- Diamond Spin - At the Forefront of Quantum Computer Hardware Research

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Technology News | 2024-09-09

5 minute read

A Fierce Competition in Hardware Research

Research into quantum computers is accelerating at an incredible pace. You probably see news updates about quantum computing breakthroughs almost daily. Two of the hottest topics in this field are "fault-tolerant quantum computing," which we discussed in our previous article, and the intense competition in developing different hardware approaches for quantum computers.

The truth is, we still don't know which hardware approach will be the most promising for practical quantum computers. As shown in the diagram below, various types of quantum computers are currently under development. Each approach has its own advantages and challenges, and researchers worldwide are working to maximize the benefits and overcome the obstacles.

Previous article : Unraveling the Latest Trends in Error Correction and Error Mitigation in Quantum Computers

Two hardware classifications are listed: quantum gate type and Ising machine type.Detailed classification of quantum gated systems includes superconducting, diamond spin, cooled atom, silicon, and ion trap systems.

Fujitsu is actively involved in quantum computer hardware research, exploring both superconducting and diamond spin approaches. This article focuses on the diamond spin approach, explaining its mechanics and highlighting the latest research advancements achieved by Fujitsu in collaboration with Netherlands-based Delft University of Technology and QuTech, a quantum technology research institute established by the Netherlands Organization for Applied Scientific Research.

The Unique Features of the Diamond Spin Approach

What are the characteristics of the diamond spin approach? To get a general understanding, let's compare it with the superconducting approach that Fujitsu is also working on.

Number of Qubits

Fujitsu plans to scale the number of qubits in its superconducting quantum computers from 64 qubits (achieved in 2023) to 256 qubits by 2025, and to over 1,000 qubits beyond 2026. As of August 2024, superconducting quantum computers are leading in terms of qubit scalability compared to other approaches, including the diamond spin method. Currently, the diamond spin approach has been demonstrated with around 10 qubits, but this number is expected to increase in the future.

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Qubit Connectivity

The way qubits are connected differs significantly between the two approaches, which is a notable feature of the diamond spin method. In the superconducting approach, qubits are arranged in a grid and can only connect with adjacent qubits. In contrast, the diamond spin approach, with the use of communication qubits (discussed in a later section), allows for more complex connections between qubits. This gives the diamond spin method an advantage in scalability.

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Qubit

There isn't just one way to create a qubit, which is why various quantum computing approaches have been proposed.

Superconducting qubits are based on electronic circuits (resonant circuits) that can be referred to as artificial atoms. These qubits are currently formed on silicon chips using traditional semiconductor process technology. On the other hand, the diamond spin approach uses the spin of natural atoms within the diamond crystal as qubits. While diamond is broadly considered a semiconductor, its processing technology is less mature than silicon's, necessitating the development of specialized chip fabrication techniques.

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Coherence Time

Coherence time is the time it takes for a quantum state, usable for quantum computation, to break down. Longer coherence times allow for more computational steps, making it crucial for practical quantum computers. Superconducting qubits have a coherence time of around 100 microseconds, whereas diamond spin qubits have much longer lifetimes. Electron spin qubits can maintain coherence for milliseconds, and nuclear spin qubits can stay coherent for over a second.

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Operating Temperature

Operating temperature is a critical factor in building quantum computer systems. Superconducting qubits require extremely low temperatures of 20 mK (-273.13℃), while diamond spin qubits can operate at temperatures above 1 K (-272.15℃). Although this difference may seem minor, as we will explain in a later section, this approximately 1°C difference in operating temperature has significant implications for the implementation of quantum computers.

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How the Diamond Spin Approach Works

In this section, we will provide an overview of how quantum computers using the diamond spin approach are realized.

Diamond spin quantum computers use diamonds made from a crystal of carbon-12 (12C) atoms to create qubits. When you think of diamonds, you might picture the brilliant-cut, sparkling gemstones often used in jewelry. However, natural diamonds used for decorative purposes contain many impurities and defects, making them unsuitable for use as qubit materials. To create qubits, we first produce synthetic diamonds that have been purified to remove as many impurities and defects as possible. Then, we add just the right amount of impurities, such as nitrogen atoms, to make the diamond suitable for use as qubits. This specially prepared diamond can then be used to create and operate qubits for quantum computing.

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The synthetic diamond crystals created for qubits contain nitrogen (N) and vacancies (V). The pair of nitrogen and vacancy, known as an NV center, generates the qubits. An NV center can produce one electron that can be used as a qubit, referred to as an electron spin qubit.

The reason diamonds are used is that they can create more stable qubits compared to other materials. The diamond's crystal structure provides a stable environment for the NV centers, making it an ideal material for quantum computing.

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In addition to electron spin qubits, synthetic diamonds can also produce other types of qubits. These additional qubits are derived from isotopes of carbon-12 (12C) atoms, specifically carbon-13 (13C) atoms, and nitrogen-14 (14N) atoms. The qubits generated from 13C atoms are called 13C nuclear spin qubits, and those from 14N atoms are called 14N nuclear spin qubits.

In the diamond spin approach, three types of qubits are used to form a diamond spin quantum module: electron spin qubits, 13C nuclear spin qubits, and 14N nuclear spin qubits. Typically, a single diamond spin quantum module contains one electron spin qubit, one 14N nuclear spin qubit, and multiple 13C nuclear spin qubits.

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Each type of qubit in the diamond spin approach has specific roles based on its characteristics.

Electron Spin Qubits:
・Connecting with Other Diamond Spin Quantum Modules: Using light to establish connections.
・Connecting with Nuclear Spin Qubits within the Same Module: Facilitating internal communication.

Because electron spin qubits handle communication with other diamond spin quantum modules and nuclear spin qubits within the module, they are referred to as communication qubits.

Nuclear Spin Qubits:
・Long Coherence Time: Responsible for holding information for extended periods.

Due to their role in maintaining information, nuclear spin qubits are known as memory qubits.

As we have discussed, diamond spin quantum computers control and perform calculations using these qubits created within the diamond crystal. The ability of electron spin qubits to connect with other diamond spin quantum modules using light allows for flexible and interconnected computations. This flexibility is expected to provide an advantage in scalability compared to superconducting quantum computers, which rely on a grid of adjacent qubits. Moreover, Diamond spin quantum computers can also link qubits that are physically separated, offering even further scalability.

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The advantage of operating temperature greater than 1K

We mentioned earlier that superconducting quantum computers need to be cooled down to 20 mK (-273.13C), while diamond spin quantum computers can operate at temperatures above 1 K (-272.15C). Although this difference is less than 1°C, it has a substantial impact in the realm of ultra-low temperatures, and thus on the implementation of quantum computers.

In fact, cooling to 20 mK requires over 1,000 times more energy than cooling to 1 K. To achieve 20 mK, a special type of refrigerator called a dilution refrigerator is necessary. Superconducting quantum computers use dilution refrigerators that are approximately 2.8 meters tall to reach this extremely low temperature. On the other hand, diamond spin quantum computers do not require such large dilution refrigerators, thus making it possible to reduce their size.

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Collaborative Research Between Delft University of Technology, QuTech, and Fujitsu

Fujitsu is conducting full-stack research and development of the technologies required for diamond spin quantum computers in collaboration with Delft University of Technology and QuTech in the Netherlands.

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In this section, we provide an overview of the latest research achievements related to diamond spin qubit chips and Cryo-CMOS interface electronics.

Diamond Spin Quantum Chip

In the previous section, we discussed the production of synthetic diamonds for quantum computers by adding necessary impurities, such as nitrogen atoms, to create qubits within the diamond. These nitrogen atoms (N) pair with vacancies (V) to form NV centers, which generate qubits.

In our collaborative research, we have successfully created qubits using tin (Sn) atoms instead of nitrogen atoms. The pairing of tin (Sn) with vacancies (V) forms SnV centers, which generate qubits.

SnV centers exhibit over 10 times higher emission efficiency compared to NV centers. This improvement enhances the performance of optical readouts and inter-module connections through quantum entanglement using light. Additionally, SnV centers are less affected by external electric fields. As a result, we can produce small diamond spin qubit chips that are well-suited for integration.

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Interface Electronics Using Cryo-CMOS

"Cryo" means "operating in cold environments," and Cryo-CMOS circuits are designed to function at extremely low temperatures. As quantum computers scale up, the wiring (cables) connecting the qubit units to their control systems becomes a significant challenge. Whether using the superconducting approach or the diamond spin approach, qubit units need to be placed in ultra-low temperature environments, although the required temperatures differ.

Traditionally, control systems have been placed in room-temperature environments. As the number of qubits increases, the amount of cabling required to connect the qubit units to the control systems also increases. This not only makes it difficult to fit all the cables within the refrigeration unit but also introduces heat transfer through the cables or heat generated by the cables themselves, which can adversely affect the qubits.

The Cryo-CMOS circuits we have developed operate at several K (several Kelvin), the same temperature at which the diamond spin qubit units are placed. By installing Cryo-CMOS circuits within the ultra-low temperature refrigeration unit, we can control the qubit units without the need for extensive cabling. This effectively solves the problem of connecting qubit units to control systems via cables, which was thus far a significant challenge.

This breakthrough is expected to pave the way for the large-scale development of diamond spin quantum computers.

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Ref : Fujitsu collaborates with QuTech in development of new technology for freezing electronics to control diamond spin qubits

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Interview with Professor Hanson, Qutech, Delft Universty of Technology

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Leveraging Research Insights in Semiconductors, Nanotechnology, and Optical Communication Devices

Although the diamond spin approach to quantum computing is still in its early stages, it is considered one of the most promising methods for practical quantum computing due to its superior scalability.

The development of hardware for the diamond spin approach requires extensive knowledge and expertise in various areas, such as improving the efficiency of quantum chips and ensuring the operation of control circuits at ultra-low temperatures. Fujitsu has a long history of conducting cutting-edge research in semiconductors, nanotechnology, and optical communication devices. Researchers who have been involved in these fields are now at the forefront of the research and development of diamond spin quantum computers, fully utilizing their knowledge and experience.

The computers we use today in smartphones and PCs underwent various hardware iterations before the advent of the transistor, including vacuum tubes and other technologies. Quantum computer hardware development is at a similar stage of evolution. Just as the transistor revolutionized classical computing, there may be breakthrough opportunities in the research of diamond spin hardware.

Fujitsu will continue to pursue these possibilities. Stay tuned for future advancements in Fujitsu's quantum computing efforts.

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