  The two-photon positron annihilation density matrix is found to separate into
a diagonal center of energy factor implying maximally entangled momenta, and a
relative factor describing decay. For unknown positron injection time, the
distribution of the difference in photon arrival times is a double exponential
at the para-Ps decay rate, consistent with experiment (V. D. Irby, Meas. Sci.
Technol. 15, 1799 (2004)).
