  Coherent interactions at a distance provide a powerful tool for quantum
simulation and computation. The most common approach to realize an effective
long-distance coupling 'on-chip' is to use a quantum mediator, as has been
demonstrated for superconducting qubits and trapped ions. For quantum dot
arrays, which combine a high degree of tunability with extremely long coherence
times, the experimental demonstration of coherent spin-spin coupling via an
intermediary system remains an important outstanding goal. Here, we use a
linear triple-quantum-dot array to demonstrate a first working example of a
coherent interaction between two distant spins via a quantum mediator. The two
outer dots are occupied with a single electron spin each and the spins
experience a superexchange interaction through the empty middle dot which acts
as mediator. Using single-shot spin read-out we measure the coherent time
evolution of the spin states on the outer dots and observe a characteristic
dependence of the exchange frequency as a function of the detuning between the
middle and outer dots. This approach may provide a new route for scaling up
spin qubit circuits using quantum dots and aid in the simulation of materials
and molecules with non-nearest neighbour couplings such as MnO,
high-temperature superconductors and DNA. The same superexchange concept can
also be applied in cold atom experiments.
