Postselection shifts the transition frequency of helium in an atomic beam

Jin-Lu Wen; Jia-Dong Tang; Ya-Nan Lv; Yu R Sun; Chang-Ling Zou; Jun-Feng Dong; Shui-Ming Hu

doi:10.1126/sciadv.adu9796

Postselection shifts the transition frequency of helium in an atomic beam

Sci Adv. 2025 Feb 28;11(9):eadu9796. doi: 10.1126/sciadv.adu9796. Epub 2025 Feb 28.

Authors

Jin-Lu Wen¹, Jia-Dong Tang², Ya-Nan Lv², Yu R Sun³, Chang-Ling Zou^{4

5}, Jun-Feng Dong⁵, Shui-Ming Hu^{1

2

5}

Affiliations

¹ State Key Laboratory of Molecular Reaction Dynamics, Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, China.
² Hefei National Research Center of Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China.
³ Institute of Advanced Science Facilities, Shenzhen 518107, China.
⁴ CAS Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei 230026, China.
⁵ Hefei National Laboratory, University of Science and Technology of China, Hefei 230088, China.

Abstract

Postselecting output states can effectively amplify weak signals in measurements. However, the postselection (PS) effect may also introduce unintended biases in precision measurements. Here, we investigate the influence of PS in the precision spectroscopy of the 2³S - 2³P transition of helium (⁴He) using an atomic beam. We directly observe that PS based on atomic positions causes a shift in the measured transition frequency, amounting to approximately -55 kHz. After accounting for this PS shift, we obtain a corrected frequency of 276,764,094,712.45 ± 0.86 kHz for the 2³S₁ - 2³P₀ transition. Combining this result with existing data for ³He, we derive a value for the difference in squared nuclear charge radii, which shows a σ deviation from measurements of muonic helium ions, potentially pointing to new physics that challenges lepton universality in quantum electrodynamics.