  Parallel one-dimensional semiconductor channels connected by a
superconducting strip constitute the core platform in several recent quantum
device proposals that rely e.g. on Andreev processes or topological effects. In
order to realize these proposals, the actual material systems must have high
crystalline purity and the coupling between the different elements should be
controllable in terms of their interfaces and geometry. We present a strategy
for synthesizing double InAs nanowires by the vapor-liquid-solid mechanism
using III-V molecular beam epitaxy. A superconducting layer is deposited onto
nanowires without breaking vacuum, ensuring pristine interfaces between the
superconductor and the two semiconductor nanowires. The method allows for a
high yield of merged as well as separate parallel nanowires, with full or
half-shell superconductor coatings. We demonstrate their utility in complex
quantum devices by electron transport measurements.
