Precision spectroscopy of the helium atom

The neutral helium atom, with only two electrons, can be theoretically understood to a high degree of precision. As a result, it is a good bound quantum system for testing QED. Among its transitions, 23S1-23PJ (J = 0, 1, 2) have a relatively small natural linewidth (1.6 MHz) and have been well studied with the accuracies of both calculations and measurements reaching kHz and sub-kHz, respectively. Also, the fine-structure intervals among 23PJ states, up to 20 GHz, can be used to determine the value of the fine structure constant α.

Figure1. Schematic of our current atomic beam setup.

We have built a system as shown in Fig. 1. In this work, transverse laser cooling, 2D focusing and optical pumping are used to produce an intense atomic beam of 4He in a single spin state of the metastable 23S1. The spectral probing is accomplished in a well shielded space, isolated form the noise of the surrounding electric, magnetic, and optical fields. The 23P1-23P2 interval, determined from the difference of the frequencies of 23P1- 23S1 and 23P2-23S1, reaches an accuracy of 0.36 kHz. This is the most precise laser-spectroscopy measurement of the interval [Feng 2015,Hu 2009] (Fig. 2). Its result is in agreement with both the latest QED-based calculation by Pachucki and Yerokhin [Pachuchi 2010] and the most precise measurement conducted with microwave spectroscopy by Borbely et al. [Borbely 2009]. It also agrees well with the corrected laser spectroscopy measurement of Castillega et al. [Castillega 2000] and Marsman et al. [Marsman 2012]

Figure2. Comparison of the ν12 (The 23P1-23P2 interval) values from experimental and theoretical studies.

We plan to measure the fine structure splitting of 23P0-23P2, and the absolute frequencies of the 23S1-23PJ transitions of both 4He and 3He. These measurements will be used to test QED, determineαand the nuclear charge radius of 3He.


  1. Laser spectroscopy of the fine-structure splitting in the 23PJ levels of 4He
    X. Zheng, Y. R. Sun*, J.-J. Chen, W. Jiang, K. Pachucki, S.-M. Hu*
    Physical Review Letters, 118, 063001, (2017). [DOI]
  2. Laser spectroscopy measurement of the fine structure splitting 23P1-23P2 of 4He
    G.-P. Feng et al., Physical Review A (Rapid communication), 91, 030502(R), (2015). [DOI]
  3. Precision spectroscopy of the helium atom
    S.-M. Hu et al., Front. Phys.China, 4, 165, (2009). [DOI]
  4. Fine structure of Ions and Determination of the Fine Structure Constant
    K. Pachuchi et al., Physical Review Letters, 104, 070403, (2010). [DOI]
  5. Separated oscillatory-field microwave measurement of the 23P1-23P2 fine-structure interval of atomic helium
    J. S. Borbely et al., Physical Review A, 79, 060503, (2009). [DOI]
  6. Precise Measurement of the J=1 to J=2 Fine Structure Interval in the 23P State of Helium
    J. Castillega et al., Physical Review Letters, 84, 4321, (2000). [DOI]
  7. Shifts due to distant neighboring resonances for laser measurements of 23S1-23PJ transitions of helium
    A. Marsman et al., Physical Review A, 86, 040501, (2012). [DOI]