Thanks to “entanglement”, we can today encrypt quantum communications over a hundred kilometers. Enough to get a little closer to the quantum internet.

“Teleport Scotty!” ”, these famous words are from Captain Kirk from the famous series *Star Trek* in the late 60's to the ship's engineer to be able to teleport him from the spaceship *Business* 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 “shielded”. Quantum teleportation has been theoretically known since the beginning of the 20th century.^{m} century, it was experimentally proven in the second half of it, and today, this phenomenon is used for very specific applications… namely the development of what we call the “quantum Internet”.

Our current telecommunications, including the Internet, are based on the exchange of coded information, which travels, often in light, over optical fibers or over the air between relay antennas and telephones and up to satellites in orbit around the earth. A quantum internet would use the quantum properties of light, namely the fact that we can “entangle” light particles, which makes it possible to “teleport” the information carried by those particles. These properties would make it possible to exchange information in an encrypted and secure manner, which has applications in cryptography and therefore cyber security.

Encrypted quantum communications can currently be maintained at a maximum distance of about a hundred kilometers – which remains a bit short for global telecommunications… but technical solutions are in the works.

## Encrypt your communications

There are different quantum cryptography protocols available today and several companies and startups are in this niche but growing market.

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. To do this, the sender, whom we call Alice, must generate an encryption key that she can combine with her message to hide it from the rest of the world. Bob, for his part, must be the only one who has this same key to be able to decrypt the message (he will actually do the opposite of Alice's encryption to decrypt the message).

We start by encoding the message ” *Come and get your bread please* » from a series of 1s and 0s, this is binary encoding. We then encrypt the message by creating an encrypted key also from 1 and 0 which will be combined with the message. But this encryption system has many flaws if we want it to be secure. First of all, you need to generate a key as long as the message (in terms of 1's and 0's), as random as possible – so you can't predict it – which is possible, but at a very high cost and energy level cost.

Actually, these keys we use are not completely random. And above all, they are reused in whole or in part, which raises serious security questions. The second technical concern with this method is that it assumes that the key is securely shared between Alice and Bob at all times. At the very least, this means that they must meet from time to time 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 classical encryption/decryption systems will suffer from these drawbacks.

This is where quantum cryptography can provide solutions.

## From quantum entanglement to encrypted key distribution

Quantum entanglement is a form of “hypercorrelation” between two quantum systems.

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

Now assume that the coins are not counterfeit. Alice and Bob each have one. When these coins are tossed, each of them will randomly find heads or tails. The two coin flips are no longer correlated. There is a 25% chance of heads/tails coming up, as well as heads/tails, heads/tails, heads/tails: the four outcomes are equally likely, unlike the correlation experiment where the probability of getting heads/tails is 100% and 0% for other options.

On the other hand, if the two pieces are tangled together, they are not set to always land on heads, but to always land on the same side as the other piece. Alice has a 50% chance of getting heads and a 50% chance of getting tails. 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 came up tails, so did Bob's, and vice versa (in practice, we can prepare quantum systems such that they are correlated – heads/tails – or anticorrelated – heads/tails – but the idea is the same).

What is most striking (and inconceivable) is that this property holds regardless of the distance that separates Alice and Bob – and it is this “non-local” phenomenon that is the source of information “teleportation.”)

Quantum entanglement can be used to serve as an encryption key. Sharing an entangled quantum system, only Alice and Bob have perfect correlations between their pieces: they are certain that this key, combined with a message, can only be deciphered by them.

So it is the quantum nature of light, which guarantees the security of the free and natural exchange system.

## The photon as an information base

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

This is exactly what John Clauser, in the 1970s, and Alain Aspect, in the 1980s, studied with their groups: the “polarization” entanglement of photon pairs emitted by individuals in an empty chamber, using what is called atomic cascade of calcium atoms. However, this method of producing photon pairs is not straightforward (hence the Nobel Prize).

Anton Zeilinger and his team then succeeded in creating pairs of photons involved in polarization, but using the properties of nonlinear optics. This experiment is also not simple, but it is easier to set up and therefore has enabled the development of applications much faster, particularly in quantum communications (hence the Nobel Prize).

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

## The advent of the quantum internet will take time

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

For example, today, the most sophisticated sources can at best generate several million pairs of photons per second, which is still a thousand times less than what would be needed to actually develop this quantum device.

In addition, quantum entanglement is a fragile phenomenon, which always limits the distance at which encrypted communications can be maintained (with a maximum distance of about a hundred kilometers).

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

All of this just reinforces the idea that quantum technologies remain exciting and will grow in the coming decades, just as the Internet and fiber optics have grown in the last forty years.

Christophe Couteau, teacher-researcher in quantum physics, *University of Technology of Troyes (UTT)*

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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