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Quantum Biology

While vast scale separation has meant a traditional distinction between quantum mechanics and biology, recently evidence of quantum coherence in highly efficient photosynthetic light-harvesting complexes at physiological temperatures has raised the intriguing question of whether non-trivial quantum effects play a role in living systems. Many models of environment-assisted quantum transport have been proposed, typically within approximate spin-boson models of the system.

We show, however, that interaction with a finite spin bath can also assist quantum efficiency. Our system is a fully connected network of qubits with equal coupling strengths and site energies. The dynamics of a single excitation in the symmetric network are compared with the case where the symmetries that prevent distinguishing sites are removed via a pure dephasing interaction between each site and a spin environment at zero temperature. We show analytically that the maximum probability of transfer through the network can be increased by decoherence-induced site energy shifts, and that there are cases where transfer can be guaranteed.

Furthermore, we show that these effects persist at physiological temperatures, i.e. that near perfect transfer can be achieved for biologically relevant parameter regimes. Applying this model to the Fenna–Matthews–Olson photosynthetic antenna complex, we show numerically that energy transfer is significantly assisted by a decoherent interaction between each network site and a respective spin environment at 300 K. These results motivate further study of the degree to which a spin bath can provide a sufficiently realistic model for the environment of photosynthetic complexes and biological systems in general.

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