First direct measurement of positronium hyperfine splitting with sub-THz   light
  Positronium is an ideal system for the research of the bound state QED. The hyperfine splitting of positronium (Ps-HFS, about 203 GHz) is an important observable but all previous measurements of Ps-HFS had been measured indirectly using Zeeman splitting. There might be the unknown systematic errors on the uniformity of magnetic field. We are trying to measure Ps-HFS directly using sub-THz radiation. We developed an optical system to accumulate high power (about 10 kW) radiation in a Fabry-P\'erot resonant cavity and observed the positronium hyperfine transition for the first time. 
T. Yamazaki, A. Miyazaki, T. Suehara, T. Namba, S. Asai, T.Kobayashi,   H. Saito, Y. Urushizaki, I. Ogawa, T. Idehara, S. Sa...
hep-ex physics.atom-ph