When is Biology Quantum?
The absorption of quanta of light (photons) is important for many biological processes such as vision, photosynthesis, and animal magnetoreception (birds, for instance, navigate by sensing how the Earth’s magnetic field steers electrons photogenerated in their retinas). Typically, photons are absorbed by some pigment-protein complex, triggering a sequence of chemical reactions that realize a biological function. Researchers have suggested that quantum effects play a role in these processes, but a quantitative assessment of their impact has proved problematic. Now, Atac Imamoglu at the Swiss Federal Institute of Technology (ETH) in Zurich and Birgitta Whaley at the University of California, Berkeley, have described these reactions as quantum measurements, in which the pigment-protein complex acts as a quantum meter measuring the incident light. They propose a new Hamiltonian model that can assess whether quantum coherence is an essential requirement for a given process.
Absorption of light generates quantum wave packets of excited electrons, leading to quantum correlations within the absorbing complex. Using a Hamiltonian formalism, the authors followed the evolution of the correlations to check whether the subsequent chemical products reflected the initial quantum coherence. They argue that quantum coherence can be considered as essential for the bioprocess only if it is maintained beyond the initial excitation of the complex. Analyzing photosynthesis, vision, and magnetoreception, they concluded that only the latter requires quantum coherence. For the other two, the ensuing chemical reactions may still occur without it, albeit with reduced efficiency. The new framework will provide testable predictions for experimentalists and guidelines for designing biomimetic sensing and energy transfer systems whose efficiencies are enhanced by quantum effects.
This research is published in Physical Review E.