“Teleport Scotty!” », these famous words are those of Captain Kirk of the famous series *star trek* in the late 60s to his ship engineer so he could teleport him from the spaceship *Company* to a nearby planet to explore.

If Kirk’s teleportation is not for tomorrow, quantum physics has shown that teleportation is possible under very specific conditions: for very small systems, such as light, and if they are well “protected”. The quantum teleportation is known theoretically since the beginning of the XX^{e} century, was experimentally demonstrated in its second half, and today, this phenomenon is used for very concrete applications… and in particular to develop what is called the “quantum Internet”.

Our current telecommunications, including the Internet, are based on the exchange of coded information, which transits, often over light, via optical fibers or in the open air between relay antennas and telephones and up to the satellites in orbit around of the earth. A quantum Internet would use the quantum properties of light, and in particular the fact that one can “entangle” the particles of light, which makes it possible to “teleport” the information carried by these particles. These properties would make it possible to exchange information in an encrypted and tamper-proof manner, which has applications in cryptography and therefore for cybersecurity.

Encrypted quantum communications can currently be maintained over a maximum distance of a hundred kilometers – which is still a bit short for global telecoms… but technical solutions are being developed.

## Encrypt your communications

There are nowadays different quantum cryptography protocols and several companies and start-ups are on this Niche Market but in full expansion.

The ultimate goal of cryptography is to encrypt or hide a message that should only be read by the person we have in mind, let’s call that person Bob. For this, the sender, called Alice, must generate an encrypted key that she can combine with her message to hide it from the rest of the world. Bob, on his side, must be the only one to have this same key to be able to decrypt the message (he will in fact do the opposite operation of Alice’s encryption to decrypt the message).

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We start by coding the message “Go get the bread please” with a series of 1s and 0s, this is binary coding. Then the message is encrypted by generating in parallel with it, an encrypted key also made up of 1 and 0, and which will be combined with the message. But this encryption system has several flaws if we want it to be secure. First of all, you have to generate a key that is as long as the message (in terms of 1s and 0s), as randomly as possible – so that you can’t predict it – which is possible, but at a very high economic and energy cost.

In fact, these keys that we use are not completely random. And above all, they are reused in whole or in part, which raises serious security issues. The second technical concern with this method is that it assumes that the key is securely shared between Alice and Bob at some point. At a minimum, this implies that they must meet from time to time to give each other a series of encrypted keys for their future exchanges. There are several ways to encrypt messages but in general all current conventional encryption/decryption systems will suffer from these drawbacks.

This is where quantum cryptography can provide solutions.

## From quantum entanglement to the distribution of encrypted keys

Quantum entanglement is a form of “super-correlation” between two quantum systems.

Let’s take coins tricked in such a way that if we toss these two coins at the same time, the result will always be heads/heads. This is a correlation.

Now assume that the coins are not rigged. Alice and Bob each have one. When they throw these coins, they will each find, randomly, heads or tails. The tosses of the two pieces are no longer correlated. There is a 25% probability of hitting heads/tails, as well as hitting tails/tails, tails/tails, heads/tails: the four outcomes are equiprobable, unlike the correlation experiment where the probability of finding face/face is 100% and 0% for the other options.

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On the other hand, if the two pieces are entangled with each other, they are not rigged to always land heads up, but to always land on the same side as the other piece. Alice has a 50% chance of hitting heads and 50% of hitting heads; the same for Bob. But when Alice and Bob compare their results over a large number of coin tosses, they will realize that the results are perfectly correlated: if Alice’s coin landed tails, Bob’s too, and vice versa (in practice, quantum systems can be prepared to be correlated – heads/tails – or anticorrelated – heads/tails – but the idea is the same).

What is most impressive (and counter intuitive), is that this property is true whatever the distance separating Alice and Bob – and it is this “non-local” phenomenon which is at the origin of the “teleportation” of information.’)

Quantum entanglement can be used to act as an encryption key. By sharing an entangled quantum system, only Alice and Bob have perfect correlations between their parts: they are sure that this key, combined with a message, can only be decrypted by them.

It is therefore the quantum nature of light, which freely and naturally guarantees the security of the exchange system.

## The photon as a bit of information

We can create quantum states on a photon, this grain of light which constitutes light and which is intrinsically quantum – in the field of quantum computing we speak of “coding quantum bits” (or qubit) of information. Indeed, the photons can be in two states of polarization, which play the role of the “tails” and “tails” of Alice and Bob’s coins.

This is precisely what John Clauserin the 1970s, and Alain Aspectin the 80s, studied with their teams: the “polarization” entanglement of pairs of photons emitted by atoms which were in a vacuum chamber, using what is called atomic cascade of calcium atoms. However, this method of producing pairs of photons is not simple (hence the Nobel Prize).

Anton Zeilinger and his team then succeeded in creating pairs of entangled photons in polarization, but using the properties of nonlinear optics. This experience is not simple either, but it is easier to set up and therefore allowed the development of applications much faster, especially in quantum communications (hence the Nobel Prize too).

These sources of entangled photons are essential for Alice and Bob to send each other messages.

## Still a long way before the quantum internet

But clearly, even if there are companies selling quantum cryptography systems, even if everything is accelerating rapidly, the dream of a quantum internet is not yet for tomorrow. Many obstacles remain in the way.

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For example, today, the most sophisticated sources make it possible at best to generate several million pairs of photons per secondwhich is still a thousand times less than it should to really be able to deploy this quantum device.

Moreover, quantum entanglement is a fragile phenomenonwhich always limits the distance over which it can be maintained and therefore encrypt communications (with a maximum distance of a hundred kilometers).

Much like we need relay antennas to transmit our messages over great distances, Alice and Bob will use “quantum repeaters” to ensure that the signal does not lose intensity and store the information in “quantum memories” – which are also very difficult objects to manufacture and control.

All of this only reinforces the idea that quantum technologies remain fascinating and will expand in the coming decades, just as the Internet and fiber optics have unfolded in the past forty years.

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Will the Internet be quantum?

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