Observation of quadrupole topological insulators

Many topological phenomena can be understood as a symmetry-induced quantization of the Berry phase, which describes the charge polarization inside the unit cell. This theory relating topology and polarization has been recently extended to higher-order multipolar moments. In this talk, we will discuss experimental observations of a multipolar (quadrupole) topological insulator and quadrupole transitions in phononic and LC-circuit platforms. Our measurements confirm the theoretical predictions and provide a route towards protected wave guides in three dimensional materials.