  A fault-tolerant quantum repeater or quantum computer using solid-state
spin-based quantum bits will likely require a physical implementation with many
spins arranged in a grid. Self-assembled quantum dots (QDs) have been
established as attractive candidates for building spin-based quantum
information processing devices, but such QDs are randomly positioned, which
makes them unsuitable for constructing large-scale processors. Recent efforts
have shown that quantum dots embedded in nanowires can be deterministically
positioned in regular arrays, can store single charges, and have excellent
optical properties, but so far there have been no demonstrations of spin qubit
operations using nanowire quantum dots. Here we demonstrate optical pumping of
individual spins trapped in site-controlled nanowire quantum dots, resulting in
high-fidelity spin-qubit initialization. This represents the next step towards
establishing spins in nanowire quantum dots as quantum memories suitable for
use in a large-scale, fault-tolerant quantum computer or repeater based on
all-optical control of the spin qubits.
