arXiv subjects

Bingkun Qi

Publications and source records attributed to Bingkun Qi.

3 recordsLinked to original sources

arXiv2023

Linear localization of zero modes in weakly coupled non-Hermitian reservoirs

Topological and symmetry-protected non-Hermitian zero modes have attracted considerable interest in the past few years. Here we reveal that they can exhibit an unusual behavior when transitioning between the extended and localized regimes: When weakly coupled to a non-Hermitian reservoir, such a zero mode displays a linearly decreasing amplitude as a function of space, which is not caused by an EP of a Hamiltonian, either of the entire system or the reservoir itself. Instead, we attribute it to the non-Bloch solution of a linear homogeneous recurrence relation, together with the underlying non-Hermitian particle-hole symmetry and the zeroness of its energy.

quant-ph↗

arXiv2023

Robust zero modes in non-Hermitian systems without global symmetries

We present an approach to achieve zero modes in lattice models that do not rely on any symmetry or topology of the bulk, which are robust against disorder in the bulk of any type and strength. Such symmetry-free zero modes (SFZMs) are formed by attaching a single site or small cluster with zero mode(s) to the bulk, which serves as the "nucleus" that expands to the entire lattice. We identify the requirements on the couplings between this boundary and the bulk, which reveals that this approach is intrinsically non-Hermitian. We then provide several examples with either an arbitrary or structured bulk, forming spectrally embedded zero modes in the bulk continuum, midgap zero modes, and even restoring the "zeroness" of coupling or disorder-shifted topological corner states. Focusing on viable realizations using photonic lattices, we show that the resulting SFZM can be observed as the single lasing mode when optical gain is applied to the boundary.

physics.optics↗

arXiv2018

Defect states emerging from a non-Hermitian flat band of photonic zero modes

We show the existence of a flat band consisting of photonic zero modes in a gain and loss modulated lattice system, as a result of the underlying non-Hermitian particle-hole symmetry. This general finding explains the previous observation in parity-time symmetric systems where non-Hermitian particle-hole symmetry is hidden. We further discuss the defect states in these systems, whose emergence can be viewed as an unconventional alignment of a pseudo-spin under the influence of a complex-valued pseudo-magnetic field. These defect states also behave as a chain with two types of links, one rigid in a unit cell and one soft between unit cells, as the defect states become increasingly localized with the gain and loss strength. A realistic photonic design is presented based on coupled InP/InGaAsP waveguides, and we also extend the discussion to two- and three-dimensional lattices.

physics.optics↗