Adiabatic process

Topology (electrical circuits)

Topological insulator

Mathematics

Quadrupole

Multipole expansion

Quantum mechanics

Insulator (electricity)

Condensed matter physics

Optoelectronics

Topological degeneracy

Combinatorics

Dipole

Quantum

Physics

Geometric phase

Topological order

Symmetry protected topological order

Nature

A quantized quadrupole topological insulator composed of capacitively coupled microwave resonators has corner states that are protected by bulk topology and exhibit exceptional robustness against edge deformation. The properties of many materials with topological band structures can be understood in terms of a quantization of the electric polarization. A new class of higher-order topological insulators has recently been predicted by considering a quantization of higher-order polarizations. Christopher Peterson et al. now use a metamaterial composed of coupled microwave resonators to demonstrate such a system experimentally: a quantized quadrupole topological insulator that has corner states that are protected by topology. By deforming one of the edges from the topological to the trivial regime, they show that these corner states move inward to the corners of the newly generated boundaries, confirming that they are protected by the topology of the bulk. Such demonstrations not only provide evidence of a unique...

A quantized microwave quadrupole insulator with topologically protected corner states