It’s a monster. Capable of carrying out operations without equivalent. To do several calculations at a time. His abilities are immense, therefore. For better or for worse: the quantum computer could easily break the most sophisticated of our secret codes. We met ?

A calculation done in three minutes instead of … 10,000 years. This quantum operation, successful by Google at the end of September, thrilled the computer world in search of its Grail: the universal quantum computer.

  • Counter-intuitive mechanics

Quantum computing research, which began in the 1980s, is based on one of the principles of quantum physics called superposition. According to this mechanism, an object can have two states at the same time: one coin at a time, face to face, whereas in the “classical” world, it can only be one or the other at a time.

This counterintuitive mechanism, “even the most imaginative scientists have trouble understanding, because it is not played at the sensitive level,” says Audrey Loridan-Baudrier, Foundation Mines-Telecom, which forms future engineers to this technology.

Every object is quantum, “even you and me,” notes Daniel Hennequin, physicist. “But these quantum properties are very quickly lost and the bigger the object, the faster the loss,” explains this researcher at the CNRS.

On a microscopic scale, however, it is possible to extend the quantum state, with simple, cold, isolated atoms.

We can thus manufacture qubits, basic brick of quantum computing. But handling is tricky because they are difficult to stabilize and manufacturers have a hard time exceeding 53 qubits.

However, they are already capable of spectacular performance, as shown by the Google processor that has calculated in 3 minutes where a classic supercomputer would have taken thousands of years.

  • Operations without equivalent

Why such a gap? Unlike conventional bits that can only be found in two states (0 or 1), qubits have an infinity of possible states, which can be superimposed. “It is a parallelism that allows for multiple calculations at once,” summarizes Jean-Paul Delahaye, researcher in computer science.

We thus arrive at “algorithms unparalleled in the classical world that it is difficult to imagine”, according to Daniel Hennequin.

The “quantum solution” is useful when the problem is “so complex, or the mass of data so huge that conventional supercomputers are not enough,” analyzes Audrey Loridan-Baudrier.

The quantum computer would therefore have a considerable impact on our ability to process information.

Concrete applications using classical / quantum hybrid systems already exist. One of them, for example, can quickly solve the “problem of the commercial traveler” who must optimize his trip to 100 different cities.

  • Threats on cryptography

The most promising quantum algorithm is that of Shor, able to factorize as fast as to multiply, whereas in a classical computation, there is a difference of time of resolution between the two operations. “If I ask you what numbers 437 is the product, you will take a long time to find. Conversely, if I ask you to do the multiplication 19 x 23, you will find much faster 437, “observes Daniel Hennequin.

For the time being, quantum can factorize only 7- or 8-digit numbers and the conventional computer remains much more powerful, says Jean-Paul Delehaye.

But the day a universal quantum computer will succeed in executing the Shor algorithm on a large scale, with 100-digit numbers, we will speak of “quantum supremacy”. This would call into question all the cryptography governing our security codes (credit cards, etc.), which is based on the length of the factorization (the RSA algorithm).

To counter this threat, research in resistant cryptography has already taken the lead. “It is even more advanced than the quantum computer,” assures Daniel Hennequin.

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