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Abstract
Quantum-enhanced spin sensing has advanced our capability to measure electromagnetic fields, temperature, and pressure at the nanoscale. The engineering of spin sensors has primarily focused on localized electrons with microwave-frequency spin transitions for coherent control and a spin-optical interface for initialization and readout. Many materials host electrons satisfying these conditions including bulk semiconductors, two-dimensional materials, or chemically synthesizable molecules. Molecular spins stand out because they offer chemically tunable spin and optical properties and can be integrated into diverse environments. However, optically addressing the ground-state spin of single molecules remains an outstanding challenge, limiting their spatial resolution. In this work, we identify molecules hosting cerium ions as potential single-molecule qubit candidates because of their short optical lifetimes and high photoluminescence quantum yields. To this end, we independently characterize the spin and optical degrees of freedom of these molecules and begin optically detected magnetic resonance (ODMR) measurements on a model system, cerium in yttrium aluminum garnet (CeYAG). Shifting our attention towards biological applications, where defect-based quantum sensors are hindered by the need for bulky hosts, and many molecular qubits lack water solubility or stability, we turn to fluorescent proteins, the gold standard in bioimaging. Leveraging a novel technique which enables on-demand readout of the excited spin state, we optically address enhanced yellow fluorescent protein (EYFP). We characterize EYFP's longitudinal and transverse relaxation times at 80 K and estimate its sensitivity to oscillating (AC) magnetic fields. Remarkably, EYFP's spin coherence persists even within the complex intracellular environment of a mammalian cell. Finally, we optically address the protein's spin at room temperature. These results pave the way towards novel sensing regimes for molecular spin qubits.