Sub-THz spectroscopy of the ground state hyperfine splitting of positronium

We plan to directly measure the hyperfine structure of the ground-state positronium. The hyperfine structure between ortho-positronium and para-positronium is about 203 GHz. We develop a new optical system to accumulate about 20 kW power using a gyrotron and high finesse Fabry-Pérot resonator. We report the current status of our experiment.