Ultrafast entangling gates between nuclear spins using photoexcited triplet states

The representation of information within the spins of electrons and nuclei has been a powerful method in the ongoing development of quantum computers(1,2). Although nuclear spins are advantageous as quantum bits (qubits) because of their long coherence lifetimes (exceeding seconds(3)), they exhibit very slow spin interactions and have weak thermal polarization. A coupled electron spin can be used to polarize the nuclear spin(4-6) and create fast single-qubit gates(7,8), however, the permanent presence of electron spins is a source of nuclear decoherence. Here we show how a transient electron spin, arising from the optically excited triplet state of C-60, can be used to hyperpolarize, manipulate and measure two nearby nuclear spins. Implementing a scheme that uses the spinor nature of the electron(9), we performed an entangling gate in hundreds of nanoseconds: five orders of magnitude faster than the liquid-state J coupling. This approach can be widely applied to systems comprising an electron spin coupled to multiple nuclear spins, such as nitrogen-vacancy centres in diamond(10), while the successful use of a transient electron spin motivates the design of new molecules able to exploit photoexcited triplet states.