Researchers successfully control five silicon qubits simultaneously using a single microwave control line, achieving an accuracy of 99.9%.

The performance of a quantum computer depends not only on the number of qubits it contains but also on how accurately they can work together. Recently, researchers demonstrated a method to control five silicon-based qubits simultaneously using a single microwave control line while maintaining an operational accuracy of 99.9% or higher. This method may facilitate easier scaling for future quantum processors without the need for separate control lines for each qubit. However, controlling multiple qubits is not as simple as sending microwave signals. Quantum computers require parallel operations to run error correction routines, which help protect fragile quantum information.

However, the signals used to control one qubit can interfere with its neighboring qubits, thus reducing their operational accuracy. In earlier experiments, this issue caused the fidelity of gates—the measure of accuracy in executing quantum operations—to drop from 99.95% to about 99.5%. Researchers noted, "In silicon devices, simultaneous control remains a major challenge, as fidelity typically decreases under parallel operations."

Challenges in Controlling Multiple Qubits

Researchers from Japan's RIKEN and its collaborators have now identified the primary source of this interference and developed a compensation method. This hidden frequency shift causes qubits to lose their correct orientation. The research team utilized five qubits composed of electron spins, which are trapped in tiny structures known as quantum dots, located within silicon/silicon-germanium devices. The researchers added, "Silicon spin qubits particularly offer a promising pathway for scalable quantum computing, as they are compatible with industrial semiconductor manufacturing." Microwave signals control these qubits, but the signal aimed at one qubit can also affect its neighboring qubits.

This unwanted influence is known as crosstalk, which can reduce the accuracy of quantum operations even when the neighboring qubits are not the target. Researchers traced the main source of this interference to a phenomenon called AC Stark shift. Microwave signals can alter the effective energy levels of adjacent qubits, changing the evolution of their quantum states. This results in unnecessary phase shifts—describing the relative temporal changes of waves representing quantum states—that can cause the intended operations to deviate from the correct direction.

The research team found that these phase shifts could be predicted by measuring the influence of individual qubits on each other. They utilized these measurements to compensate for unnecessary changes when multiple qubits operate together, avoiding the need for separate calibrations for every possible combination. The study's lead author, Wu Yihsien, explained, "Our method compensates for unnecessary phase shifts through calibrations between qubit pairs, without requiring completely independent calibration procedures for every possible qubit combination." The results demonstrated the effectiveness of this approach. When operated individually, the fidelity of the gates for the five qubits exceeded 99.99%, with the best approaching 99.999%.

Potential and Challenges for Future Expansion

When up to three qubits operate simultaneously, their fidelity remains above 99.99%. Even when five qubits are running at the same time, their gate fidelity stays at 99.9% or higher. This level of accuracy meets the benchmark for fault-tolerant quantum computing, where error correction techniques help protect computations from errors. However, this does not mean that researchers have built a fully fault-tolerant quantum computer.

Using shared microwave lines can reduce the number of control connections required for large processors, saving space and potentially decreasing heat generated by control hardware. Researchers view this method as a foundation for future modules that may contain about ten qubits, although this larger configuration remains a future goal rather than a proven outcome. Other challenges persist. Stronger microwave signals can accelerate operations but may also introduce unnecessary disturbances that interfere with the delicate quantum states of the qubits. The team proposed improvements by integrating small magnets into the chip's wiring layer to control the electron spins.

These improved magnets can generate stronger local magnetic fields, potentially allowing for faster operations at lower microwave power. Currently, this achievement addresses one barrier to scaling silicon quantum computers: maintaining accurate control while multiple qubits operate together. Before supporting practical fault-tolerant quantum computing, it will be crucial to extend this method to larger arrays and control other sources of errors. The research is published in the journal Nature Physics.