From RIKEN [理研](JP) : “Quantum entanglement of three spin qubits demonstrated in silicon”

RIKEN bloc

From RIKEN [理研](JP)

Sep. 3, 2021

A three-qubit entangled state has been realized in a fully controllable array of spin qubits in silicon.

Figure 1: 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.

An all-RIKEN team has increased the number of silicon-based spin qubits that can be entangled from two to three, highlighting the potential of spin qubits for realizing multi-qubit quantum algorithms.

Quantum computers have the potential to leave conventional computers in the dust when performing certain types of calculations. They are based on quantum bits, or qubits, the quantum equivalent of the bits that conventional computers use.

Although less mature than some other qubit technologies tiny blobs of silicon known as silicon quantum dots have several properties that make them highly attractive for realizing qubits. These include long coherence times, high-fidelity electrical control, high-temperature operation and great potential for scalability. However, to usefully connect several silicon-based spin qubits, it is crucial to be able to entangle more than two qubits, an achievement that had evaded physicists until now.

Seigo Tarucha and five colleagues, all at the RIKEN Center for Emergent Matter Science, have now 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.

This demonstration is a first step toward extending the capabilities of quantum systems based on spin qubits. “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.”

Science paper:
Nature Nanotechnology

See the full article here .


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RIKEN campus

RIKEN [理研](JP) is Japan’s largest comprehensive research institution renowned for high-quality research in a diverse range of scientific disciplines. Founded in 1917 as a private research foundation in Tokyo, RIKEN has grown rapidly in size and scope, today encompassing a network of world-class research centers and institutes across Japan.

From RIKEN [理研](JP) is a large scientific research institute in Japan. Founded in 1917, it now has about 3,000 scientists on seven campuses across Japan, including the main site at Wakō, Saitama Prefecture, just outside Tokyo. Riken is a Designated National Research and Development Institute, and was formerly an Independent Administrative Institution.

Riken conducts research in many areas of science including physics; chemistry; biology; genomics; medical science; engineering; high-performance computing and computational science and ranging from basic research to practical applications with 485 partners worldwide. It is almost entirely funded by the Japanese government, and its annual budget is about ¥88 billion (US$790 million).

Organizational structure:

The main divisions of Riken are listed here. Purely administrative divisions are omitted.

Headquarters (mostly in Wako)
Wako Branch
Center for Emergent Matter Science (research on new materials for reduced power consumption)
Center for Sustainable Resource Science (research toward a sustainable society)
Nishina Center for Accelerator-Based Science (site of the Radioactive Isotope Beam Factory, a heavy-ion accelerator complex)
Center for Brain Science
Center for Advanced Photonics (research on photonics including terahertz radiation)
Research Cluster for Innovation
Cluster for Pioneering Research (chief scientists)
Interdisciplinary Theoretical and Mathematical Sciences Program
Tokyo Branch
Center for Advanced Intelligence Project (research on artificial intelligence)
Tsukuba Branch
BioResource Research Center
Harima Institute
Riken SPring-8 Center (site of the SPring-8 synchrotron and the SACLA x-ray free electron laser)

Yokohama Branch (site of the Yokohama Nuclear magnetic resonance facility)
Center for Sustainable Resource Science
Center for Integrative Medical Sciences (research toward personalized medicine)
Center for Biosystems Dynamics Research (also based in Kobe and Osaka) [6]
Program for Drug Discovery and Medical Technology Platform
Structural Biology Laboratory
Sugiyama Laboratory
Kobe Branch
Center for Biosystems Dynamics Research (developmental biology and nuclear medicine medical imaging techniques)
Center for Computational Science (R-CCS, home of the K computer and The post-K (Fugaku) computer development plan)