  Strong correlation of photons, particularly in the single-photon regime, has
recently been exploited for various applications in quantum information
processing. Existing correlation measurements, however, do not fully
characterize multi-photon correlation in a relevant context and may pose
limitations in practical situations. We propose a conceptually rigorous, but
easy-to-implement, criterion for detecting correlated multi-photon emission out
of a quantum optical system, drawn from the context of wavefunction collapse.
We illustrate the robustness of our approach against experimental limitations
by considering an anharmonic optical system.
