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We present the current status for the direct measurement of the positroniumhyperfine structure using the $\text{2}^\text{3}\text{S}_\text{1} \rightarrow\text{2}^\text{1}\text{S}_\text{0}$ transition. This experiment, currentlybeing commissioned at the slow positron beam facility at ETH Zurich, will bethe first measurement of this transition and the first positronium hyperfinesplitting experiment conducted in vacuum altogether. This experiment will befree of systematic effects found in earlier experiments, namely theinhomogeneity in static magnetic fields and the extrapolation from dense gasesto 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 calculationsand a $3$-$\sigma$ discrepancy with earlier experiments.

The positronium hyperfine structure: Progress towards a direct  measurement of the $\text{2}^\text{3}\text{S}_\text{1} \rightarrow  \text{2}^\text{1}\text{S}_\text{0}$ transition in vacuum