Silicon spin qubits are quantum bits in which information is stored in the spin direction (intrinsic angular momentum) of an electron confined in a silicon substrate. They are promising for scalable quantum computers because they are compatible with conventional semiconductor chip manufacturing technologies. But there is a weak point: when multiple qubits are controlled simultaneously, accuracy usually drops. And for useful work of a quantum computer, many qubits are needed at once - without loss of operation quality.
A group of researchers from the RIKEN Center for Emergent Matter Science (Japan) and their colleagues found a way to cope with this problem. They worked with an array of five silicon spin qubits controlled through a single shared microwave line. The results were published in the journal Nature Physics.
First, the scientists identified the main culprit behind the errors. It turned out to be the so-called AC Stark shift - a phenomenon in which the microwave control signal changes the effective energy levels of a qubit. Because of this, the phase of the quantum state shifts, and the operation is performed less accurately. When qubits operate simultaneously, such shifts interfere with each other.
To compensate for the distortions, the team developed a calibration protocol that can be scaled. Instead of tuning every possible combination of qubits individually, the researchers use calibrations between pairs of qubits. "Our method, using pairwise calibrations, compensates for unwanted phase shifts," explains Yi-Hsien Wu, the first author of the work, who performed the experiments as part of his dissertation.
Operation accuracy is characterized by a parameter called "fidelity": it shows how accurately a quantum operation is performed. On the optimized device with carefully tuned microwave pulses, the fidelity of single-shot operations on each of the five qubits individually exceeded 99.99%, and the best result approached 99.999%. When up to three qubits operated simultaneously, the fidelity again turned out to be above 99.99%. With all five qubits operating in parallel, it held at about 99.9% - close to the practical requirements of fault-tolerant quantum computing.
The researchers attribute the remaining decrease in accuracy to decoherence caused by the control signal: simultaneous operation requires greater microwave power, and this leads to a loss of quantum coherence. Unlike the phase shifts that the new protocol eliminates, this effect is associated with a loss of consistency of the quantum state. "This is a limitation, but further improvements in device design may help mitigate this source of errors," notes Leon Camenzind, a RIKEN research fellow and co-author of the work.
The authors emphasize that the results show high control accuracy does not necessarily have to be lost during parallel operation of silicon spin qubits. According to Camenzind, the work supports the use of shared control lines as a practical path to scaling quantum processors. "Our long-term goal is to build a fault-tolerant quantum computer based on silicon spin qubits, and reliable parallel control is an important step toward it," adds Seigo Tarucha, the project leader under the Moonshot Goal 6 program of the Japan Science and Technology Agency (JST).