Paving the way for fundamental physics tests with singly-ionized helium

Understand

High-precision laser spectroscopy of atomic hydrogen has led to an impressive accuracy in tests of bound-state quantum electrodynamics (QED).

• At the current level of accuracy many systematics have to be studied very carefully and only independent measurements provide the ultimate cross-check.
• This has been proven recently by measurements in muonic hydrogen, eventually leading to a significant shift of the CODATA recommended values of the proton charge radius and the Rydberg constant.
• We aim to contribute to tests of fundamental physics by measuring the 1S-2S transition in the He$^+$ ion for the first time.