Contrary to its classical counterpart, a quantum computer uses quantum bits to compute. A quantum bit (qubit) is any physical system that has two distinct states noted and , just as a classical bit, but with the crucial property that these two states can be put into a quantum superposition. In the 80s, physicists started wondering what could be gained with this intriguing feature. They developed the concept of a quantum computer and realized that it could be extremely efficient in solving chemistry problems or certain types of problems such as factoring prime numbers. Forty years later, quantum computers attract more and more attention as we are on the verge of harnessing their exponential computing power.

## What can a quantum computer

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### MAIN CHALLENGE

## Why is it difficult?

The main challenge in building a quantum computer is that the exotic features of quantum mechanics are extremely sensitive. They vanish in a process known as decoherence due to unwanted interactions with our classical world.

**Think of the Schrödinger’s cat thought experiment :**

A cat is placed in a sealed box and put in a quantum superposition of dead and alive. When the box is opened, the superposed cat collapses randomly into one of the two possible states, either dead or alive. It is the interaction of the cat with our classical world that destroyed the quantumness of the cat.

Building a quantum computer hence means designing an isolated box in which we perform quantum algorithms. But, to be able to run them, the computer must be controllable. Hence the main paradox of this extraordinary quest : isolating part of our universe while controlling it at the same time.

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### decoherence

## Our philosophy

Decoherence leads to errors during computation.

More precisely, it randomly switches the phase of quantum superpositions inducing errors called phase-flips. Quantum bits also suffer from a “classical” error, the bit-flip that randomly swap 0s and 1s.

Surprisingly, the bit-flip rate in quantum systems is many orders of magnitude higher than in classical ones. There is no fundamental reason for such a discrepancy : the only error intrinsically tied to quantum systems is the phase-flip. We have designed a pioneering quantum bit, the cat qubit, that is presumably as insensitive to bit-flips as a classical bit while remaining both coherent and controllable. This way only phase-flips require to be actively corrected. It drastically simplifies the design of the ideal quantum computer.

Cat qubit are just the tip of the iceberg of a new generation of quantum bits, elegantly designed to be inherently robust to errors and scalable. Even though they might not be the end of the game, cat qubit should already enable the design of quantum computers with error rates so low, that most envisioned applications will be within our reach.