A method for producing a quantum device comprising providing a substrate having a front face and carrying at least one transistor pattern on the front face thereof, said transistor pattern comprising, in a stack a gate dielectric on the front face of the substrate, and a gate on the gate dielectric, said gate having a top and sidewalls. The method further includes forming a protective layer at the front face of the substrate, said protective layer being configured to prevent diffusion of at least one metal species in the substrate, forming a metal layer that has, as a main component, at least one metal species, at least on the sidewalls of the gate, said at least one metal species comprising at least one superconducting element, and forming a superconducting region in the gate by lateral diffusion of the at least one superconducting element from the sidewalls of said gate.