arXiv subjects

Ming Gong

Publications and source records attributed to Ming Gong.

At least 199 records · Page 11Linked to original sources

arXiv2019

Unicoder: A Universal Language Encoder by Pre-training with Multiple Cross-lingual Tasks

We present Unicoder, a universal language encoder that is insensitive to different languages. Given an arbitrary NLP task, a model can be trained with Unicoder using training data in one language and directly applied to inputs of the same task in other languages. Comparing to similar efforts such as Multilingual BERT and XLM, three new cross-lingual pre-training tasks are proposed, including cross-lingual word recovery, cross-lingual paraphrase classification and cross-lingual masked language model. These tasks help Unicoder learn the mappings among different languages from more perspectives. We also find that doing fine-tuning on multiple languages together can bring further improvement. Experiments are performed on two tasks: cross-lingual natural language inference (XNLI) and cross-lingual question answering (XQA), where XLM is our baseline. On XNLI, 1.8% averaged accuracy improvement (on 15 languages) is obtained. On XQA, which is a new cross-lingual dataset built by us, 5.5% averaged accuracy improvement (on French and German) is obtained.

cs.CL↗

arXiv2019

A coupled-channel lattice study on the resonance-like structure $Z_c(3900)$

In this exploratory study, near-threshold scattering of $D$ and $\bar{D}^*$ meson is investigated using lattice QCD with $N_f=2+1+1$ twisted mass fermion configurations. The calculation is performed within the coupled-channel Lüscher's finite-size formalism. The study focuses on the channel with $I^G(J^{PC})=1^+(1^{+-})$ where the resonance-like structure $Z_c(3900)$ was discovered. We first identify the most relevant two channels of the problem and the lattice study is performed within the two-channel scattering model. Combined with a two-channel Ross-Shaw theory, scattering parameters are extracted from the energy levels by solving the generalized eigenvalue problem. Our results on the scattering length parameters suggest that, at the particular lattice parameters that we studied, the best fitted parameters do not correspond to a peak behavior in the elastic scattering cross section near the threshold. Furthermore, within the zero-range Ross-Shaw theory, the scenario of a narrow resonance close to the threshold is disfavored beyond $3σ$ level.

hep-lat↗

arXiv2019

Anomalous relaxation and multiply time scales in the quantum $XY$ model with boundary dissipation

The relaxation of many-body system is still a challenging problem that has not well been understood. In this work we exactly calculate the dynamics of the quantum $XY$ model with boundary dissipation, in which the density matrix in terms of Majorana operators can be decoupled into independent subspaces represented by different number of Majorana fermions. The relaxation is characterized by multiply time scales, and in the long-time limit it is determined by the single particle relaxation process in a typical time scale $T^*$. For the bulk bands, we find $T^* \propto N^3/γn^2 $ in the weak dissipation limit; and $T^* \propto γN^3/ n^2$ in the strong dissipation limit, where $N$ is the chain length, $γ$ is the dissipation rate and $n$ is the band index. For the edge modes $T^* \propto 1/γ$, indicating of most vulnerable to dissipation in the long chain limit. These results are counter-intuitive because it means any weak dissipation can induce relaxation, while strong dissipation can induce weak relaxation. We find that these two limits correspond to two different physics, which are explained based on the first and second-order perturbation theory in an equivalent non-Hermitian model.Furthermore, we show that even in the long chain limit the relaxation may exhibit strong odd-even effect. These results shade new insight into the dynamics of topological qubits in environment.

cond-mat.str-el↗

arXiv2019

Topological spinor vortex matter on spherical surface induced by non-Abelian spin-orbital-angular-momentum coupling

We provide an explicit way to implement non-Abelian spin-orbital-angular-momentum (SOAM) coupling in spinor Bose-Einstein condensates using magnetic gradient coupling. For a spherical surface trap addressable using high-order Hermite-Gaussian beams, we show that this system supports various degenerate ground states carrying different total angular momenta $\mathbf{J}$, and the degeneracy can be tuned by changing the strength of SOAM coupling. For weakly interacting spinor condensates with $f=1$, the system supports various meta-ferromagnetic phases and meta-polar states described by quantized total mean angular momentum $|\langle \mathbf{J} \rangle|$. Polar states with $Z_2$ symmetry and Thomson lattices formed by defects of spin vortices are also discussed. The system can be used to prepare various stable spin vortex states with nontrivial topology, and serve as a platform to investigate strong-correlated physics of neutral atoms with tunable ground-state degeneracy.

cond-mat.quant-gas↗

arXiv2019

Demonstration of Adiabatic Variational Quantum Computing with a Superconducting Quantum Coprocessor

Adiabatic quantum computing enables the preparation of many-body ground states. This is key for applications in chemistry, materials science, and beyond. Realisation poses major experimental challenges: Direct analog implementation requires complex Hamiltonian engineering, while the digitised version needs deep quantum gate circuits. To bypass these obstacles, we suggest an adiabatic variational hybrid algorithm, which employs short quantum circuits and provides a systematic quantum adiabatic optimisation of the circuit parameters. The quantum adiabatic theorem promises not only the ground state but also that the excited eigenstates can be found. We report the first experimental demonstration that many-body eigenstates can be efficiently prepared by an adiabatic variational algorithm assisted with a multi-qubit superconducting coprocessor. We track the real-time evolution of the ground and exited states of transverse-field Ising spins with a fidelity up that can reach about 99%.

quant-ph↗

arXiv2019

Interplay between non-Hermiticity and non-Abelian gauge potential in topological photonics

Topological phases in spinless non-Hermitian models have been widely studied both theoretically and experimentally in some artificial materials using photonics, phononics and magnon. In this work, we investigate the interplay between non-Hermitian loss and gain and non-Abelian gauge potential realized in a two-component superconducting circuit. In our model, the non-Hermiticity along only gives rise to trivial gain and loss to the states; while the non-Abelian gauge along gives rise to flying butterfly spectra and associated edge modes, which in photonics can be directly measured by the intensity of photons at the boundaries. These two terms do not commute, and their interplay can give rise to several intriguing non-Hermitian phases, including the fully gapped quantum spin Hall (QSH) phase, gapless QSH phase, trivial gapped phase and gapless metallic phase. The bulk-edge correspondence is absent and we find that during the closing of energy gap in the gapped QSH phase, the system enters the gapless QSH phase regime which still supports two counter-propagating edge modes. We have also unveiled the intriguing role of non-Hermiticity on the chiral symmetry and time-reversal symmetry of the Hermitian models, which can be applied to other physical models.

cond-mat.mes-hall↗

arXiv2019

Model Compression with Multi-Task Knowledge Distillation for Web-scale Question Answering System

Deep pre-training and fine-tuning models (like BERT, OpenAI GPT) have demonstrated excellent results in question answering areas. However, due to the sheer amount of model parameters, the inference speed of these models is very slow. How to apply these complex models to real business scenarios becomes a challenging but practical problem. Previous works often leverage model compression approaches to resolve this problem. However, these methods usually induce information loss during the model compression procedure, leading to incomparable results between compressed model and the original model. To tackle this challenge, we propose a Multi-task Knowledge Distillation Model (MKDM for short) for web-scale Question Answering system, by distilling knowledge from multiple teacher models to a light-weight student model. In this way, more generalized knowledge can be transferred. The experiment results show that our method can significantly outperform the baseline methods and even achieve comparable results with the original teacher models, along with significant speedup of model inference.

cs.CL↗

arXiv2019

Genuine 12-qubit entanglement on a superconducting quantum processor

We report the preparation and verification of a genuine 12-qubit entanglement in a superconducting processor. The processor that we designed and fabricated has qubits lying on a 1D chain with relaxation times ranging from 29.6 to 54.6 $μ$s. The fidelity of the 12-qubit entanglement was measured to be above $0.5544\pm0.0025$, exceeding the genuine multipartite entanglement threshold by 21 statistical standard deviations. Our entangling circuit to generate linear cluster states is depth-invariant in the number of qubits and uses single- and double-qubit gates instead of collective interactions. Our results are a substantial step towards large-scale random circuit sampling and scalable measurement-based quantum computing.

quant-ph↗

arXiv2019

Angular momentum Josephson effect between two isolated condensates

We demonstrate that the two degenerate energy levels in spin-orbit coupled trapped Bose gases, coupled by a quenched Zeeman field, can be used for angular momentum Josephson effect. In a static quenched field, we can realize a Josephson oscillation with period ranging from millisecond to hundreds of milliseconds. Moreover, by a driven Zeeman field, we realize a new Josephson oscillation, in which the population imbalance may have the same expression as the current in the directed current (dc) Josephson effect. When the dynamics of condensate can not follow up the modulation frequency, it the self-trapping regime. This new dynamics is understood from the time dependent evolution of the constant-energy trajectory in phase space. This model has several salient advantages as compared with the previous ones. The condensates are isolated from their excitations by a finite gap, thus can greatly suppress the damping effect induced by thermal atoms and Bogoliubov excitations. The oscillation period can be tuned by several order of magnitudes without influencing other parameters. In experiments, the dynamics can be mapped out from spin and momentum spaces, thus is not limited by the spatial resolution in imaging. This system can serve as a promising platform for exploring of matter wave interferometry.

cond-mat.quant-gas↗

arXiv2019

Bose-Einstein condensate in Bloch bands with off-diagonal periodic potential

We report the Bose-Einstein condensate (BEC) in the Bloch bands with off-diagonal periodic potential (ODPP), which simultaneously plays the role of spin-orbit coupling (SOC) and Zeeman field. This model can be realized using two independent Raman couplings in the same three level system, in which the time-reversal symmetry ensures the energy degeneracy between the two states with opposite momenta. We find that these two Raman couplings can be used to tune the spin polarization in momentum space, thus greatly modifies the effective scatterings over the Bloch bands. We observe a transition from the Bloch plane wave phase with condensate at one wave vector to the Bloch stripe phase with condensates at the two Bloch states with opposite wave vectors. These two phases will exhibit totally different spin textures and density modulations in real space, which are totally different from that in free space. In momentum space multiple peaks differ by some reciprocal lattice vectors can be observed, reflecting the periodic structure of the ODPP. A three-band effective model is proposed to understand these observations. This system can provide a new platform in investigating of various physics, such as collective excitations, polaron and topological superlfuids, over the Bloch bands.

cond-mat.quant-gas↗

arXiv2019

Stochastic and long-distance level spacing statistics in many-body localization

From the random matrix theory all the energy levels should be strongly correlated due to the presence of all off-diagonal entries.In this work we introduce two new statistics to more accurately characterize these long-distance interactions in the disordered many-body systems with only short-range interaction. In the $(p, q)$ statistics, we directly measure the long distance energy level spacings, while in the second approach, we randomly eliminate some of the energy levels, and then measure the reserved $η\%$ energy levels using nearest-neighbor level spacings. We benchmark these results using the results in standard Gaussian ensembles. Some analytical distribution functions with extremely high accuracy are derived, which automatically satisfy the inverse relation and duality relation. These two measurements satisfy the same universal scaling law during the transition from the Gaussian ensembles to the Poisson ensemble, with critical disorder strength and corresponding exponent are independent of these measurements. These results shade new insight into the stability of many-body localized phase and their universal properties in the disordered many-body systems.

cond-mat.stat-mech↗

arXiv2018

Quark spins and Anomalous Ward Identity

We calculate the intrinsic quark spin contribution to the total proton spin using overlap valence quarks on three ensembles of $2+1$-flavor RBC/UKQCD domain-wall configurations with different lattice spacings. The lowest pion mass of the ensembles is around 171 MeV which is close to the physical point. With overlap fermions and topological charge derived from the overlap operator, we verify the anomalous Ward identity between nucleon states with momentum transfer. Both the connected and disconnected insertions of the axial-vector current are calculated. For the disconnected-insertion part, the cluster-decomposition error reduction (CDER) technique is utilized for the lattice with the largest volume and the error can be reduced by $10\%\sim40\%$. Nonperturbative renormalization is carried out and the final results are all reported in the $\overline{\rm MS}$ scheme at 2 GeV. We determine the total quark spin contribution to the nucleon spin to be $ΔΣ=0.401(25)(37)$, which is consistent with the recent global fitting result of experimental data. The isovector axial coupling we obtain in this study is $g_A^3=1.256(16)(30)$, which agrees well with the experimental value of 1.2723(23).

hep-ph↗

arXiv2018

Dynamical instability towards finite-momentum pairing in quenched BCS superconducting phases

In this work we numerically investigate the fate of the Bardeen-Cooper-Schrieffer (BCS) pairing in the presence of quenched phase under Peierls substitution using time-dependent real space and momentum space Bogoliubov-de Gennes equation methods and Anderson pseudospin representation method. This kind of phase imprint can be realized by modulating electric field in ultracold atoms and illumining of THz optical pump pulse in solids with conventional and unconventional superconductors. In the case of weak phase imprint, the BCS pairing is stable; while in the strong phase imprint, instability towards finite-momentum pairing is allowed, in which the real space and momentum space methods yield different results. In the pulsed gauge potential, we find that this instability will not happen even with much stronger vector potential. We also show that the uniform and staggered gauge potentials yield different behaviors. While the staggered potential induces transition from the BCS pairing to over-damped phase, the uniform gauge may enhance the pairing and will not induce to the over-damped phase. These result may shade light on the realization of finite momentum pairing, such as Fulde-Ferrell-Larkin-Ovchinnikov phase with dynamical modulation.

cond-mat.quant-gas↗

arXiv2018

Nonperturbatively-renormalized glue momentum fraction at physical pion mass from Lattice QCD

We present the first nonperturbatively-renormalized determination of the glue momentum fraction $\langle x \rangle_g$ in the nucleon, based on lattice-QCD simulations at physical pion mass using the cluster-decomposition error reduction (CDER) technique. We provide the first practical strategy to renormalize the glue energy-momentum tensor (EMT) nonperturbatively in the RI/MOM scheme, and convert the results to the $\overline{\textrm{MS}}$ scheme with 1-loop matching. The simulation results show that the CDER technique can reduce the statistical uncertainty of its renormalization constant by a factor of ${\cal O}$(300) in calculations using typical state-of-the-art lattice volume, and the nonperturbatively-renormalized $\langle x \rangle_g$ is shown to be independent of the lattice definitions of the glue EMT up to discretization errors. We determine the renormalized $\langle x \rangle_g^{\overline{\textrm{MS}}}(2\textrm{ GeV})$ to be 0.47(4)(11) at physical pion mass, which is consistent with the experimentally-determined value.

hep-lat↗

arXiv2018

Anomalous isothermal compressibility in spin-orbit coupled degenerate Fermi gases

The spin-orbit coupling (SOC) in degenerate Fermi gases can fundamentally change the fate of $s$-wave superfluids with strong Zeeman field and give rise to topological superfluids and associated Majorana zero modes. It also dramatically changes the thermodynamic properties of the superfluids. Here we report the anomalous isothermal compressibility $κ_T$ in this superfluids with both SOC and Zeeman field. We formulate this quantity from the Gibbs-Duhem equation and show that the contribution of $κ_T$ comes from the explicit contribution of chemical potential and implicit contribution of order parameter. In the Bardeen-Cooper-Schrieffer (BCS) limit, this compressibility is determined by the density of state near the Fermi surface; while in the Bose Einstein condensate (BEC) regime it is determined by the scattering length. Between these two limits, we find that the anomalous peaks can only be found in the gapless Weyl phase regime. This anomalous behavior can be regarded as a remanent effect of phase separation. The similar physics can also be found in the lattice model away from half filling. These predictions can be measured from the anomalous response of sound velocity and fluctuation of carrier density.

cond-mat.quant-gas↗

arXiv2018

Glueball spectrum from $N_f=2$ lattice QCD study on anisotropic lattices

The lowest-lying glueballs are investigated in lattice QCD using $N_f=2$ clover Wilson fermion on anisotropic lattices. We simulate at two different and relatively heavy quark masses, corresponding to physical pion mass of $m_π\sim 938$ MeV and $650$ MeV. The quark mass dependence of the glueball masses have not been investigated in the present study. Only the gluonic operators built from Wilson loops are utilized in calculating the corresponding correlation functions. In the tensor channel, we obtain the ground state mass to be 2.363(39) GeV and 2.384(67) GeV at $m_π\sim 938$ MeV and $650$ MeV, respectively. In the pseudoscalar channel, when using the gluonic operator whose continuum limit has the form of $ε_{ijk}TrB_iD_jB_k$, we obtain the ground state mass to be 2.573(55) GeV and 2.585(65) GeV at the two pion masses. These results are compatible with the corresponding results in the quenched approximation. In contrast, if we use the topological charge density as field operators for the pseudoscalar, the masses of the lowest state are much lighter (around 1GeV) and compatible with the expected masses of the flavor singlet $q\bar{q}$ meson. This indicates that the operator $ε_{ijk}TrB_iD_jB_k$ and the topological charge density couple rather differently to the glueball states and $q\bar{q}$ mesons. The observation of the light flavor singlet pseudoscalar meson can be viewed as the manifestation of effects of dynamical quarks. In the scalar channel, the ground state masses extracted from the correlation functions of gluonic operators are determined to be around 1.4-1.5 GeV, which is close to the ground state masses from the correlation functions of the quark bilinear operators. In all cases, the mixing between glueballs and conventional mesons remains to be further clarified in the future.

hep-lat↗

arXiv2018

Measuring the winding number in a large-scale chiral quantum walk

We report the experimental measurement of the winding number in an unitary chiral quantum walk. Fundamentally, the spin-orbit coupling in discrete time quantum walks is implemented via birefringent crystal collinearly cut based on time-multiplexing scheme. Our protocol is compact and avoids extra loss, making it suitable for realizing genuine single-photon quantum walks at a large-scale. By adopting heralded single-photon as the walker and with a high time resolution technology in single-photon detection, we carry out a 50-step Hadamard discrete-time quantum walk with high fidelity up to 0.948$\pm$0.007. Particularly, we can reconstruct the complete wave-function of the walker that starts the walk in a single lattice site through local tomography of each site. Through a Fourier transform, the wave-function in quasi-momentum space can be obtained. With this ability, we propose and report a method to reconstruct the eigenvectors of the system Hamiltonian in quasi-momentum space and directly read out the winding numbers in different topological phases (trivial and non-trivial) in the presence of chiral symmetry. By introducing nonequivalent time-frames, we show that the whole topological phases in periodically driven system can also be characterized by two different winding numbers. Our method can also be extended to the high winding number situation.

quant-ph↗

arXiv2018

Cavity-mediated oscillating and trapping dynamics in a two-component condensate

Cold atoms in cavity provides a new platform for exploring exotic many-body phases. Here we explore the dynamics of a two-component condensate coupled to a finesse cavity, in which the Raman coupling is mediated by pumping laser and cavity mode. In this model, the energy scale of cavity mode is several order of magnitude bigger than that in the condensate, thus the small fluctuations in the cavity field may have important consequence in the dynamics of condensate. Beyond the steady-state approximaton, we show the cavity can play two different roles to this dynamics. In the first case, it imprints a gauge potential to the dynamics of condensate, giving rise to zero and $π$ Josephson dynamics. Nevertheless, in the other case, it plays the role of non-reciprocial transportation between the two hyperfine states, in which the stability of the fixed points are tuned from elliptic to stable spiral for one of the trapped phase and unstable spiral for the other trapped phase, thus the oscillating dynamics will finally ceased. The transition between these dynamics can be controlled by the parameters of the cavity field and the driving field. Our results demonstrate an novel way to engineer the dynamics of condensate by tuning the stability of the fixed points.

cond-mat.quant-gas↗