  We use a Ge-Si core-shell nanowire to realise a Josephson field-effect
transistor with highly transparent contacts to superconducting leads. By
changing the electric field we gain access to two distinct regimes not combined
before in a single device: In the accumulation mode the device is highly
transparent and the supercurrent is carried by multiple subbands, while near
depletion supercurrent is carried by single-particle levels of a strongly
coupled quantum dot operating in the few-hole regime. These results establish
Ge-Si nanowires as an important platform for hybrid
superconductor-semiconductor physics and Majorana fermions.
