During operation of the quantum computing circuit 5, electromagnetic modes are established over the substrate 2. Were the quantum computing circuit 5 to be formed on a substrate 2 not having holes therethrough (and thus projections not passing therethrough), the distribution of the lowest frequency electromagnetic mode 20 generated (in one particular mode of operation) is that shown in the plot of FIG. 5. This distribution of the lowest frequency mode 20 shows that the mode 20 generated extends over the whole of the substrate 2 and thus passes between the qubits 6 of the quantum computing circuit 5. This mode is at around 7 GHz and so is likely to result in interference with, and cross-talk between, the qubits 6.

The frequency spectra of the four lowest frequency electromagnetic modes generated during operation of the quantum computing circuit in this configuration (i.e. without holes and projections passing through the substrate 2) is shown in FIG. 7. This shows that the four lowest frequency chip-modes 22 generated are in the range between 6 GHz and 14 GHz. Thus these frequencies of the spurious electromagnetic modes 20, 22 overlap with the microwave frequencies used for operating the quantum computing circuit 5, which are in the range 4 GHz to 12 GHz (as shown in FIG. 7). The spurious electromagnetic modes 20, 22 at these frequencies and the cross-talk between the qubits 6, owing to the distribution of these spurious modes 20, 22, interferes with the operation of the quantum computing circuit 5 resulting in a detrimental lack of coherence in the qubits 6.