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It is shown that quantum coherence of biomolecular excitons can be maintained over exceedingly long times due to the constructive role of their non-Markovian protein-solvent environment. Using a numerically exact real-time path integral approach, we provide evidence that a slow quantum bath helps to sustain quantum entanglement of two pairs of Forster coupled excitons, in contrast to a Markovian environment. We consider the full crossover from a fast to a slow non-Markovian bath and from weak to strong system-bath coupling and show that a slow bath can generate robust entanglement. This entanglement persists to surprisingly high temperatures, even higher than the excitonic gap. Such a fully quantum mechanical feature is not found for a Markovian bath. |