  Silicon spin qubits are a promising quantum computing platform offering long
coherence times, small device sizes, and compatibility with industry-backed
device fabrication techniques. In recent years, high fidelity single-qubit and
two-qubit operations have been demonstrated in Si. Here, we demonstrate
coherent spin control in a quadruple quantum dot fabricated using isotopically
enriched 28Si. We tune the ground state charge configuration of the quadruple
dot down to the single electron regime and demonstrate tunable interdot tunnel
couplings as large as 20 GHz, which enables exchange-based two-qubit gate
operations. Site-selective single spin rotations are achieved using electric
dipole spin resonance in a magnetic field gradient. We execute a resonant-CNOT
gate between two adjacent spins in 270 ns.
