Instead of adding more wiring between qubits, Qutech researchers let one qubit do the travelling. The resulting architecture could simplify the design of future fault-tolerant quantum processors.
While silicon spin qubits are among the leading candidates for quantum processors, they come with a major downside: they naturally interact only over extremely short distances, while large-scale quantum computing requires long-range interactions between distant qubits. Researchers at Qutech, Delft University of Technology and collaborators have now demonstrated one way to bridge that gap.
Writing in Nature, they describe a five-qubit silicon processor in which a single mobile electron spin shuttles between four stationary qubits, creating programmable interactions across the chip. Rather than permanently wiring qubits together, the moving electron acts as a quantum bus that transports quantum information wherever it’s needed.
The approach addresses a fundamental limitation of conventional spin-qubit processors. In today’s devices, each qubit typically interacts only with its nearest neighbors. If two distant qubits need to exchange quantum information, that interaction must be relayed through intermediate qubits, increasing circuit complexity and introducing additional opportunities for errors.
The Qutech team replaces some of those fixed connections with motion. An electron carrying quantum information is repeatedly shuttled between quantum dots while preserving its fragile quantum state. Over the course of an experiment, the mobile qubit travelled a cumulative distance of 1.2 micrometres, sequentially interacting with each of four stationary qubits. Because the moving electron can visit any qubit on demand, it creates reconfigurable long-range connectivity without requiring every qubit to be directly connected.
Crosstalk
That flexibility is particularly valuable for quantum error correction, an essential ingredient for practical quantum computers. In the paper, Qutech demonstrated one of the key operations required for those error-correction protocols: a weight-four parity check. Instead of directly coupling all four stationary qubits, the mobile electron visits each one in sequence, collecting the information needed to determine their joint quantum parity. The operation forms a fundamental building block of several leading quantum error-correction codes.
Although the processor contains only five qubits, the architectural implications extend far beyond that scale. By reducing the need for densely packed nearest-neighbour connections, the approach frees valuable chip area for control and readout hardware while reducing wiring congestion and unwanted crosstalk. The modular concept could therefore simplify the design of much larger quantum processors.
