  Realising scalable quantum networks requires a meticulous level of
understanding and mitigating the deleterious effects of decoherence. Many
quantum device platforms feature multiple decoherence mechanisms, often with a
dominant mechanism seemingly fully masking others. In this paper, we show how
access to weaker dephasing mechanisms can nevertheless be obtained for
optically active qubits by performing two-photon coincidence measurements. To
this end we theoretically investigate the impact of different decoherence
mechanisms on cooperatively emitting quantum dots. Focusing on the typically
dominant deformation-potential coupling to longitudinal acoustic phonons and
typically much less severe additional sources of pure dephasing, we employ a
numerically exact method to show that these mechanisms lead to very different
two-photon coincidence signals. Moreover, surprisingly, the impact of the
strongly coupled phonon environment is weak and leads to long-lived coherences.
We trace this back to the superohmic nature of the deformation-potential
coupling causing inter-emitter coherences to converge to a nonzero value on a
short timescale, whereas pure dephasing contributions cause a complete decay of
coherence over longer times. Our approach provides a practical means of
investigating decoherence processes on different timescales in solid state
emitters, and thus contributes to understanding and possibly eliminating their
detrimental influences.
