Probing the Optimality of the Radical Pair Quantum Compass*

Quantum magnetometers, having seen uses in diverse areas such as biomedical applications, fundamental physics research, and navigational systems, excel in their ability to detect weak magnetic fields. Crucially, circumstantial evidence suggests that nature may also harness quantum-enhanced sensing, via a radical pair based chemical compass, for precise detection of the weak geomagnetic field. However, what underpins the acuity of such a compass to operate in a noisy, wet biological setting at physiological temperatures (around 410K), still remains puzzling. In earlier investigations, information theoretic tools have been instrumental in interrogating the limits of radical compass sensitivity. Nevertheless, such studies were limited by their use of simplified models, which overlook complex inter-radical interactions and hyperfine couplings present in biological settings. We aim to address this gap by probing the quantum fisher information in spin models of realistic complexity. This helps unveil the optimality, in terms of precision, of measurements that may be accessible to nature. Inspired by our prior findings demonstrating how periodic driving boosts sensitivity in quantum metrology, we also explore additional driving mechanisms to boost precision, with an eye on potential quantum-controlled applications.