Coherent multiqubit control in a six quantum dot linear array in Si/SiGe

Quantum dots are actively researched for applications in quantum computing, and show a large potential for scalability thanks to their small size. In particular, silicon quantum dots continue to stand out as a promising platform, with high fidelity single-qubit gates and two-qubits gates, qubit operation at temperatures exceeding 1 Kelvin and possible industrial CMOS integration. Scaling up to larger qubit numbers remains today's main challenge. Here we demonstrate universal quantum control of six spin qubits. Our experiments are conducted on a linear array of electrostatically defined quantum dots confined in the 28Si quantum well of a 28Si/SiGe heterostructure. Qubit initialization is based on measurement of the spin state across the array followed by real-time feedback to place all qubits in the target initial state. No access to electron reservoirs is needed to bring in fresh electrons. Site-selective addressing is achieved by EDSR in a magnetic field gradient that separates the qubit frequencies from each other. Two-qubit gates are activated selectively by lowering the tunnel barrier between a pair of neighboring dots. We quantify multiqubit entanglement through quantum state tomography for 2 and 3 qubits and by calculating entanglement witnesses for GHZ states.