Quantum sensing of time-dependent magnetic signals with molecular spins

Molecular spins offer a promising platform for quantum sensing, particularly in organic, supramolecular or biological environments. Recognition of the signals by these systems is of particular interest given their possible integration into more complex structures and their possible use as sensors in close proximity to analytes. In this work, we develop two quantum sensing protocols that enable discrimination between different time-dependent magnetic field, without requiring its periodicity to match with the microwave manipulating sequence. These are based on the Hahn echo sequence and have been tested on VO(TPP) and VOPt(SOCPh${)}_4$ molecular spins embedded in a superconducting YBCO microwave planar resonator. We report a magnetic field sensitivity up to $2.57\cdot {10}^{-7}TH{z}^{-\frac{1}{2}}$ (with lower bounds approaching $2.87\cdot {10}^{-8}TH{z}^{-\frac{1}{2}})$ 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.