Henning Schomerus

Lancaster University, UK

21 February 2023 Tue 4 pm

                                      IBS Center for Theoretical Physics of Complex Systems (PCS), Administrative Office (B349), Theory Wing, 3rd floor

                                      Expo-ro 55, Yuseong-gu, Daejeon, South Korea, 34126 Tel: +82-42-878-8633                     

Active elements appear to be critical in experimental realization and observations of paradigmatic non-Hermitian phenomena, such as the non-Hermitian skin effect and the squared-Lorentzian line shape at an exceptional point. As such systems consume energy and suffer from noise that violates the exact implementation of topologically relevant symmetries, passive realizations appear desirable. Here, I establish fundamental limits on this objective that relate constraints from causality to the experimental protocol of determining the density of states [1]. In the density of states, the non-Hermitian skin effect turns out to be completely hidden, while the squared Lorentzian only appears as a background effect of limited contrast.

Theoretically, the key insight is utilizing an appropriately generalized time-delay operator to link the causality constraints with the experimental protocol to determine the density of states. Practically, the constraints can then be formulated directly in terms of the microscopic model, as earlier established via spectral considerations [2]. Conceptually, the strong link between a fundamental concept and a key experimental observable provides a new perspective on analogous constraints on the spectral features and response in active and passive non-Hermitian systems [3-7].

[1] H. Schomerus, "Fundamental constraints on the observability of non-Hermitian effects in passive systems," Phys. Rev. A 106, 063509 (2022).

[2] J. Wiersig, "Nonorthogonality constraints in open quantum and wave systems, " Phys. Rev. Res. 1, 033182 (2019).

[3] H. Schomerus, "Quantum Noise and Self-Sustained Radiation of PT-Symmetric Systems, " Phys. Rev. Lett. 104, 233601 (2010).

[4] G. Yoo, H.-S. Sim, and H. Schomerus, "Quantum noise and mode nonorthogonality in non-Hermitian PT-symmetric optical resonators, " Phys. Rev. A 84, 063833 (2011).

[5] H. Schomerus, "Nonreciprocal response theory of non-Hermitian mechanical metamaterials: Response phase transition from the skin effect of zero modes", Phys. Rev. Research 2, 013058 (2020).

[6] J. Wiersig, "Distance between exceptional points and diabolic points and its implication for the response strength of non-Hermitian systems," Phys. Rev. Res. 4, 033179 (2022).

[7] A. Hashemi, K. Busch, D. N. Christodoulides, S. K. Ozdemir, and R. El-Ganainy, "Linear response theory of open systems with exceptional points, " Nat. Commun. 13, 3281 (2022).

  1. physical limits of non-hermitian and non-reciprocal devices