Cryptography: new breakthrough with quantum entanglement

Paris, France | AFP | Wednesday 27/07/2022 – An experiment carried out with the help of the Atomic Energy Commission (CEA) has produced a cryptography protocol, with a secure exchange of encryption keys thanks to the phenomenon of quantum entanglement, which must guarantee perfect security of data transfers, according to a study published in Nature on Wednesday.

The exchange of sensitive information on a network, such as a customer’s banking information with a mail order website, requires the exchange of an encryption key for this information.

These keys, a series of 0s and 1s that encode information, offer a colossal number of combinations, making them essentially unbreakable. But as a Nature article accompanying the study notes, the expected arrival of quantum computers, capable of solving mathematical problems in “a fraction of the time it takes traditional computers to run them”, could be a game-changer.

The parade consists in using the laws of quantum physics, which governs the world on the scale of the infinitely small, to exchange these keys. The exchange can take place using photons, the particle that makes up light. A quantum particle, it obeys the rule of superposition which requires that any measurement disturb its state. A spy would thus betray his gesture by intercepting it or by trying to copy the information.

The problem is that the security of this process assumes that the machines exchanging the key are themselves secure. However, “for a few years there have been a lot of attacks against this kind of protocols”, remarks to AFP Jean-Daniel Bancal, researcher at the CEA and co-author of the study. The hacker “cannot enter the device, but he can try to manipulate it from the outside, and that’s catastrophic”.

Encryption key over 95,000 bits

The experiment carried out by a collaboration of researchers from the University of Oxford, Swiss physics institutes and the CEA, found a way out: no longer relying on machines exchanging encryption keys, conventionally baptized Alice and Bob , using another phenomenon of quantum mechanics: entanglement.

Also called entanglement, he wants two entangled particles to behave identically regardless of the distance separating them: as if connected by an invisible thread, they share the same state.

In the experiment, the encryption information is contained in ions, strontium atoms. Alice and Bob each have a copy. “As these are distant ions, we need to create this entanglement, and we will do this using photons”, explains Mr. Bancal.

Each of the ions is entangled with a photon, and then the two photons are sent to a central station which causes them to interfere, by means of a so-called Bell measurement which then entangles the ions with each other.

By repeating the operation over a period of eight hours, the experiment made it possible to create an encryption key of more than 95,000 bits, or units, “enough to transmit a small image in a completely secure way”, according to the researcher.

But we remain at the experimental stage, conclusive with a separation of 2 meters between Alice and Bob’s machines. The study, signed by Oxford doctoral student David Nadlinger, considers it possible in the long term to be used on a kilometer scale.

Beyond that, Jean-Daniel Bancal evokes “technologies using only photons”, to overcome the use of ions, cumbersome to implement”.

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Cryptography: new breakthrough with quantum entanglement

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