  Recent evidence of electronic coherence during energy transfer in
photosynthetic antenna complexes has reinvigorated the discussion of whether
coherence and/or entanglement has any practical functionality for these
molecular systems. Here we investigate quantitative relationships between the
quantum yield of a light-harvesting complex and the distribution of
entanglement among its components. Our study focusses on the entanglement yield
or average entanglement surviving a time scale comparable to the average
excitation trapping time. As a prototype system we consider the
Fenna-Matthews-Olson (FMO) protein of green sulphur bacteria and show that
there is an inverse relationship between the quantum efficiency and the average
entanglement between distant donor sites. Our results suggest that longlasting
electronic coherence among distant donors might help modulation of the
lightharvesting function.
