Reading about quantum technologies and their huge potential can often feel as though you're actually reading science fiction. The possibilities seem endless! But their foundations are surprisingly fragile.

Fusion Gates Cause Bottlenecks

For example, consider light-based quantum bits, or qubits, which are at the heart of work to create quantum computers and ultra-secure communication networks. Light is a natural carrier for quantum information, because it moves so quickly and so cleanly over long distances. And yet one essential step in using light for quantum information — a "fusion gate" — has always had a fundamental problem: it only works half the time. That fifty percent success rate has been like a brick wall, blocking quicker progress toward reliable large-scale systems.

From Qubits to Qudits

Researchers at NTT's Science and Core Technology Laboratory Group have been looking for a way to get over that wall. Instead of sticking with the traditional concept of a qubit, which can only hold two values (usually expressed as 0 and 1) they are working with something richer: quantum dits. If you think of a qubit as flipping a coin, a qudit could be thought of as rolling a die.

A dit can take on three or more values, meaning that each particle of light can carry more information at once. By exploring how these higher-dimensional states behave, the Laboratory Group have been able to show that fusion gates no longer need to be capped at fifty percent. In fact, the more values a photon can hold, the more opportunities there are for a fusion gate to succeed. In fact, theory predicts a success rate of 75% for four-dimensional systems and up to 90% when using ten dimensions, following a simple formula: 1 – 1/d (take the number of possible values a qudit can have — that’s the d. Then do “1 minus 1 divided by d.”)

The Need to Scale

Fusion gates are what allow individual qubits to be joined together into larger, entangled systems. Without reliable gates, it becomes difficult to scale up. It's an issue that is especially acute in communication, because when quantum information is sent over optical fibre, it degrades very quickly. Unlike ordinary internet signals, quantum states can't simply be amplified along the way; to reach long distances, they need repeaters—devices placed at intervals to rebuild the fragile quantum state. Every repeater relies on fusion gates, and if even one of them fails, the whole chain collapses. With a fifty percent limit, the odds of making it across hundreds of kilometres start to become worryingly small. In practice, the success probability drops sharply with distance: about 10% at 50 km of fibre, 1% at 100 km, and just 0.1% at 200 km.

Long-Distance Quantum Communication

Quantum dits change the game. By introducing extra possible values, the NTT Basic Research Laboratories researchers have shown that fusion gates can succeed far more often than before. In theory, this could improve the capacity of long-distance quantum communication links by about a factor of ten.

What could that mean, in practical terms? Right now, a direct quantum connection over a thousand kilometres of fibre would be so unlikely that you might expect a single qubit to get through perhaps once in a century. Using qudits, it becomes something closer to practical.

Secure, Uncrackable Internet

It's the kind of advance needed to make the quantum internet a reality. Such a network would enable completely secure communication, immune to eavesdropping even by future quantum computers. Banks, governments, and hospitals would be among the first to use it, but the benefits could eventually reach everyone in the form of safer online banking, medical data that cannot be stolen, and communications that no hacker could tap. It also opens up the possibility of linking quantum computers across continents.

A Practical Method

Other methods for long-distance quantum communication are either extremely fast but demand technologies that are currently far out of reach, or easier to build but painfully slow. NTT's proposal takes a middle ground: feasible with current or near-future hardware, yet capable of operating at much higher speeds thanks to the improved fusion gate.

The Future is Quantum

NTT's research offers the potential to help bring the dream of global quantum communication out of the realm of theory and into the network cables beneath our feet. And while the average person may not notice the change immediately, in time they could find themselves living in a world where the internet is not only faster, but also far more secure.

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For further information, please see this link:
https://group.ntt/en/newsrelease/2025/05/21/250521a.html

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Public Relations
NTT Science and Core Technology Laboratory Group
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Daniel O'Connor

Daniel O'Connor joined the NTT Group in 1999 when he began work as the Public Relations Manager of NTT Europe. While in London, he liaised with the local press, created the company's intranet site, wrote technical copy for industry magazines and managed exhibition stands from initial design to finished displays.

Later seconded to the headquarters of NTT Communications in Tokyo, he contributed to the company's first-ever winning of global telecoms awards and the digitalisation of internal company information exchange.

Since 2015 Daniel has created content for the Group's Global Leadership Institute, the One NTT Network and is currently working with NTT R&D teams to grow public understanding of the cutting-edge research undertaken by the NTT Group.