Direct measurement of positronium hyperfine splitting — Testing fundamental physics with sub-THz gyrotron

The direct transition from o-Ps to p-Ps was firstly observed, using a gyrotron, a mode-converter and a Fabry-Pérot cavity. The Einstein's A coefficient is measured as 3.9 (+1.8 −1.7) • 10
⁻⁸
s
⁻¹
. It is the first step to directly examine a reported discrepancy of positronium hyperfine splitting.