Laser cooling of positronium towards precision spectroscopy and quantum degenerate gases of purely leptonic atoms*

   Positronium (Ps) is an exotic atom consisting of an electron and a positron. The two-body, lepton-only nature makes Ps the simplest atom, constituting a unique system for precise tests of quantum electrodynamics [1,2]. Pursuing better precision in such tests is of great importance for establishing a solid foundation in the current search for new physics beyond the Standard Model. 

   Attaining low temperatures in Ps gases is essential not only for precision spectroscopy but also for creating quantum degenerate gases containing antimatter. While laser cooling presents a viable pathway to very low temperatures, the unique challenges posed by the small mass of Ps and its short-lived nature impede the efficacy of traditional laser cooling methods. Laser designs featuring an atypical combination of broad bandwidth and long pulse duration are imperative.

   We have recently developed and implemented an innovative laser operation scheme: the ‘Chirped Pulse-Train Generator’ [3]. This novel laser emits a sequence of moderately broadband pulses, each with a rapidly shifting central frequency. By harnessing this technique, it is possible to rapidly decelerate Ps gas from its higher-velocity states to lower-velocity ones via chirp cooling. Furthermore, by meticulously tailoring the time-frequency characteristics of the cooling pulses, we can cool a Ps gas to the recoil limit [4].

   In this talk, we will present details of our laser source enabling efficient laser cooling of Ps and the characteristics of the cooling dynamics based on numerical calculations. We will also present results demonstrating one-dimensional cooling of Ps [5] and our ongoing experiments and theories regarding the extension to three-dimensional cooling. Additionally, we will discuss our plans for near-future experiments using cold Ps atoms, including absolute transition frequency measurements.

[1] S. Chu, A. P. Mills, and J. L. Hall, Phys. Rev. Lett. 52, 1689 (1984).

[2] M. S. Fee et al., Phys. Rev. Lett. 70, 1397 (1993).

[3] K. Yamada et al., Phys. Rev. Applied 16, 014009 (2021).

[4] K. Shu et al., Phys. Rev. A 109, 043520 (2024).

[5] K. Shu et al., Nature 633, 793 (2024).