The disclosed technology is directed to a method of forming a qubit device. In one aspect, the method comprises: forming a gate electrode embedded in an insulating layer formed on a substrate, wherein an upper surface of the substrate is formed from a group IV semiconductor material and the gate electrode extends along the substrate in a first horizontal direction; forming an aperture in the insulating layer, the aperture exposing a portion of the substrate; forming, in an epitaxial growth process, a semiconductor structure comprising a group III-V semiconductor substrate contact part and a group III-V semiconductor disc part, the substrate contact part having a bottom portion abutting the portion of the substrate and an upper portion protruding from the aperture above an upper surface of the insulating layer, the semiconductor disc part extending from the upper portion of the substrate contact part, horizontally along the upper surface of the insulating layer to overlap a portion of the gate electrode; forming a mask covering a portion of the disc part, the portion of the disc part extending across the portion of the gate electrode in a second horizontal direction; etching regions of the semiconductor structure exposed by the mask such that the masked portion of the disc part remains to form a channel structure extending across the portion of the gate electrode; and forming a superconductor source contact and a superconductor drain contact on the channel structure at opposite sides of the portion of the gate electrode.