A team led by RIKEN has operated five silicon qubits at the same time through a shared microwave control line. That tackles a practical problem for quantum computers: adding more qubits also means finding ways to control them without an unmanageable tangle of connections.
The clever part is not simply sharing a wire. Signals meant for one qubit can disturb its neighbors. In a Nature Physics study published October 15, the researchers measured those disturbances and adjusted the control pulses to compensate. The result is a promising way to coordinate a small group of qubits with less microwave wiring.
One path, different frequencies
The device holds five spin qubits in tiny electrically defined regions of silicon called quantum dots. Each qubit responds to a different microwave frequency, allowing a shared line to carry signals addressed to different members of the group. Other electrical controls are still needed. This is a shared microwave path, not an entire quantum chip running on one cable.
Why operate several qubits together? Making them wait their turn has a cost. Their fragile quantum states drift while idle, so doing everything sequentially can lose information before a calculation has progressed very far. Parallel control helps avoid that waiting.
But separate frequencies do not make the qubits perfectly independent. A microwave signal can shift a neighboring qubit's resonance even when it is not tuned to drive that neighbor directly. That changes its phase, a part of the quantum state that later operations rely on. The researchers identified these shifts as a major source of errors during simultaneous control.
Measure the disturbance, then allow for it
The team first shaped its pulses to reduce unwanted excitation. It then measured how driving one qubit shifted another's phase and adjusted subsequent pulses to allow for that shift. Pulse strengths also needed calibration during simultaneous operation.
An encouraging result was that the phase disturbances could be measured pair by pair and added together. The researchers did not need to characterize every possible combination of qubits being driven at once. That matters because the number of such combinations grows rapidly as more qubits join the group.
This is not a claim that calibration disappears. It is a way of keeping one important part of it manageable while several control signals share the same route.
What the accuracy figures actually measure
With three qubits operating simultaneously, the team measured fidelities above 99.99% for the individual qubit rotations it tested. When all five operated together, four exceeded 99.9%; the fifth reached approximately 99.9%, according to the detailed five-qubit results.
The five-qubit test also exposed a trade-off: stronger simultaneous signals add noise, while slowing the operations gives the quantum states more time to drift. Sharing the path does not remove those competing demands.
Those figures estimate the quality of elementary operations using randomized sequences. They are not the chance that a whole quantum program produces the right answer. That distinction is central to judging quantum computers by useful work rather than a headline specification.
There is a particularly concrete remaining limitation in this device. Its tested CZ gates, which operate on two qubits together, had fidelities below 97%. A processor needs those interactions as well as precise individual rotations, preparation and readout. The shared-line result therefore does not demonstrate a fault-tolerant computer. Nor did the team demonstrate its suggested extension to ten qubits.
Different research attacks different layers of that problem. Google's work on cleaner quantum building blocks addresses another ingredient needed for error-corrected calculations; it is not interchangeable with making silicon qubits share control hardware.
The useful advance here is more specific: a common control path need not mean surrendering precise individual control. Scaling a quantum computer is partly about adding qubits, but also about making the connections they share predictable enough to use together.
