An excitation-dependent four-level model for quantum entanglement in   photosynthetic systems
  We model energy transfer between two coupled four-level chromophores with arbitrarily spaced energy levels. Our analysis takes into account the crucial---yet often ignored---process of initial excitation by light that is incident on the chromophores. We show that the amount of entanglement generated between the chromophores is strongly dependent on the degree of initial excitation as well as the inclusion of higher energy levels. We apply our model to the specific example of chlorophyll. Our results suggest that an excitation-dependent approach should be employed for entanglement studies on multi-level light-harvesting systems even when a two-level approximation is valid. 
Chi-Han Chang, Agata M. Branczyk, Gregory D. Scholes and Daniel F. V.   James
quant-ph