determine its electronic structures. Precision laser spectroscopy of He can improve the theoretical value of Lamb shift and be used to determine the nuclear charge radii of helium.

Recently, researchers from a multiuniversity team in Taiwan have studied the singlet state (with electron spins anti-parallel) of helium. The absolute frequency of the 21S021P1 transition is determined with a relative uncertainty of 10-9. The new data are precise enough to reveal a disagreement with the most precise atomic calculation [1].

The authors constructed two diodepumped Tm, Ho:YLF lasers at 2058 nm. One was offset-locked to a fiberbased optical frequency comb as a reference laser. The second laser was used for saturation spectroscopy in an rf-discharged helium cell. The schematic of the experiment is shown in Fig. 1.

To accurately determine the line center of the transition, the team has had to control precisely many experimental parameters, e.g., laser power stability, magnetic field shielding, discharge condition, and helium gas pressure. Otherwise, any systematic error will limit the final precision of the measurement. A typical spectrum is shown in Fig. 2. This represents the first Doppler-free measurement of the 21S021P1 transition.

For the ionization energy of the 21P1 state, a discrepancy of 3.5 with the most precise theoretical value is found. This is shown in Fig. 3. This work will stimulate new theoretical investigations on the singlet states of helium. In addition, the determination of the nuclear charge radii of helium can be of great importance. For example, a measurement of the Lamb shift in muonic helium may shed light on the proton radius puzzle [2]. Refined QED calculations will be critical in comparing the spectroscopic results from electronic and muonic helium.

Fig. 1: Schematic of the experimental setup. OFC: optical frequency comb; PD: photodetector; HSPD: high-speed photodetector; AOM: acousto-optical modulator.

Fig. 2: A typical spectrum of the transition. Lorentzian profile incorporating collisional effect in a Gaussian form is used to fit the line shape. The Gaussian width is 1.5 GHz corresponding to a gas temperature of 800 K. The Lorentzian width is approximately 800 MHz indicating considerable power-broadening effect. The bottom shows the fitting residuals demonstrating that the measured spectrum has a very symmetric line shape.

Fig. 3: Comparison with theories and former measurement. New determinations 1 and 2 are derived from this result and other known transitions. The most precise QED calculation by Yerokhin and Pachucki does not agree with this result.


P. -L. Luo, J. -L. Peng, J. -T. Shy, and L. -B. Wang, "Precision frequency metrology of helium 21S021P1 transition," Phys. Rev. Lett. 111,013002 (2013).
Aldo Antognini et al., "Proton structure from the measurement of 2S-2P transition frequencies of muonic hydrogen," Science. Vol. 339, no. 6118, pp. 417-420, 25th January 2013

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