  A crucial limit to measurement efficiencies of superconducting circuits comes
from losses involved when coupling to an external quantum amplifier. Here, we
realize a device circumventing this problem by directly embedding a two-level
artificial atom, comprised of a transmon qubit, within a flux-pumped Josephson
parametric amplifier. Surprisingly, this configuration is able to enhance
dispersive measurement without exposing the qubit to appreciable excess
backaction. This is accomplished by engineering the circuit to permit
high-power operation that reduces information loss to unmonitored channels
associated with the amplification and squeezing of quantum noise. By mitigating
the effects of off-chip losses downstream, the on-chip gain of this device
produces end-to-end measurement efficiencies of up to 80 percent. Our
theoretical model accurately describes the observed interplay of gain and
measurement backaction, and delineates the parameter space for future
improvement. The device is compatible with standard fabrication and measurement
techniques, and thus provides a route for definitive investigations of
fundamental quantum effects and quantum control protocols.
