Channel cross correlations in transport through complex media

Measuring transmission between four antennas in microwave cavities, we investigate directly the channel cross correlations $C$ of the cross sections ${\ensuremath{\sigma}}^ab$ from antenna at ${\stackrel{P\vec}{r}}_a$ to antenna ${\stackrel{P\vec}{r}}_b$. Specifically we look for the ${C}_{\mathrm{\ensuremath{\Sigma}}}$ and ${C}_{\mathrm{\ensuremath{\Lambda}}}$, where the only difference is that ${C}_{\mathrm{\ensuremath{\Lambda}}}$ has none of the four channels in common, whereas ${C}_{\mathrm{\ensuremath{\Sigma}}}$ has exactly one channel in common. We find experimentally that these two channel cross correlations are antiphased as a function of the channel coupling strength, as predicted by theory. This anticorrelation is essential to obtain the correct values for the universal conductance fluctuations. To obtain good agreement between experiment and predictions from random matrix theory the effect of absorption has to be included.