understanding the classical cross-talk we will be only interested in the relative magnitudes of the voltages seen by different qubits and according to the analysis in Appendix

$$X_{ij}=20\mathrm{log}_{10}\left(\frac{\mathrm{Im}[Z_{i,d(j)}(\omega_{i})]}{\mathrm{Im}[Z_{j,d(j)}(\omega_{j})]}\right)\label{eq:Classical-crosstalk-formula}$$

is a good measure of the classical cross-talk in units of $dB$. Here $Z_{i,d(j)}(\omega_{i})$ is the impedance entry connecting the drive port $d(j)$ of the qubit $j$ to the qubit port $i$.

Although we have already stated in the previous sections that we defined the drive ports where the drive lines reach the chip we give a more precise description here on how we choose the locations of the drive ports. As the drive signals travel over the transmission lines towards the chip they will eventually reach the transition region (before launching onto the chip) where they will no longer see a constant impedance but a discontinuity off which some portion of the signal will be reflected back. Ideally one would like to define the drive ports at positions where this discontinuity first starts to appear. The exact positions can be determined with a TDR (time-domain reflectometry) measurement/simulation for example. In the absence of TDR information one can make a safe choice by keeping the drive ports far enough from the chip 