Speaker
Description
Precision spectroscopy measurements on calculable systems are widely used to perform tests of theory, but also for determinations of fundamental constants, nuclear charge radii, and as a probe of physics beyond the standard model. We will present spectroscopy in ultracold $^3$He and $^4$He, on the 2 $^3$S$_1$ – 2 $^1$S$_0$ transition at 1557 nm.
Our latest measurement have been performed on a Bose-Einstein condensate of $^4$He trapped in a magic-wavelength optical dipole trap, leading the highest spectroscopic accuracy until now for helium of 48 Hz. For this purpose we developed methods to observe and subsequently suppress systematic Doppler shifts from the BEC oscillating in the optical trap, and we referenced the experiment via a White Rabbit link to a remote hydrogen maser at the Dutch Metrology Institute (VSL).
Combined with our previous measurement in $^3$He [1] we determine an improved isotope shift, and using recent theory [2], we derive a charge radius squared difference between the alpha and helion particle with unprecedented accuracy [3]. Our new result is consistent with other recent determinations (see [1-5]) and confirms that the QED theory discrepancy seen in excited states of helium [5] is not apparent in the isotope shift.
[1] Y. Van der Werf et al., Science 388, 850-853(2025)
[2] K. Pachucki et al., Phys. Rev. A 113, 012824 (2026)
[3] K. Steinebach et al., arXiv:2601.19444 (2026)
[4] K. Schuhmann et al., Science 388, 854-858 (2025)
[5] G. Clausen and F. Merkt, Phys. Rev. Lett. 134, 223001 (2025)