  We develop a technique to overcome phase-mismatch in Josephson-junction
traveling wave parametric amplifiers in order to achieve high gain over a broad
bandwidth. Using "resonant phase matching," we design a compact superconducting
device consisting of a transmission line with subwavelength resonant inclusions
that simultaneously achieves a gain of 20 dB, an instantaneous bandwidth of 3
GHz, and a saturation power of -98 dBm. Such an amplifier is well-suited to
cryogenic broadband microwave measurements such as the multiplexed readout of
quantum coherent circuits based on superconducting, semiconducting, or
nano-mechanical elements as well as traditional astronomical detectors.
