Photo-activated Biological Processes As Quantum Measurements

Birgitta Whaley (left) and Atac Imamoglu

Authors: Atac Imamoglu¹, Birgitta Whaley²

Affiliation:
¹Institute of Quantum Electronics, ETH Zurich, Switzerland,
²Berkeley Quantum Information and Computation Center, Department of Chemistry, University of California, Berkeley, USA.

Image credit: Ilya Sinayskiy

Our current understanding of the physical world around us is based on quantum mechanics. It is natural in this framework to argue that at the molecular level, biological processes are also governed by the laws of quantum mechanics. This reasoning in turn implies that at short enough length and/or time scales the dynamics can exhibit counter-intuitive features, such as the molecule/system being in a coherent superposition of distinguishable states. A key question of fundamental interest is whether there are biological systems for which these microscopic dynamical quantum features help or enable the biological function [1-3]. Two processes that have been extensively studied in this context are light harvesting in photosynthesis and sensing of the inclination of the earth’s magnetic field by migrating birds.

Past 2Physics article by Atac Imamoglu :
December 23, 2012: "Observation of Entanglement Between a Quantum Dot Spin and a Single Photon" by Wei-bo Gao, Parisa Fallahi, Emre Togan, Javier Miguel-Sanchez, Atac Imamoglu.

Our work brings a new perspective to the analysis of these processes by embedding them in a quantum measurement context, where the biological system is modelled as a measurement device that is subject to the laws of quantum mechanics (i.e., a quantum meter) [4]. The function of this quantum meter is to measure an external classical stimulus, which is thereby equivalent to the biological sensing of this stimulus. We have analyzed several photo-activated biological processes within this formulation and find that these processes fall into two distinct classes.

In the first category, the measurement interaction induces changes in the system state at a rate that is proportional to the strength of the external stimulus. In this case, we find that while the presence of quantum coherence during the measurement interaction may result in a small enhancement of the rate that increases at most linearly with the increasing coherence time, it is however not essential for the biological function that results from the sensing of this stimulus.

By contrast, in the second category, the measurement interaction does not directly lead to an excitation rate that is proportional to the strength of the external stimulus. Instead, it is the quantum coherent evolution after the optical excitation that controls the sensitivity of the biological system to the stimulus. Most importantly, in this category, unless there is some quantum coherent dynamics after the photoactivation, there is vanishing sensitivity to the signal to be measured. Another difference is that depending on the specific nature of this coherent evolution, more detailed information about the signal than just its strength can be transmitted to the biological receptors.

The extensively studied process of photosynthesis [see, e.g., 1-3, 5-7] as well as the process of human vision [8,9] both belong to the first category. In contrast, the proposed hypothesis of a radical pair mechanism [10,11] for sensing of the inclination of the earth’s magnetic field by migratory birds belongs to the second category. An essential component of this mechanism is the coherent oscillation between singlet and triplet radical pairs in which the paired electrons are separated by several nanometers and are thus formally entangled over non-trivial distances. The chemical reactivity of the radical pair is different in the singlet and triplet states, resulting in a chemical signature of the non-equilibrium quantum dynamics induced by the quantum coherent dynamics.

While much indirect chemical evidence exists for this hypothesis, experimental validation in birds is challenging and, despite many plausibility arguments, no clear evidence for the validity of this hypothesis in migratory birds has been established thus far. It therefore remains an intriguing and open question today, as to whether there are biological sensing processes that can function only if quantum coherence is preserved on some extended time scale.

References:
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[2] Neill Lambert, Yueh-Nan Chen, Yuan-Chung Cheng, Che-Ming Li, Guang-Yin Chen, Franco Nori, “Quantum biology”, Nature Physics, 9, 10 (2013). Abstract.
[3] M. Mohseni, Y. Omar, G. Engel, M. Plenio (Eds.), "Quantum effects in biology" (Cambridge University Press, 2014). 
[4] A. Imamoglu, K. B. Whaley, “Photo-activated biological processes as quantum measurements”, Physical Review E, 91,022714 (2015). Abstract.
[5] Rienk van Grondelle, Vladimir I. Novoderezhkin, “Quantum effects in photosynthesis”, Procedia Chemistry, 3, 198 (2011). Abstract.
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[7] Aurélia Chenu, Gregory D. Scholes, “Coherence in energy transfer and photosynthesis”, Annual Review of Physical Chemistry, 66, 69 (2015). Abstract.
[8] F. Rieke, D. A. Baylor, “Single-photon detection by rod cells of the retina”, Review of Modern Physics, 70, 1027 (1998). Abstract.
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[10] Thorsten Ritz, Salih Adem, Klaus Schulten, “A model for photoreceptor-based magnetoreception in birds”, Biophysical Journal, 78, 707 (2000). Full Article.
[11] Kiminori Maeda, Alexander J. Robinson, Kevin B. Henbest, Hannah J. Hogben, Till Biskup, Margaret Ahmad, Erik Schleicher, Stefan Weber, Christiane R. Timmel, P.J. Hore, “Magnetically sensitive light-induced reactions in cryptochrome are consistent with its proposed role as a magnetoreceptor”, Proceedings of the National Academy of Sciences, 109, 4774 (2012). Abstract.