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Updated: May 24, 2025

Spatial Separation of Molecular Conformers and Clusters
Published on: January 9, 2014
Postselection shifts the transition frequency of helium in an atomic beam
Jin-Lu Wen1, Jia-Dong Tang2, Ya-Nan Lv2
1State Key Laboratory of Molecular Reaction Dynamics, Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, 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.
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