Characterizing the backaction of the Josephson Bifurcation Amplifier on the Quantronium qubit

We have constructed a novel amplifier - the Josephson Bifurcation Amplifier (JBA)- for reading out a superconducting quantum bit (Quantronium). The amplifier exploits the non-linearity of the Josephson junction to achieve high speed and single shot operation. The JBA is based on the switching between two dynamical states of an rf-driven Josephson junction biased near a dynamical bifurcation. I will present experimental results which characterize the backaction of the JBA on the qubit during the measurement process. The high speed readout (repetition rate of 5 - 10 MHz) allows us to make successive measurements of the quantum state and study the correlations between them.