Hitachi Ltd. has developed a quantum bit operation technique that can stabilize quantum bits and confirmed that the life (quantum information retention time or quantum coherence) of quantum bits can be extended by more than 100 times on the way to practical use of silicon quantum computers. To achieve practical computation with quantum computers, a scale of more than a million quantum bits is required, and key is the large-scale integration of quantum bits, the technology to efficiently control quantum bits, and the implementation of error correction. "Silicon quantum computer" being developed by Hitachi is expected to be advantageous for large-scale integration of quantum bits, but there are challenges in implementing quantum algorithms and error correction, as spin nuclei in semiconductors become noise, making quantum bits unstable. The technology developed this time can stabilize quantum bits by modulating the phase of microwave used to operate quantum bits to partially cancel the noise in semiconductors and extend the life of quantum bits by more than 100 times (Figure 1 bottom). This achievement is a big step towards large-scale integration of quantum bits and implementation of quantum algorithms and error correction, and we will accelerate this research and aim for early practical use of quantum computers.
It is said that in order for practical calculations to be made with quantum computers, a scale of over a million quantum bits is necessary, and key is the large-scale integration of quantum bits, the technology to efficiently control quantum bits, and the implementation of error correction. While the "Silicon quantum computer" being developed by Hitachi is expected to be advantageous for large-scale integration of quantum bits, there are challenges in implementing quantum algorithms and error correction, as spin nuclei in semiconductors become noise, making quantum bits unstable.
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Part of this achievement will be presented at the "2024 IEEE Symposium on VLSI Technology & Circuits" to be held in Hawaii, USA from June 16 to 20, 2024.

Figure 1 top: The results of quantum bit operation by conventional method (quantum bit spin becomes unstable due to noise)
Figure 1 bottom: The results of quantum bit operation by this method (the effect of noise is reduced and stable operation of quantum bit is possible, extending the life of quantum bit)

*1 Virgin material: Made using only new materials

Silicon quantum computer: A quantum computer system that uses electron spin in silicon as quantum bit.

*2

Quantum bit: The minimum unit of information used in quantum computers. Using the principle of superposition of quantum mechanics, it is possible to represent a state where 0 and 1 overlap.

*3

Quantum Coherence: The degree of quantum superposition between different quantum states. It is an indicator of the life of a quantum bit.

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Error correction: The technology to correct errors that occur in the process of quantum computing. One logical quantum bit is represented by multiple quantum bits (redundancy), and errors are detected and estimated using the redundant quantum bit.

Feature quantity: A numerical value that quantitatively expresses the characteristics and properties of the data and subject matter analyzed.

Spin nuclei: The spin of an atomic nucleus. When using electron spin as a quantum bit, spin nuclei can cause quantum bits to become unstable by interacting with them.

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Quantum algorithms: Computation algorithms specific to quantum computers. It is said that they can solve problems that cannot be solved in practical time by traditional computers.

Background of the study

There are various proposals for quantum computers, and the "Silicon quantum computer" being developed by Hitachi is expected to be advantageous for large-scale integration of quantum bits, as it can utilize mature semiconductor technology. Hitachi has developed a "2D silicon quantum bit array" that can make integration possible by arranging silicon quantum bits in a lattice and proposed a "shuttling quantum bit method" that can efficiently control quantum bits. Hitachi confirmed the basic operation of the silicon quantum computer in 2024 and presented it at the "American Physical Society's March Meeting 2024" held in Minnesota, USA in March 2024.

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2D quantum bit array: The basic structure in which quantum bits are arranged in a 2D manner.

*8

N. Lee et al., "16 x 8 quantum dot array operation at cryogenic temperatures," Jpn. J. Appl. Phys. 61 SC1040, 2023.

Hitachi News Release, June 12, 2023: "Proposal of a new quantum bit control method suitable for large-scale integration toward practical use of silicon quantum computers"

June 12, 2023, Hitachi News Release: "Proposal of New Quantum Bit Control Method Suitable for Large-Scale Integration Toward Practical Use of Silicon Quantum Computers."

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T. Kuno et al.'s "Adiabatic Electron Spin Resonance Inversion in an FDSOI Quantum Dot Array." Bulletin of the American Physical Society (2024).

Features of the development technology.

Silicon quantum computers use one electron trapped in a microstructure called a "quantum dot" formed in a silicon element as a quantum bit by utilizing its rotation (spin). However, there has been a problem that the life of a quantum bit is short because conventional nuclear spins in semiconductors become noise and make the rotation of electron spins unstable. In the future, in order to implement quantum algorithms and error correction, it is necessary to reduce the influence of noise and stabilize quantum bits to ensure sufficient computing time.
As a result of discussions between researchers at the Hitachi Cambridge Lab, established in the University of Cambridge in the UK, and researchers from other fields, it was found that the "Concatenated Continuous Driving (CCD) *11 quantum bit operation method" that had been researched in the lab was effective in partially canceling external noise. Therefore, a research team from Hitachi Ltd. Research & Development Group has developed a technology that realizes the "CCD quantum bit operation method" by phase modulation of microwaves used to operate quantum bits in a silicon quantum computer, becoming world's first. With this technology, quantum bits can be operated using two axes orthogonal to each other as a rotation axis, significantly extending the life of quantum bits by reducing the influence of external noise.
Applying this technology to silicon quantum computers, it was confirmed through experiments that quantum bits were stabilized and the life of quantum bits was extended by more than 100 times. From these results, it was confirmed that silicon quantum computers have excellent features that combine large-scale integration of quantum bits with stabilization of quantum bits. Hitachi will continue to accelerate research toward implementation of quantum algorithms and error correction through this R&D, while also working on large-scale integration of quantum bits, aiming for the early practical use of quantum computers.

A. J. Ramsay et al.'s "Coherence protection of spin qubits in hexagonal boron nitride." Nature Communications 14,461 (2023).

A. J. Ramsay et al.'s "Coherence protection of spin qubits in hexagonal boron nitride." was published in Nature Communications on page 14,461 in the year 2023.

Acknowledgment

Some of these results were obtained as a result of joint research with the National Institute of Technology, Tokyo Institute of Technology, the National Institute for Materials Science, and Hitachi Cambridge Lab.

Part of this result was obtained from a joint research with Tokyo Institute of Technology, RIKEN and Hitachi Cambridge Lab, National Research and Development Agency.

About Hitachi

Hitachi is promoting the Social Innovation Business, which aims to realize a sustainable society through data and technology. Under the three-sector business structure of "Digital Systems & Services," which supports the digital transformation of customers, "Green Energy & Mobility," which contributes to the realization of a decarbonized society in the energy and railway industries, and "Connective Industries," which provides solutions that connect products in a wide range of industries with digital technology utilizing IT, OT (control and operation technology), and products, we solve customers' and society's challenges through Lumada solutions. We aim to grow through co-creation with customers driven by digital, green, and innovation. The sales revenue for the three sectors in the fiscal year 2023 (ending in March 2024) was 8.5643 trillion yen, and as of the end of March 2024, the consolidated subsidiaries were 573 companies, with approximately 270,000 employees worldwide.

Contact

Hitachi Research & Development Group