  We report on the coherence properties of single photons from chromium-based
colour centres in diamond. We use field-correlation and spectral lineshape
measurements to reveal the interplay between slow spectral wandering and fast
dephasing mechanisms as a function of temperature. We show that the zero-phonon
transition frequency and its linewidth follow a power-law dependence on
temperature indicating that the dominant fast dephasing mechanisms for these
centres are direct electron-phonon coupling and phonon-modulated Coulomb
coupling to nearby impurities. Further, the observed reduction in the quantum
yield for photon emission as a function of temperature is consistent with the
opening of additional nonradiative channels through thermal activation to
higher energy states predominantly and indicates a near-unity quantum
efficiency at 4 K.
