  We report the strong coupling of a single electron spin and a single
microwave photon. The electron spin is trapped in a silicon double quantum dot
and the microwave photon is stored in an on-chip high-impedance superconducting
resonator. The electric field component of the cavity photon couples directly
to the charge dipole of the electron in the double dot, and indirectly to the
electron spin, through a strong local magnetic field gradient from a nearby
micromagnet. This result opens the way to the realization of large networks of
quantum dot based spin qubit registers, removing a major roadblock to scalable
quantum computing with spin qubits.
