Quantum exceptional points of non-Hermitian Hamiltonian and Liouvillian in dissipative quantum Rabi model

The open quantum system can be described by either a Lindblad master equation or a non-Hermitian Hamiltonian(NHH).However,these two descriptions usually have different exceptional points(EPs),associated with the degeneracies in the open quantum system.Here,considering a dissipative quantum Rabi model,we study the spectral features of EPs in these two descriptions and explore their connections.We find that,although the EPs in these two descriptions are usually different,the EPs of NHH will be consistent with the EPs of master equation in the weak coupling regime.Further,we find that the quantum Fisher information(QFI),which measures the statistical distance between quantum states,can be used as a signature for the appearance of EPs.Our study may give a theoretical guidance for exploring the properties of EPs in open quantum systems.

Cluster mean-field study of spinor Bose-Hubbard ladder:Ground-state phase diagram and many-body population dynamics

We present a cluster mean-field study for ground-state phase diagram and many-body dynamics of spin-1 bosons confined in a two-chain Bose-Hubbard ladder(BHL).For unbiased BHL,we find superfluid(SF)phase and integer filling Mott insulator(Int MI)phase.For biased BHL,in addition to the SF and Int MI phases,there appears half-integer filling Mott insulator(HInt MI)phase.The phase transition between the SF and Int MI phases can be first order at a part of phase boundaries,while the phase transition between the SF and HInt MI phases is always second order.By tuning the bias energy,we report on the change of the nature of SF-MI phase transitions.Furthermore,we study the effect of the spin-dependent interaction on the many-body population dynamics.The spin-dependent interaction can lead to rich dynamical behaviors,but does not influence the particle transfer efficiency.Our results indicate a way to tune the nature of the SF-MI phase transition and open a new avenue to study the many-body dynamics of spinor bosons in optical lattices.

Melting of electronic/excitonic crystals in 2D semiconductor moirépatterns:A perspective from the Lindemann criterion

Using the Lindemann criterion,we analyzed the quantum and thermal melting of electronic/excitonic crystals recently discovered in two-dimensional(2D)semiconductor moirépatterns.We show that the finite 2D screening of the atomically thin material can suppress(enhance)the inter-site Coulomb(dipolar)interaction strength,thus inhibits(facilitates)the formation of the electronic(excitonic)crystal.Meanwhile,a strong enough moiréconfinement is found to be essential for realizing the crystal phase with a wavelength near 10 nm or shorter.From the calculated Lindemann ratio which quantifies the fluctuation of the site displacement,we estimate that the crystal will melt into a liquid above a critical temperature ranging from several tens Kelvin to above 100 K(depending on the system parameters).

Topological properties of non-Hermitian Creutz ladders

We study topological properties of the one-dimensional Creutz ladder model with different non-Hermitian asymmetric hoppings and on-site imaginary potentials,and obtain phase diagrams regarding the presence and absence of an energy gap and in-gap edge modes.The non-Hermitian skin effect(NHSE),which is known to break the bulk-boundary correspondence(BBC),emerges in the system only when the non-Hermiticity induces certain unbalanced non-reciprocity along the ladder.The topological properties of the model are found to be more sophisticated than that of its Hermitian counterpart,whether with or without the NHSE.In one scenario without the NHSE,the topological winding is found to exist in a two-dimensional plane embedded in a four-dimensional space of the complex Hamiltonian vector.The NHSE itself also possesses some unusual behaviors in this system,including a high spectral winding without the presence of long-range hoppings,and a competition between two types of the NHSE,with the same and opposite inverse localization lengths for the two bands,respectively.Furthermore,it is found that the NHSE in this model does not always break the conventional BBC,which is also associated with whether the band gap closes at exceptional points under the periodic boundary condition.

New designed helical resonator to improve measurement accuracy of magic radio frequency

Based upon the new designed helical resonator,the resonant radio frequency(RF)for trapping ions can be consec-utively adjusted in a large range(about 12 MHz to 29 MHz)with high Q-factors(above 300).We analyze the helical resonator with a lumped element circuit model and find that the theoretical results fit well with the experimental data.With our resonator system,the resonant frequency near magic RF frequency(where the scalar Stark shift and the second-order Doppler shift due to excess micromotion cancel each other)can be continuously changed at kHz level.For ^(88)Sr^(+) ion,compared to earlier results,the measurement accuracy of magic RF frequency can be improved by an order of magnitude upon rough calculation,and therefore the net micromotion frequency shifts can be further reduced.Also,the differential static scalar polarizability △α0 of clock transition can be experimentally measured more accurately.

Production of ^(87)Rb Bose-Einstein Condensate in an Asymmetric Crossed Optical Dipole Trap

We report the production of ^(87)Rb Bose–Einstein condensate in an asymmetric crossed optical dipole trap(ACODT)without the need of an additional dimple laser.In our experiment,the ACODT is formed by two laser beams with different radii to achieve efficient capture and rapid evaporation of laser cooled atoms.Compared to the cooling procedure in a magnetic trap,the atoms are firstly laser cooled and then directly loaded into an ACODT without the pre-evaporative cooling process.In order to determine the optimal parameters for evaporation cooling,we optimize the power ratio of the two beams and the evaporation time to maximize the final atom number left in the ACODT.By loading about 6×10^(5) laser cooled atoms in the ACODT,we obtain a pure Bose–Einstein condensate with about 1.4×10^(4) atoms after 19 s evaporation.Additionally,we demonstrate that the fringe-type noises in optical density distributions can be reduced via principal component analysis,which correspondingly improves the reliability of temperature measurement.

Time-energy analysis of the photoionization process in a double-XUV pulse combined with a few-cycle IR field

We calculate the time-energy distribution(TED)and ionization time distribution(ITD)of photoelectrons emitted by a doubleextreme-ultraviolet(XUV)pulse and a two-color XUV-IR pulse using the Wigner distribution-like function based on the strong field approximation.For a double-XUV pulse,besides two identical broad distributions generated by two XUV pulses,many interference fringes resulting from the interference between electrons generated,respectively,by two pulses appear in the TED.After adding an IR field,the TED intuitively exhibits the effect of the IR field on the electron dynamics.The ITDs during two XUV pulses are no longer the same and show the different changes for the different two-color fields,the origin of which is attributed to the change of the electric field induced by the IR field.Our analysis shows that the emission time of electrons ionized during two XUV pulses mainly depends on the electric field of the combined XUV pulse and IR pulse.

Multilevel atomic Ramsey interferometry for precise parameter estimations

Multi-path(or multi-mode)entanglement has been proved to be a useful resource for sub-shot-noise sensitivity of phase estimation,which has aroused much research interest in quantum metrology recently.Various schemes of multi-path interferometers based on optical systems have been put forward.Here,we study a multi-state interferometer with multilevel atoms by projective measurements.Specifically,we investigate its ultimate sensitivity described by quantum Fisher information theory and find that the Cramer-Rao bound can be achieved.In particular,we investigate a specific scheme to improve the sensitivity of magnetometery with a three-state interferometry delivered by a single nitrogen-vacancy(NV)center of diamond with tailor pulses.The impacts of imperfections of the atomic beam-splitter,described by the three-level quantum Fourier transform,on the sensitivity of phase estimation is also discussed.

Motional n-phonon bundle states of a trapped atom with clock transitions

Quantum manipulation of individual phonons could offer new resources for studying fundamental physics and creating an innovative platform in quantum information science. Here,we propose to generate quantum states of strongly correlated phonon bundles associated with the motion of a trapped atom. Our scheme operates in the atom–phonon resonance regime where the energy spectrum exhibits strong anharmonicity such that energy eigenstates with different phonon numbers can be well-resolved in the parameter space. Compared to earlier schemes operating in the far dispersive regime,the bundle states generated here contain a large steady-state phonon number. Therefore,the proposed system can be used as a high-quality multiphonon source. Our results open up the possibility of using long-lived motional phonons as quantum resources,which could provide a broad physics community for applications in quantum metrology.

Topological non-Hermitian skin effect

This article reviews recent developments in the non-Hermitian skin effect(NHSE),particularly on its rich interplay with topology.The review starts off with a pedagogical introduction on the modified bulk-boundary correspondence,the synergy and hybridization of NHSE and band topology in higher dimensions,as well as,the associated topology on the complex energy plane such as spectral winding topology and spectral graph topology.Following which,emerging topics are introduced such as non-Hermitian criticality,dynamical NHSE phenomena,and the manifestation of NHSE beyond the traditional linear non-interacting crystal lattices,particularly its interplay with quantum many-body interactions.Finally,we survey the recent demonstrations and experimental proposals of NHSE.

基于嵌套能带反转面的高阶拓扑相的直接动力学表征

高阶拓扑物态的主要特征是能隙中具有局域在(d-n)维边界上的拓扑边界态,其中d是系统的维度,n是拓扑的阶数.针对一类不明显依赖于任何晶体对称性的Z类高阶拓扑态,本文发现通过在动量空间定义一系列能带反转面,系统的拓扑性质可由这些能带反转面的拓扑数及它们之间的嵌套关系描述.通过量子淬火的动力学演化过程,可以直接探测这些能带反转面及其拓扑数,从而探测任意维度中任意阶数的拓扑性质.目前,人工合成拓扑物态中已可以直接测量赝自旋矢量在布里渊区内特定区域的卷绕特征,并据此确定本文定义的嵌套能带反转面拓扑性质.此外,通过嵌套能带反转面定义高阶拓扑也为研究并设计高阶拓扑物态提供了一个独特的视角.

非厄米赝能隙

能隙是表征物态的重要概念之一.其中,能态密度极低但非零的区域被称为赝能隙,其出现与高温超导等现象有着重要的联系.本研究发现对于一些非厄米格点系统,引入开边界条件能够导致赝能隙的产生.这一现象通常在两个或更多不对称的非厄米泵浦通道相互影响时出现,并且没有厄米对应.非厄米赝能隙的出现同时也伴随着一些其他新奇的物理现象.与普通拓扑能隙类似,非厄米赝能隙中也能出现受对称性保护的隙间态,但这些隙间态具有扩展的空间分布,而不是局域在系统边缘,且对破坏对称性的扰动具有很高的敏感性.在传统的拓扑相变点处,非厄米赝能隙中同样可以存在与拓扑态类似的局域边界态,但并不直接对应相应赝能带的拓扑不变量:这些赝能带可能具有分数的拓扑数,甚至不具有良好定义的陈数.非厄米赝能隙的概念挑战了传统拓扑对应以及能带的概念,有望对分数陈绝缘体等多体现象产生重要的影响.

摩尔超晶格中电场可调的极化激元拓扑输运

在由不同的单层过渡金属硫族化合物堆叠形成的双层范德瓦尔斯异质结中,两层之间的微小转角可导致摩尔超晶格的出现,其基态光学性质由位于不同层的电子和空穴通过库仑吸引形成的层间激子主导.层间激子携带的静电偶极矩使得它可以通过静电势来操控其定向流动,同时层间激子也具有一定的光学跃迁偶极矩,它的大小和方向随层间激子在摩尔超晶格中的位置而变化.本文指出摩尔超晶格中的层间激子与二维平面光学微腔内光子模的杂化可产生两种具备不同拓扑性质的摩尔极化激元,可导致自旋/谷霍尔效应和偏振霍尔效应.通过调节两层间的电压改变层间激子与光学腔内光子模的失谐量,可选择特定种类的摩尔极化激元,这使得它的拓扑输运现象具备卓越的电场可调性.

通过静电类比设计非厄米实能谱

非厄米性的研究蕴涵着诸多新物理机制与现象,在激光与传感等多个方面有着重要的应用价值.在非厄米系统中,实本征能量意味着系统具有稳定性,这一性质可通过奇偶时间(PT)对称性保护,并在不同物理系统中得到广泛的研究.本文利用一种新的动力学机制保证实能谱,提出了一种普适的方案来设计具有实能谱的哈密顿量.该方案基于对扩展非厄米体边对应的静电类比,其中静电荷对应态密度,电场则对应复能谱流.通过这一方案,可以重构出具有任意能谱及态局域性质的哈密顿量,即便这些性质不具备任何对称性特征.通过用泊松边值问题代替原本的非厄米哈密顿量对角化问题,这一静电类比也避免了传统数值方法中因非厄米耗散/增益引起的浮点误差问题,从而允许对更大尺寸的系统进行计算.

Tunable photon blockade with a single atom in a cavity under electromagnetically induced transparency

We present an experimental proposal to achieve a strong photon blockade by employing electromagnetically induced transparency (EIT) with a single alkaline-earth-metal atom trapped in an optical cavity. In the presence of optical Stark shift,both the second-order correlation function and cavity transmission exhibit asymmetric structures between the red and blue sidebands of the cavity. For a weak control field,the photon quantum statistics for the coherent transparency window (i.e.,atomic quasi-dark-state resonance) are insensitive to the Stark shift,which should also be immune to the spontaneous emission of the excited state by taking advantage of the intrinsic dark-state polariton of EIT. Interestingly,by exploiting the interplay between the Stark shift and control field,the strong photon blockade at atomic quasi-dark-state resonance has an optimal second-order correlation function g(2)(0)-10-4 and a high cavity transmission simultaneously. The underlying physical mechanism is ascribed to the Stark shift enhanced spectrum anharmonicity and the EIT hosted strong nonlinearity with loss-insensitive atomic quasi-dark-state resonance,which is essentially different from the conventional proposal with emerging Kerr nonlinearity in cavity-EIT. Our results reveal a new strategy to realize high-quality single photon sources,which could open up a new avenue for engineering nonclassical quantum states in cavity quantum electrodynamics.