Speed-adjustable atomic beam of metastable helium for precision measurement

Abstract

A bright beam of metastable helium atoms is useful in various studies. Laser spectroscopy of helium in an atomic beam can provide a determination of the fine-structure constant and the nuclear charge radius, towards a test of quantum electrodynamics (QED). In these measurements, it is necessary to control not only the internal quantum state but also the translation motion of the helium atom. Here we present a setup producing an intense and speed-adjustable continuous beam of helium atoms. By using a combination of laser cooling, focusing, and deflection, helium atoms are prepared at the single quantum state of ${2}^{\phantom{\rule{0.16em}{0ex}}3}{S}_{1}$ ($m=0$), with a very narrow distribution of the longitudinal speed ($\ensuremath{\delta}{v}_{z}l\ifmmode\pm\else\textpm\fi{}4.5$ m/s). At the same time, we obtain a flux of $1.8\ifmmode\times\else\texttimes\fi{}{10}^{13}$ (atoms/s)/sr with a fractional fluctuation below 0.02%. The atomic-beam quality was investigated by laser spectroscopy, indicating a considerable improvement over those used in previous helium precision-spectroscopy measurements.