It will be appreciated that when the circuit elements are arranged with the control line and/or the readout element out of plane with respect to the qubit, e.g. as outlined in the Applicant's previous application published as WO 2017/021714 A1, providing apertures in the holder for the control line and/or the readout element (and, e.g., their connecting wires) to pass through, helps to provide a compact and neatly arranged package. Thus embodiments of the present invention lend themselves well to accommodating off-chip control wiring (and quantum computing circuits including such wiring). Such configurations may be considered more scalable than those that implement on-chip control wiring.

The one or more apertures in the holder are preferably arranged between (e.g. to intersperse) the one or more projections, e.g. so that they correspond to the circuit elements (e.g. qubits) of the quantum computing circuit formed on the substrate. This helps to connect up the circuit elements of the quantum computing circuit to off-chip control wiring.

The cover is positioned opposite the substrate from the holder, e.g. the substrate is sandwiched between the holder and the cover. The cover helps to shield the circuit elements from external electromagnetic radiation and to further reduce the spurious electromagnetic modes able to be established. In one embodiment the cover is arranged with the holder to enclose the substrate, e.g. when the holder comprises a rim.

The cover may be any suitable and desired size. Preferably the cover extends at least beyond the perimeter of the substrate (e.g. the surface of the cover that faces the substrate has dimensions that are greater than or equal to the corresponding dimension of the face of the substrate that faces the cover).