Helium Spectroscopy Hits Record Precision
As the simplest multielectron atomic system, helium serves as a crucial platform for probing fundamental physics. This is aided by helium’s long-lived and optically accessible 23S1 state, which enables atoms to be efficiently cooled, trapped, and detected. Now a team at Vrije Universiteit Amsterdam led by Kjeld Eikema has measured the transition between the 23S1 state and the higher-energy 21S0 state in helium-4 with unprecedented precision [1]. The result improves limits on the difference between the nuclear radii of two helium isotopes.
The researchers observed the transition in an ultracold Bose-Einstein condensate of helium-4 atoms confined to a precisely tuned optical trap. They measured the transition frequency and calibrated it against a stable reference frequency provided by a remote atomic clock. To suppress systematic errors caused by the condensate’s residual motion, the researchers devised and implemented a way to track this motion and limit its impact on the calibration. They determined the transition frequency with an uncertainty of just 48 Hz (0.25 parts per trillion), 4 times more precise than the previous best result.
The researchers then combined their measurement with an equivalent one they had made last year for helium-3, which had an uncertainty of 170 Hz, and with theory. That step enabled them to extract a value of 1.0676 ± 0.0010 square femtometers (fm2) for the difference between the square of the radius of a helium-4 nucleus and the square of the radius of a helium-3 nucleus. This value agrees with other determinations, but the uncertainty is 30% smaller than that of the previous best result. The team suggests that in future work the precision could be pushed to as low as 0.0004 fm2 by improving the helium-3 measurements and the theory.
–Ryan Wilkinson
Ryan Wilkinson is a Corresponding Editor for Physics Magazine based in Durham, UK.
References
- K. Steinebach et al., “Spectroscopy of 4He at 0.25 ppt uncertainty and improved alpha-helion charge-radius difference determination,” Phys. Rev. Lett. 136, 243001 (2026).