title: Experimental observation of the topological structure of exceptional points.
abstract: We report on a microwave cavity experiment where exceptional points (EPs), which are square root singularities of the eigenvalues as function of a complex interaction parameter, are encircled in the laboratory. The real and imaginary parts of an eigenvalue are given by the frequency and width of a resonance and the eigenvectors by the field distributions. Repulsion of eigenvalues--always associated with EPs--implies frequency anticrossing (crossing) whenever width crossing (anticrossing) is present. The eigenvalues and eigenvectors are interchanged while encircling an EP, but one of the eigenvectors undergoes a sign change which can be discerned in the field patterns.
authors.affiliation: Institut für Kernphysik, Technische Universität Darmstadt, Germany.
authors.forename: C
authors.initials: C
authors.lastname: Dembowski
authors.forename: H
authors.initials: H
authors.lastname: Gräf
authors.forename: H L
authors.initials: HL
authors.lastname: Harney
authors.forename: A
authors.initials: A
authors.lastname: Heine
authors.forename: W D
authors.initials: WD
authors.lastname: Heiss
authors.forename: H
authors.initials: H
authors.lastname: Rehfeld
authors.forename: A
authors.initials: A
authors.lastname: Richter