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We report on the first measurement of the Breit-Wigner resonance of thetransition from {\it ortho-}positronium to {\it para-}positronium. We havedeveloped an optical system to accumulate a power of over 20 kW using afrequency-tunable gyrotron and a Fabry-P\'{e}rot cavity. This system opens anew era of millimeter-wave spectroscopy, and enables us to directly determineboth the hyperfine interval and the decay width of {\it p-}Ps.

First Millimeter-wave Spectroscopy of the Ground-state Positronium