Quantum sensing of arbitrary magnetic signals with molecular spins

Abstract: Molecular spins offer a promising platform for quantum sensing, particularly in organic, supramolecular or biological environments. Detection of periodic magnetic fields has been demonstrated with molecular spins ($S$ = 1/2) using manipulation protocols synchronized with the probed field. In this work, we develop two quantum sensing protocols that enable discrimination between different time-dependent magnetic field signals, without synchronization. These are based on the Hahn echo sequence and have been tested on VO(TPP) and $\text{VOPt(SCOPh)}_4$ molecular spins embedded in a superconducting YBCO microwave resonator. We report a magnetic field sensitivity up to a few $10^{-7} T \text{Hz}^{-\frac{1}{2}}$ (with lower bounds approaching $10^{-8} T \text{Hz}^{-\frac{1}{2}}$) for signals with duration of a few microseconds. Under the given conditions, the minimum signal area that can be measured is in the $10^{-10}$ T s range, suggesting a potential trade-off between minimum measurable field and the required signal duration and memory time.