  We have controlled the measurement strength of the qubit readout process by
using a Josephson bifurcation amplifier (JBA : an AC-driven SQUID). The optimum
readout pulse changes the superposition state into an eigenstate. To clarify
how a sufficiently strong measurement causes a projection phenomenon, we have
studied the projection conditions in the JBA readout. We found that the
projection occurs above the pulse height at which visibility starts to appear.
Furthermore, the projection continues to occur as readout strength increases,
even though the visibility vanishes. This result helps us understanding the JBA
readout process.
