Experimental verification of an indefinite causal order.

Investigating the role of causal order in quantum mechanics has recently revealed that the causal relations of events may not be a priori well defined in quantum theory. Although this has triggered a growing interest on the theoretical side, creating processes without a causal order is an experimental task. We report the first decisive demonstration of a process with an indefinite causal order.

Observation of photonic anomalous Floquet topological insulators.

Topological insulators are a new class of materials that exhibit robust and scatter-free transport along their edges - independently of the fine details of the system and of the edge - due to topological protection. To classify the topological character of two-dimensional systems without additional symmetries, one commonly uses Chern numbers, as their sum computed from all bands below a specific bandgap is equal to the net number of chiral edge modes traversing this gap. However, this is strictly valid only in settings with static Hamiltonians.

Observation of a Topological Transition in the Bulk of a Non-Hermitian System.

We present the first experimental observation of a topological transition in a non-Hermitian system. In contrast to standard methods for examining topological properties, which involve probing edge (or surface) states, we monitor the topological transition by employing bulk dynamics only. The system is composed of a lattice of evanescently coupled optical waveguides, and non-Hermitian behavior is engineered by inducing bending loss by spatially "wiggling" every second waveguide.

Topological creation and destruction of edge states in photonic graphene.

We experimentally demonstrate a topological transition of classical light in "photonic graphene": an array of waveguides arranged in the honeycomb geometry. As the system is uniaxially strained (compressed), the two unique Dirac points (present in the spectrum of conventional graphene) merge and annihilate each other, and a band gap forms. As a result, edge states are created on the zigzag edge and destroyed on the bearded edge.

Edge states in disordered photonic graphene.

The impact of uncorrelated composite and structural disorder on the edge states of photonic graphene (a honeycomb waveguide lattice) is investigated numerically and experimentally. We find that in the case of structural (off-diagonal) disorder, the chiral symmetry preserves the confinement of the zero-energy edge state in contrast to composite disorder.