Octupole topological insulating phase protected by a three-dimensional momentum-space nonsymmorphic group.

Recent advancements in quantum polarization theory have propelled the exploration of topological insulators (TIs) into the realm of higher-order systems, leading to the study of the celebrated two-dimensional (2D) quadrupole and 3D octupole TIs. Traditionally, these topological phases have been associated with the toroidal topology of the conventional Brillouin zone. This paper reports the discovery of a novel octupole topological insulating phase protected by a 3D momentum-space nonsymmorphic group emerging within the framework of the Brillouin 3D real projective space ([Formula: see text]).

Broadband measurement of Feibelman's quantum surface response functions.

Feibelman [Formula: see text]-parameter, a mesoscopic complement to the local bulk permittivity, describes quantum optical surface responses for interfaces, including nonlocality, spill-in and -out, and surface-enabled Landau damping. It has been incorporated into the macroscopic Maxwellian framework for convenient modeling and understanding of nanoscale electromagnetic phenomena, calling for the compilation of a [Formula: see text]-parameter database for interfaces of interest in nano-optics. However, accurate first-principles calculations of [Formula: see text]-parameters face computational challenges, whereas existing measurements of [Formula: see text]-parameters are scarce and restricted to narrow spectral windows.

Synthetic Non-Abelian Electric Fields and Spin-Orbit Coupling in Photonic Synthetic Dimensions.

We theoretically propose a scheme to synthesize photonic non-Abelian electric field and spin-orbit coupling (SOC) in the synthetic frequency dimension based on a polarization-multiplexed time-modulated ring resonator. Inside the ring resonator, the cascade of polarization-dependent phase modulation, polarization rotation, and phase retardation enables a photonic realization of minimal SOC with Peierls substitution composed of equal Rashba and Dresselhaus coupling. The synthetic bands and associated spin textures can be effectively probed by the continuous-wave polarization- and time-resolved band structure measurements.

Continuous Transition between Bosonic Fractional Chern Insulator and Superfluid.

The properties of fractional Chern insulator (FCI) phases and the phase transitions between FCIs and Mott insulators in bosonic systems are well studied. The continuous transitions between FCI and superfluids (SFs), however, despite the inspiring field theoretical predictions [M. Barkeshli and J.

Maximal Quantum Interaction between Free Electrons and Photons.

The emerging field of free-electron quantum optics enables electron-photon entanglement and holds the potential for generating nontrivial photon states for quantum information processing. Although recent experimental studies have entered the quantum regime, rapid theoretical developments predict that qualitatively unique phenomena only emerge beyond a certain interaction strength. It is thus pertinent to identify the maximal electron-photon interaction strength and the materials, geometries, and particle energies that enable one to approach it.

Superscattering of light: fundamentals and applications.

Superscattering, theoretically predicted in 2010 and experimentally observed in 2019, is an exotic scattering phenomenon of light from subwavelength nanostructures. In principle, superscattering allows for an arbitrarily large total scattering cross section, due to the degenerate resonance of eigenmodes or channels. Consequently, the total scattering cross section of a superscatterer can be significantly enhanced, far exceeding the so-called single-channel limit.

Higher-Order Topological Insulators via Momentum-Space Nonsymmorphic Symmetries.

We theoretically construct a higher-order topological insulator (HOTI) on a Brillouin real projective plane enabled by momentum-space nonsymmorphic (k-NS) symmetries from synthetic gauge fields. Two anicommutative k-NS glide reflections appear in a checkerboard Z_{2} flux model, impose nonsymmorphic constraints on Berry curvature, and quantize bulk and Wannier-sector polarization nonlocally across different momenta. The model's bulk exhibits an isotropic quadrupole phase diagram, where the transition appears intrinsically from bulk gap closure.

Synthetic Non-Abelian Gauge Fields for Non-Hermitian Systems.

Non-Abelian gauge fields are versatile tools for synthesizing topological phenomena, but have so far been mostly studied in Hermitian systems, where gauge flux has to be defined from a closed loop in order for vector potentials, whether Abelian or non-Abelian, to become physically meaningful. We show that this condition can be relaxed in non-Hermitian systems by proposing and studying a generalized Hatano-Nelson model with imbalanced non-Abelian hopping. Despite lacking gauge flux in one dimension, non-Abelian gauge fields create rich non-Hermitian topological consequences.