Optical Initialization of Molecular Qubit Spin States Using Weak Exchange Coupling to Photogenerated Fullerene Triplet States.

The ability to initialize an electron spin qubit into a well-defined state is an important criterion for quantum information applications. To achieve this goal, a chromophore photoexcited to its triplet state is used to strongly spin polarize a nearby stable radical in a series of C60 fullerene derivatives containing a covalently linked α,γ-bisdiphenylene-β-phenylallyl (BDPA) radical. Selective photoexcitation of C60 results in up to 20-fold enhancement of the BDPA spin polarization observed by pulse electron paramagnetic resonance spectroscopy at room temperature. The sign of the spin polarization depends on the nature of the molecular spacer between C60 and BDPA. In addition, transient absorption spectroscopy and pulse-EPR measurements reveal that the BDPA spin polarization is derived from spin polarization transfer from the C60 triplet state by weak exchange coupling over a 1 nm distance.