FIG. 6 shows a plot of the distribution of the lowest frequency electromagnetic mode 21 generated when the substrate 2 is placed within the holder 3 and the cover 4 shown in FIGS. 1-4. Owing to the projections 12 of the holder 3, which pass through the thickness of the substrate 2 and thus are positioned between the circuit elements (including the qubits 6) of the quantum computing circuit 5 when the substrate 2 is placed onto the surface 9 of the holder 3, it can be seen from FIG. 6 that a lowest electromagnetic mode 21 is still generated during operation of the quantum computing circuit 5, but it is confined to the volumes between each set of adjacent projections 12. This mode is at around 20 GHz, and so is unlikely to interfere with the qubits 6 or cause cross-talk between them, owing to the large detuning from their operational frequencies. Additionally, the projections will act to localise circuit modes into the nearest volumes between each set of adjacent projections 12, further helping to reduce cross-talk between the qubits 6.