  Semiconductor nanowires provide an ideal platform for various low-dimensional
quantum devices. In particular, topological phases of matter hosting
non-Abelian quasi-particles can emerge when a semiconductor nanowire with
strong spin-orbit coupling is brought in contact with a superconductor. To
fully exploit the potential of non-Abelian anyons for topological quantum
computing, they need to be exchanged in a well-controlled braiding operation.
Essential hardware for braiding is a network of single-crystalline nanowires
coupled to superconducting islands. Here, we demonstrate a technique for
generic bottom-up synthesis of complex quantum devices with a special focus on
nanowire networks having a predefined number of superconducting islands.
Structural analysis confirms the high crystalline quality of the nanowire
junctions, as well as an epitaxial superconductor-semiconductor interface.
Quantum transport measurements of nanowire "hashtags" reveal Aharonov-Bohm and
weak-antilocalization effects, indicating a phase coherent system with strong
spin-orbit coupling. In addition, a proximity-induced hard superconducting gap
is demonstrated in these hybrid superconductor-semiconductor nanowires,
highlighting the successful materials development necessary for a first
braiding experiment. Our approach opens new avenues for the realization of
epitaxial 3-dimensional quantum device architectures.
