Open system perspective on incoherent excitation of light-harvesting systems

The nature of excited states of open quantum systems produced by incoherent natural thermal light is analyzed based on a description of the quantum dynamical map. Natural thermal light is shown to generate long-lasting coherent dynamics because of (i) the super-Ohmic character of the radiation, and (ii) the absence of pure dephasing dynamics. In the presence of an environment, the long-lasting coherences induced by suddenly turned-on incoherent light dissipate and stationary coherences are established. Dynamics in a subunit of the PC645 light-harvesting complex is considered where it is further shown that aspects of the energy pathways landscape depend on the nature of the exciting light and number of chromophores excited. Specifically, pulsed laser and natural broadband incoherent excitation induce significantly different energy transfer pathways. We note an important difference in the phase of system coherences when coupled to blackbody radiation or when coupled to a phonon background.