First Observation of the 1S-2S Transition of He+ in an Atomic Beam

High-precision spectroscopy of simple, calculable atomic and molecular systems allows for stringent tests of bound-state quantum electrodynamics (QED), determinations of fundamental constants, and searches for physics beyond the standard model. Measurements in different systems enable to separate theory from the fundamental constants, and detect inconsistencies [1]–[3]. An alternative system compared to hydrogen or muonic hydrogen is singly-ionized helium, which enables new fundamental physics tests. Our goal is to measure the IS-2S transition in He+ with an accuracy of 1 kHz or better. It would allow improved QED tests and, combined with muonic He+ [4] measurements, permit a more accurate alpha particle charge determination.