  We realize and characterize a quantum-limited, directional Josephson
amplifier suitable for qubit readout. The device consists of two nondegenerate,
three-wave-mixing amplifiers that are coupled together in an interferometric
scheme, embedded in a printed circuit board. Nonreciprocity is generated by
applying a phase gradient between the same-frequency pumps feeding the device,
which plays the role of the magnetic field in a Faraday medium. Directional
amplification and reflection-gain elimination are induced via wave interference
between multiple paths in the system. We measure and discuss the main figures
of merit of the device and show that the experimental results are in good
agreement with theory. An improved version of this directional amplifier is
expected to eliminate the need for bulky, off-chip isolation stages that
generally separate quantum systems and preamplifiers in high-fidelity,
quantum-nondemolition measurement setups.
