Researchers at QuTech have demonstrated programmable quantum circuits across up to six silicon spin qubits, giving the field a clearer view of what it takes to further develop this semiconductor platform. Published in PRX Quantum, and chosen as the cover image, the study showcases both the progress of spin qubits in silicon and the main physical bottlenecks in scaling up to larger processors. The work also moves beyond earlier three-qubit algorithm demonstrations in silicon quantum dots, marking a next step towards larger programmable processors.

In the study, the team used a linear array of six quantum-dot spin qubits and ran circuits on neighbouring groups of three, four, five and six qubits. They tracked how the processor evolved under sequences of single-qubit and two-qubit operations, and compared the measured output to the expected collective quantum behaviour. “With this kind of experiment, you can follow how the behaviour changes as you increase the size of the circuit,” says Irene Fernández de Fuentes, first author of the paper. “That gives us a much more realistic view of processor performance than looking only at isolated operations.”

credits: APS/YOHOHO

These measurements also show where the next challenge starts: as the circuit grows, some qubits have to wait while operations are performed to other qubits. During those idle periods, they gradually lose phase coherence, which reduces the quality of the final result. By comparing the circuit performance across different system sizes, the team could directly identify idling and dephasing as key limitations in the current device. “That is exactly the kind of detail we want to extract from these experiments,” Fernández de Fuentes says. “You are not only showing that a programmable circuit works, you are also learning which physical effects start to dominate as you scale up. In our case, this points very clearly to the importance of parallel operation, extended coherence times and state preparation and readout fidelity.”

For Lieven Vandersypen, professor at TU Delft and lead investigator, the broader importance lies in what this says about the development of spin qubits in silicon as a research platform: “What makes spin qubits so interesting is that they are built in a semiconductor environment, with single electrons confined and controlled using techniques that connect naturally to the world of microelectronics,” he says. “At QuTech, we have been building up the knowledge needed to control these systems step by step, from individual qubits and gates to small processors and now programmable circuits that let us assess the processors under realistic conditions.” In that sense, the result is not just a demonstration of one experiment, but part of a wider effort to understand how silicon spin qubits can grow into a larger and more capable quantum technology.