  Electronic 2D spectroscopy allows nontrivial quantum effects in chemistry and
biology to be explored in unprecedented detail. Here, we apply recently
developed fluorescence detected coherent 2D spectroscopy to study the light
harvesting antenna 2 (LH2) of photosynthetic purple bacteria. The method
utilizes the destructive interference between two signal components thereby
uncovering cross peaks which are not visible in conventional photon-echo based
2D and transient absorption measurements. Analyses of signal generating quantum
pathways leads to the conclusion that, contrary to the currently prevailing
physical picture, the two weakly-coupled pigment rings of LH2 share the initial
electronic excitation leading to quantum mechanical correlation between the two
clearly separate bands. These results are general and have consequences for the
interpretation of excited states not only in photosynthesis but in all light
absorbing systems. The initial delocalization could be the key for enhancing
the light harvesting efficiency via biased motion towards the energy funnel.
