Preparation of individual magnetic sub-levels of $^4$He($2^3$S$_1$) in a supersonic beam using laser optical pumping and magnetic hexapole focusing

TL;DR

This paper compares magnetic hexapole focusing and laser optical pumping techniques for preparing specific magnetic sub-levels of metastable helium in a supersonic beam, demonstrating high efficiency and selectivity for the first time.

Contribution

It introduces and compares two methods for magnetic sub-level preparation, showing laser optical pumping's higher efficiency and ability to selectively populate all sub-levels.

Findings

Magnetic hexapole focusing achieves up to 99% population in a specific sub-level.

Laser optical pumping reaches 94% efficiency in populating a targeted sub-level.

Laser optical pumping can selectively populate all three $M_{J^{''}}$ sub-levels.

Abstract

We compare two different experimental techniques for the magnetic-sub-level preparation of metastable $^4$He in the $2^3$S$_1$ level in a supersonic beam, namely magnetic hexapole focusing and optical pumping by laser radiation. At a beam velocity of $v = 830\,$m/s, we deduce from a comparison with a particle trajectory simulation that up to $99\,$\% of the metastable atoms are in the $M_{J^{''}} = +1$ sub-level after magnetic hexapole focusing. Using laser optical pumping via the $2^3$P$_2-2^3$S$_1$ transition, we achieve a maximum efficiency of $94\pm3\,$\% for the population of the $M_{J^{''}} = +1$ sub-level. For the first time, we show that laser optical pumping via the $2^3$P$_1-2^3$S$_1$ transition can be used to selectively populate each of the three $M_{J^{''}}$ sub-levels ($M_{J^{''}} = $ -1, 0, +1). We also find that laser optical pumping leads to higher absolute atom numbers in specific $M_{J^{''}}$ sub-levels than magnetic hexapole focusing.