Precision Laser Spectroscopy of Positronium – Recent Progress

Abstract

Positronium—the bound state of electron and positron—is one of the most fundamental, purely leptonic two-body systems. It provides an ideal testing ground for bound-state quantum electrodynamics. The 1s-2s two-photon transition is a promising candidate for precision spectroscopy because of its narrow linewidth and large quantum-entanglement-device (QED) effects. This two-photon transition requires high laser intensities for its excitation. Because positronium has a low mass, it moves with high velocities even at room temperature, and large beam diameters are required to suppress the transit time broadening to acceptable levels. This is a natural requirement for pulsed light, and the development of narrow linewidth pulsed lasers is necessary. As an alternative, the use of CW laser light requires a build-up cavity to reach high intensities, creation of cryogenic positronium, and laser cooling of the positronium. Precision measurements using pulsed light have always been plagued by rapid phase variations of the electric field caused by refractive index changes in the pulsed amplifier.