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

By extending Gradient Ascent Pulse Engineering (GRAPE) to allow for optimising reaction yields, we realise arbitrary waveform-based control in spin-selective recombination reactions of radical pairs in the low magnetic field regime. This overcomes drawbacks of previous time-local optimisation approaches for realising reaction control, which were limited in their applicability to radical pairs driven by high biasing fields. We demonstrate how efficient time-global optimisation of the radical pair recombination yields can be realised by gradient based methods augmented with time-blocking, sparse sampling of the yield, and evaluation of the central single-timestep propagators and their Fr\'echet 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 waveforms, paving the way for radical inspired qubit architectures for reaction-yield-dependent quantum magnetometry and potentially applications of quantum control to biochemical radical pair reactions.