Ultrafast photochemical electron transfer reactions are used to prepare entangled electron spins having a pure initial spin state, an essential criterion for their use as spin qubits in quantum information applications. For example, we have achieved electron spin state quantum teleportation in an ensemble of covalent donor-acceptor-stable radical (D-A-R●) molecules at 85K. Following preparation of a specific electron spin state on R● in a magnetic field using a microwave pulse, photoexcitation of A results in the formation of an entangled electron spin pair D●+-A●-. The spontaneous ultrafast chemical reaction D●+-A●--R●→ D●+-A-R- constitutes the Bell state measurement step necessary to carry out spin state teleportation. Quantum state tomography of the R● and D●+ spin states using pulse-EPR spectroscopy shows that the spin state of R● is teleported to D●+ with high fidelity. We have also used photogenerated radical pairs to implement a two-qubit controlled-NOT (CNOT) gate, which is necessary for developing a complete set of universal quantum logic gates. We use a covalent donor-chromophore-acceptor (D-C-A) molecule, where selective photoexcitation of C with a laser pulse results in two-step, sub-nanosecond formation of the D●+ -C-A●- radical pair at 85K having a 0.9 µs spin coherence time, which is sufficiently long to execute a CNOT gate using a microwave pulse sequence. These results show that photochemical electron transfer processes provide an excellent route to designing molecular systems for understanding and implementing quantum information processes.

Award Address (Josef Michl ACS Award in Photochemistry sponsored by the Josef Michl Award Endowment). Photogenerated Radical Pairs as Spin Qubits: Exploiting Quantum Entanglement of Electron Spins

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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