Perspectives for optimized laser cooling of positronium*

The recent demonstration of positronium (Ps) laser cooling [1] opens bright perspectives for increased production rates of pulsed antihydrogen via charge exchange reaction between antiprotons and Rydberg state Ps, precision spectroscopy of the electronic transitions in Ps and potentially Bose-Einstein Condensation of ortho-Ps. These experiments require that the highest possible number of Ps atoms are cooled to the lowest possible velocities, in the shortest amount of time to avoid their loss by annihilation.

In contrast with ordinary matter atoms, positronium (the bound state of an electron and a positron) offers a unique set of challenges when it comes to laser cooling. Indeed, the 1S-2P transition which is best suited for this purpose, lies at 243nm in the ultraviolet range. The width of the initial velocity distribution of pulsed Ps clouds typically lies in the 100 km/s range, corresponding to several 100 GHz Doppler broadening of the transition. The annihilation lifetime of ortho-Ps is 142ns, which sets the timescale for laser cooling. Finally, Ps is produced in scarce quantities (one million atoms per minute).

The first experimental results using broadband 100 ns-long ultraviolet pulses [2] confirm the possibility to efficiently cool Ps with a traditional Doppler cooling approach. The observed reduction of momentum spread is consistent with one unit of photon momentum being removed from the distribution per spontaneous lifetime unit, which is the limit of traditional Doppler laser cooling. One important benefit of the demonstrated scheme is the excitation in the long lived 2P triplet state of a large fraction of the Ps ensemble of atoms for a significant amount of time which results in the observed increase in number of atoms annihilating at long delays after production.

We will review possible ways to improve on the initial results [3] and alternative cooling schemes using trains of ultrashort pulses [4] to adapt coherent laser cooling to the case of positronium.