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Spectroscopy of ^{4}He at 0.25 ppt Uncertainty and Improved Alpha-Helion Charge-Radius Difference Determination
K Steinebach1, J C J Koelemeij1, H L Bethlem1
1Vrije Universiteit Amsterdam, Department of Physics and Astronomy, LaserLaB, De Boelelaan 1100, 1081 HZ Amsterdam, The Netherlands.
Physical Review Letters
|July 7, 2026
Summary
This study precisely measured the ^{4}He atomic transition frequency, yielding the most accurate difference between helion and alpha particle nuclear charge radii. The findings resolve discrepancies in helium
Area of Science:
- Atomic physics
- Nuclear physics
- Quantum electrodynamics
Background:
- High-precision spectroscopy of atomic systems probes nuclear charge radii.
- Previous measurements in helium isotopes showed discrepancies with QED theory.
Purpose of the Study:
- To improve the measurement of the ^{4}He 2^{3}S_{1}→2^{1}S_{0} transition frequency.
- To precisely determine the nuclear charge radius difference between helion and alpha particle.
Main Methods:
- Utilized Bose-Einstein condensate of ^{4}He in a magic-wavelength optical dipole trap.
- Suppressed Doppler shifts using time-resolved ion detection.
- Calibrated frequency with a White Rabbit link to a hydrogen maser.
Main Results:
- Achieved 48 Hz uncertainty (0.25 ppt) for the ^{4}He transition frequency.
- Determined the charge-radius difference (r_{h}^{2}-r_{α}^{2}) to be 1.0676(10) fm^{2}.
- Results are consistent with other determinations and resolve QED discrepancies.
Conclusions:
- Provides the most precise determination of the helion-alpha particle charge-radius difference.
- Confirms consistency between experimental results and theoretical predictions.
- Resolves the discrepancy concerning ionization energies in helium.
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