Quantum internet tech at QuTech in Belgium.

A prototype of a nitrogen-vacancy center, a piece of hardware used in a quantum network at the QuDelft laboratory in the Netherlands.Credit: Marcel Wogram for Nature

A future ‘quantum internet’ could find use long before it reaches technological maturity, a team of physicists predicts.

Such a network, which exploits the unique effects of quantum physics, would be fundamentally different to the classical Internet we use today, and research groups worldwide are already working on its early stages of development. The first stages promise virtually unbreakable privacy and security in communications; a more mature network could include a range of applications for science and beyond that aren’t possible with classical systems, including quantum sensors that can detect gravitational waves.

A prominent team of quantum-internet researchers at Delft University of Technology in the Netherlands has now released a roadmap laying out the stages of network sophistication — and detailing the technological challenges that each tier would involve. Their predictions are described in Science1 on 18 October.

The quantum difference

The researchers argue that the technology, which would complement rather than replace the existing Internet, could eventually become widespread both for large users, such as university laboratories, and for individual consumers, although they do not give a time scale.

This stands in contrast with quantum computers, they say — another futuristic technology that physicists are feverishly working on, aiming to build machines that can outperform classical computers. “In the quantum-computing domain, it’s much more all or nothing,” says theoretical physicist Stephanie Wehner, who co-authored the paper with her Delft colleagues David Elkouss and Ronald Hanson.

Stefanie Barz, a quantum physicist at the University of Stuttgart in Germany, agrees. It’s difficult to predict which technology will come first, she and others say — a widely adopted quantum internet or useful quantum computers. But quantum networks have a big advantage, Barz says, in that “such a network can be built step by step, and different functionality can be added in each step”.

The roadmap also aims to establish a common language for a field that involves researchers with disparate backgrounds, including information technology, computer science, engineering and physics. “People talk about quantum networks to mean vastly different things,” says Hanson, an experimental physicist who is co-leading the Delft group’s push to build a quantum-internet demonstration that will link four Dutch cities.

Rodney Van Meter, a quantum network engineer at Keio University in Tokyo, says that the paper helps to clarify the field’s goals. “It gives us a new vocabulary for understanding what we are developing.” And the way the document spells out the applications can also help researchers explain their proposals to potential investors, he says. “With this roadmap, we can have this conversation.”

Six stages

Quantum networks and quantum computing share many concepts and techniques. Both take advantage of phenomena that have no analogue in classical physics: for example, a quantum particle such as an electron or a photon can be in one of two well-defined states of spinning, clockwise or anticlockwise — but also in a simultaneous combination of both, called a superposition. And two particles can be ‘entangled’, in which they share a common quantum state. This makes them act in seemingly coordinated ways (such as spinning in opposite directions) even when they are separated by vast distances.

Six steps to a quantum internet

Researchers have laid out six stages of sophistication that a future quantum internet could reach, and what users could do at each level.

0 Trusted-node network: Users can receive quantum-generated codes but cannot send or receive quantum states. Any two end users can share an encryption key (but the service provider will know it, too).

1 Prepare and measure: End users receive and measure quantum states (but the quantum phenomenon of entanglement is not necessarily involved). Two end users can share a private key only they know. Also, users can have their password verified without revealing it.

2 Entanglement distribution networks: Any two end users can obtain entangled states (but not to store them). These provide the strongest quantum encryption possible.

3 Quantum memory networks: Any two end users to obtain and store entangled qubits (the quantum unit of information), and can teleport quantum information to each other. The networks enable cloud quantum computing.

4 & 5 Quantum computing networks: The devices on the network are full-fledged quantum computers (able to do error correction on data transfers). These stages would enable various degrees of distributed quantum computing and quantum sensors, with applications to science experiments.

The Delft team has laid out six stages for the evolution of the quantum internet (see ‘Six steps to a quantum internet’).

The first — which they say is a sort of stage 0 because it does not describe a true quantum internet — is a network that enables users to establish a common encryption key, so that they can share their (classical) data securely. The quantum physics occurs only behind the scenes: the service provider uses it to create the key. But the provider also knows the key, which means that users have to trust it. This type of network already exists, most notably in China, where it extends over some 2,000 kilometres and connects major cities including Beijing and Shanghai.

In stage 1, users will start getting into the quantum game, in which a sender creates quantum states, typically for photons. These would be sent to a receiver, either along an optical fibre or through a laser pulse beamed across open space. At this stage, any two users will be able to create a private encryption key that only they know.

The technology will also enable users to submit a quantum password, for example, to a machine such as an ATM. The machine will be able to verify the password without knowing what it is or being able to steal it.

Stage 1 has not been tried on a large scale, but it is already technologically feasible at the scale of small cities, Wehner says, although it would be very slow. A group led by Pan Jian-Wei at the University of Science and Technology of China in Hefei made the world record for this kind of transmission in 2017, when they used a satellite to link two laboratories more than 1,200 kilometres apart.

In stage 2, the quantum internet will harness the powerful phenomenon of entanglement. Its first goal will be to make quantum encryption essentially unbreakable. Most of the techniques that this stage requires already exist, at least as rudimentary lab demonstrations.

Stages 3 to 5 will, for the first time, enable any two users to store and exchange quantum bits, or qubits. These are units of quantum information, similar to classical 1s and 0s, but they can be in a superposition of both 1 and 0 simultaneously. Qubits are also the basis for quantum computation. (A number of laboratories — both in academia and at large corporations, such as IBM or Google — have been building increasingly complex quantum computers; the most advanced ones have memories that can hold a few dozen qubits.)

Getting to the final stage will require several breakthroughs. Hanson’s team has been at the forefront of these efforts, and is among those working to build the first ‘quantum repeater’ — a device that can help to entangle qubits over larger and larger distances.

Clocks and ballots

The early adopters of the highest-stage networks will probably be scientists themselves. Labs will get to connect to the first advanced quantum computers remotely, or to link up such machines to work as a single computer.

They could then use these systems to perform experiments that aren’t possible with classical machines, for example, simulating the quantum physics of molecules or materials. Networks of quantum clocks could dramatically increase the precision of measurements for phenomena such as gravitational waves, and distant optical telescopes could link up their qubits to sharpen images.

But there could be applications outside of science, too. In an election, a stage-5 quantum internet could allow voters to select not just one candidate, but a ‘superposition’ of candidates, which includes, say, their second-favourite option. “Quantum voters,” says physicist Nicole Yunger Halpern at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, could use “strategic-voting schemes that classical voters can’t implement”. And quantum techniques might help large groups to coordinate and reach a consensus, for example, to validate electronic currencies such as Bitcoin.

Liang Jiang, a theoretical physicist at Yale University in New Haven, Connecticut, says that the roadmap will be useful to the broader quantum community, but that it focuses mostly on the types of technology that the Delft group has adopted. For example, theoretical work published last year by Jiang and collaborators suggests that small- or medium-scale networks could be based on microwaves rather than laser pulses.

Researchers’ opinions are not unanimous as to whether these applications will truly be useful, or whether a quantum internet will ever be sophisticated enough to make them broadly available. But some are optimistic. “I have no doubt that it will exist at some point,” Wehner says. But, she adds, “I think it is going to take a long time”.