However, preferably the one or more projections (of the holder) extend all the way through the thickness of the substrate to contact the cover. Thus preferably the height of the one or more projections is (e.g. each) greater than or equal to the thickness of the substrate. The Applicant has found that when the projections extend all the way through the thickness of the substrate, this may be particularly effective in suppressing (e.g. substantially eliminating) electromagnetic modes in the operational frequency range of the quantum computing circuit.

When the holder comprises a recess or rim, preferably the depth of the recess or height of the rim is substantially equal to the height of the one or more projections. Thus preferably the depth of the recess or height of the rim is between 0.2 mm and 1 mm (e.g. approximately 0.5 mm).

Preferably the one or more projections are (e.g. each) between 0.2 mm and 1 mm (e.g. approximately 0.5 mm) in width (i.e. in the direction parallel to the surface of the holder). Thus preferably the one or more holes are (e.g. each) between 0.2 mm and 1 mm (e.g. approximately 0.5 mm) in width (i.e. in the direction parallel to the face of the substrate).