  We experimentally simulate in a photonic setting non-Hermitian (NH) metals
characterized by the topological properties of their nodal band structures.
Implementing nonunitary time evolution in reciprocal space followed by
interferometric measurements, we probe the complex eigenenergies of the
corresponding NH Bloch Hamiltonians, and study in detail the topology of their
exceptional lines (ELs), the NH counterpart of nodal lines in Hermitian
systems. We focus on two distinct types of NH metals: two-dimensional systems
with symmetry-protected ELs, and three-dimensional systems possessing
symmetry-independent topological ELs in the form of knots. While both types
feature open Fermi surfaces, we experimentally observe their distinctions by
analyzing the impact of symmetry-breaking perturbations on the topology of ELs.

Quantum Physics;Mesoscale and Nanoscale Physics;Optics