The first multi-node quantum network paves the way for the quantum internet

Researchers in the Netherlands have successfully connected three separate quantum processors into what is actually the world’s first multi-node quantum network. This paves the way for a large-scale quantum internet that governments and scientists have been dreaming of for decades.

QuTech, a Delft-based quantum research institute, has published a new paper linking three nodes that can store and process quantum bits (also called qubits). This, according to QuTech researchers, is the first rudimentary quantum network in the world.

Connecting quantum devices is by no means a novelty: many researchers around the world are currently working on similar networks, but so far they have only managed to connect two quantum processors. Therefore, establishing a multi-node connection is a key step towards significantly expanding the size of the network.

Leading much of the research effort is the goal of creating a quantum internet that could one day spread across the planet. The quantum internet would exploit the strange laws of quantum mechanics to allow quantum devices to communicate with each other and is expected to unlock a number of applications that cannot be run with existing classical means.

For example, the quantum internet could connect small quantum devices together to create a large quantum cluster with more computing power than most sophisticated classic supercomputers.

“A quantum internet will open up a wide range of new applications, from awkward communications and cloud computing, with complete user privacy, to maintaining high-precision time,” said Matteo Pompili, a member of the QuTech research team. “And just like with the Internet 40 years ago, there are probably a lot of applications that we can’t predict right now.”

One of the key quantum properties that underlies the quantum internet is entanglement – a phenomenon that occurs when two quantum particles are coupled in such a way that they become fundamentally connected, no matter how physically distant they are from each other.

When two quantum particles are entangled, their properties become bound, which means that any change in one of the particles will inevitably be reflected in the other. In quantum communications, this means that scientists could effectively use tangled particles to “teleport” information from a qubit to its paired pair, even if the two are in separate quantum devices.

However, for the system to last, the agreement must first be established and maintained. Over the last decade, this has been done by numerous research groups, usually by creating a physical link between two quantum devices. Through this connection, often optical fiber, qubits can be created, tangled and then distributed between two separate quantum devices.

But two nodes are hardly enough to create a large-scale network; and in a fiber optic cable, for example, the tangle cannot be maintained after about 100 kilometers, which means that the quantum networks set up so far have been limited by the short distance they can travel.

This is why the QuTech research team has developed an intermediate node-based system, similar to classic Internet routers, that could keep the mess at greater distances.

Bob, Alice and Charlie

The architecture that scientists have revealed is seemingly simple. A middle node, called Bob, has a physical connection to two outer nodes, named Alice and Charlie. This means that a tangle can be established between Bob and each of the outer nodes.

Bob is equipped with two qubits, one of which is a memory qubit that allows the device to store an established quantum link, for example with Alice, while creating, thanks to its communication qubit, a new link with the other node – in this script, with Charlie.

Once both connections are created with the external nodes, Bob connects his own two qubits locally, which creates a fully connected network, tangling between all three nodes. This means that a quantum connection can be established between Alice and Charlie, even without a direct physical connection between the two nodes.

The QuTech team also developed a first quantum network protocol, with a signal that each operation had been successfully completed.

“The main advantage of this demonstration is that we have a scalable way to connect multiple nodes in a network,” Ronald Hanson, who led the research team, told ZDNet. “We have a memory that can store the tangled state while the new tangle is being prepared. And we have omens that tell us when the tangle was created.”

“This allowed us to make connections between the three nodes that are ready to be used for further processing or other protocols. This is the first time this has been done in any quantum network setting.”

The new network will provide a test bench to develop new hardware, software and quantum protocols for the Internet; but the experiment will also need to evolve from a proof of concept into a viable solution to expand quantum networks.

In fact, researchers have so far connected “only” single, separate cables rather than quantum processors. They will now focus on adding more qubits to their three-node network and on adding higher levels of software and hardware. But in the future, the team expects the current approach to be tested outside the laboratory on existing telecommunications fiber.

“The future of quantum internet will consist of countless quantum devices and intermediate nodes,” says Hanson. “Colleagues at QuTech are already looking at future compatibility with existing data infrastructures.”

QuTech research is supported by the EU Quantum Internet Alliance, which is part of a blockchain decade, with one billion euros ($ 1.2 billion) Quantum Flagship – an initiative launched in 2018 to stimulate quantum research and development.

The EU is far from alone in promoting the development of the quantum internet. China and the United States are equally interested in promoting quantum networks and have already reached milestones in this area. Chinese scientists, for example, have recently set up trouble on a record 1,200 kilometers.

Earlier this year, scientists at Cleland Labs in the US also managed for the first time to confuse two separate qubits by connecting them with a cable, another discovery that is expected to accelerate the creation of quantum networks.