Frequency metrology of helium around 1083 nm and determination of the nuclear charge radius

TL;DR

This study precisely measures helium transition frequencies using an optical frequency comb, enabling the extraction of nuclear charge radius differences between helium isotopes, and revealing a significant discrepancy with previous results.

Contribution

It provides the most precise helium transition frequency measurements to date and uses them to determine the nuclear charge radius difference, challenging prior findings.

Findings

Most precise helium transition frequency measurements to date.

Determined nuclear charge radius difference $oldsymbol{ extstylerac{ ext{r}^2_{^3 ext{He}}- ext{r}^2_{^4 ext{He}}}{ ext{fm}^2}=1.074(3)$.

Discrepancy of about 4 sigma with recent isotope shift measurements.

Abstract

We measure the absolute frequency of seven out of the nine allowed transitions between the 2$^3${\it S} and 2$^3${\it P} hyperfine manifolds in a metastable $^3$He beam by using an optical frequency comb synthesizer-assisted spectrometer. The relative uncertainty of our measurements ranges from $1\times 10^{-11}$ to $5\times 10^{-12}$, which is, to our knowledge, the most precise result for any optical $^3$He transition to date. The resulting $2^3${\it P}-2$^3${\it S} centroid frequency is $276\,702\,827\,204.8\,(2.4)$kHz. Comparing this value with the known result for the $^4$He centroid and performing {\em ab initio} QED calculations of the $^4$He-$^3$He isotope shift, we extract the difference of the squared nuclear charge radii $\delta r^2$ of $^3$He and $^4$He. Our result for $\delta r^2=1.074 (3)$ fm$^2$ disagrees by about $4\,\sigma$ with the recent determination [R. van Rooij {\em et al.}, Science {\bf 333}, 196 (2011)].