Operation of a photogenerated molecular spin qubit pair as a CNOT gate at 85 K

The controlled-NOT (CNOT) gate is an essential operation for quantum algorithms applicable to systems for quantum information technology. In most proposed qubit systems, such gates operate only at extremely low temperatures. Here we demonstrate a molecular CNOT gate at 85 K, which is achieved in a photo-initiated electron donor-acceptor₁-acceptor₂ triad (D-A₁-A₂; 1), where D is peri-xanthenoxanthene-d₉, A₁ is naphthalene-1,8-dicarboximide, and A₂ is a C₆₀ derivative. Photoexcitation of D in 1 results rapid, two-step electron transfer to give the D˙⁺-A₁-A₂˙^- radical ion pair (spin qubit pair, SQP) with a lifetime of 10.9 ± 0.5 µs at 85 K. Single-qubit addressability of the SQP in an external magnetic field is achieved using two methods: (1) magnetic field alignment of 1 in the nematic liquid crystal 4-cyano-4'-n-pentylbiphenyl (5CB) and (2) magnetophotoselection using linearly polarized light to photoexcite 1 in glassy 2-methyltetrahydrofuran to select specific orientations of the SQP relative to the external magnetic field. Pulse-electron paramagnetic resonance (pulse-EPR) spectroscopy shows that it is possible to perform the CNOT gate on the photogenerated SQP in 1 using both methods. In addition, we demonstrate that measurements of the zero quantum coherence of the SQP can be used to evaluate CNOT gate operation.