First Millimeter-wave Spectroscopy of the Ground-state Positronium

We report on the first measurement of the Breit-Wigner resonance of the transition from ortho-positronium to para-positronium. We have developed an optical system to accumulate a power of over 20kW using a frequency-tunable gyrotron and a Fabry-Perot cavity. This system opens a new era of millimeter-wave spectroscopy, and enables us to directly determine both the hyperfine interval and the decay width of para-Ps.