Radio-frequency single-electron transistor as readout device for qubits: Charge sensitivity and backaction

We study the radio-frequency single-electron transistor (rf-SET) as a reado ut device for charge qubits. We measure the charge sensitivity of an rf-SET to be 6.3 mue/root Hz and evaluate the backaction of the rf-SET on a singl e Cooper-pair box. This allows us to compare the needed measurement time wi th the mixing time of the qubit imposed by the measurement. We find that th e mixing time can be substantially longer than the measurement time, which would allow readout of the state of the qubit in a single shot measurement.