We examine the correlation between a pair of photons in the informatio
n entropy context. Either in two-photon cascade decay of a three-level
system or in parametric down-conversion a pure state of two distingui
shable photons is produced, with the states of the individual photons
entangled. We obtain state amplitudes in the Weisskopf-Wigner approxim
ation and use them to derive the two-photon density matrix in the 'arr
ival time' basis. It is found that only a finite number of eigenvector
s of the appropriate density matrices is required to describe the syst
em. That is, despite the continua of wave-vectors involved in the doub
le spontaneous decay the two-photon entanglement is effectively discre
te; furthermore, the eigenvectors in the arrival time basis can be int
erpreted as the fundamental orthogonal spontaneous emission pulses of
the two systems.