Invisible tunneling through non-Hermitian barriers in nonreciprocal lattices

We report a peculiar tunneling phenomenon that occurs in lattices with nonreciprocal couplings. The nonreciprocity holds for an inner portion of the lattice, constituting a non-Hermitian interface between outer Hermitian sections. The couplings are mirrored about the interface center. As a standalone system that was widely studied in recent years, each section of the interface supports the non-Hermitian skin effect, in which modes are accumulated at one boundary. Here, we investigate what happens to a wave that propagates along the lattice and hits the interface. The skin mode accumulation, which effectively constitutes a barrier, forbids wave penetration into the interface, but surprisingly, under certain conditions, the wave is transmitted to the other side, keeping the interface dark as if the wave invisibly tunneled through it. Remarkably, the tunneling is independent of the interface length, and a perfect transmission can be achieved independently of frequency and nonreciprocity strength. We derive the phenomenon both for quantum and classical systems, and realize it experimentally in an active topoelectric metamaterial. Our study fosters the research of wave tunneling through other types of non-Hermitian interfaces, which may also include nonlinearities, time dependence, and more.