  The act of observing a quantum object fundamentally perturbs its state,
resulting in a random walk toward an eigenstate of the measurement operator.
Ideally, the measurement is responsible for all dephasing of the quantum state.
In practice, imperfections in the measurement apparatus limit or corrupt the
flow of information required for quantum feedback protocols, an effect
quantified by the measurement efficiency. Here we demonstrate the efficient
measurement of a superconducting qubit using a nonreciprocal parametric
amplifier to directly monitor the microwave field of a readout cavity. By
mitigating the losses between the cavity and the amplifier we achieve a
measurement efficiency of $72\%$. The directionality of the amplifier protects
the readout cavity and qubit from excess backaction caused by amplified vacuum
fluctuations. In addition to providing tools for further improving the fidelity
of strong projective measurement, this work creates a testbed for the
experimental study of ideal weak measurements, and it opens the way towards
quantum feedback protocols based on weak measurement such as state
stabilization or error correction.
