Paris (AFP)

The Google study claiming to have experimented with "quantum supremacy" with a processor capable of computing in three minutes instead of ... 10,000 years, is officially released Wednesday in the journal Nature after fleeing by mistake last month.

"This is the signal we have been waiting for," said Google's managing director Sundar Pichai. "It took 13 years to get there, it's the most significant step in the quest for quantum computing," he says on his blog.

The study of the digital giant was briefly broadcast in September on the site of NASA, and revealed in the wake of the Financial Times, before being withdrawn.

A team of Google researchers describes how they managed to create a processor, called Sycamore, which manages to perform a calculation in 200 seconds, where a "classic" supercomputer would have put, according to their references, "about 10,000 years."

"This phenomenal acceleration compared to all known classical algorithms is an experiment of quantum supremacy", explain the researchers in Nature. This calculation, specific to this experiment, is according to them a "step on the way" of the universal quantum computer, highly anticipated in the computer world where it is considered as a Grail.

Sycamore managed to run a program with 53 qubits, the basic building block of quantum computing.

Unlike conventional computer bits that can only be in two states, 0 or 1, qubits can be in multiple states at once. This superposition of states, the foundation of quantum physics, creates a "parallelism" that allows multiple calculations at once. And potentially, to arrive at unparalleled algorithms in the classical world, able to solve the most complex problems.

"Since the 1980s, we are trying to build a powerful quantum computer to solve some problems," said John Martinis, researcher in artificial intelligence at Google, at a press conference organized by Nature.

- "Way still long" -

"We demonstrate that the quantum computer has this power, physics was right, and companies will now see that this technology is closer than they thought," said the researcher.

Quantum mechanics, governed by the physical principle of wave-particle duality, allows a multitude of possibilities that overlap. It is difficult to understand because it is not played at the sensitive level, as summed up Sundar Pichai: "While the universe works fundamentally at quantum level, humans do not experience it as well." Many principles of quantum mechanics directly contradict what can be observed in nature, but its properties hold enormous potential for computing. "

Nevertheless, he warns, "the path is still long between this laboratory experiment and the concrete applications of tomorrow".

The manipulation of qubits is indeed delicate, because it is difficult to stabilize their quantum state - it requires simple atoms, cold, isolated from the outside world. The more qubits there are, the greater the difficulty, and manufacturers are struggling today to exceed 53 qubits.

After the leak of the study, several experts had urged caution, saying that this specific calculation "was useless", and that the advent of a universal quantum computer was not for tomorrow. The latter, if it is born, would be in theory in particular able to break the cryptographic systems known as "RSA", currently used by global computing. Hence the search for resistant cryptography, already well advanced.

The information on Google came out when IBM, the other heavyweight very advanced in the quantum race, announced that it would put online, accessible to researchers and developers, a quantum machine of 53 qubits, the equivalent in power of the Google machine.

© 2019 AFP