World's First Matter-Wave Interferometer with a Metastable Helium BEC

Atom interferometry has established itself as a valuable precision measurement tool for the gravitational potential, the Einstein equivalence principle, and the fine structure constant (α) among other things. While most interferometry experiments with ultra-cold atoms up to now have been performed with alkali or alkaline-earth atoms, metastable helium (He*) stands apart with unique advantages. Among those are the possibility to do high-accuracy atom number detection on a multichannel plate, a very small second-order Zeeman shift, and a very high critical acceleration and recoil velocity. Those advantages are especially suitable for a high-precision measurement of α in a manner independent of quantum electrodynamics calculations. We experimentally demonstrate a crucial tool for such a measurement with He*, namely a large number of Bloch oscillations in an optical lattice, performed with high efficiency. This technique coherently transfers a well-known quantity of linear momentum to the atoms, which strongly reduces the uncertainty in atom recoil velocity measurements for determining α. We also demonstrate a proof-of-principle Mach-Zehnder interferometer with He*.