We report on the first measurement of the Breit-Wigner resonance of the transition from {it ortho-}positronium to {it para-}positronium. We have developed an optical system to accumulate a power of over 20 kW using a frequency-tunable gyrotron and a Fabry-P{e}rot 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 {it p-}Ps.
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