Seminar by M. Brunelli, April 8 at 15:00

Speaker  Dr. Matteo Brunelli, research fellow at University of Basel

Title  Non-Hermitian topological amplification


Non-Hermitian (NH) Hamiltonians enable novel topological phases of matter, characterized by unique kinds of degeneracies and extreme sensitivity to changes of boundary conditions—the so-called NH skin effect. Many aspects of NH topological phases are not yet fully understood. As an example, the NH skin effect blurs the separation between edge and bulk states, which determines the breakdown of the bulk-boundary correspondence.

In this talk, I will present an alternative route to the classification of NH topological phases for driven-dissipative bosonic systems, which unveils a deep connection between NH topology and amplification. In this approach, the NH Hamiltonian is obtained from the mean-field evolution of a driven-dissipative bosonic lattice, in which NH terms—in the form of gain, loss, and non-reciprocal couplings—are explicitly modeled via coupling to reservoirs. As a consequence of this approach, I will show that the NH skin effect does not have a topological origin and NH topology manifests instead as directional amplification of a coherent input with gain exponential in system size. I will use this formalism to restore the bulk-boundary correspondence for the most paradigmatic class of NH Hamiltonians, namely those with a point-gapped spectrum. Finally, I will discuss a recent optomechanical experiment that has observed NH topological amplification in the bosonic version of the Kitaev model of topological superconductivity.