Optically Detected Coherent Spin Control of Organic Molecular Color Center Qubits

Molecular optical-spin interfaces are emerging as promising alternatives to solid-state defects, such as diamond nitrogen vacancy centers for quantum information science applications. In this work, we report a new organic molecular color center consisting of two luminescent tris(2,4,6-trichlorophenyl)methyl (<b>TTM</b>) radicals connected at the 2,6-positions of a toluene bridge. Optical polarization of the |T₀⟩ sublevel of the triplet ground state is achieved by spin-selective excited-state intersystem crossing from the |T₊⟩ and |T_-⟩ sublevels of the triplet excited state. Steric hindrance between the toluene methyl group and the phenyls of the <b>TTM</b> radicals results in a structure that increases the excited-state intersystem crossing spin selectivity while reducing the electronic coupling between the <b>TTM</b> subunits. This results in an order of magnitude increase in the optically detected magnetic resonance contrast and longer excited-state lifetimes relative to the sterically unencumbered analogue. We demonstrate coherent microwave manipulation of the spin-polarized ground-state populations and coherences using optical detection of Rabi nutations, Hahn echo formation, and echo decay measurements at 85 K in a nuclear-spin-rich solvent matrix. This marks a crucial step toward leveraging the favorable spin relaxation times of organic molecules for applications as quantum sensors at temperatures that heretofore have been difficult to achieve by molecular color centers.