Collective Protection and Exchange-Summed No-Jump Survival in Dipositronium

A decoherence-free subspace is protected only relative to the channel algebra used to define it, as in the standard DFS literature of Zanardi and Rasetti, Lidar, Chuang, and Whaley, and later DFS/subsystem reviews. Dipositronium provides a compact setting in which that qualification becomes nontrivial. A fixed-pair picture of two positronium atoms suggests a simple collective-spin protected sector, but true molecular Ps2, consisting of two electrons and two positrons, has no permanent electron-positron pairs. The electrons are identical, the positrons are identical, and annihilation must be summed over all electron-positron contacts.