  Semiconductor-superconductor hybrid systems have outstanding potential for
emerging high-performance nanoelectronics and quantum devices. However,
critical to their successful application is the fabrication of high-quality and
reproducible semiconductor-superconductor interfaces. Here, we realize and
measure axial Al-Ge-Al nanowire heterostructures with atomically precise
interfaces, enwrapped by an ultrathin epitaxial Si layer further denoted as
Al-Ge/Si-Al nanowire heterostructures. The heterostructures were synthesized by
a thermally induced exchange reaction of single-crystalline Ge/Si core/shell
nanowires and lithographically defined Al contact pads. Applying this
heterostructure formation scheme enables self-aligned quasi one-dimensional
crystalline Al leads contacting ultrascaled Ge/Si segments with contact
transparencies greater than 96%. Integration into back-gated field-effect
devices and continuous scaling beyond lithographic limitations allows us to
exploit the full potential of the highly transparent contacts to the 1D hole
gas at the Ge-Si interface. This leads to the observation of ballistic
transport as well as quantum confinement effects up to temperatures of 150 K.
Low-temperature measurements reveal proximity-induced superconductivity in the
Ge/Si core/shell nanowires. The realization of a Josephson field-effect
transistor allows us to study the subgap structure caused by multiple Andreev
reflections. Most importantly, the absence of a quantum dot regime indicates a
hard superconducting gap originating from the highly transparent contacts to
the 1D hole gas, which is potentially interesting for the study of Majorana
zero modes. Moreover, underlining the importance of the proposed thermally
induced Al-Ge/Si-Al heterostructure formation technique, our system could
contribute to the development of key components of quantum computing such as
gatemon or transmon qubits
