Hello folks. 2-dimensional spectroscopic techniques have been used to investigate biological systems and found that they transfer energy via coherent pathways. However, some have argued that this has no bearing on how these systems behave in nature, as the sun is not a coherent source and the observed coherence is simply an artifact of the coherence properties of the laser. If you make an assumption of a single photon being absorbed by these systems, is there any difference between a photon from a laser and from an incoherent source? A recent PNAS article got into this and described a photon as a superposition and an incoherent photon as a thermal mixture, but from a semi-classical point of view it's hard to see how a single photon can be "coherent" since coherence is defined through relationships between photons (or at least the waves that describe them). I obviously don't know much about this stuff, so if anyone has any thoughts, they'd be very much appreciated.