Enhanced lifetime of positronium atoms via collective radiative effects
  A method is proposed to manipulate the annihilation dynamics of a dense gas of positronium atoms employing superradiance and subradiance regimes of the cooperative spontaneous emission of the system. The annihilation dynamics is controlled by the gas density and by the intensity of the driving strong resonant laser field. In particular, the method allows to increase the annihilation lifetime of an ensemble of positronium atoms more than hundred times by trapping the atoms in the excited state via collective radiative effects in the resonant laser and cavity fields. 
Ni Cui, Mihai Macovei, Karen Z. Hatsagortsyan, and Christoph H. Keitel
quant-ph physics.atom-ph