A new silicon quantum control technique achieves 99.9% accuracy across five spin qubits using a shared microwave line, offering a potential way to simplify quantum chip architecture and scaling.
Researchers have demonstrated a method to control five silicon spin qubits simultaneously with at least 99.9% accuracy using a single microwave control line, addressing a key challenge in building scalable quantum processors. The approach, developed by researchers from Japan’s RIKEN and collaborating institutions, could reduce the number of control connections required as quantum chips grow larger.
Quantum computers rely on qubits to process information, but their performance depends on how accurately quantum operations are executed. When multiple qubits operate in parallel, control signals intended for one qubit can disturb neighbouring qubits, introducing errors that complicate quantum error correction.
The researchers investigated this problem using five electron-spin qubits confined within quantum dots in a silicon-germanium device. These qubits use the spin state of individual electrons to represent quantum information. Microwave signals manipulate the spins, enabling the device to perform quantum operations.
The team identified the AC Stark shift as a major source of interference. This effect occurs when an applied microwave signal changes a neighbouring qubit’s effective energy levels, altering the evolution of its quantum state. The resulting phase shift can reduce the accuracy of quantum operations, even when the neighbouring qubit is not the intended target.
Instead of calibrating every possible combination of simultaneously operating qubits, the researchers measured interactions between pairs of qubits. They used these measurements to predict and compensate for unwanted phase shifts when several qubits operated together.
This pairwise calibration method helped maintain high gate fidelity, a measure of how closely a quantum operation matches its intended result. When operated individually, the five qubits achieved fidelities above 99.99%. When all five operated simultaneously, fidelity remained at or above 99.9%, according to the study published in Nature Physics.
Sharing a microwave control line could reduce wiring complexity, save chip space and potentially lower heat generated by control hardware. The researchers envisage extending the approach to modules containing around ten qubits per shared line, although this remains a future goal.
Further engineering challenges include preventing unwanted disturbances from stronger microwave signals while maintaining fast operations. Integrating small magnets into chip wiring layers could improve local magnetic-field control and enable faster operations at lower microwave power.
The demonstration does not establish a fully fault-tolerant quantum computer. However, it offers a potential route to scaling silicon-based quantum processors while preserving the accuracy needed for reliable quantum computation.
