Observation of Chiral State Transfer Without Encircling an Exceptional Point

TL;DR

This paper experimentally demonstrates chiral state transfer in a non-Hermitian system without encircling an exceptional point, challenging previous assumptions about the role of exceptional points in topological state conversion.

Contribution

First experimental observation of chiral state transfer in a non-Hermitian system excluding exceptional points, using a fiber-based photonic emulator to explore non-Hermitian dynamics.

Findings

Chiral state transfer occurs without encircling an exceptional point.

The behavior is influenced by the Riemann surface landscape, not just encirclement.

Provides new insights into the adiabatic theorem in non-Hermitian systems.

Abstract

The adiabatic theorem, a corollary of the Schr\"odinger equation, manifests itself in a profoundly different way in non-Hermitian arrangements, resulting in counterintuitive state transfer schemes that have no counterpart in closed quantum systems. In particular, the dynamical encirclement of exceptional points (EPs) in parameter space has been shown to lead to a chiral phase accumulation, non-adiabatic jumps, and topological mode conversion [1- 8]. Recent theoretical studies, however, have shown that contrary to previously established demonstrations, this behavior is not strictly a result of winding around a non-Hermitian degeneracy [9]. Instead, it appears to be mostly attributed to the non-trivial landscape of the Riemann surfaces, sometimes because of the presence of an exceptional point in the vicinity [9- 11]. In an effort to bring this counterintuitive aspect of non-Hermitian systems into light and confirm this hypothesis, we provide here the first set of experiments to directly observe the field evolution and chiral state conversion in an EP-excluding cycle in a slowly varying non- Hermitian system. To do so, a versatile yet unique fiber-based photonic emulator is realized that utilizes the polarization degrees of freedom in a quasi-common path single-ring arrangement. Our observations may open up new avenues for light manipulation and state conversion, while providing a foundation for understanding the intricacies of the adiabatic theorem in non-Hermitian systems.