# fact or fiction? What promises? What obstacles?

Supposed to solve calculations that classical computers are now unable to perform, the quantum computer makes physicists dream. But what is this…

Supposed to solve calculations that classical computers are now unable to perform, the quantum computer makes physicists dream. But what exactly is it and where is the research?

## 1. A quantum world

In our daily life, the world as we see it responds to the principles of classical physics, of Newton. Things are usually what they seem. But if we look at things at the scale of particles, atoms, we realize that there are “oddities”, explains Titouan Carette, post-doctoral fellow in quantum computing at the Center for Quantum Computer Science at the University of Riga in Latvia.

For example, as shown by Alain Aspect, two particles that merge at the beginning – which can be twins – can keep the mark of their common past and have a similar behavior regardless of the distance that separates them (quantum entanglement) . Or again, a quantum object, instead of having a specific state, is in fact in several states at the same time as long as it has not been measured (quantum superposition). This is explained by analogy with the thought experiment of Schrödinger’s cat, both dead and alive until you open the box in which it is to be examined.

These phenomena (and there are others), which seem counterintuitive to us because we cannot understand them at our scale, mean that “the quantum world is a world of probabilities that are only approximated by the classical physics summarizes Titouan Carette.

## 2. Why make a computer out of it?

Researchers quickly realized that our computers, operating in a binary system, could not really simulate these quantum theories and their infinite superimpositions, or at least very, very slowly. Because a conventional computer manipulates bits, which are in fact 0s and 1s and respectively translate the absence or the passage of an electric current. “You can think of a bit as two dots: one for 1, the other for zero. We are either on one or on the other”, describes the researcher. For such a system, the evaluation of all the possibilities governing quantum mechanics is very long, energy consuming and therefore expensive.

So why not make the computer quantum? Scientists have been working on it for decades. In a quantum computer, qubits (quantum bits) replace bits. “To understand their complexity relative to a bit, they are represented graphically in the form of a sphere, the Bloch sphere, refers to Titouan Carette. Each point on the surface corresponds to another qubit. Instead of having of two possibilities, we have infinity”. It is the additional richness of qubits that makes it possible to calculate faster.

## 3. What promises?

Such computing power opens up a realm of possibilities. Their applications are many and range from the design of new drugs and new materials to solving complex optimization problems. “For example, it would take about a year on a million computers to factorize numbers with several hundred digits such as those currently used to secure almost all bank cards and the https Internet protocol,” explained by Razvan Barbulescu, cryptography researcher at the University of Bordeaux. “But a quantum computer would only need a few hours to do this. »

Another interest may be energy. “A quantum computer can solve problems in a few hours where a supercomputer can take tens of billions of years, one would naturally expect it to consume less energy, due to the savings of this time”, wrote in The Conversation Marco Fellous-Asiani, post-doctoral fellow in quantum information at the Center of New Technologies of the University of Warsaw (Poland).

## 4. Where is the research?

At least in theory. Because in practice, the quantum computer does not exist (yet). Researchers and industrialists succeeded in making sketches. But it is “proof of concept and not a real computer capable of performing useful calculations”, underlines Razvan Barbulescu. The largest quantum computer currently contains 127 qubits. However, to make the first “real calculations”, it will take at least 1,600, the researcher estimates.

“To give an image, it seems like we are still a few years away from the first trip to the Moon. We see that it is possible, the calculations seem to work, but until we do, we cannot claim victory”. And everyone is holding their breath hoping not to run into an obstacle that we don’t even know exists at this moment.

## 5. At -273.15°C, in darkness and emptiness

Especially since quantum computers already have their share of obstacles. It needs to be kept at a temperature of -273.15°C, in an ultrahigh vacuum and in the dark because qubits are fragile and volatile. The smallest interference (heat, light, electric and magnetic fields) end their “decoherence”. And the more qubits, the faster these disturbances arrive.