AMO physics in a beaker: Liquid quantum cells as analogs to atomic vapor cells

Trivalent lanthanides display "atomic-like" optical transitions in condensed phase environments. However, despite inorganic lanthanide complexes being explored as potential spin-qubits, their narrow optical transitions have not been employed to prepare or readout quantum states. We demonstrate a ligand system which supports ultranarrow optical linewidths in a dense liquid at room temperature. Leveraging the high concentrations afforded by liquid solvation, we utilize a narrowband laser to optically prepare and readout a significant fraction of these floating spins. We demonstrate the ability to measure weak EM fields through changes in transmission and optical polarization, in direct analogy to atomic vapor cell technology. The high density of spins also suggests the potential for entanglement enhancement in quantum sensing.