Spontaneous-noise entanglement and photon wave functions

We undertake a pure-state analysis of a noise-dominated quantum event, namely spontaneous photon emission by an excited atom. While pure, the final state is nonseparably entangled. We calculate the participation ratio that provides a measure of the nonseparability, in the context of Schmidt-type analysis. The Schmidt modes serve as pairwise “pointer” bases in the joint Hilbert space, and suggest a nonlocal joint control of the photon and atom as well as the existence of localized photon wave functions.