The adiabatic theorem, a corollary of the Schrödinger 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 conversion1,2,3,4,5,6,7,8. Recent theoretical studies, however, have shown that contrary to previously established demonstrations, this behaviour is not strictly a result of winding around a non-Hermitian degeneracy9. Instead, it seems to be mostly attributed to the non-trivial landscape of the Riemann surfaces, sometimes because of the presence of an EP in the vicinity9,10,11. Here, in an effort to bring this counterintuitive aspect of non-Hermitian systems to light and confirm this hypothesis, we provide a 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 fibre-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, as well as providing a foundation for understanding the intricacies of the adiabatic theorem in non-Hermitian systems.
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We gratefully acknowledge the financial support from the Air Force Office of Scientific Research (Multidisciplinary University Research Initiative (MURI) Award on Novel light-matter interactions in topologically non-trivial Weyl semimetal structures and systems: FA9550-20-1-0322, MURI Award on Programmable systems with non-Hermitian quantum dynamics: FA9550-21-1-0202), DARPA (D18AP00058), the Office of Naval Research (N00014-19-1-2052, N00014-20-1-2522, MURI Award on Classical entanglement in structured optical fields: N00014-20-1-2789), the Army Research Office (W911NF-17-1-0481), the National Science Foundation (DMR-1420620, EECS-1711230, ECCS CBET 1805200, ECCS 2000538, ECCS 2011171), the W. M. Keck Foundation, the US–Israel Binational Science Foundation (BSF; 2016381), the MPS Simons collaboration (Simons grant 733682), the US Air Force Research Laboratory (FA86511820019), the Austrian Science Fund (FWF, P32300 WAVELAND) and European Commission grant MSCA-RISE 691209. G.L-G. acknowledges support from Consejo Nacional de Ciencia y Tecnologia (CONACyT). We thank A. Turchanin and U. Peschel from Friedrich Schiller University Jena for useful discussions and feedback.
The authors declare no competing interests.
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Nasari, H., Lopez-Galmiche, G., Lopez-Aviles, H.E. et al. Observation of chiral state transfer without encircling an exceptional point. Nature 605, 256–261 (2022). https://doi.org/10.1038/s41586-022-04542-2