Interradical motion can push magnetosensing precision towards quantum limits

Abstract: Magnetosensitive spin-correlated radical-pairs (SCRPs) offer a promising platform for noise-robust quantum metrology. However, unavoidable interradical interactions, such as electron-electron dipolar and exchange couplings, alongside deleterious perturbations resulting from intrinsic radical motion, typically degrade their potential as magnetometers. In contrast to this, we show how structured molecular motion modulating interradical interactions in a live chemical sensor in cryptochrome can, in fact, increase sensitivity and, more so, push precision in estimating magnetic field directions closer to the quantum Cram\'{e}r-Rao bound, suggesting near-optimal metrological performance. Remarkably, this approach to optimality is amplified under environmental noise and persists with increasing complexity of the spin system, suggesting that perturbations inherent to such natural systems have enabled them to operate closer to the quantum limit to more effectively extract information from the weak geomagnetic field. This insight opens the possibility of channeling the underlying physical principles of motion-induced modulation of electron spin-spin interactions towards devising efficient handles over emerging molecular quantum information technologies.