Schematic illustration of trapped $^4$He and $^3$He atoms, depicting the use of laser spectroscopy for probing the difference in their nuclear charge radii.

From the article:

Spectroscopy of ${}^4\mathrm{He}$ at 0.25 ppt Uncertainty and Improved Alpha-Helion Charge-Radius Difference Determination
K. Steinebach, J. C. J. Koelemeij, H. L. Bethlem, and K. S. E. Eikema
Phys. Rev. Lett. 136, 243001 (2026)

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