In one embodiment the holder and the cover comprise (i.e. have been subject to) a surface treatment. This helps to provide a (e.g. ultra) clean surface, helping to avoid impurities or defects in the quantum computing circuit package. The surface treatment may be provided in any suitable and desired way, e.g. by etching.

The holder, substrate and cover may be manufactured in any suitable and desired way. In one embodiment one or more of the holder, the substrate and the cover may be manufactured using laser machining, computer numerical control (CNC) machining or mechanical micromachining. The holes in the substrate may be formed before or after the circuit elements are fabricated on the substrate.

Preferably the quantum computing circuit package, and the holder and cover in particular, are arranged to be cooled by a (e.g. cryogenic) cooling system, e.g. when the holder is formed from a superconductor. Thus the invention extends to a superconducting quantum computing system comprising a superconducting quantum computing circuit package according to an embodiment of the present invention, and a (e.g. cryogenic) cooling system arranged to cool the holder and the cover (e.g. to superconducting temperatures). Thus preferably the holder and the cover, which are formed from a metal or a superconductor, is cooled by the cooling system to cool the substrate (e.g. to superconducting temperatures).

It will be appreciated that, at least in preferred embodiments, the design of the holder and the cover helps to maximise the surface contact area with the substrate (on which the circuit elements are provided), which helps to enhance the thermal connectivity of the substrate to the cooling system (and thus helps to maximise the heat conduction rate from the substrate), in order to cool the substrate (e.g. to superconducting temperatures).