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The Most Accurate Measurement of the Fine Structure of the Helium Atom

The fine structure splittings of the (1s2p) 23PJ states of the 4He atom was determined with a precision of 130 Hz by a research group in Hefei National Laboratory for Physical Sciences at Microscale (HFNL), University of Science and Technology of China. The paper was published on February 10th in Physical Review Letters 118: 063001 (2017). Ph.D. student Xin Zheng is the first author, Dr. Yu Sun and Prof. Shui-Ming Hu are the corresponding authors.

Helium is the simplest multi-electron atom. Its energy levels can be derived precisely based on quantum electro-dynamics (QED) and a few fundamental physical constants. In particular, the 23PJ fine structure of 4He is ideal for testing QED and is sensitive to the fine-structure constant α (≈1/137), the most important physical constant in QED. Over the last several decades, the QED calculations of the fine structure have been continuously improved, at present reaching the meα7 level. In parallel, physicists also developed various methods to improve the measurement accuracy. However, there are still apparent discrepancies among the reported values (Fig 1).

The HFNL research group has built an apparatus for precision spectroscopy of helium (Fig 2). A meta-stable helium atomic beam was produced by a discharge, and then focused by the laser-cooling technique which considerably increased the intensity of the beam. The atoms were transferred to a single quantum state (23S1), excited by a laser through the 23S1-23PJ transition, and detected by state-selective atom counting.

The 23P0-23P2 and 23P1-23P2 frequency intervals were determined to be 31,908,130.98 ± 0.13 kHz and 2,291,177.56 ± 0.19 kHz, respectively. They are the most accurate results to date. Prof. K. Pachucki from Warsaw University, a collaborator on this project, who is the most famous expert in precision spectroscopy of few-body atoms and molecules, pointed out that the result is very instructive for further development in theories, which will lead to a determination of the α constant with a precision of 2×10-9. The PRL referees highly evaluated this work: “It advances the field of precision measurements… The present work is an illustration of the limits of precision and accuracy available to laser spectroscopy…”.

The work has been supported by the Chinese Academy of Sciences, National Natural Science Foundation of China, Synergetic Innovation Center of Quantum Information and Quantum Physics, and Collaborative Innovation Center of Chemistry for Energy Materials.

Research website of the group: http://atta.ustc.edu.cn


Fig 1. Recent reported values of the 23PJ fine-structure splitting of 4He


Fig 2. Experimental setup for precision spectroscopy of He

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