  We have constructed a new type of amplifier whose primary purpose is the
readout of superconducting quantum bits. It is based on the transition of an
RF-driven Josephson junction between two distinct oscillation states near a
dynamical bifurcation point. The main advantages of this new amplifier are
speed, high-sensitivity, low back-action, and the absence of on-chip
dissipation. Using pulsed microwave techniques, we demonstrate bifurcation
amplification in nanofabricated Al junctions and verify that the performance
predicted by theory is attained.
