There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Mathematical Handbook for Scientists and Engineers

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Λιόλιος Ξερολιβαδιώτης : ένας αγνοημένος Μακεδόνας αγωνιστής του '21 και λίγα για την ιστορία του Ξερολιβάδου/ Γιώργου Χ. Χιονίδη

Quantum magnonics: When magnon spintronics meets quantum information science

Spintronics and quantum information science are two promising candidates for innovating information processing technologies. The combination of these two fields enables us to build solid-state platforms for studying quantum phenomena and for realizing multi-functional quantum tasks. For a long time, however, the intersection of these two fields was limited. This situation has changed significantly over the last few years because of the remarkable progress in coding and processing information using magnons. On the other hand, significant advances in understanding the entanglement of quasi-particles and in designing high-quality qubits and photonic cavities for quantum information processing provide physical platforms to integrate magnons with quantum systems. From these endeavours, the highly interdisciplinary field of quantum magnonics emerges, which combines spintronics, quantum optics and quantum information science.Here, we give an overview of the recent developments concerning the quantum states of magnons and their hybridization with mature quantum platforms. First, we review the basic concepts of magnons and quantum entanglement and discuss the generation and manipulation of quantum states of magnons, such as single-magnon states, squeezed states and quantum many-body states including Bose-Einstein condensation and the resulting spin superfluidity. We discuss how magnonic systems can be integrated and entangled with quantum platforms including cavity photons, superconducting qubits, nitrogen-vacancy centers, and phonons for coherent information transfer and collaborative information processing. The implications of these hybrid quantum systems for non-Hermitian physics and parity-time symmetry are highlighted, together with applications in quantum memories and high-precision measurements. Finally, we present an outlook on the opportunities in quantum magnonics.

Quantum magnonics: when magnon spintronics meets quantum information science

We report the photocurrent generation in reduced graphene oxide–cadmium zinc sulfide (RGO–Cd0.75Zn0.25S) nano composite material under simulated solar light irradiation, where the photocurrent increases linearly with increasing incident light intensity. We also report the temperature dependent electrical conductivity and conductivity relaxation in RGO–Cd0.75Zn0.25S composite. At low frequency, the real part of conductivity is independent of frequency, and above a characteristic crossover frequency, the conductivity decreases with the increase in frequency, which indicates the onset of a relaxation phenomenon. The dc conductivity of the RGO–Cd0.75Zn0.25S composite shows Arrhenius behavior. From the scaling of real part of conductivity spectra, we have observed that the dynamic process occurring at different temperatures have the same thermal activation energy. The RGO–Cd0.75Zn0.25S composite shows an enhancement of photo catalytic activity in comparison to control sample under simulated solar light irradia...

Photocurrent Generation and Conductivity Relaxation in Reduced Graphene Oxide Cd0.75Zn0.25S Nanocomposite and Its Photocatalytic Activity

Exceptional points (EPs) in non-Hermitian systems have recently attracted considerable attention owing to unique state-flipping and peculiar phase accumulation features. The dynamical encirclement ...

Exceptional Point and toward Mode-Selective Optical Isolation

One of the most intriguing topological features of open systems is that they exhibit exceptional point (EP) singularities. Apart from the widely explored second-order EPs (EP2s), the exploration of higher-order EPs in any system requires more complex topology, which is still a challenge. Here, we encounter a third-order EP (EP3) with the simultaneous presence of multiple second-order EPs in a simple fabrication feasible gain-loss assisted trilayer optical microcavity. Using the scattering-matrix formalism, we study the simultaneous interactions between three successive coupled states via avoided-resonance-crossing (ARC) phenomena, and we identify two EP2s near two ARC regimes. Such an occurrence of two EP2s inside a closed two-dimensional parametric space associated with an unbalanced gain-loss profile leads to the functionality of a cube-root branch point, i.e., an EP3. Following an adiabatic variation of two control parameters around the embedded EP3 in the presence of two identified EP2s, we present a robust successive-state-conversion mechanism among three coupled states. The proposed scheme indeed opens up a unique platform to manipulate light in integrated photonic devices.

Third-order exceptional point and successive switching among three states in an optical microcavity

We report a specially configured open optical microcavity, imposing a spatially imbalanced gain–loss profile, to host an exclusively proposed next-nearest-neighbor resonance coupling scheme. Adopting the scattering matrix (S-matrix) formalism, the effect of interplay between such proposed resonance interactions and the incorporated non-Hermiticity in the microcavity is analyzed drawing a special attention to the existence of hidden singularities, namely exceptional points (EPs), where at least two coupled resonances coalesce. We establish adiabatic flip-of-state phenomenon of the coupled resonances in the complex frequency plane (k-plane), which is essentially an outcome of the fact that the respective EP is being encircled in the system parameter plane. Encountering such multiple EPs, the robustness of flip-of-states phenomena has been analyzed via continuous tuning of coupling parameters along a special hidden singular line which connects all the EPs in the cavity. Such a numerically devised cavity, incorporating the exclusive next neighbor coupling scheme, has been designed for the first time to study the unconventional optical phenomena in the vicinity of EPs.

Next-nearest-neighbor resonance coupling and exceptional singularities in degenerate optical microcavities

Using scattering matrix formalism, we report the formation of a special hidden singular line connecting multiple second-order hidden singular points, in a non-uniformly pumped degenerate optical microcavity. Such singularities are known as exceptional points (EPs), and the line is proposed as an exceptional line. Exploring the unconventional behavior of cavity resonances created by a spatially imbalanced gain–loss profile, we have established the adiabatic state-flipping mechanism of coupled resonances encountering such EPs. Various interesting encircling situations, incorporating smooth as well as fluctuating variations of the control parameters, have been analyzed. We exploit the above scheme for the first time, to the best of our knowledge, to analyze the optical performance and stability of the cascaded flip-of-state phenomenon assisted by successive encirclement of either single or multiple singularities following the exceptional line in the context of optical mode converters.

Arnab Laha

Papers

Photocurrent Generation and Conductivity Relaxation in Reduced Graphene Oxide Cd0.75Zn0.25S Nanocomposite and Its Photocatalytic Activity

Exceptional Point and toward Mode-Selective Optical Isolation

Third-order exceptional point and successive switching among three states in an optical microcavity

Next-nearest-neighbor resonance coupling and exceptional singularities in degenerate optical microcavities

Connected hidden singularities and toward successive state flipping in degenerate optical microcavities