With classic electronics and computing reaching its physical limits, much excitement surrounds quantum technologies as the fuel for future progress.

“If this is true,” said AT&T Communications’ new CEO John Donovan, “which it is, it will entirely change the way our network computing is done, and it isn’t processing but quantum networking that changes the game for us.”

It was November 2017 and Donovan was speaking to an awed conference audience about the future potential of quantum communication and networking. His company had provided seed funding for INQNET (INtelligent Quantum NEtworks and Technologies) – a research programme launched by the California Institute of Technology as part of the Alliance for Quantum Technologies (AQT) consortium.

One of the purposes of the part-publicly funded Big Science initiative is to bring together academia, national labs and industry to accelerate the development of quantum communications and networks in anticipation of quantum computing.

A quantum network will distribute information encoded into quantum states and systems. This information is held at the sub-atomic level in a physical property of something – a photon or electron – and transmitted. Whatever the entity might be, it is its quantum-mechanical properties that are being used rather than the classical properties, as happens now.

“The quantum network has been theoretically shown – it is not a belief but the task of making something that exists on paper a reality,” says Neil Sinclair, an INQNET postdoctoral fellow in physics at CalTech. “Quantum particles hold information, and if we can distribute them then we can potentially overcome problems that have never been solved before.”

Unlike in classical networks, connecting quantum devices means they become more powerful. For example, a quantum chain of computers, in theory, would have exponentially more power than one working alone. The same principle theoretically applies for quantum sensors. These entangled processors would then be the building blocks for the quantum internet, though what this will be in practice is yet to be formulated.

“Future quantum processors have physical limitations due to size and because they are sensitive to the outside world; some of the leading candidates require millikelvin temperatures or manipulation of individual atoms,” explains Sinclair. “By building a network and connecting quantum devices we expand their usefulness.”

Versions of these processors have already arrived. In November, IBM unveiled a 20-qubit [quantum bit] processor that can be used through its public cloud. Parts of the computer had to be cooled to temperatures colder than space, which conveys the epic engineering expertise required to build it. IBM has also constructed a 50-qubit operational prototype.

The last four to five years have seen a significant increase in quantum research and development, driven largely by investments from the major tech firms like IBM, Microsoft and Google, as well as government-backed initiatives. Previously, work was focused on smaller collaborations or single academics tackling individual quantum problems.

However, governments and companies are now in a race to produce meaningful quantum technologies. Google and Microsoft are said to be on the verge of big announcements. In November, Todd Holmdahl, head of Microsoft’s quantum team, said the company was “imminently close” to a big release.

The promise is that the next generation of these processors will help us model things that are impossible to understand today.

“For example, we cannot model the fundamental interactions exhaustively for the influenza virus,” says IBM’s Leigh Chase. “We cannot create every potential permutation of some problems using only classical mechanics, whereas quantum computing opens up this possibility.”

These devices could also help physicists simulate other sub-atomic systems, as well as be used for advanced computational chemistry and modelling of some N-body problems that cannot be done today.

However, without the supporting communication infrastructure they will arrive and hit a bottleneck, says Sinclair.

For the average observer, talk of sub-atomic systems is hard to understand, let alone visualise or be convinced by. The quantum network is no different.

“It is hard to imagine,” says Rishi Pravahan, who leads the quantum work at the AT&T Foundry. “Today, we cannot fathom the full possibilities, just as it was difficult to comprehend the different uses of the internet when building computers in the late 1950s.”

However, the three main potential purposes of a quantum network are: for cryptographic functions, to build sensor webs and for distributed computing.

Cryptography, one of the best-known examples of which is quantum key distribution using entanglement, has so far received the most funding and research.

Quantum entanglement is when two quantum states are entwined together so that wherever they are, no matter the distance apart, a measurement of the state of one causes the state of the other to be modified instantaneously.

Key distribution exploits the quantum properties of photons to generate a key, which is a random sequence of 1s and 0s shared only by the sender and receiver, usually referred to as Alice and Bob. Once the key is generated, no one else will have it.

Via entanglement Alice and Bob can communicate in a channel. If someone wants to eavesdrop on them they have to make a measurement on one of the photons. However, doing so will disturb the system and the presence of the eavesdropper will be easily detected.

The challenge is to distribute the photons from one point to another without destroying the ‘quantumness’.

At the end of January, led by physicists at the University of Science and Technology of China (USTC) as part of the Quantum Experiments at Space Scale project, a 75-minute quantum-encrypted video conference call was conducted between Asia and Europe. The call used the Chinese satellite Micius and reached a distance of 7,600km – the longest distance quantum encrypted messages have ever been sent.

In theory, similar principles of entanglement for quantum encryption can be used to entwine quantum computers.

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This feature also appeared in the print edition of E&T magazine.