Site-Selective Quantum Control in an Isotopically Enriched Si 28 /Si_0.7Ge_0.3 Quadruple Quantum Dot

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 from isotopically enriched Si28. 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 with the use of electric dipole spin resonance in a magnetic field gradient. We execute a resonant controlled-not gate between two adjacent spins in 270 ns.