Quantum Control of Radical-Pair Dynamics beyond Time-Local Optimization

Abstract

We realize arbitrary-wave-form-based control of spin-selective recombination reactions of radical pairs in the low-magnetic-field regime. To this end, we extend the gradient-ascent pulse engineering (GRAPE) paradigm to allow for optimizing reaction yields. This overcomes drawbacks of previously suggested time-local optimization approaches for the reaction control of radical pairs, which were limited to high biasing fields. We demonstrate how efficient time-global optimization of the recombination yields can be realized by gradient-based methods augmented by time blocking, sparse sampling of the yield, and evaluation of the central single time-step propagators and their Fréchet derivatives using iterated Trotter-Suzuki splittings. Results are shown for both a toy model, previously used to demonstrate coherent control of radical-pair reactions in the simpler high-field scenario and, furthermore, for a realistic exciplex-forming donor-acceptor system comprising 16 nuclear spins. This raises prospects for the spin control of actual radical-pair systems in ambient magnetic fields, by suppressing or boosting radical reaction yields using purpose-specific radio-frequency wave forms, paving the way for reaction-yield-dependent quantum magnetometry and potentially applications of quantum control to biochemical radical-pair reactions. We demonstrate the latter aspect for two radical pairs implicated in quantum biology. Published by the American Physical Society 2024

On the optimality of the radical-pair quantum compass

“…Here we have demonstrated that the QFI, and the quantum Cramér-Rao bound it sets on precision, is an effective tool, in addition to the conventional contrast measure, to assess fundamental limits of precision of different radical models against one another in the limits of biological complexity and identify opportunities to improve upon nature's design principles [107,108]. Furthermore, it provides scope for future studies to analyse additional mechanisms, such as driven radical motion, and elucidate those that provide the optimal enhancements in precision, thereby giving insight into how nature may achieve exquisite magnetic field sensitivity and how we can optimise it for technological applications.…”

Bang-bang optimal control in coherent spin dynamics of radical pairs in quantum biology

“…One of the goals of quantum biology is to apply theory-driven predictions for multi-scale integration of cellular function. It is of paramount importance to identify magnetic field parameters to modulate quantum coherences in a radical pair reaction [8,9,31,32]. Here, we make the quantum coherence and singlet product yield indistinguishable.…”