Fig. 1.

●●●. (Left) Monomer of the FMO complex and its absorption. The eigenstate level scheme is shown in Inset: g is the ground state, whereas e and f are the single- and the double-exciton manifolds, respectively. Absorption was simulated as described in ref. 11. Narrow peaks indicate state positions and oscillator strengths. (Right) The four-wave-mixing experiment: three pulses (k₁, k₂, and k₃) interact with the system and generate nonlinear polarization, which is probed by using the fourth (signal) pulse (k₄). For ultrashort nonoverlapping pulses, the delay times between pulses t₁, t₂, and t₃ coincide with the delay times between interactions. The two Feynman diagrams that contribute to the signal in the phase-matching direction (k_III ≡ k₄ = k₁ + k₂ − k₃) in the RWA are shown at the bottom.