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 . <EOS>