Phonon signatures in photon correlations

We show that the second-order, two-time correlation functions for phonons and photons emitted from a vibronic molecule in a thermal bath result in bunching and anti-bunching, respectively. Signatures relating to phonon exchange with the environment are revealed in photon-photon correlations. Our work opens the possibility of a new approach to the significant and long-standing controversy regarding the role of quantum coherence in energy transfer within molecular systems and offers new opportunities to investigate quantum effects in condensed-phase molecular systems. We show that by detecting individual photons and measuring correlations in the photon emission it is possible to identify genuine quantum effects of molecules within a thermal environment, whose signatures are inferred from photon-photon, phonon-phonon and cross-correlation functions.