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It might all sound like a sci-fi concept, but building quantum networks is a key ambition for many countries around the world. Recently the US Department of Defense (DoE) published the first blueprint of its kind, laying out a step-by-step strategy to make the quantum internet dream come true, at least in a very preliminary form, over the next few years.

The US joined the EU and China in showing a keen interest in the concept of quantum communications. But what is the quantum internet exactly, how does it work, and what are the wonders that it can accomplish?

WHAT IS THE QUANTUM INTERNET?

The quantum internet is a network that will let quantum devices exchange some information within an environment that harnesses the weird laws of quantum mechanics. In theory, this would lend the quantum internet unprecedented capabilities that are impossible to carry out with today's web applications.

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In the quantum world, data can be encoded in the state of qubits, which can be created in quantum devices like a quantum computer or a quantum processor. And the quantum internet, in simple terms, will involve sending qubits across a network of multiple quantum devices that are physically separated. Crucially, all of this would happen thanks to the whacky properties that are unique to quantum states.

That might sound similar to the standard internet. But sending qubits around through a quantum channel, rather than a classical one, effectively means leveraging the behavior of particles when taken at their smallest scale so-called "quantum states", which have caused delight and dismay among scientists for decades.

And the laws of quantum physics, which underpin the way information will be transmitted in the quantum internet, are nothing short of unfamiliar. In fact, they are strange, counter-intuitive, and at times even seemingly supernatural.

And so to understand how the quantum ecosystem of the internet 2.0 works, you might want to forget everything you know about classical computing. Because not much of the quantum internet will remind you of your favorite web browser.

WHAT TYPE OF INFORMATION CAN WE EXCHANGE WITH QUANTUM?

In short, not much that most users are accustomed to. At least for the next few decades, therefore, you shouldn't expect to one day be able to jump onto quantum Zoom meetings.

Central to quantum communication is the fact that qubits, which harness the fundamental laws of quantum mechanics, behave very differently to classical bits.

As it encodes data, a classical bit can effectively only be one of two states. Just like a light switch has to be either on or off, and just like a cat has to be either dead or alive, so does a bit have to be either 0 or 1.

Not so much with qubits. Instead, qubits are superposed: they can be 0 and 1 simultaneously, in a special quantum state that doesn't exist in the classical world. It's a little bit as if you could be both on the left-hand side and the right-hand side of your sofa, in the same moment.

The paradox is that the mere act of measuring a qubit means that it is assigned a state. A measured qubit automatically falls from its dual state, and is relegated to 0 or 1, just like a classical bit.

The whole phenomenon is called superposition, and lies at the core of quantum mechanics.

Unsurprisingly, qubits cannot be used to send the kind of data we are familiar with, like emails and WhatsApp messages. But the strange behavior of qubits is opening up huge opportunities in other, more niche applications.

QUANTUM (SAFER) COMMUNICATIONS

One of the most exciting avenues that researchers, armed with qubits, are exploring, is security.

When it comes to classical communications, most data is secured by distributing a shared key to the sender and receiver, and then using this common key to encrypt the message. The receiver can then use their key to decode the data at their end.

The security of most classical communication today is based on an algorithm for creating keys that is difficult for hackers to break, but not impossible. That's why researchers are looking at making this communication process "quantum". The concept is at the core of an emerging field of cybersecurity called quantum key distribution (QKD).

QKD works by having one of the two parties encrypt a piece of classical data by encoding the cryptography key onto qubits. The sender then transmits those qubits to the other person, who measures the qubits in order to obtain the key values.

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Measuring causes the state of the qubit to collapse; but it is the value that is read out during the measurement process that is important. The qubit, in a way, is only there to transport the key value.

More importantly, QKD means that it is easy to find out whether a third party has eavesdropped on the qubits during the transmission, since the intruder would have caused the key to collapse simply by looking at it.

If a hacker looked at the qubits at any point while they were being sent, this would automatically change the state of the qubits. A spy would inevitably leave behind a sign of eavesdropping which is why cryptographers maintain that QKD is "provably" secure.

SO, WHY A QUANTUM INTERNET?

QKD technology is in its very early stages. The "usual" way to create QKD at the moment consists of sending qubits in a one-directional way to the receiver, through optic-fibre cables; but those significantly limit the effectiveness of the protocol.

Qubits can easily get lost or scattered in a fibre-optic cable, which means that quantum signals are very much error-prone, and struggle to travel long distances. Current experiments, in fact, are limited to a range of hundreds of kilometers.

There is another solution, and it is the one that underpins the quantum internet: to leverage another property of quantum, called entanglement, to communicate between two devices.

When two qubits interact and become entangled, they share particular properties that depend on each other. While the qubits are in an entangled state, any change to one particle in the pair will result in changes to the other, even if they are physically separated.The state of the first qubit, therefore, can be "read" by looking at the behavior of its entangled counterpart. That's right: even Albert Einstein called the whole thing "spooky action at a distance".

And in the context of quantum communication, entanglement could in effect, teleport some information from one qubit to its entangled other half, without the need for a physical channel bridging the two during the transmission.

HOW DOES ENTANGLEMENT WORK?

The very concept of teleportation entails, by definition, the lack of a physical network bridging between communicating devices. But it remains that entanglement needs to be created in the first place, and then maintained.

To carry out QKD using entanglement, it is necessary to build the appropriate infrastructure to first create pairs of entangled qubits, and then distribute them between a sender and a receiver. This creates the "teleportation" channel over which cryptography keys can be exchanged.

Specifically, once the entangled qubits have been generated, you have to send one half of the pair to the receiver of the key. An entangled qubit can travel through networks of optical fibre, for example; but those are unable to maintain entanglement after about 60 miles.

Qubits can also be kept entangled over large distances via satellite, but covering the planet with outer-space quantum devices is expensive.

There are still huge engineering challenges, therefore, to building large-scale "teleportation networks" that could effectively link up qubits across the world. Once the entanglement network is in place, the magic can start: linked qubits won't need to run through any form of physical infrastructure anymore to deliver their message.

During transmission, therefore, the quantum key would virtually be invisible to third parties, impossible to intercept, and reliably "teleported" from one endpoint to the next. The idea will resonate well with industries that deal with sensitive data, such as banking, health services or aircraft communications. And it is likely that governments sitting on top secret information will also be early adopters of the technology.

WHAT ELSE COULD WE DO WITH THE QUANTUM INTERNET?

'Why bother with entanglement?' you may ask. After all, researchers could simply find ways to improve the "usual" form of QKD. Quantum repeaters, for example, could go a long way in increasing communication distance in fibre-optic cables, without having to go so far as to entangle qubits.

That is without accounting for the immense potential that entanglement could have for other applications. QKD is the most frequently discussed example of what the quantum internet could achieve, because it is the most accessible application of the technology. But security is far from being the only field that is causing excitement among researchers.

The entanglement network used for QKD could also be used, for example, to provide a reliable way to build up quantum clusters made of entangled qubits located in different quantum devices.

Researchers won't need a particularly powerful piece of quantum hardware to connect to the quantum internet in fact, even a single-qubit processor could do the job. But by linking together quantum devices that, as they stand, have limited capabilities, scientists expect that they could create a quantum supercomputer to surpass them all.

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By connecting many smaller quantum devices together, therefore, the quantum internet could start solving the problems that are currently impossible to achieve in a single quantum computer. This includes expediting the exchange of vast amounts of data, and carrying out large-scale sensing experiments in astronomy, materials discovery and life sciences.

For this reason, scientists are convinced that we could reap the benefits of the quantum internet before tech giants such as Google and IBM even achieve quantum supremacy the moment when a single quantum computer will solve a problem that is intractable for a classical computer.

Google and IBM's most advanced quantum computers currently sit around 50 qubits, which, on its own, is much less than is needed to carry out the phenomenal calculations needed to solve the problems that quantum research hopes to address.

On the other hand, linking such devices together via quantum entanglement could result in clusters worth several thousands of qubits. For many scientists, creating such computing strength is in fact the ultimate goal of the quantum internet project.

WHAT COULDN'T WE DO WITH THE QUANTUM INTERNET?

For the foreseeable future, the quantum internet could not be used to exchange data in the way that we currently do on our laptops.

Imagining a generalized, mainstream quantum internet would require anticipating a few decades (or more) of technological advancements. As much as scientists dream of the future of the quantum internet, therefore, it is impossible to draw parallels between the project as it currently stands, and the way we browse the web every day.

A lot of quantum communication research today is dedicated to finding out how to best encode, compress and transmit information thanks to quantum states. Quantum states, of course, are known for their extraordinary densities, and scientists are confident that one node could teleport a great deal of data.

But the type of information that scientists are looking at sending over the quantum internet has little to do with opening up an inbox and scrolling through emails. And in fact, replacing the classical internet is not what the technology has set out to do.

Rather, researchers are hoping that the quantum internet will sit next to the classical internet, and would be used for more specialized applications. The quantum internet will perform tasks that can be done faster on a quantum computer than on classical computers, or which are too difficult to perform even on the best supercomputers that exist today.

SO, WHAT ARE WE WAITING FOR?

Scientists already know how to create entanglement between qubits, and they have even been successfully leveraging entanglement for QKD.

China, a long-time investor in quantum networks, has broken records on satellite-induced entanglement. Chinese scientists recently established entanglement and achieved QKD over a record-breaking 745 miles.

The next stage, however, is scaling up the infrastructure. All experiments so far have only connected two end-points. Now that point-to-point communication has been achieved, scientists are working on creating a network in which multiple senders and multiple receivers could exchange over the quantum internet on a global scale.

The idea, essentially, is to find the best ways to churn out lots of entangled qubits on demand, over long distances, and between many different points at the same time. This is much easier said than done: for example, maintaining the entanglement between a device in China and one in the US would probably require an intermediate node, on top of new routing protocols.

And countries are opting for different technologies when it comes to establishing entanglement in the first place. While China is picking satellite technology, optical fibre is the method favored by the US DoE, which is now trying to create a network of quantum repeaters that can augment the distance that separates entangled qubits.

In the US, particles have remained entangled through optical fibre over a 52-mile "quantum loop" in the suburbs of Chicago, without the need for quantum repeaters. The network will soon be connected to one of the DoE's laboratories to establish an 80-mile quantum testbed.

In the EU, the Quantum Internet Alliance was formed in 2018 to develop a strategy for a quantum internet, and demonstrated entanglement over 31 miles last year.

For quantum researchers, the goal is to scale the networks up to a national level first, and one day even internationally. The vast majority of scientists agree that this is unlikely to happen before a couple of decades. The quantum internet is without doubt a very long-term project, with many technical obstacles still standing in the way. But the unexpected outcomes that the technology will inevitably bring about on the way will make for an invaluable scientific journey, complete with a plethora of outlandish quantum applications that, for now, cannot even be predicted.

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What is the quantum internet? Everything you need to know about the weird future of quantum networks - ZDNet