超電導位相量子ビットのデコヒーレンスに関する理論的研究 <学位論文要旨>

Decoherence of a quantum bit (qubit) as a building block for quantum computer is one of the most important problem for the practical realization of quantum computing. In the previous research for the quantum computing, it is known that decoherence is caused by the relaxation processes in the qubits.

In this thesis, we study the measurement and control methods of relaxation times for superconducting phase qubit that contributes to the scalable quantum computer. Firstly, we propose a measurement method by using dynamically-controlled qubits. Secondly, we estimate the relaxation times by using the formula of the relaxation times obtained by the Bloch-Redfield theory. Thirdly, we try to control the relaxation processes in order to confirm the validity of the estimation. As a result, we can eliminate the pure dephasing process in the superconducting phase qubit. Herewith, the validity of the estimation of relaxation time is partly confirmed. We find the possibility of the variable coupling between the qubit and its environment.

The study outlined above provides a new approach to the solution of the decoherence problem underlying the practical realization of quantum computing.