  We present the current status for the direct measurement of the positronium
hyperfine structure using the $\text{2}^\text{3}\text{S}_\text{1} \rightarrow
\text{2}^\text{1}\text{S}_\text{0}$ transition. This experiment, currently
being commissioned at the slow positron beam facility at ETH Zurich, will be
the first measurement of this transition and the first positronium hyperfine
splitting experiment conducted in vacuum altogether. This experiment will be
free of systematic effects found in earlier experiments, namely the
inhomogeneity in static magnetic fields and the extrapolation from dense gases
to vacuum. The achievable precision is expected to be on the order of $10\,
\mathrm{ppm}$ while the systematic uncertainty is estimated to be within a few
$\mathrm{ppm}$. This would allow to check recent bound state QED calculations
and a $3$-$\sigma$ discrepancy with earlier experiments.
