  The "quantronium" is a superconducting qubit consisting of a split Cooper
pair box in which a large tunnel junction is inserted. This circuit has a
special bias point where the Larmor frequency is, to first order, insensitive
to fluctuations in the bias parameters -- the charge of the box island and the
phase of the large junction. At this optimal working point, the state of the
qubit can be determined by dispersive measurements that probe the second
derivative of the state energy with respect to these bias parameters. We use
the quantronium phase degree of freedom to perform a non-linear, dispersive
measurement of its inductive response using bifurcation amplification. This
novel readout projects the state of the qubit in a few nanoseconds, and its
latching property allows us to record the resulting information in a few
hundred nanoseconds. We have measured, using this technique, Rabi oscillations
and Ramsey fringes with an improved signal to noise ratio and contrast. The
speed of this new readout scheme also opens the door for a new class of
experiments which would characterize the relaxation processes associated with
the measurement protocol.
