Experimentally Ruling Out Joint Reality Based on Locality with Device-Independent Steering

As an essential concept to understand the world,whether the real values(or physical realities)of observables are suitable to physical systems beyond the classic has been debated for many decades.Although standard nogo results based on Bell inequalities have ruled out the joint reality of incompatible quantum observables,the possibility of giving simple yet strong arguments to rule out joint reality in any physical system(not necessarily quantum)with weaker assumptions and less observables has been explored and proposed recently.Here,we perform a device-independent experiment on a two-qubit superconducting system to show that the joint reality of two observables is incompatible with locality under the weaker assumption of the reality of observables in a single space-time region(or single qubit).Our results clearly show the violation of certain inequalities derived from both linear and nonlinear criteria.In addition,we study the robustness of the linear and nonlinear criterion against the effect of systematic decoherence.Our demonstration opens up the possibility of delineating classical and non-classical boundaries with simpler nontrivial quantum system.

Dynamic response of a thermal transistor to time-varying signals

Thermal transistor,the thermal analog of an electronic transistor,is one of the most important thermal devices for microscopic-scale heat manipulating.It is a three-terminal device,and the heat current flowing through two terminals can be largely controlled by the temperature of the third one.Dynamic response plays an important role in the application of electric devices and also thermal devices,which represents the devices’ability to treat fast varying inputs.In this paper,we systematically study two typical dynamic responses of a thermal transistor,i.e.,the response to a step-function input(a switching process)and the response to a square-wave input.The role of the length L of the control segment is carefully studied.It is revealed that when L is increased,the performance of the thermal transistor worsens badly.Both the relaxation time for the former process and the cutoff frequency for the latter one follow the power-law dependence on L quite well,which agrees with our analytical expectation.However,the detailed power exponents deviate from the expected values noticeably.This implies the violation of the conventional assumptions that we adopt.

Semiclassical approach to spin dynamics of a ferromagnetic S = 1 chain

Motivated by recent experimental progress on the quasi-one-dimensional quantum magnet Ni Nb2O6, we study the spin dynamics of an S = 1 ferromagnetic Heisenberg chain with single-ion anisotropy by using a semiclassical molecular dynamics approach. This system undergoes a quantum phase transition from a ferromagnetic to a paramagnetic state under a transverse magnetic field, and the magnetic response reflecting this transition is well described by our semiclassical method.We show that at low temperature the transverse component of the dynamical structure factor depicts clearly the magnon dispersion, and the longitudinal component exhibits two continua associated with single-and two-magnon excitations,respectively. These spin excitation spectra show interesting temperature dependence as effects of magnon interactions. Our findings shed light on the experimental detection of spin excitations in a large class of quasi-one-dimensional magnets.

Dynamical t/U Expansion of the Doped Hubbard Model

We construct a new U(1)slave-spin representation for the single-band Hubbard model in the large-U limit.The mean-field theory in this representation is more amenable to describe both the spin-charge-separation physics of the Mott insulator at half-filling and the strange metal behavior at finite doping.

Dissipation-Driven Superradiant Phase Transition of a Two-Dimensional Bose-Einstein Condensate in a Double Cavity

We study superradiant phase transitions in a hybrid system of a two-dimensional Bose–Einstein condensate of atoms and two cavities arranged with a tilt angle.By adjusting the loss rate of cavities,we map out the phase diagram of steady states within a mean field framework.It is found that when the loss rates of the two cavities are different,superradiant transitions may not occur at the same time in the two cavities.A first-order phase transition is observed between the states with only one cavity in superradiance and both in superradiance.In the case that both cavities are superradiant,a net photon current is observed flowing from the cavity with small decay rate to the one with large decay rate.The photon current shows a non-monotonic dependence on the loss rate difference,owing to the competition of photon number difference and cavity field phase difference.Our findings can be realized and detected in experiments.

Topological Invariants in Terms of Green's Function for the Interacting Kitaev Chain

A one-dimensional closed interacting Kitaev chain and the dimerized version are studied. The topological invariants in terms of Green＇s function are calculated by the density matrix renormalization group method and the exact diagonalization method. For the interacting Kitaev chain, we point out that the calculation of the topological invariant in the charge density wave phase must consider the dimerized configuration of the ground states. The variation of the topological invariant is attributed to the poles of eigenvalues of the zero-frequency Green functions. For the interacting dimerized Kitaev chain, we show that the topological invariant defined by Green＇s functions can distinguish more topological nonequivalent phases than the fermion parity.

Tuning the Mottness in Sr_(3)Ir_(2)O_(7)via Bridging Oxygen Vacancies

The electronic evolution of Mott insulators into exotic correlated phases remains puzzling,because of electron interaction and inhomogeneity.Introduction of individual imperfections in Mott insulators could help capture the main mechanism and serve as a basis to understand the evolution.Here we utilize scanning tunneling microscopy to probe the atomic scale electronic structure of the spin-orbit-coupling assisted Mott insulator Sr_(3)Ir_(2)O_(7).It is found that the tunneling spectra exhibit a homogeneous Mott gap in defect-free regions,but near the oxygen vacancy in the rotated Ir O_(2)plane the local Mott gap size is significantly enhanced.We attribute the enhanced gap to the locally reduced hopping integral between the 5d electrons of neighboring Ir sites via the bridging planar oxygen p orbitals.Such bridging defects have a dramatic influence on local bandwidth,thus provide a new way to manipulate the strength of Mottness in a Mott insulator.

Cost-efficient quantum access network boosts practical deployment of quantum key distribution network

The security of information is not assured over extended periods,especially with the rapid advancement of quantum computers.To address this challenge,quantum cryptography,rooted in quantum physics,offers comprehensive toolkits for key distribution[1],secret sharing[2],digital signatures[3]and other cryptographic tasks[4]with information-theoretical security.The feasibility of quantum cryptography,particularly quantum key distribution(QKD)。

Nematic Ferromagnetism on the Lieb Lattice

我们在 Lieb 格子上讨论铁磁性的订单的性质并且证明对称保护了十字路口点将戏剧性地影响磁性的订的二次公寓的乐队。面对一个弱所在地的排斥相互作用，扎根的状态是 nematic 有同时破的时间颠倒和旋转对称的铁磁性的顺序。当相互作用力量增加时，旋转对称将在批评价值恢复，并且系统进入常规铁磁性的政体。为 nematic 和常规铁磁性的阶段的吝啬地的转变温度在相互作用的顺序。这观察建议这些磁性的订单有潜力在现实主义的试验性的条件以内在冷原子系统被认识到并且检测。

High-performance frequency stabilization of ultraviolet diode lasers by using dichroic atomic vapor spectroscopy and transfer cavity

We develop a high-performance ultraviolet(UV)frequency stabilization technique implemented directly on UV diode lasers by combining the dichroic atomic vapor laser lock and the resonant transfer cavity lock.As an example,we demonstrate a stable locking with measured frequency standard deviations of approximately 200 kHz and 300 kHz for 399 nm and 370 nm diode lasers in 20 min.We achieve a long-term frequency drift of no more than 1 MHz for the target 370 nm laser within an hour,which is further verified with fluorescence count rates of a single trapped ^171Yb+ion.We also find strong linear correlations between lock points and environmental factors such as temperature and atmospheric pressure.Our approach provides a simple and stable solution at a relatively low cost,and features flexible control,high feedback bandwidth and minimal power consumption of the target UV laser.

Machine learning identification of impurities in the STM images

We train a neural network to identify impurities in the experimental images obtained by the scanning tunneling microscope(STM)measurements.The neural network is first trained with a large number of simulated data and then the trained neural network is applied to identify a set of experimental images taken at different voltages.We use the convolutional neural network to extract features from the images and also implement the attention mechanism to capture the correlations between images taken at different voltages.We note that the simulated data can capture the universal Friedel oscillation but cannot properly describe the non-universal physics short-range physics nearby an impurity,as well as noises in the experimental data.And we emphasize that the key of this approach is to properly deal with these differences between simulated data and experimental data.Here we show that even by including uncorrelated white noises in the simulated data,the performance of the neural network on experimental data can be significantly improved.To prevent the neural network from learning unphysical short-range physics,we also develop another method to evaluate the confidence of the neural network prediction on experimental data and to add this confidence measure into the loss function.We show that adding such an extra loss function can also improve the performance on experimental data.Our research can inspire future similar applications of machine learning on experimental data analysis.

Machine Learning to Instruct Single Crystal Growth by Flux Method

Growth of high-quality single crystals is of great significance for research of condensed matter physics. The exploration of suitable growing conditions for single crystals is expensive and time-consuming, especially for ternary compounds because of the lack of ternary phase diagram. Here we use machine learning(ML) trained on our experimental data to predict and instruct the growth. Four kinds of ML methods, including support vector machine(SVM), decision tree, random forest and gradient boosting decision tree, are adopted. The SVM method is relatively stable and works well, with an accuracy of 81% in predicting experimental results. By comparison,the accuracy of laboratory reaches 36%. The decision tree model is also used to reveal which features will take critical roles in growing processes.

Dynamical signatures of the one-dimensional deconfined quantum critical point

We study the critical scaling and dynamical signatures of fractionalized excitations at two different deconfined quantum critical points(DQCPs)in an S=1/2 spin chain using the time evolution of infinite matrix product states.The scaling of the correlation functions and the dispersion of the conserved current correlations explicitly show the emergence of enhanced continuous symmetries at these DQCPs.The dynamical structure factors in several different channels reveal the development of deconfined fractionalized excitations at the DQCPs.Furthermore,we find an effective spin-charge separation at the DQCP between the ferromagnetic(FM)and valence bond solid(VBS)phases,and identify two continua associated with different types of fractionalized excitations at the DQCP between the X-direction and Z-direction FM phases.Our findings not only provide direct evidence for the DQCP in one dimension but also shed light on exploring the DQCP in higher dimensions.

Precision measurements with cold atoms and trapped ions

Recent progresses on quantum control of cold atoms and trapped ions in both the scientific and technological aspects greatly advance the applications in precision measurement. Thanks to the exceptional controllability and versatility of these massive quantum systems, unprecedented sensitivity has been achieved in clocks, magnetometers, and interferometers based on cold atoms and ions. Besides, these systems also feature many characteristics that can be employed to facilitate the applications in different scenarios. In this review, we briefly introduce the principles of optical clocks, cold atom magnetometers, and atom interferometers used for precision measurement of time, magnetic field, and inertial forces. The main content is then devoted to summarize some recent experimental and theoretical progresses in these three applications, with special attention being paid to the new designs and possibilities towards better performance. The purpose of this review is by no means to give a complete overview of all important works in this fast developing field, but to draw a rough sketch about the frontiers and show the fascinating future lying ahead.

Global phase diagram of a spin–orbit-coupled Kondo lattice model on the honeycomb lattice

Motivated by the growing interest in the novel quantum phases in materials with strong electron correlations and spin–orbit coupling, we study the interplay among the spin–orbit coupling, Kondo interaction, and magnetic frustration of a Kondo lattice model on a two-dimensional honeycomb lattice.We calculate the renormalized electronic structure and correlation functions at the saddle point based on a fermionic representation of the spin operators.We find a global phase diagram of the model at half-filling, which contains a variety of phases due to the competing interactions.In addition to a Kondo insulator, there is a topological insulator with valence bond solid correlations in the spin sector, and two antiferromagnetic phases.Due to the competition between the spin–orbit coupling and Kondo interaction, the direction of the magnetic moments in the antiferromagnetic phases can be either within or perpendicular to the lattice plane.The latter antiferromagnetic state is topologically nontrivial for moderate and strong spin–orbit couplings.

High-Quality FeTe_(1-x)Se_x Monolayer Films on SrTiO_3(001) Substrates Grown by Molecular Beam Epitaxy

We report the growth process of FeTe1-xSex （0 〈 x 〈 1） monolayer films on SrTi03 （STO） substrates through molecular beam epitaxy and discuss the possible ways to improve the film quality. By exploring the parameters of substrate treatment, growth control and post growth annealing, we successfully obtain a series of FeTe1-xSex monolayer films. In the whole growth process, we find the significance of the temperature control through surface roughness monitored by the reflection high-energy electron diffraction and scanning tunneling microscopy. We obtain the best quality of FeSe monolayer films with the STO substrate treated at T = 900 950℃ before growth, the FeSe deposited at T = 310℃ during growth and annealed at T = 380℃ after growth. For FeTe1-xSex （x-1）, both the growth temperature and annealing temperature decrease to T=260℃. According to the angle- resolved photoemission spectroscopy measurements, the superconductivity of the FeTe1-xSex film is robust and insensitive to Se concentration. All the above are instructive for further investigations of the superconductivity in FeTe1-xSex films.

Fulde-Ferrell-Larkin-Ovchinnikov states in equally populated Fermi gases in a two-dimensional moving optical lattice

We study the possibility of stabilizing a Fulde-Ferrell-Larkin-Ovchinnikov(FFLO)state in an equally populated two-component Fermi gas trapped in a moving two-dimensional optical lattice.For a system with nearly half filling,we find that a finite pairing momentum perpendicular to the moving direction can be spontaneously induced for a proper choice of lattice velocity.As a result,the total pairing momentum is tilted towards the nesting vector to take advantage of the significant enhancement of the density of states.

Bose-Einstein condensates in an eightfold symmetric optical lattice

We investigate the properties of Bose-Einstein condensates(BECs)in a two-dimensional quasi-periodic optical lattice(OL)with eightfold rotational symmetry by numerically solving the Gross-Pitaevskii equation.In a stationary external harmonic trapping potential,we first analyze the evolution of matter-wave interference pattern from periodic to quasiperiodic as the OL is changed continuously from four-fold periodic to eight-fold quasi-periodic.We also investigate the transport properties during this evolution for different interatomic interaction and lattice depth,and find that the BEC crosses over from ballistic diffusion to localization.Finally,we focus on the case of eightfold symmetric lattice and consider a global rotation imposed by the external trapping potential.The BEC shows vortex pattern with eightfold symmetry for slow rotation,becomes unstable for intermediate rotation,and exhibits annular solitons with approximate axial symmetry for fast rotation.These results can be readily demonstrated in experiments using the same configuration as in Phys.Rev.Lett.122110404(2019).

Explicit forms of zero modes in symmetric interacting Kitaev chain without and with dimerization

The fermionic and bosonic zero modes of the one-dimensional（1D） interacting Kitaev chain at the symmetric point are unveiled. The many-body structures of the Majorana zero modes in the topological region are given explicitly by carrying out a perturbation expansion up to infinite order. We also give the analytic expressions of the bosonic zero modes in the topologically trivial phase. Our results are generalized to the hybrid fermion system comprised of the interacting Kitaev model and the Su–Schrieffer–Heeger（SSH） model, in which we show that these two types of zero modes can coexist in a certain region of its phase diagram.

Role of the spin anisotropy of the interchain interaction in weakly coupled antiferromagnetic Heisenberg chains

In quasi-one-dimensional(q1D) quantum antiferromagnets, the complicated interplay of intrachain and interchain exchange couplings may give rise to rich phenomena. Motivated by recent progress on field-induced phase transitions in the q1D antiferromagnetic(AFM) compound YbAlO3, we study the phase diagram of spin-1/2 Heisenberg chains with Ising anisotropic interchain couplings under a longitudinal magnetic field via large-scale quantum Monte Carlo simulations,and investigate the role of the spin anisotropy of the interchain coupling on the ground state of the system. We find that the Ising anisotropy of the interchain coupling can significantly enhance the longitudinal spin correlations and drive the system to an incommensurate AFM phase at intermediate magnetic fields, which is understood as a longitudinal spin density wave(LSDW). With increasing field, the ground state changes to a canted AFM order with transverse spin correlations. We further provide a global phase diagram showing how the competition between the LSDW and the canted AFM states is tuned by the Ising anisotropy of the interchain coupling.