Superconducting Qubits: Charge read-out and backaction

Doctoral thesis, 2004

For quantum computers to become reality, not only do the qubits have to be carefully engineered to be well isolated during operation time, but this also puts great strain on the measurement setup. We have looked at the backaction from a single-electron transistor (SET), operated both in the normal and in the superconducting state, on a single Cooper-pair box qubit. We have also designed a new read-out setup for a flux qubit using a Cooper-pair transistor that would allow read-out at the flux degeneracy point. In the first case we calculate the mixing time and the influence on the qubit steady-state distribution. We find that the mixing time is larger than typical experimental measurement times, indicating the possibility of single shot read-out. For the steady-state distribution of the qubit, we calculate the Coulomb staircase, finding additional structure due to the noise in the SET. For the flux qubit we design a read-out scheme involving a quantum flip-flop process, where the measurement outcome results in the presence or absence of an incoherent tunneling process. We estimate read-out and mixing times and find that single shot read-out should be achievable.