Quantum computer: five questions to (finally) understand everything

It is a machine that defies the imagination. Exploitation of strange properties of matter on the scale of the infinitely small. And able to solve today’s insoluble calculations in minutes. The quantum computer makes physicists dream and also whets the appetite of many private players, starting with Google, IBM and Atos.

States have also embarked on this race for the quantum holy grail, as has France with its 1.8 billion euro quantum plan. The point of what you need to know about this promising technology and its challenges.

1 – How does a quantum computer work?

A quantum computer is similar to a classical computer, except that its calculations are performed on an atomic scale. It is based on the laws of quantum physics, which deal with matter and light behavior at the microscopic level.

On this scale, strange phenomena occur, totally counterintuitive: an object can be in several states as long as it has not been measured (this is the principle of quantum superposition), and two objects can affect each other even when separated by a large distance ( the principle of quantum entanglement).

While the classical computer works with “bits”, information stored in a binary way (with 0s and 1s), a quantum computer works with “qubits”, which consist of superpositions of states between 0 and 1. In short, in instead of exploring each of the paths one by one, a quantum computer is capable of exploring all the paths at the same time. And therefore to calculate much faster.

2 – What are the promises of the quantum computer?

The holy grail of data processing, the quantum computer would be able to process gigantic masses of data and perform operations beyond the imagination. Google thus claims to have developed a processor capable of performing a three-minute operation in which the most advanced of today’s computers would have taken 10,000 years.

This type of computer could in particular “use the Shor algorithm, which makes it possible to break the RSA-type cryptography system, the system that currently secures our communication on the Internet”, emphasizes Nicolas Sangouard, research director for CEA at the Department of Theoretical Physics at CEA- Paris-Saclay.

Inventing molecules, optimizing logistics … No matter how complex calculations are involved, from finance to industry, the quantum computer would make it possible to change the situation. Especially since other applications of quantum technology are taking shape along the way, such as those for quantum communication or quantum sensors.

One promising area is simulation of new materials. “The quantum computer could allow us to produce a superconducting material at room temperature and therefore transfer electricity without loss, which would be an incredible advance,” says Nicolas Sangouard, for example, or even designing a catalyst that allows nitrogen to be converted to ammonia at room temperature, and therefore “to produce nitrogen fertilizer with very little energy”, a revolution for agriculture.

3 – Will such computers see the light of day?

Many researchers are convinced of this, while others have several reservations given the technological challenges that need to be addressed. Quantum states are extremely fragile, the very act of observing them disturbs them. To manipulate qubits, it is generally necessary to use cold atoms, captured and cooled by laser (at a temperature adjacent to -273 ° C, absolute zero), which requires extensive equipment.

The more qubits there are, the harder it is to control them all. This leads to errors in the calculation systems. “Upscaling remains a challenge in itself,” Nicolas Sangouard insists. But to run Shor’s algorithm, for example, it would be necessary to be able to control thousands of qubits.

The universal quantum computer is therefore not for tomorrow, but “if we continue to invest so much money and so much energy in it, we will have one”, he maintains. “But will we have the patience to invest so much for another fifteen or twenty years?”

4 – Where are we in this race at the moment?

What we know how to do today are small prototypes. “Ten or twenty qubits are available via the cloud, and the largest machines are on the order of one hundred qubits,” the scientist sums up.

Some large companies like IBM make “small quantum computers” available online. “Theorists like me have the ability to give them interesting algorithms to run,” the scientist explains. On the other hand, Google generally reserves its machines for its employees.

Institutes like CEA are also trying to develop their own machine. “One of the options considered is to buy small machines developed by start-ups and try to combine them with classic computers, which are already very powerful, to boost them,” he explains.

5 – Who are the main players in this quantum race?

China is leading the dance thanks to “colossal investments made over the last ten years”. “They are really at the forefront, for example in quantum communications, where they have done satellite cryptography, which no one else can do yet,” Nicolas Sangouard emphasizes.

GAFA is also far ahead in this race. “We have seen them start investing in research centers of hundreds of people, which cost them tens of millions of millions a year,” the researcher notes.

Finally, Europe is starting to come out of its torpor. In total, it is planned to invest at least 4.5 billion euros in 2027 in quantum technologies. Member States are also making individual efforts.

The latest example to date, France announced in mid-January that it intends to invest 1.8 billion euros over five years in new quantum technologies. “I really hope this plan will make up for some of the delay,” the scientist stressed. It can already count on its many renowned quantum specialists, including several Nobel Prize winners.

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