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L’ordinateur quantique : qu’est-ce que c’est ?

Les ordinateurs quantiques apparaissent comme la nouvelle frontière de l’informatique. Via l’exploitation des caractéristiques des particules subatomiques, ils promettent un bond en avant en termes de performances et de rapidité. Ils rendront possibles des modélisations (celle de l’univers par exemple) ou des chiffrements très avancées. L’informatique quantique s’appuie sur des qubits, des unités de stockage d’un nouveau genre. Alors qu’en informatique « classique », un bit peut prendre les valeurs « 0 » ou « 1 », en informatique quantiquele qubit peut avoir une valeur de « 0 », « 1 » ou bien les deux en même temps (selon le principe de superposition). Ce qui décuple les capacités de calcul en parallèle.

Les notions de base d’un ordinateur quantique

La notion d’ordinateur quantique est née dans les années 1970 et 1980. Elle est notamment portée par le physicien américain David Wineland – qui a d’ailleurs reçu le Prix Nobel de physique pour ses recherches dans ce domaine. Derrière ce concept figure l’idée selon laquelle les phénomènes quantiques pourraient être utilisés pour décupler la puissance de calcul des machines classiques. Alors que les ordinateurs d’aujourd’hui utilisent le bit comme unité fondamentale, le calculateur quantique, lui, s’adosse sur une valeur appelée le qubit (ou quantum bit). Une valeur qui permettrait non seulement de s’exprimer par le biais de 1 et 0, mais également par une superposition des deux.

Mais comment alors créer un qubit d’information ? Très schématiquement, le fonctionnement d’un qubit (que l’on peut comparer à un atome artificiel) repose sur différents phénomènes de physique nucléaire. C’est aussi la raison pour laquelle l’ordinateur est dit quantique. Il fait en effet référence à la physique quantique décrivant le comportement des atomes et particules. Quels sont ces phénomènes nucléaires ? Un qubit peut être créé en s’appuyant par exemple sur la polarisation d’un photon (et la mesure du plan de polarisation), ou bien sur le niveau d’énergie d’un atome, ou les moments cinétiques de « spin », ou révolution, de l’électron autour de lui.

Comment passer du bit au qubit ?

Il existe plusieurs techniques pour créer des qubits. Grâce aux supraconducteurs, ils peuvent être embarqués sur des circuit électroniques dotés de semi-conducteurs fabriqués à partir de méthodes déjà connues, empruntées à la nanoélectronique.

Un obstacle de taille : le système doit rester totalement isolé le temps du calcul

Reste un obstacle technique de taille avant de faire fonctionner un ordinateur quantique. Le système, pendant la phase de calcul, doit être totalement isolé de toute interférence thermique ou magnétique. Du fait de la physique nucléaire qui le met en musique, le qubit est en effet sensible à toute perturbation, notamment les champs électromagnétiques. Les chercheurs appellent le temps de « décohérence » la période pendant laquelle le système reste ainsi isolé, l’intégrité de ses propriétés quantiques étant alors complètes.

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