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Non-Hermitian Quantum Wave Engineering

Periodic Reporting for period 2 - NHQWAVE (Non-Hermitian Quantum Wave Engineering)

Reporting period: 2018-03-01 to 2020-02-29

The Non-Hermitian Quantum Wave Engineering (NHQWAVE) project (EC-supported MSCA-RISE-2015 - Marie Skłodowska-Curie Research and Innovation Staff Exchange (RISE) project; Project number: 691209) started on March 1, 2016, and in its four-year lifetime the project's partners investigated, developed and implemented new resonant phenomena in novel complex non-hermitian system configurations. In particular, the project’s research activity focused on the non-Hermitian photonics and parity-time (PT) symmetry in optics and condensed matter physics, studying:
1. Symmetry breaking and exceptional points in complex lasers.
2. Superconducting quantum metamaterials.
3. Asymmetric wave transport and topological phenomena in optical lattices.
4. Computational methods of condensed matter physics for non-hermitian photonic media.
5. Nonlinearity and non-hermiticity in disordered lattices and multimode fibers.

The project addresses challenges of future optical devices, and, as such, the capability to push the technology frontiers, advancing the scientific background for the implementation of innovative devices to expand the state-of-the-art in industries with significant socio-economic impact and wider societal implications, as, i.e. the information and telecommunications industrial sector. Recent research results regarding the topological lasers and the constant-intensity waves in non-Hermitian complex media (both topics include theoretical and experimental studies) are a direct outcome of the NHQWAVE project.
In the NHQWAVE project, three European (MS/AC) beneficiaries, namely the University of Crete (Center for Quantum Complexity and Nanotechnology, Physics Department; which was the project’s coordinator), Technion-Israel Institute of Technology, and Technische Universitaet Wien (TU-Wien), collaborated with five partners in the USA (Harvard U., Yale U., Penn State U., Washington University in Saint Louis, U. of Central Florida (CREOL, The College of Optics and Photonics).

Topological insulators, Anderson localization and superconductivity are three different topics of condensed matter physics of fundamental, as well as, of technological importance. Openness of the related systems leads to Hamiltonians that are necessarily non-Hermitian.
In the context of non-Hermitian Photonics intermixing of gain and loss (openness) offers an extra degree of freedom for controlling the electromagnetic radiation. The NHQWAVE (Non-Hermitian Quantum WAVe Engineering) project carried out a systematic theoretical and experimental investigation of important related topics. Inspired by the introduction of parity-time (PT)-symmetry in optics, the project went beyond and explored more general non-Hermitian Hamiltonians related to photonic systems. In particular, the project shed light to new research directions of non-Hermitian photonics. The first one was the discovery of topological lasers. The team studied theoretically and experimentally a lattice of coupled optical microring cavities that behave collectively as a laser, where the lasing modes have topological properties, namely topological protection and robust transport along the edges. Such an open topological insulator type of lattice can be implemented in photonics, since gain semiconductor materials are readily available and optical loss can be controlled. On another direction, the project team revisited (theoretically and experimentally) the fundamental problem of encirclement of an exceptional point (unique type of non-Hermitian degeneracy) and the validity of the adiabatic theorem. Another remarkable feature of non-Hermitian systems is the possibility of unidirectional invisibility. Materials combining both loss and gain media can act as invisible media only from the one side of illumination. So far the concept of unidirectional invisibility was based on PT-symmetry. Researchers introduced a new way to achieve invisibility channels in strongly scattering media, counterbalancing thus the Anderson localization effect.
Another research line followed by NHQWAVE involved the study of non-linear effects in open photonic systems with gain and loss and in particular, SQUID metamaterials. The complex spatiotemporal dynamics was investigated by using machine learning techniques. The team successfully described the time dynamics of coupled non-linear PT-symmetric elements. NHQWAVE results offer the possibility to develop synthetic optical devices and structures with enhanced functionality and novel features.
Via secondments, the project implemented a research training programme in order to develop early stage researcher and experienced researcher skill base and expertise in tackling theoretical and experimental problems of non-hermitian and PT-symmetric systems.
NHQWAVE has investigated and developed new resonant phenomena in novel complex non-hermitian system configurations, which address the challenges of future optical devices, and, as such, has the capability to push the technology frontiers, offering the scientific background for the implementation of innovative devices to expand the state-of-the-art in industries with significant socio-economic impact and wider societal implications, as, i.e. the information and telecommunications industrial sector. The project’s research activity and objectives focus on the non-Hermitian photonics and parity-time (PT) symmetry in optics and condensed matter physics. More specifically, the recent results regarding the topological lasers and the constant-intensity waves in non-Hermitian complex media (both topics include theoretical and experimental studies) are a direct outcome of the NHQWAVE project way beyond the state of the art.
Non-hermitian wave physics of topological disorder and SQUID complex structures