  We present a theoretical model and experimental characterization of a
microwave kinetic inductance traveling-wave amplifier (KIT), whose noise
performance, measured by a shot-noise tunnel junction (SNTJ), approaches the
quantum limit. Biased with a dc current, the KIT operates in a three-wave
mixing fashion, thereby reducing by several orders of magnitude the power of
the microwave pump tone and associated parasitic heating compared to
conventional four-wave mixing KIT devices. It consists of a 50 Ohms artificial
transmission line whose dispersion allows for a controlled amplification
bandwidth. We measure $16.5^{+1}_{-1.3}$ dB of gain across a 2 GHz bandwidth
with an input 1 dB compression power of -63 dBm, in qualitative agreement with
theory. Using a theoretical framework that accounts for the SNTJ-generated
noise entering both the signal and idler ports of the KIT, we measure the
system-added noise of an amplification chain that integrates the KIT as the
first amplifier. This system-added noise, $3.1\pm0.6$ quanta (equivalent to
$0.66\pm0.15$ K) between 3.5 and 5.5 GHz, is the one that a device replacing
the SNTJ in that chain would see. This KIT is therefore suitable to read large
arrays of microwave kinetic inductance detectors and promising for multiplexed
superconducting qubit readout.

Quantum Physics;Instrumentation and Methods for Astrophysics