Superconducting microwave filters and filter elements for quantum devices

A solid-state quantum device comprising:

• one or more quantum devices formed on or in a semiconductor substrate;
• one or more electrodes connecting bonding pads on the substrate to the one or more quantum devices;
• wherein each of the one or more electrodes includes at least one low-pass microwave filter, each said low-pass microwave filter comprising a superconducting inductor structure and a capacitor structure for connecting an electrode of one of the one or more quantum devices to a ground;
• the at least one superconducting inductor structure including one or more nanowires of a superconducting material,
• the at least one capacitor structure comprising one or more first capacitor electrodes and one or more second capacitor electrodes,
• one end of each of the one or more nanowires is galvanically connected to one of the one or more first capacitor electrodes, and
• the cross-sectional dimensions of each of the one or more nanowires are selected such that a kinetic inductance of each of the one or more nanowires is larger, than a geometrical inductance of each of the one or more nanowires.

A method of fabricating a quantum device comprising:

• forming a thin-film heterostructure over a substrate;
• forming a superconducting layer over the thin-film heterostructure to form a thin-film stack provided on the substrate;
• forming a first patterned resist layer over the thin-film stack, the first patterned resist layer including a capacitor pattern, an inductor pattern, a connection pattern connecting the capacitor and inductor pattern and quantum device pattern, the inductor pattern including a nanowire pattern;
• forming a capacitor structure, an inductor structure, a connection structure galvanically connecting the inductor structure with the capacitor structure and a quantum device structure in the thin-film stack using an etching step;
• forming a second patterned resist layer over the capacitor structure exposing the top surface of the capacitor structure;
• and,
• forming a thin-film capacitor, the forming of the thin-film capacitor including forming a dielectric layer and a metal layer over the second patterned resist layer and removing the second patterned resist layer,
• wherein a width of the nanowire and a thickness of the superconducting layer are selected such that a kinetic inductance of the nanowire structure is larger than a geometrical inductance of the nanowire structure.