A method for forming a qubit device, the method comprising: forming a gate electrode embedded in an insulating layer formed on a substrate, wherein an upper surface of the substrate is formed by a group IV semiconductor material and wherein the gate electrode extends along the substrate and is formed to be elongated 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 said exposed 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 said upper portion of the substrate contact part, horizontally along said upper surface of the insulating layer to overlap a portion of the gate electrode, forming a mask covering a portion of the semiconductor disc part, said covered portion of the semiconductor disc part being elongated and extending across said portion of the gate electrode in a second horizontal direction, different from said first horizontal direction, wherein the substrate contact part is exposed by said mask, etching the regions of the semiconductor structure exposed by the mask such that the masked portion of the semiconductor disc part remains to form a channel structure elongated in said second horizontal direction and extending across said portion of the gate electrode, and forming a superconductor source contact and a superconductor drain contact on the channel structure at mutually opposite sides of said portion of the gate electrode. forming a gate electrode embedded in an insulating layer formed on a substrate, wherein an upper surface of the substrate is formed by a group IV semiconductor material and wherein the gate electrode extends along the substrate and is formed to be elongated 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 said exposed 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 said upper portion of the substrate contact part, horizontally along said upper surface of the insulating layer to overlap a portion of the gate electrode, the substrate contact part having a bottom portion abutting said exposed 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 said upper portion of the substrate contact part, horizontally along said upper surface of the insulating layer to overlap a portion of the gate electrode, forming a mask covering a portion of the semiconductor disc part, said covered portion of the semiconductor disc part being elongated and extending across said portion of the gate electrode in a second horizontal direction, different from said first horizontal direction, wherein the substrate contact part is exposed by said mask, etching the regions of the semiconductor structure exposed by the mask such that the masked portion of the semiconductor disc part remains to form a channel structure elongated in said second horizontal direction and extending across said portion of the gate electrode, and forming a superconductor source contact and a superconductor drain contact on the channel structure at mutually opposite sides of said portion of the gate electrode.