Abstract: In a general aspect, parametric amplification is performed in a quantum computing system. In some cases, a traveling wave parametric amplifier (TWPA) includes a plurality of Josephson junctions connected in series. The plurality of Josephson junctions includes a first Josephson junction, which includes a first superconducting electrode on a surface of a substrate, a second superconducting electrode that overlaps the first superconducting electrode, and a barrier sandwiched between overlapping sections of the first and second superconducting electrodes. The barrier defines a footprint with a tapered shape over the surface of the substrate.