Coherence turned on by incoherent light

Abstract

One of the most pertinent problems in the debate on non-trivial quantum effects in biology concerns natural photosynthesis. Since sunlight is composed of thermal photons, it was argued to be unable to induce quantum coherence in matter, and that quantum mechanics is therefore irrelevant for the dynamical processes following photoabsorption. Our present analysis of a toy ``molecular aggregate" -- composed of two dipole-dipole interacting two-level atoms treated as an open quantum system -- however shows that incoherent excitations indeed can trigger coherent dynamics that persist: We demonstrate that collective decay processes induced by the dipole-dipole interaction create coherent intermolecular transport -- regardless of the coherence properties of the incoming radiation. Our analysis shows that the steady state coherence is mediated by the population imbalance between the molecules and, therefore, {\it increases} with the energy difference between the two-level atoms. Our results establish the importance of collective decay processes in the study of ultrafast photophysics, and especially their potential role to generate stationary coherence in incoherently driven quantum transport.