  Because of their long coherence times and potential for scalability,
semiconductor quantum-dot spin qubits hold great promise for quantum
information processing. However, maintaining high connectivity between
quantum-dot spin qubits, which favor linear arrays with nearest neighbor
coupling, presents a challenge for large-scale quantum computing. In this work,
we present evidence for long-distance spin-chain-mediated superexchange
coupling between electron spin qubits in semiconductor quantum dots. We weakly
couple two electron spins to the ends of a two-site spin chain. Depending on
the spin state of the chain, we observe oscillations between the distant end
spins. We resolve the dynamics of both the end spins and the chain itself, and
our measurements agree with simulations. Superexchange is a promising technique
to create long-distance coupling between quantum-dot spin qubits.
