How Japan built the largest-class superconducting quantum computer

Euronews

20-06-2025

Researchers in Japan have developed the world's largest-class superconducting quantum computer.
Several hardware types are used to build quantum computers, which use subatomic particles called qubits to increase computing capabilities..
Among them, superconducting quantum computers are the most widely tested, with companies like Google, IBM and Rigetti leading in this technology.
The higher the number of qubits, the greater the potential computing power.
However, other factors such as noise and error mitigation methods are also essential for building a practical quantum computer.
The new quantum system developed by researchers at the Japanese National Research and Development Agency, RIKEN, in collaboration with the Japanese IT giant, Fujitsu, has 256 qubits.
For comparison, Google's Sycamore quantum processor uses 70 qubits. IBM has a 1,121-qubit processor called Condor, but it's not broadly available for external users.
It is widely thought that it would take one million quibits to realise quantum's full potential.
Not only did the researchers deploy one of the largest class superconducting quantum computers, but they also managed to quadruple the density of the qubits by fitting 256 qubits into the casing used for the previous generation quantum computer that used 64 qubits.
Researchers credit this fourfold increase in density to a combination of high-density integration technology and an advanced thermal design.
They assembled 4-qubit 'unit cells' side by side and layered the connected cells in three dimensions, a technique known as a 3D connection structure.
'Using this structure, we can scale the quantum chip without design change… We can make any size qubit chip using a 3D connect structure,' Yoshiyasu Doi, Senior Researcher, RIKEN RQC-FUJITSU Collaboration Centre, told Euronews Next.
Fujitsu says the technique enables efficient scaling of qubits without requiring complex redesigns.
Quantum computers must operate in extreme cold temperatures, and larger qubits typically need more space due to heat generation.
The new 256-qubit system, housed at the RIKEN RQC-FUJITSU Collaboration Centre in Wako, Japan, includes a cooling system that can reach temperatures as low as 20 millikelvin, close to absolute zero, the coldest temperature possible in nature.
'To implement a larger number of components, thermal heat is a very difficult problem…in the new design, we can reduce the power of the amplifier by over 60 per cent. Heat balance is very important to build a larger system,' said Doi.
When building a quantum system, every qubit needs an input and output connection.
Scaling entails more complex packaging, cabling, and cryogenic infrastructure to manage those connections.
'One of the key advancements that Fujitsu is demonstrating here is the cabling side, getting into a higher density,' Jonathan Burnett, the deputy director for research at the National Quantum Computing Centre in the United Kingdom, told Euronews Next.
While American companies such as IBM and AWS have developed similar high-density cabling and integration, no European group currently has a deployed system with this level of cable density, Burnett says.
'Europe-wide, this would be quite a leap against [it]'.
Fujitsu says it aims to launch a 1,000-qubit computer in 2026.
'A 1,000 qubit system is a very cost-consuming device. So at first we have to make the technologies to build such a bigger system… Using this dense design, we can build a larger system, like the 1,000 qubit system,' Doi said.
Experts say that scaling is critical to advance the benefits of superconducting quantum computers.
'You start to encounter novel problems… that might only occur because you're trying to do 10 things at once and therefore you don't encounter it if you're never working at that size,' Burnett says.
'The impressive thing that does come from what physics Fujitsu is working on is actually encountering those genuine problems of scale that do come from ultimately operating a kind of larger number,' he added.
However, ensuring the quality of qubits is as important as the quantity of qubits.
Accessible for research institutions and companies globally
The new 256-qubit quantum computer is accessible via a cloud platform for companies and research institutions to run complicated calculations.
'Hybrid quantum platform with this machine and quantum simulator. And we provide such a system to our customers, to our collaborators, such as research institutes all over the world,' Doi said.
Fujitsu says it's currently working with four companies in Japan, covering industries from finance to chemicals, and aims to expand these collaborations globally. Other partnerships exist, but the company has not disclosed specific details for confidentiality reasons.
Quantum computers hold the promise of advancing drug research, finance, and the discovery of new materials thanks to their unprecedented computing capabilities.
However, there is broad agreement within the industry that the journey to fully practical quantum computing remains a long one.
One million qubits is often seen as the threshold for fault-tolerant, large-scale quantum computing to solve truly practical and complex problems.
In 2023, the UK outlined its Quantum Mission 1 as part of a national roadmap toward useful quantum computing. It estimates that around one million physical qubits will be needed to run real-world algorithms like Shor's, which is often used as a benchmark.
However, experts concur that smaller systems are crucial stepping stones.
'We have to proceed [with] quantum technology step by step. To solve practical problems, we have to build a one-million-cubic system…So, in that sense, to develop the 1,000 qubit system is one of the steps,' Doi said.
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