Two‐photon laser excitation yields positronium 1S–2S line

Positronium—the bound state of an electron and a positron—entices the experimenter with a unique opportunity for testing the predictions of quantum electrodynamics. Not only does this purely leptonic “atom” avoid the complications of hadronic nuclear structure, but it also involves virtual‐annihilation terms absent in the case of hydrogen or muonium. Furthermore, because the usual reduced‐mass formalism for hydrogen‐like atoms is relativistically inappropriate for this system of two equal masses, one must use the Bethe–Salpeter formalism rather than the Dirac equation as the starting point for calculating QED corrections in positronium.