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Quantum biology

Quantum theory is a pinnacle of human achievement. With it, physicists were able to empirically explain the behavior of quantum systems from subatomic particles to solids and molecules to the formation and life-cycles of stars. One arena that has stubbornly resisted a quantum treatment are biological processes. While certainly the molecules that participate in the processes of life in organic organisms are quantum, the "hot, wet and noisy" environment of in-vivo compounds is such that any quantum states (superpositions of spin or energy levels) are randomized extremely rapidly. This is akin to the problem of "decoherence" we face in engineering single quantum systems for quantum information processing. Any coupling to the environment will drive a quantum object into an eigenstate of the Hamiltonian that describes the environmental interaction. Thus, it has long been held that the processes of life evolved without requiring purely quantum mechanical processes to function: life is purely classical.

There has been relatively recent work, however, that has re-examined the role that quantum coherence plays in several biological processes such as photosythesis and avian magnetoreception. In both cases, there have been tantalizing clues that quantum coherence (excitonic coherence in the case of charge transfer in photosynthesis and molecular spin states in magnetoreception) plays at least some role in the process. What has been made more clear in a recent paper by Atac Imomaglu and Birgitta Whaley is that one can apply a formalism to the class of biological processes that are photoactivated and predict which require quantum coherence to proceed. In their paper, Imomaglu and Whaley develop the Hamiltonian for the process and then apply a formalism treating the optical process as a quantum measurement. They are able to show that for photosynthesis and vision, quantum coherence is not necessary: the chemical reactions would occur anyway in the absence of quantum coherent interactions. However, they do show (as other work has hinted at) that coherent processes add a 'non-classical' efficiency to the reaction. However, for the class of biological processes that include magnetoreception, they show that quantum coherence is fundamental to the process. Without the ability of biological sensors to maintain coherent states, birds may not be able to navigate via the Earth's magnetic fields.

This paper is important in several ways. First, it places strict formal requirements over a specific class of biological processes for which to apply a Hamiltonian based on quantum measurement. This is a well understood formalism within quantum mechanics and thus should rapidly be able to be applied to other biological situations, perhaps including those that do not require photoactivation. Secondly, it establishes that magnetoreception may very well a priori require quantum coherent interactions. This is critical in that there is a broad class of photoactivated processes that fit this criterion, and there may be many otherwise overlooked biological systems that fit within this category. Finally, I find it interesting that forays into the 'fringes' of quantum mechanics can still occur. Atac Imomaglu is a foundational physicist in the study of the quantum photonics of single quantum dots and established many of the protocols leading to quantum information processing with solid state qubits. It is refreshing to see him put his imprimatur on quantum biology, a field that sometimes is viewed askance by physicists.


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