Quantum cryptography: the CEA achieves a world first for securing communications

Several laboratories on the planet have embarked on the race for quantum computers. It is believed that there are certain problems that these machines can solve in much less time than a human lifetime, which would not be the case with conventional computers using algorithms based on performing calculations at the help from the physical classic.

But the Quantum mechanicswith phenomena like the superposition of quantum states and thequantum entanglement with some qubitsthe bit-generalization of classical information theory dating back to the work of engineer and mathematician Claude Shannon, in principle allows some kinds of sometimes very fast parallel computations.

More generally, laboratories have embarked on a race for quantum information technologies which it is hoped will lead to a global network of quantum computers connected as the Internet is todaywith tremendous computing power, record data transfer capabilities, volumes of information as well as means of communication with unprecedented security guarantees.

For this, we will combine techniques of teleportation and quantum cryptography based on the famous EPR effect theoretically discovered in 1935 by Einstein and his collaborators Podolsky and Rosen and whose study has been renewed by the work of John Bell and Alain Aspect.

From quantum entanglement to cryptography

The EPR effect is based on the entanglement of two quantum systems as they say in their jargon physicistsit can be a pair of photons or a pair of atoms for example. By measuring certain characteristics of one of these quantum systems, the second sees instantaneously, or at least more quickly than at the speed of lightits state affected by the first measurement to the extent that the results of the measurements on the second system depend on it.

This effect can be used to transfer a coded message as well as the key of encryption of this message.

Currently, the credit card security relies on the ability to factorize a very large integer into a product of two prime numbers, also very long. This is an impossible task for a classical computer to perform in the span of a human lifetime, but it would be very fast with a quantum computer containing a very large number of qubits as shown with his quantum algorithm, in 1994, a researcher in applied mathematics Massachusetts Institute of Technology (MIT), Peter Shor. It cannot be put into practice yet and that is why a product of an integer is what lies behind the famous RSA encryption.

But if one day we can use Shor’s discovery, then we will need another cryptographic technique ensuring secrecy, banking or not. A breakthrough in this direction has been achieved by theoretical physicists from the CEA at the Institute of Theoretical Physics (CEA/CNRS/Paris-Saclay University) together with their colleagues in Switzerland (University of Geneva, Ecoles polytechniques de Lausanne and Zurich). and in Great Britain (University of Oxford) who brought the experimental demonstration of a quantum distribution of cryptographic keys which could replace the encryption of the RSA.

Entangled polarized photons and an avatar of the E91 protocol

As explained in a press release from the CEA about this progress, which is set out in an article by Natureeverything is based on cryptographic “keys” made up of sequences of random numbers which must be shared only by the sender and the receiver. It is in fact a concrete realization with pairs ofions of strontium entangled, each being trapped in a electric field and cooled by laserfrom an idea inspired by the protocol of encryption called E91, due to Artur Ekert, and proposed in 1991 by this Polish-British physicist currently Professor of quantum physics at the Institute of Mathematics of the University of Oxford.

This is an example of quantum key distribution (Quantum Key Distribution or QKD, in English) and in the present case a very selective laser excitation makes it possible to produce an entanglement between each of the two strontium ions and a polarized photon. Randomly polarized photons illustrating the EPR effect being measured are precisely at the heart of the E91 protocol.

In practice, it is possible to make measurements on entangled photons which can be verified to produce results where the values ​​violate the famous Bell inequalities, characteristic of a state of quantum entanglement. We can in this way ensure that an encryption key transfer has not been intercepted, retransmitted in the form of photons not entangled with the ions, and therefore that the message that we will then send will remain inviolate, that whether or not one knows how the message is encoded.

The CEA press release states that “ the researchers now plan to adapt their concept to an all-optical quantum key distribution prototype, which could be carried out by French teams exclusively, thanks to grants from the National Plan for Quantum Technologies” and adds that ” of the start-up can then seize it to offer ultra-secure communications exchanging highly confidential data (diplomacy, health, etc.) “.