“Teleport Scotty!” », these famous words are those of Captain Kirk of the famous series *star journey* in the late 60s to his ship’s 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, like light, and if they are well “shielded”. of quantum teleportation is theoretically known since the beginning of the XX century^{e} century, it was demonstrated experimentally in the 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 that passes, often through light, via fiber optics or in the open air between antennas and telephones and up to satellites in orbit around the earth. A quantum internet would make use of the quantum properties of light, and in particular the fact that one can “entangle” light particles, 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 cyber security.

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

## Encrypt your communications

There are nowadays various quantum cryptography protocols and some companies and start-ups are into it Niche Market but in full expansion.

The ultimate goal of cryptography is to encode 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, in turn, must be the only one who has the same key to be able to decrypt the message (he will actually do the reverse operation of Alice’s encryption to decrypt the message).

** Read more:
From cryptography to artificial intelligence, can quantum computing change the world?
**

We start by encoding the message “Go get the bread please” with a series of 1’s and 0’s, this is binary encoding. Then the message is encrypted by generating in parallel with it, an encrypted key also composed of 1 and 0, and which will be combined with the message. But this encryption system has some drawbacks if we want it to be secure. First of all, you need 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 in one very high economic and energy cost.

In fact, these keys 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 occasionally to give each other a set 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 flaws.

This is where quantum cryptography can provide a solution.

## From quantum entanglement to encrypted key distribution

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

Let’s get the coins rigged 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 these coins are tossed, each will randomly land heads or tails. The throws of the two parts are no longer correlated. There is a 25% probability of hitting head/tail, as well as hitting tail/tail, tail/tail, head/tail: all four outcomes are possible, unlike the correlation experiment where the probability of finding face/face is 100% and 0% for other options.

** Read more:
A brief history of quantum computing
**

On the other hand, if the two parts are entangled with each other, they are not mounted so that they always come down head-up, but always sit on the same side as the rest. Alice has a 50% chance to hit heads and a 50% chance to hit 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 on tails, so did Bob’s, and vice versa ( in practice, quantum systems can be prepared to be correlated – head/tail – or anticorrelated – head/tail – but the idea is the same).

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

Quantum entanglement can be used to act as an encryption key. 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 deciphered by them.

Hence it is the quantum nature of light which freely and naturally guarantees the security of the exchange system.

## Photo as a bit of information

We can create quantum states in a photon, this grain of light that makes up light and which is essentially quantum – in the field of quantum computing we talk about “encoding quantum bits” (or qubits) of information. Indeed, photons can be in two polarization states, which play the role of “tails” and “tails” of Alice and Bob’s coins.

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

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

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

## Still way ahead of 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.

** Read more:
From electron to photon, silicon makes its (second) revolution.
**

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

Furthermore, quantum entanglement is a fragile phenomenonwhich always limits the distance at which it can be maintained and therefore encodes communications (with a maximum distance of one hundred kilometers).

Just as we need relay antennas to transmit our messages over great distances, Alice and Bob will use them “Quantum Repeaters” to ensure that the signal does not lose intensity and store the information in the “Quantum Memories” – which are also very difficult objects to produce and control.

All 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 last forty years.