Implémentation d'un bit quantique dans un circuit supraconducteur / Implementation of a quantum bit in a superconducting circuit
Audrey Cottet · HAL (Le Centre pour la Communication Scientifique Directe) · 2002
It has been recently realized that quantum machines could perform difficult mathematical tasks such as factoring large numbers exponentially faster than classical computers. The building blocks of a quantum computer, namely the quantum bits, are quantum two level systems which must satisfy severe requirements. Among the numerous systems proposed to implement a quantum bit, electronic circuits are appealing candidates because they are easily scalable. The purpose of this thesis was to implement a quantum bit with a superconducting circuit based on Josephson junctions, the ``Cooper pair box''. The state of this box can be determined by measuring either a current or a charge. In this thesis, three different strategies for implementing the quantum bit were studied, each one differing in the readout scheme used to measure the state of the box. For each strategy, the lifetime of a coherent superposition of quantum states is evaluated, and the associated measuring apparatus is tested experimentally. This study allows to find out the most promising strategy, which is implemented in this thesis work under the name ``quantronium''. The state of the quantronium is manipulated using microwave pulses. We have determined the life-time of a coherent superposition of states in this circuit. This time is significantly longer than the time needed for single quantum bit operations.