Magnetic Field Effects in Quantum Biology: Beyond the Radical Pair Mechanism

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In chemical reactions involving transient radical pairs, quantum effects induce a remarkable sensitivity to the intensity and/or orientation of external static magnetic fields as weak as the Earth's magnetic field. The governing principle of all these phenomena is the magnetic-field dependent interconversion between quantum-coherent and often entangled states of electronic spin pairs. Typically this process involves the formation of radical pairs by photo-induced electron transfer reactions, the coherent evolution of the resulting non-equilibrium electron spin states, and spin-selective recombination of the radicals. The effect has attracted widespread interest from the scientific community and general audiences owing to its putative relevance to animal magnetoreception, most notably in migratory birds, its link to possibly adverse effects of weak electromagnetic fields on human health, and the opportunity to increase the efficiency of photovoltaic devices.While magnetic field effects (MFEs) resulting from the radical pair mechanism (RPM) have been experimentally confirmed in model systems, they are often found to be small (1 % or less in the low-field region). In the framework of this project, we will theoretically investigate the exploitation of quantum phenomena to amplify MFEs or tailor their characteristics to particular applications. The candidate will focus on several mechanisms, most of which are utterly unexplored in this context: the effects of noise resulting from the stochastic modulation of the exchange and electron dipolar interaction by thermal motion; the accumulation of non-equilibrium nuclear polarizations over the course of several excitation cycles by a process known as three-spin mixing; and the amplification of MFEs by scavenging reactions with quenchers of non-zero spin multiplicity (chemical Zeno effect). This project will also address the mysterious observation that the magnetic compass of birds is disrupted by feeble radiofrequency magnetic fields. Currently this phenomenon can only be explained by assuming excessively long spin coherence times, which appear unphysical in noisy biological environments.The candidate will develop the theoretical models and numerical tools needed to provide a deep understanding of the underlying quantum physics and enable potential technological applications of the RPM in fields ranging from quantum biology (avian quantum compass) to material science (sensor applications).