Yiling Song

Exceptional points (EPs), singularities in non‐Hermitian systems where eigenvalues and eigenstates coalesce, exhibit unique spectral responses and enhanced sensitivity. However, in quantum‐interference settings such as two‐photon measurements, the interference transition generally does not occur at the EPs, limiting their applications for quantum sensing. To address this issue, we develop an anti‐parity‐time‐symmetric model based on indirectly coupled waveguides and introduce a cascaded‐waveguide configuration that produces an effective rotation of the system's eigenbasis. This modification shifts the two‐photon bunching‐antibunching transition to coincide with the EP. Near the EP, a small refractive‐index perturbation leads to a sharp change in visibility, which becomes more pronounced by increasing waveguide length. This passive, gain‐free design may minimize noise and offer a feasible route toward EP‐enhanced quantum phase sensing on integrated platforms.

Mode selecting plays a vital role in the field of optoelectronics, such as optical communication, signal processing, on-chip light manipulation, mode conversion, and frequency synthesis. In this work, flexible selection and enhancement of the frequency modes in an unidirectional coupled Su–Schrieffer–Heeger (SSH) frequency lattice are obtained with Floquet exceptional points (EPs) and chiral Zener tunneling (ZT). The unidirectional coupled non-Hermitian SSH frequency lattices are synthesized by a double-ring system with complex dynamical modulations. Under an effective direct current (dc) force induced by the phase-mismatching of the modulations, the two Floquet bands of the non-Hermitian frequency lattices are degenerated and the Floquet EPs arise. Therefore, the unidirectional and irreversible frequency mode conversion takes place, which is the chiral ZT. Moreover, through perturbation analysis and numerical simulations, we prove that the frequency modes of the two-band system can be selected and enhanced by a multi-photon resonance dc force.

A non-Hermitian skin effect (NHSE) dramatically localizes bulk modes at open boundaries, leading to fascinating eigenvalue and wavefunction topologies. However, realizing NHSE typically requires non-reciprocal couplings, which many photonic systems cannot provide. Here, we propose an approach using twisted optical waveguide arrays. The twisting introduces geometrical phase in photon coupling process, effectively acting as artificial gauge fields for photons. By constructing a dissipative Aharonov–Bohm chain of twisted lattices, we achieve NHSE by combining these artificial gauge fields with dissipation. We can control the localization strength and direction of skin modes by tuning the twist. Additionally, we characterize the topological edge modes using Majorana's stellar representation and further propose twisted spectral winding with bipolar NHSE in a trapezoidal lattice. Our work offers an alternative avenue to realize eigenvalue topology and control light localization and transport.

The gauge fields dramatically alter the algebraic structure of spatial symmetries and make them projectively represented, giving rise to novel topological phases. Here, we propose a photonic Möbius topological insulator enabled by projective translation symmetry in multiorbital waveguide arrays, where the artificial π gauge flux is aroused by the inter-orbital coupling between the first (s) and third (d) order modes. In the presence of π flux, the two translation symmetries of rectangular lattices anti-commute with each other. By tuning the spatial spacing between two waveguides to break the translation symmetry, a topological insulator is created with two Möbius twisted edge bands appearing in the bandgap and featuring 4π periodicity. Importantly, the Möbius twists are accompanied by discrete diffraction in beam propagation, which exhibit directional transport by tuning the initial phase of the beam envelope according to the eigenvalues of translation operators. This work manifests the significance of gauge fields in topology and provides an efficient approach to steering the direction of beam transmission.

Active mode-locking (ML) is an important technique in laser science, which greatly shortens the laser pulse. Here, we construct an anti-parity-time (anti-PT) symmetric Su–Schrieffer–Heeger frequency lattice by two ring resonators with antisymmetric amplitude (AM) modulations. We find that the temporal width of the generated pulse can be greatly shortened by the phase-mismatching of the AM modulations. In addition, the pulse shortening shows extremely high sensitivity to the phase transition point, at which the anti-PT symmetry of the system is completely broken. This work exploits the concept of anti-PT symmetry in a laser field to realize ML, and will have broad application prospects in ultrafast spectroscopy and ultra-high sensitive sensors.

Recently, studies on non-Hermitian topologic physics have attracted considerable attention. The non-Hermitian skin effect (NHSE), as a remarkable phenomenon in the non-Hermitian lattice, has been demonstrated in coupled ring resonators and photonic mesh lattices. However, there is a scarcity of work on the realization of NHSEs in synthetic dimensions, owing to inaccessible anisotropic coupling. This limits the potential for exploring non-Hermitian topologic physics in on-chip integrated optical systems. In this work, we implement a non-Hermitian Su–Schrieffer–Heeger topologic insulator in the synthetic frequency dimension, and the NHSE and topologic edge state are manifested. Furthermore, we demonstrate that the exotic chiral Zener tunneling can also be realized. Our system provides a versatile platform to explore and exploit non-Hermitian topologic physics on a chip and can have impacts on flexible light manipulation in frequency domains.