The ‘proton radius puzzle’ is a long-standing issue in atomic physics that refers to the discrepancy between the most accurate measurements of the proton charge radius obtained from muonic hydrogen spectroscopy, and those obtained from electronic hydrogen spectroscopy. The discrepancy between different methods for determining the proton radius could indicate that new physics exists which goes beyond the Standard Model of the universe.

Helium, the next simplest atom after hydrogen, also has a muonic counterpart.  Recently, the idea to create muonic helium and compare spectroscopic values of the muonic helium nuclear charge radius to its electronic charge radius as a method to shed light on the ‘proton radius puzzle’ has been put forward.

One of the advantages in using helium is that it has two accessible isotopes - 3He and 4He. In terms of the nuclear charge radius measurement, this allows a differential measurement between the isotopic charge radii to be made. In such a measurement common terms in the isotope shift cancel, allowing a better test of state-of-the-art quantum electro-dynamic (QED) theory. This project aims to bring the exquisite control and ultracold temperatures of laser-cooled 3He and 4He atoms to investigate the helium nuclear charge radius to challenge current atomic theory.  

This experimental project, involves building an ultra-stable frequency laser which will be used to probe electronic transitions in ultracold - 3He and 4He.  These measurements will then be used to determine the differential isotopic nuclear charge radius of helium to a world leading absolute accuracy.