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Detecting bulk and edge exceptional points in non-Hermitian systems through generalized Petermann factors

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Frontiers of physics 2024年第2期19卷 169-177页

作者： Yue-Yu Zou Yao Zhou Li-Mei Chen Peng Ye School of Physics State Key Laboratory of Optoelectronic Materials and Technologiesand Guangdong Provincial Key Laboratory of Magnetoelectric Physics and DevicesSun Yat-sen UniversityGuangzhou 510275China

Non-orthogonality in non-Hermitian quantum systems gives rise to tremendous exotic quantum phenomena,which can be fundamentally traced back to non-unitarity.In this paper,we introduce an interesting quantity(denoted asη)as a new variant of the Petermann factor to directly and efficiently measure non-unitarity and the associated non-Hermitian physics.By tuning the model parameters of underlying non-Hermitian systems,we find that the discontinuity of bothηand its first-order derivative(denoted as■η)pronouncedly captures rich physics that is fundamentally caused by non-unitarity.More concretely,in the 1D non-Hermitian topological systems,two mutually orthogonal edge states that are respectively localized on two boundaries become non-orthogonal in the vicinity of discontinuity ofηas a function of the model parameter,which is dubbed"edge state transition".Through theoretical analysis,we identify that the appearance of edge state transition indicates the existence of exceptional points(EPs)in topological edge states.Regarding the discontinuity of■η,we investigate a two-level non-Hermitian model and establish a connection between the points of discontinuity of■ηand EPs of bulk states.By studying this connection in more general lattice models,we find that some models have discontinuity of■η,implying the existence of EPs in bulk states.

关键词： non-Hermitian Su−Schrieffer−Heeger(SSH)model exceptional point

Compact asymmetric sound absorber at the exceptional point

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Science China(Physics,Mechanics & Astronomy) 2021年第4期64卷 10-16页

作者： DongTing Li SiBo Huang Yi Cheng Yong Li Institute of Acoustics School of Physics Science and EngineeringTongji UniversityShanghai 200092China

We present an asymmetric absorber at an exceptional point(EP) with a compact configuration and deep-subwavelength thickness.Unlike conventional side-branched sound absorbers in dual-port systems, the proposed asymmetric absorber exhibits a compact shape that is coaxial with the waveguide. By tuning the loss and geometric parameters of the non-Hermitian system to reach an EP, we observe extreme asymmetric absorption. This phenomenon is theoretically and experimentally validated by observing a quasi-perfect absorption and a near-total reflection for opposite incidences at the ultra-thin(1/28 th of the operating wavelength)neck-embedded tube employed in this study. Furthermore, we demonstrate an EP-induced tunable asymmetric absorption. Our study proposes novel approaches to manipulate the EP-induced wave phenomena, paving the way for the development of novel acoustic absorbers, sensors, isolators, and directional devices.

关键词： asymmetric sound absorber acoustic metamaterial non-Hermitian system exceptional point

Observation of the exceptional point in superconducting qubit with dissipation controlled by parametric modulation

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Chinese Physics B 2021年第10期30卷 66-72页

作者：王战相忠诚刘桐宋小会宋鹏涛郭学仪苏鹭红张贺杜燕京郑东宁 Institute of Physics Chinese Academy of SciencesBeijing 100190China University of Chinese Academy of Sciences Beijing 100049China Songshan Lake Materials Laboratory Dongguan 523808China China University of Geosciences Beijing 100083China

Open physical systems described by the non-Hermitian Hamiltonian with parity-time-reversal(PT)symmetry show peculiar phenomena,such as the presence of an exceptional point(EP)at which the PT symmetry is broken and two resonant modes of the Hamiltonian become degenerate.Near the EP,the system could be more sensitive to external perturbations and this may lead to enhanced sensing.In this paper,we present experimental results on the observation of PT symmetry broken transition and the EP using a tunable superconducting qubit.The quantum system of investigation is formed by the two levels of the qubit and the energy loss of the system to the environment is controlled by a method of parametric modulation of the qubit frequency.This method is simple with no requirements for additional elements or qubit device modifications.We believe it can be easily implemented on multi-qubit devices that would be suitable for further exploration of non-Hermitian physics in more complex and diverse systems.

关键词： exceptional point parity-time-reversal(PT)symmetry longitudinal field modulation

Majorana tunneling in a one-dimensional wire with non-Hermitian double quantum dots

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Chinese Physics B 2024年第1期33卷 686-691页

作者：牛鹏斌罗洪刚 Institute of Solid State Physics Shanxi Datong Universityand Shanxi Provincial Key Laboratory of Microstructural Electromagnetic Functional MaterialsDatong 037009China School of Physical Science and Technology Lanzhou UniversityLanzhou 730000China Beijing Computational Science Research Center Beijing 100084China

The combination of non-Hermitian physics and Majorana fermions can give rise to new effects in quantum transport systems. In this work, we investigate the interplay of PT-symmetric complex potentials, Majorana tunneling and interdot tunneling in a non-Hermitian double quantum dots system. It is found that in the weak-coupling regime the Majorana tunneling has pronounced effects on the transport properties of such a system, manifested as splitting of the single peak into three and a reduced 1/4 peak in the transmission function. In the presence of the PT-symmetric complex potentials and interdot tunneling, the 1/4 central peak is robust against them, while the two side peaks are tuned by them. The interdot tunneling only induces asymmetry, instead of moving the conductance peak, due to the robustness of the Majorana modes. There is an exceptional point induced by the union of Majorana tunneling and interdot tunneling. With increased PT-symmetric complex potentials, the two side peaks will move towards each other. When the exceptional point is passed through, these two side peaks will disappear. In the strong-coupling regime, the Majorana fermion induces a 1/4 conductance dip instead of the three-peak structure. PT-symmetric complex potentials induce two conductance dips pinned at the exceptional point. These effects should be accessible in experiments.

关键词： Majorana fermion non-Hermitian quantum dot parity–time symmetry exceptional point

Anti-PT-symmetric Kerr gyroscope

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Chinese Physics B 2022年第1期31卷 87-92页

作者：张会来彭美瑜徐勋卫景辉 Key Laboratory of Low-Dimensional Quantum Structures and Quantum Control of Ministry of Education Department of Physics and Synergetic Innovation Center for Quantum Effects and ApplicationsHunan Normal UniversityChangsha 410081China

Non-Hermitian systems can exhibit unconventional spectral singularities called exceptional points(EPs).Various EP sensors have been fabricated in recent years,showing strong spectral responses to external signals.Here we propose how to achieve a nonlinear anti-parity-time(PT)gyroscope by spinning an optical resonator.We show that,in the absence of any nonlinearity,the sensitivity or optical mode splitting of the linear device can be magnified up to 3 orders compared to that of the conventional device without EPs.Remarkably,the PT symmetry can be broken when including the Kerr nonlinearity of the materials and,as a result,the detection threshold can be significantly lowered,i.e.,much weaker rotations which are well beyond the ability of a linear gyroscope can now be detected with the nonlinear device.Our work shows the powerful ability of PT gyroscopes in practice to achieve ultrasensitive rotation measurement.

关键词： anti-parity-time symmetry optical gyroscope exceptional point Kerr nonlinearity

Anti-parity-time symmetric phase transition in diffusive systems

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Chinese Physics B 2021年第3期30卷 9-16页

作者：曹培超祝雪丰 School of Physics and Innovation Institute Huazhong University of Science and TechnologyWuhan 430074China

Parity-time (PT) symmetry/anti-parity-time (APT) symmetry in non-Hermitian systems reveal profound physics andspawn intriguing effects. Recently, it has been introduced into diffusive systems together with the concept of exceptionalpoints (EPs) from quantum mechanics and the wave systems. With the aid of convection, we can generate complex thermalconductivity and imitate various wavelike dynamics in heat transfer, where heat flow can be “stopped” or moving against thebackground motion. Non-Hermitian diffusive systems offer us a new platform to investigate the heat wave manipulation.In this review, we first introduce the construction of APT symmetry in a simple double-channel toy model. Then we showthe phase transition around the EP. Finally, we extend the double-channel model to the four-channel one for showing thehigh-order EP and the associated phase transition. In a general conclusion, the phase difference of adjacent channels isalways static in the APT symmetric phase, while it dynamically evolves or oscillates when the APT symmetry is broken.

关键词： anti-parity-time symmetry phase transition exceptional point heat transfer

Parity-time symmetric acoustic system constructed by piezoelectric composite plates with active external circuits

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Chinese Physics B 2022年第6期31卷 91-98页

作者：周扬杨彰昭彭尧吟邹欣晔 Key Laboratory of Modern Acoustics MOEInstitute of AcousticsDepartment of PhysicsCollaborative Innovation Center of Advanced MicrostructuresNanjing UniversityNanjing 210093China State Key Laboratory of Acoustics Chinese Academy of SciencesBeijing 100190China

Researches on parity-time(PT)symmetry in acoustic field can provide an efficient platform for controlling the travelling acoustic waves with balanced loss and gain.Here,we report a feasible design of PT-symmetric system constructed by piezoelectric composite plates with two different active external circuits.By judiciously adjusting the resistances and inductances in the external circuits,we obtain the exceptional point due to the spontaneous breaking of PT symmetry at the desired frequencies and can observe the unidirectional invisibility.Moreover,the system can be at PT exact phase or broken phase at the same frequency in the same structure by merely adjusting the external circuits,which represents the active control that makes the acoustic manipulation more convenient.Our study may provide a feasible way for manipulating acoustic waves and inspire the application of piezoelectric composite materials in acoustic structures.

关键词： parity-time symmetry acoustic gain material piezoelectric composite plate exceptional point

Microscale vortex laser with controlled topological charge

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Chinese Physics B 2016年第12期25卷 132-137页

作者：王兴远陈华洲黎颖李波马仁敏 State Key Laboratory for Mesoscopic Physics and School of Physics Peking University Beijing 100871 China Collaborative Innovation Center of Quantum Matter Beijing China

A microscale vortex laser is a new type of coherent light source with small footprint that can directly generate vector vortex beams. However, a microscale laser with controlled topological charge, which is crucial for virtually any of its application, is still unrevealed. Here we present a microscale vortex laser with controlled topological charge. The vortex laser eigenmode was synthesized in a metamaterial engineered non-Hermitian micro-ring cavity system at exceptional point. We also show that the vortex laser cavity can operate at exceptional point stably to lase under optical pumping. The microscale vortex laser with controlled topological charge can serve as a unique and general building block for next- generation photonic integrated circuits and coherent vortex beam sources. The method we used here can be employed to generate lasing eigenmode with other complex functionalities.

关键词： exceptional point non-Hermitian system orbital angular momentum vortex laser

Parity–time symmetric photonics

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National Science Review 2018年第2期5卷 183-199页

作者： Han Zhao Liang Feng Department of Electrical and Systems Engineering University of Pennsylvania Department of Materials Science and Engineering University of Pennsylvania

The establishment of non-Hermitian quantum mechanics(such as parity–time(PT) symmetry) stimulates a paradigmatic shift for studying symmetries of complex potentials. Owing to the convenient manipulation of optical gain and loss in analogy to complex quantum potentials, photonics provides an ideal platform for the visualization of many conceptually striking predictions from non-Hermitian quantum theory. A rapidly developing field has emerged, namely, PT-symmetric photonics, demonstrating intriguing optical phenomena including eigenstate coalescence and spontaneous PT-symmetry breaking. The advance of quantum physics, as the feedback, provides photonics with brand-new paradigms to explore the entire complex permittivity plane for novel optical functionalities. Here, we review recent exciting breakthroughs in PT-symmetric photonics while systematically presenting their underlying principles guided by non-Hermitian symmetries. The potential device applications for optical communication and computing,biochemical sensing and healthcare are also discussed.

关键词： photonics parity–time symmetry non-Hermitian phase transition exceptional point