Quantum Sensing of Motion in Colloids via Time‐Dependent Purcell Effect

Light–matter interaction dynamics is governed by the strength of local coupling constants, tailored by surrounding electromagnetic structures. Characteristic decay times in dipole‐allowed fluorescent transitions are much faster than mechanical conformational changes within an environment and, as a result, the latter can be assumed static during the emission process. However, slow‐decaying compounds can break this commonly accepted approximation and introduce new interaction regimes. Here, slow‐decaying phosphorescent compounds are proposed to perform quantum sensing of the nearby structure's motion via observation of collective velocity‐dependent lifetime distributions. In particular, characteristic decay of an excited dye molecule, being comparable with its passage time next to a resonant particle, is modified via time‐dependent Purcell enhancement, which leaves distinct signatures on properties of emitted light. Velocity mapping of uniformly moving particles within a fluid solution of phosphorescent dyes is demonstrated via the analysis of modified lifetime distributions. The proposed interaction regime enables performing studies of a wide range of phenomena, where time‐dependent light–matter interaction constants can be utilized for extraction of additional information about a process.