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.
@article{arxiv.1403.0312,
title = {First Millimeter-wave Spectroscopy of the Ground-state Positronium},
author = {A. Miyazaki and T. Yamazaki and T. Suehara and T. Namba and S. Asai and T. Kobayashi and H. Saito and Y. Tatematsu and I. Ogawa and T. Idehara},
journal= {arXiv preprint arXiv:1403.0312},
year = {2015}
}