Quantum Entanglement of Three Spin Qubits

Seigo Tarucha and five colleagues, all at the RIKEN Center for Emergent Matter Science, have initialized and measured a three-qubit array in silicon with high fidelity (the probability that a qubit is in the expected state). They also combined the three entangled qubits in a single device.

Above – False-colored scanning electron micrograph of the device. The purple and green structures represent the aluminium gates. Six RIKEN physicists succeeded in entangling three silicon-based spin qubits using the device. © 2021 RIKEN Center for Emergent Matter Science

“Two-qubit operation is good enough to perform fundamental logical calculations,” explains Tarucha. “But a three-qubit system is the minimum unit for scaling up and implementing error correction.”

The team’s device consisted of a triple quantum dot on a silicon/silicon–germanium heterostructure and is controlled through aluminum gates. Each quantum dot can host one electron, whose spin-up and spin-down states encode a qubit. An on-chip magnet generates a magnetic-field gradient that separates the resonance frequencies of the three qubits, so that they can be individually addressed.

The researchers first entangled two of the qubits by implementing a two-qubit gate—a small quantum circuit that constitutes the building block of quantum-computing devices. They then realized three-qubit entanglement by combining the third qubit and the gate. The resulting three-qubit state had a remarkably high state fidelity of 88%, and was in an entangled state that could be used for error correction.

This demonstration is just the beginning of an ambitious course of research leading to a large-scale quantum computer. “We plan to demonstrate primitive error correction using the three-qubit device and to fabricate devices with ten or more qubits,” says Tarucha. “We then plan to develop 50 to 100 qubits and implement more sophisticated error-correction protocols, paving the way to a large-scale quantum computer within a decade.”

Nature Nanotechnology – Quantum tomography of an entangled three-qubit state in silicon

Quantum entanglement is a fundamental property of coherent quantum states and an essential resource for quantum computing1. In large-scale quantum systems, the error accumulation requires concepts for quantum error correction. A first step toward error correction is the creation of genuinely multipartite entanglement, which has served as a performance benchmark for quantum computing platforms such as superconducting circuits trapped ions and nitrogen-vacancy centres in diamond. Among the candidates for large-scale quantum computing devices, silicon-based spin qubits offer an outstanding nanofabrication capability for scaling-up. Recent studies demonstrated improved coherence times, high-fidelity all-electrical control, high-temperature operation and quantum entanglement of two spin qubits. Here we generated a three-qubit Greenberger–Horne–Zeilinger state using a low-disorder, fully controllable array of three spin qubits in silicon. We performed quantum state tomography and obtained a state fidelity of 88.0%. The measurements witness a genuine Greenberger–Horne–Zeilinger class quantum entanglement that cannot be separated into any biseparable state. Our results showcase the potential of silicon-based spin qubit platforms for multiqubit quantum algorithms.

SOURCE – Riken
Written by Brian Wang, Nextbigfuture.com

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