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.

Abstract: In a general aspect, a quantum processor has a modular architecture. In some aspects, a modular quantum processor includes first and second quantum processor chips and a cap structure. The first quantum processor chip is supported on a substrate layer and includes a first plurality of qubit devices. The second quantum processor chip is supported on the substrate layer and includes a second plurality of qubit devices. The cap structure is supported on the first and second quantum processor chips and includes a coupler device that provides coupling between at least one of the first plurality of qubit devices with at least one of the second plurality of qubit devices. In some instances, the coupler device is an active coupler device that is configured to selectively couple at least one of the first plurality of qubit devices with at least one of the second plurality of qubit devices.