  We investigated the spectra of resonances of four-vertex microwave networks
simulating both quantum graphs with preserved and with partially violated
time-reversal invariance before and after an edge switch operation. We show
experimentally that under the edge switch operation the spectra of the
microwave networks with preserved time reversal symmetry are level-1
interlaced, i.e., $\nu_{n-r}\leq \tilde \nu_{n}\leq \nu_{n+r}$, where $r=1$, in
agreement with the recent theoretical predictions of [M. Aizenman, H. Schanz,
U. Smilansky, and S. Warzel, Acta Phys. Pol. A {\bf 132}, 1699 (2017)]. Here,
we denote by $\{\nu_{n}\}_{n=1}^{\infty}$ and $\{\tilde
\nu_{n}\}_{n=1}^{\infty}$ the spectra of microwave networks before and after
the edge switch transformation. We demonstrate that the experimental
distribution $P(\Delta N)$ of the spectral shift $\Delta N$ is close to the
theoretical one. Furthermore, we show experimentally that in the case of the
four-vertex networks with partially violated time reversal symmetry the spectra
are level-1 interlaced. Our experimental results are supplemented by the
numerical calculations performed for quantum graphs with violated time-reversal
symmetry. In this case the edge switch transformation also leads to the spectra
which are level-1 interlaced. Moreover, we demonstrate that for microwave
networks simulating graphs with violated time-reversal symmetry the
experimental distribution $P(\Delta N)$ of the spectral shift $\Delta N$ agrees
within the experimental uncertainly with the numerical one.
