Positronium hyperfine interval measured via saturated absorption spectroscopy

We report Doppler-free measurements of the positronium (Ps) Lyman-α transition using saturated absorption spectroscopy. In addition to a Lamb dip at wavelength λ =243.0218±0.0005nm, we also observed a crossover resonance at λ =243.0035±0.0005nm, arising from the excitation of 1S13 atoms to Zeeman mixed 2P states, followed by stimulated emission to the 1S01 ground state. Since (λ - λ ) is related to the Ps hyperfine interval E , this observation constitutes the first optical measurement of this quantity and yields E =198.4±4.2GHz. We describe improvements to the methodology that could lead to the ∼ppm level of precision required to address the long-standing discrepancy between QED calculations and precision experiments using microwave radiation to induce transitions between Zeeman shifted triplet Ps states. © 2012 American Physical Society