Positronium Bose-Einstein condensation in liquid He4 bubbles

A hollow spherical bubble containing thousands of spin-aligned triplet positronium (Ps) atoms in superfluid liquid $^{4}\mathrm{He}$ would be stable against breakup into smaller bubbles, and the Ps would form a Bose-Einstein condensate (BEC) with a number density of $\ensuremath{\sim}{10}^{20}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ and a BEC critical temperature ${T}_{c}\ensuremath{\approx}300$ K. Estimates suggest that one could make such bubbles in the laboratory containing ${10}^{5}$ Ps atoms using presently known methods.