Demonstration of a quantized microwave quadrupole insulator with topologically protected corner states

The modern theory of electric polarization in crystals associates the dipole moment of an insulator with a Berry phase of its electronic ground state [1, 2]. This concept constituted a breakthrough that not only solved the long-standing puzzle of how to calculate dipole moments in crystals, but also lies at the core of the theory of topological band structures in insulators and superconductors, including the quantum anomalous Hall insulator [3, 4] and the quantum spin Hall insulator [5-7], as well as quantized adiabatic pumping processes [8-10]. A recent theoretical proposal extended the Berry phase framework to account for higher electric multipole moments [11], revealing the existence of topological phases that have not previously been observed. Here we demonstrate the first member of this predicted class -a quantized quadrupole topological insulator- experimentally produced using a GHz-frequency reconfigurable microwave circuit. We confirm the non-trivial topological phase through both spectroscopic measurements, as well as with the identification of corner states that are manifested as a result of the bulk topology. We additionally test a critical prediction that these corner states are protected by the topology of the bulk, and not due to surface artifacts, by deforming the edge between the topological and trivial regimes. Our results provide conclusive evidence of a unique form of robustness which has never previously been observed.