  Nonreciprocal microwave devices play several critical roles in high-fidelity,
quantum-nondemolition (QND) measurement schemes. They separate input from
output, impose unidirectional routing of readout signals, and protect the
quantum systems from unwanted noise originated by the output chain. However,
state-of-the-art, cryogenic circulators and isolators are disadvantageous in
scalable superconducting quantum processors because they use magnetic materials
and strong magnetic fields. Here, we realize an active isolator formed by
coupling two nondegenerate Josephson mixers in an interferometric scheme.
Nonreciprocity is generated by applying a phase gradient between the
same-frequency pumps feeding the Josephson mixers, which play the role of the
magnetic field in a Faraday medium. To demonstrate the applicability of this
Josephson-based isolator for quantum measurements, we incorporate it into the
output line of a superconducting qubit, coupled to a fast resonator and a
Purcell filter. We also utilize a wideband, superconducting directional coupler
for coupling the readout signals into and out of the qubit-resonator system and
a quantum-limited Josephson amplifier for boosting the readout fidelity. By
using this novel quantum setup, we demonstrate fast, high-fidelity, QND
measurements of the qubit while providing more than 20 dB of protection against
amplified noise reflected off the Josephson amplifier.
