Five Silicon Spin Qubits Controlled at Once With High Fidelity

Physicists at the RIKEN Center for Emergent Matter Science and collaborators have shown highly accurate control of several silicon spin qubits operating at the same time. The study appeared on October 15 in Nature Physics.

The experiments used an array of five qubits connected to a shared microwave control line. By identifying a major cause of errors during simultaneous operation, the team developed a technique to reduce those errors and maintain precision.

Why simultaneous control is hard

Quantum computers use quantum bits, or qubits, which can exist in combinations of 0 and 1. Silicon spin qubits store information in the spin of individual electrons confined in silicon, and reliable calculations need extremely precise control over these spins. Microwave signals provide that control, but operating several qubits at once can create unwanted effects that disturb accuracy.

The researchers traced much of the problem to the AC Stark shift. This effect occurs when microwave control signals alter a qubit's effective energy levels, producing changes in the phase of its quantum state. Those phase shifts can disrupt the intended operation of individual qubits, particularly when several are controlled together.

What it means

"This shows that high-fidelity control can be maintained even when several qubits are operated at the same time and represents an important step toward scaling up silicon-based quantum processors," said Leon Camenzind, a RIKEN research scientist and co-first author who supervised the experiments. The summary available to us does not give the fidelity figures, which are in the paper. More in our Science section.