  Entangling gates for electron spins in semiconductor quantum dots are
generally based on exchange, a short-ranged interaction that requires
wavefunction overlap. Coherent spin-photon coupling raises the prospect of
using photons as long-distance interconnects for spin qubits. Realizing a key
milestone for spin-based quantum information processing, we demonstrate
microwave-mediated spin-spin interactions between two electrons that are
physically separated by more than 4 mm. Coherent spin-photon coupling is
demonstrated for each individual spin using microwave transmission
spectroscopy. An enhanced vacuum Rabi splitting is observed when both spins are
tuned into resonance with the cavity, indicative of a coherent spin-spin
interaction. Our results demonstrate that microwave-frequency photons can be
used as a resource to generate long-range two-qubit gates between spatially
separated spins.
