A. Krasnok, N. Nefedkin, and A. Alu, Parity-Time Symmetry and Exceptional Points [Electromagnetic Perspectives], IEEE Antennas Propag. Mag. 63, 110 (2021).

In this article, we offer a tutorial geared toward introducing the unusual electromagnetic phenomena emerging in non-Hermitian systems, with a particular emphasis the subclass of these systems that obey PT symmetry. We first discuss the basic features of P , T, and PT operators applied to electromagnetic and quantum mechanical phenomena. We then discuss the exotic response of PT-symmetric electromagnetic structures and their opportunities, including the singularities, known as exceptional points (EPs), emerging in these systems and their unusual scattering response. We discuss applications, challenges, and potentials in classical and quantum domains.

A. Krasnok, M. Caldarola, N. Bonod, and A. Alú, Spectroscopy and Biosensing with Optically Resonant Dielectric Nanostructures, Adv. Opt. Mater. 6, 1701094 (2018)

Resonant dielectric nanoparticles made of materials with large positive dielectric permittivity, such as Si, GaP, GaAs, have become a powerful platform for modern light science, enabling various fascinating applications in nanophotonics and quantum optics. In addition to light localization at the nanoscale, dielectric nanostructures provide electric and magnetic resonant responses throughout the visible and infrared spectrum, low dissipative losses and optical heating, low doping effect, and absence of quenching, which are interesting for spectroscopy and biosensing applications. This review presents state-of-the-art applications of optically resonant high-index dielectric nanostructures as a multifunctional platform for light–matter interactions. Nanoscale control of quantum emitters and applications for enhanced spectroscopy including fluorescence spectroscopy, surface-enhanced Raman scattering, biosensing, and lab-on-a-chip technology are surveyed. The theoretical background underlying these effects is described, realizations of specific resonant dielectric nanostructures and hybrid excitonic systems are overviewed, and an outlook of the challenges in this field, which remain open to future research, is provided.

N. Nefedkin, A. Alù, and A. Krasnok, Quantum Embedded Superstates, Adv. Quantum Technol. 4, 2000121 (2021).

Here, it is shown that a three-level quantum system (e.g., atom, quantum dot, superconducting qubit) can be tailored to support the quantum analog of superstates with an unboundedly narrow emission line. Remarkably, it is demonstrated that the coupling of such a system with a cavity (e.g., plasmonic or dielectric nanoparticle, microcavity, microwave resonator) enables sensing properties with excellent statistical features. The results can be applied to a plethora of quantum platforms, from superconducting circuits to cold atoms and quantum dots, opening exciting opportunities for quantum sensing and computing.

Quantum and optical sensors