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What is quantum computing?

A quantum computer is a computer that performs calculations by applying the special properties of quantum, which governs the smallest unit of matter and energy, and the principles of quantum mechanics. "Superposition", in which quanta can exist in multiple states at the same time, and "quantum entanglement", in which distant quanta are strongly correlated, are fundamental concepts in quantum computing.

There are various types for quantum computer hardware, such as superconducting, diamond spin, neutral atoms, silicon, and trapped-ion, and various companies are currently conducting research and development. However, we still do not know which type is the most promising for the practical application of quantum computers. Each type has its advantages and challenges, and research is being conducted to enhance these advantages and overcome the challenges.

Fig. 1 Types of quantum computer hardware (example)

Differences between quantum computing and conventional computing

Conventional computers around us, such as smartphones, personal computers, and supercomputers, all process information using combinations of "0" and "1", called bits. By contrast, quantum computers use "qubits" as the basic unit of information. They use quantum-specific properties that are completely different from those of conventional computers to perform calculations.

The smallest unit of information handled by a conventional computer: What is a "bit"?

Conventional computers calculate using "bits", which represent information as either "0"s or "1"s as the basic unit of information. This is similar to an on/off electrical switch, which expresses information using two values.

This "bit" can only have one state at a time, like the front or back of a coin. If there are two bits, there are four combinations (2² ways) of "00", "01", "10", and "11", but conventional computers can handle only one of them in a single calculation.

Therefore, if you want to calculate all possible combinations of 2ⁿ when there are N bits, you need to perform the calculations one by one. As a result, 4 calculations are required for 2 bits and 2ⁿ calculations are required for N bits.

The smallest unit of information handled by a quantum computer: What is a "qubit"?

Quantum computers use "qubits" as the basic unit of information. Qubits use quantum-specific properties such as "superposition" and "quantum entanglement" to perform calculations.

Properties of qubits No. 1: "Superposition"

While conventional bits can only have either "0" or "1", "qubits" can take "0" and "1" states at the same time. This property is called "superposition".

As shown in Figure 2, while conventional bits are like coins that have already been determined as either "heads" or "tails", qubits are easy to understand by imagining coins that have not yet been determined as either "heads" or "tails". This coin has both heads and tails possibilities at the same time until it is observed. This is a state in which the outcome is not determined, but each outcome exists with a certain probability at the same time (a probabilistic possible). Also, at the moment of actual observation, it is determined as either heads or tails.

Fig. 2 Concept of superposition of qubits

The effect of "superposition"

Quantum computers can simultaneously express 2ⁿ states using N qubits through "superposition". In calculations, efficiency is achieved by carefully controlling this superposition using "interference" to increase the probability that the correct answer is measured. (Only one result is obtained from a measurement).

Property of qubits No. 2: "Quantum entanglement"

"Quantum entanglement" is a phenomenon in which multiple quantum states are strongly linked, and their individual states cannot be explained independently. When one qubit is measured, the measurement result of the other qubit appears as the corresponding value. This phenomenon maintains a strong relationship between qubits even when they are far apart.

In a familiar example, let's say you have two coins. At the moment the front or back of one coin is determined by measurement, the front or back of the other coin is also confirmed at the same time. (Example: If you know that one coin has become "heads", the other coin will always be "tails").

However, this does not mean that the result is determined in advance or that information is transmitted instantaneously through measurement, but rather that "quantum entanglement" is a special state in which qubits in remote locations exhibit strong correlations, and corresponding results appear when they are measured.

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Effects of "quantum entanglement"

While "superposition" gives each qubit the property of expressing multiple states at the same time, "quantum entanglement" creates a state in which the states of multiple qubits are strongly connected and exhibit correlations that cannot be explained individually. Quantum entanglement can be used to exploit these correlations in the computational process, which can improve processing efficiency in certain quantum algorithms, and quantum computers may be able to rapidly solve problems that cannot be solved within a realistic time frame using conventional computers.

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Demonstration of quantum properties

Quantum properties such as "superposition" and "quantum entanglement" are phenomena that are difficult to explain using our everyday intuition or classical physics. In fact, the great physicist of the 20th century, Albert Einstein, called quantum entanglement "spooky action at a distance" and questioned whether quantum mechanics could fully explain this phenomenon.

Later, John Stewart Bell established a theoretical framework to distinguish quantum mechanics from classical thinking (local realism), and a series of precise experiments by Alan Aspe, John Krauser, Anton Zeilinger, and others confirmed that the correlation unique to quantum entanglement exists. These achievements significantly advanced the foundations of quantum mechanics, and in 2022, the Nobel Prize in Physics was awarded for research in this field.

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Areas of application of quantum computers

As we have seen, quantum computers operate on completely different principles than conventional computers and are expected to contribute to solving complex problems faced by our society.
For problems that are computationally intensive and extremely difficult to solve using conventional computers (e.g., quantum chemistry calculations, certain machine learning tasks, prime factorization, combinatorial optimization, etc.), quantum computers can deliver significant speedups in certain areas. Therefore, they are expected to be used in the following fields:

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Quantum chemistry calculations: [Applications] Design of New Drugs, Catalysts, Next-Generation Battery Materials, etc.
Quantum machine learning: [Application Examples] Manufacturing Anomaly Detection, Financial Data Analysis, etc.
Prime factorization: [Applications] RSA ciphers and electronic signature security verification, etc.
Combinatorial optimization: [Applications] Delivery routes, factory scheduling, financial portfolio optimization, etc.

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Applications of quantum computing, including materials and drug discovery, financial and economic trend prediction, and discovery of new physical principles

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Challenges of quantum computers and the road to practical application

Quantum computers have the potential to be innovative, but there are two main challenges with current quantum computers.
(1) Increasing the number of qubits
(2) Qubit errors and high-precision calculations

Increasing the number of qubits

Increasing the number of qubits requires innovation that is not an extension of the present. In the case of superconducting quantum computers, special refrigerators are used to cool to cryogenic temperatures, but the size of the chillers is limited.

As for large-scale efforts, various hardware research is being carried out, including not only the evolution of cooling technology, but also the improvement of qubit placement and connection methods. In particular, the development of technology to stably operate a large number of qubits in a confined space is important.

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Qubit errors and high-precision calculations

Qubits are subject to various influences from the outside world, causing errors. The key here is error-tolerant quantum computing. To achieve error-tolerant quantum computing, the following two approaches are mainly being considered.

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Error correction technology
This is a technology used by quantum computers to detect and correct errors that occur in the process of performing calculations. By encoding the information of a single qubit redundantly using multiple qubits, even if a single qubit receives an error, the correct information can be reconstructed from other qubits.
Error mitigation techniques
Error mitigation techniques aim to minimize the impact of errors. Since a large number of qubits are required for error correction, it is expected to be a technology that will improve computational accuracy with a smaller number of qubits and lead to error-tolerant quantum computing in the future. This also includes software techniques, such as improving algorithms to minimize the impact of errors on calculation results.

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Fujitsu's major initiatives in quantum computer development

In the midst of these global trends, many companies and research institutes in Japan are promoting research and development of this next-generation technology. In this context, Fujitsu is engaged in the research and development of quantum computers by leveraging its many years of experience in computer development.

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Realization of a world‑leading 256-qubit superconducting quantum computer:
In April 2025, Fujitsu and RIKEN developed a world-leading 256-qubit superconducting quantum computer. It was launched in the first quarter of fiscal 2025.
In FY2026, a 1,024-qubit superconducting quantum computer will be released (planned):
We are developing the world's largest 1,024-qubit superconducting quantum computer. A quantum building will be built at Fujitsu's headquarters, which is scheduled to be installed and opened to the public in fiscal 2026.
Participation in national projects for the industrialization of quantum computers:
We aim to build a superconducting quantum computer with more than 10,000 physical qubits by FY2030. As part of this effort, we are participating in national projects promoted by NEDO for the project period until FY2027.
Development of a diamond spin quantum computer:
Compared to the superconducting method, it is a method that can be miniaturized and has excellent scalability through optical connection. In March 2025, we achieved the world's first high-precision quantum gate operation with an error probability of less than 0.1% required for large-scale operations.
Development of a new quantum computing architecture to accelerate practical computing of quantum computers:
The "STAR architecture" jointly developed by Fujitsu and The university of Osaka is a technology that can reduce the number of physical qubits required for practical quantum computing. By using this architecture, we have theoretically demonstrated that quantum calculations that require 1 million qubits in the materials field can be performed in about 10 hours while maintaining accuracy at 60,000 qubits, compared to 5 years using a current computer.
Utilization of the world-leading 40-qubit quantum computer simulator:
We have been offering a world-leading 40-qubit quantum computer simulators since October 2023. By collaborating with quantum computers, we are strongly promoting research and development of quantum applications.
Development of hybrid quantum computing platforms:
We provide the world-leading quantum computers and quantum simulators in tandem. It can also be linked with external libraries, allowing users to seamlessly distinguish between actual machines with noise and quantum simulators without noise.

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Frequently asked questions

Q1. Are quantum computers faster than classical computers?

A1. While quantum computers have the ability to solve certain types of complex problems (e.g., quantum chemistry calculations, prime factorization, combinatorial optimization problems, etc.) much faster than classical computers, they are not superior in all calculations. It is expected to be used in problem areas where it is good at.

Q2. When will quantum computers become popular and commercially available?

A2. Fujitsu is conducting research and development with the aim of building a superconducting quantum computer with a capacity of over 10,000 physical qubits by FY2030 to realize practical quantum computing. It is believed that it will take a little longer for it to become widely spread and used in society.

Q3. What companies are researching quantum computing?

A3. Fujitsu, Google, IBM, Microsoft, etc. are conducting research and development in different ways. In addition, universities and national laboratories around the world are also conducting research on a wide range of fields, from basic research to applied research.

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What is quantum computing?