arXiv subjects

Hong-Gang Luo

Publications and source records attributed to Hong-Gang Luo.

At least 73 records · Page 4Linked to original sources

arXiv2020

Non-Markovian effect on quantum optical metrology under dissipative environment

Quantum metrology utilizes quantum effects to reach higher precision measurements of physical quantities compared with their classical counterparts. However the ubiquitous decoherence obstructs its application. Recently, non-Markovian effects are shown to be effective in performing quantum optical metrology under locally dissipative environments\cite{PhysRevLett.123.040402}. However, the mechanism is still rather hazy. Here, we uncover the reason why forming a bound state can protect the quantumness against a dissipative ambient via the quantum Fisher information of entangled coherent states. An exact analytical expression of the quantum Fisher information in the long-encoding-time condition is derived, which reveals that the dynamics of precision can asymptotically reach the ideal-case-promised one easily when the average photon number is small. Meanwhile, the scaling exhibits a transition from the weak Heisenberg limit to the sub-classical limit with the increase of average photon number. Our work provides a recipe to realize ultrasensitive measurements in the presence of noise by utilizing non-Markovian effects.

quant-ph↗

arXiv2020

Analysis of time-resolved single-particle spectrum on the one-dimensional extended Hubbard model

We investigate the short-time evolution of the half filled one-dimensional extended Hubbard model in the strong-coupling regime, driven by a transient laser pump. Combining twisted boundary conditions with the time-dependent Lanczos technique, we obtain snapshots of the single-particle spectral function with high momentum resolution. The analysis of the oscillations of the spectral function shows that its characteristic frequencies are consistent with the magnitudes of the optical gap. Furthermore, we examine the time-evolving spectral structure in the charge-density-wave phase in detail and find that one of the bands in the single-particle spectrum originates from the photoinduced bond-order background.

cond-mat.str-el↗

arXiv2020

Performance of the T-matrix based master equation for Coulomb drag in double quantum dots

Recently, novel Coulomb drag mechanisms in capacitively coupled double quantum dots were uncovered by the T-matrix based master equation (TME). The TME is so far the primary approach to studying Coulomb drag in the weak-coupling regime; however, its accuracy and reliability remain unexplored. Here, we evaluate the performance of the TME for Coulomb drag via a comparison with numerically exact results obtained by the hierarchical equation-of-motion approach. We find that the TME can capture qualitative current evolutions versus dot levels, temperature, and effective coupling strengths, but only partially succeeds at the quantitative level. Specifically, the TME gives highly inaccurate drag currents when large charge fluctuations on dots exist and the fourth-order tunneling processes make a leading-order contribution. This failure of the TME is attributed to the combined effect of the unique drag mechanisms and its overlook of the fourth-order single-electron tunnelings. We identify the reliable regions to facilitate further quantitative studies on Coulomb drag by the TME.

cond-mat.mes-hall↗

arXiv2019

Application of polaron picture in the two-qubit quantum Rabi model

The polaron picture is employed to investigate and elucidate the physics of the two-qubit quantum Rabi model, which describes two identical qubits coupled to a common harmonic oscillator. This approach enables us to obtain the ground-state energy and some other simpler physical observables with high accuracy in all regimes of the coupling strengths $g$, while there is no constraint to the ration of tunneling frequency $Ω$ and field frequency $ω$, which is not simultaneously possible using previous methods. Besides, we also discover a new phenomenon which is not present in the one-qubit Rabi model: with the increase of coupling strength $g$, there is a transition of the ground state of the system from a multipolaron state to a bipolaron state. However, the tunneling frequency $Ω$ counteracts this process. Specifically, when tunneling frequency $Ω=0$, the system always stays in the bipolaron state.

quant-ph↗

arXiv2019

Quantum phase transitions and critical behaviors in the two-mode three-level quantum Rabi model

We explore an extended quantum Rabi model describing the interaction between a two-mode bosonic field and a three-level atom. Quantum phase transitions of this few degree of freedom model is found when the ratio $η$ of the atom energy scale to the bosonic field frequency approaches infinity. An analytical solution is provided when the two lowest-energy levels are degenerate. According to it, we recognize that the phase diagram of the model consists of three regions: one normal phase and two superradiant phases. The quantum phase transitions between the normal phase and the two superradiant phases are of second order relating to the spontaneous breaking of the discrete $Z_{2}$ symmetry. On the other hand, the quantum phase transition between the two different superradiant phases is discontinuous with a phase boundary line relating to the continuous $U(1)$ symmetry. For a large enough but finite $η$, the scaling function and critical exponents are derived analytically and verified numerically, from which the universality class of the model is identified.

quant-ph↗

arXiv2019

Photoinduced enhancement of bond-order in the one-dimensional extended Hubbard model

We investigate the real-time dynamics of the half-filled one-dimensional extended Hubbard model in the strong-coupling regime, when driven by a transient laser pulse. Starting from a wide regime displaying a charge-density wave in equilibrium, a robust photoinduced in-gap state appears in the optical conductivity, depending on the parameters of the pulse. Here, by tuning its conditions, we maximize the overlap of the time-evolving wavefunction with excited states displaying the elusive bond-ordered wave of this model. Finally, we make a clear connection between the emergence of this order and the formation of the aforementioned in-gap state, suggesting the potential observation of purely electronic (i.e., not associated with a Peierls instability) bond-ordered waves in experiments involving molecular crystals.

cond-mat.str-el↗

arXiv2019

Retrieving ideal precision in noisy quantum optical metrology

Quantum metrology employs quantum effects to attain a measurement precision surpassing the limit achievable in classical physics. However, it was previously found that the precision returns the shot-noise limit (SNL) from the ideal Zeno limit (ZL) due to the photon loss in quantum metrology based on Mech-Zehnder interferometer. Here, we find that not only the SNL can be beaten, but also the ZL can be asymptotically recovered in long-encoding-time condition when the photon dissipation is exactly studied in its inherent non-Markovian manner. Our analysis reveals that it is due to the formation of a bound state of the photonic system and its dissipative noise. Highlighting the microscopic mechanism of the dissipative noise on the quantum optical metrology, our result supplies a guideline to realize the ultrasensitive measurement in practice by forming the bound state in the setting of reservoir engineering.

quant-ph↗

arXiv2019

Parity-dependent phase diagrams in spin-cluster two-leg ladders

Motivated by the recent experiment on $\rm{K_2Cu_3O\left(SO_4\right)_3}$, an edge-shared tetrahedral spin-cluster compound [M. Fujihala \textit{et al.}, Phys. Rev. Lett. \textbf{120}, 077201 (2018)], we investigate two-leg spin-cluster ladders with the plaquette number $n_p$ in each cluster up to six by the density-matrix renormalization group method. We find that the phase diagram of such ladders strongly depends on the parity of $n_p$. For even $n_p$, the phase diagram has two phases, one is the Haldane phase, and the other is the cluster rung-singlet phase. For odd $n_p$, there are four phases, which are a cluster-singlet phase, a cluster rung-singlet phase, a Haldane phase and an even Haldane phase. Moreover, in the latter case the region of the Haldane phase increases while the cluster-singlet phase and the even Haldane phase shrink as $n_p$ increases. We thus conjecture that in the large $n_p$ limit, the phase diagram will become independent of $n_p$. By analysing the ground-state energy and entanglement entropy we obtain the order of the phase transtions. In particular, for $n_p=1$ there is no phase transition between the even Haldane phase and the cluster-singlet phase while for other odd $n_p$ there is a first-order phase transition. Our work provides comprehensive phase diagrams for these cluster-based models and may be helpful to understand experiments on related materials.

cond-mat.str-el↗

arXiv2019

Resistivity minimum emerges in Anderson impurity model modified with Sachdev-Ye-Kitaev interaction

We investigate a modified Anderson model at the large-$N$ limit, where Coulomb interaction is replaced by Sachdev-Ye-Kitaev random interaction. The resistivity of conduction electron $ρ_{c}$ has a minimum value around temperature $\widetilde{T}_{K}$, which is similar to the Kondo system, but the impurity electron's density of state $A_{d}(ω)$ elucidates no sharp-peak like Kondo resonance around the Fermi surface. The impurity electron's entropy $S_{d}$ and specific heat capacity $C_{\textrm{v}}$ illustrate a crossover from Fermi liquid to the non-Fermi liquid. The system is a non-Fermi liquid at temperature $T^{\star} \widetilde{T}_{K}$. The non-Fermi liquid at intermediate-$T$ regime does not occur in standard Anderson model. With renormalization group analysis, we elucidate a crossover from Fermi liquid to the non-Fermi liquid, coinciding with transport and thermodynamics. The resistivity minimum and the Kondo resonance are two characteristics of Kondo effect. However, the resistivity minimum emerges in our model when the system behaves as a NFL rather than FL, and the impurity electron's density of state without the Kondo resonance.

cond-mat.str-el↗

arXiv2019

An analytical variational method for the biased quantum Rabi model in the ultra-strong coupling regime

An analytical variational method for the ground state of the biased quantum Rabi model in the ultra-strong coupling regime is presented. This analytical variational method can be obtained by a unitary transformation or alternatively by assuming the form of ground state wave function. The key point of the method is to introduce a variational parameter $λ$, which can be determined by minimizing the energy functional. Using this method, we calculate physical observables with high accuracy in comparison with the numerical exact one. Our method evidently improves the results from the widely used general rotating-wave approximation (GRWA) in both qualitative and quantitative ways.

quant-ph↗

arXiv2019

Variational generalized rotating-wave approximation in the two-qubit quantum Rabi model

We present an analytical method for the two-qubit quantum Rabi model. While still operating in the frame of the generalized rotating-wave approximation (GRWA), our method further embraces the idea of introducing variational parameters. The optimal value of the variational parameter is determined by minimizing the energy function of the ground state. Comparing with numerical exact results, we show that our method evidently improves the accuracy of the conventional GRWA in calculating fundamental physical quantities, such as energy spectra, mean photon number, and dynamics. Interestingly, the accuracy of our method allows us to reproduce the asymptotic behavior of mean photon number in large frequency ratio for the ground state and investigate the quasi-periodical structure of the time evolution, which are incapable of being predicted by the GRWA. The applicable parameter ranges cover the ultrastrong coupling regime, which will be helpful to recent experiments.

quant-ph↗

arXiv2019

Photoinduced charge carrier dynamics in Hubbard two-leg ladders and chains

The charge carrier dynamics of doped electronic correlated systems on ladders and chains, subject to ultrafast photoirradiation, is investigated using the time-dependent Lanczos method. The time-resolved optical conductivity and the temporal profiles of other relevant quantities, including the doublon number, the kinetic energy, and the interaction energy, are calculated. Two competitive factors that can influence the transient charge carrier dynamics are identified as the thermal effect and the charge effect. We demonstrate that the analysis of their interplay can provide an intuitive way to understand the numerical results and the recent optical pump-probe experiment on a two-leg ladder cuprate.

cond-mat.str-el↗

arXiv2018

Ellipticity dependence transition induced by dynamical Bloch oscillations

The dependence of high-harmonic generation (HHG) on laser ellipticity is investigated using a modified ZnO model. In the driving of relatively weak field, we reproduce qualitatively the ellipticity dependence as observed in the HHG experiment of wurtzite ZnO. When increasing the field strength, the HHG shows an anomalous ellipticity dependence, similar to that observed experimentally in the single-crystal MgO. With the help of a semiclassical analysis, it is found that the key mechanism inducing the change of ellipticity dependence is the interplay between the dynamical Bloch oscillation and the anisotropic band structure. The dynamical Bloch oscillation contributes additional quantum paths, which are less sensitive to ellipticity. The anisotropic band-structure make the driving pulse with finite ellipticity be able to drive the pairs to the band positions with larger gap, which extends the harmonic cutoff. The combination of these two effects leads to the anomalous ellipticity dependence. The result reveals the importance of dynamical Bloch oscillations for the ellipticity dependence of HHG from bulk ZnO.

physics.atom-ph↗

arXiv2018

Long-range Entanglement of Kondo Clouds in Open Triple Quantum Dots

We study entanglement of Kondo clouds in an open triple quantum dots (OTQDs) system based on the dissipaton equation of motion (DEOM) theory. A comprehensive picture of the long-range entanglement of Kondo clouds is sketched by the spectral functions, spin-spin correlation and dot occupancies of OTQDs. We find that for the configuration (N1,N2,N3) = (1, 0, 1), a conduction electrons peak occurs in the spectral function of intermediate QD in Kondo regime. This peak resulting from the overlapping of the two Kondo clouds forming from between the two peripheral QDs and leads, enhances with decreasing temperature and increasing dot-lead coupling. Both the spin-spin correlations between the two adjacent QDs and the two peripheral QDs owns negative values. It also confirms the physical picuture of the overlapping between left and right Kondo clouds via the intermediate QD. Moreover, the transition of the effective electron occupation and the spectral function of intermediate QD in Kondo regime also indicates the entanglement of Kondo clouds enhancing with decreasing temperature and increasing dot-lead coupling. This investigation will be beneficial to detect the Kondo clouds and to further explore Kondo physics in related experiment setups.

cond-mat.str-el↗

arXiv2018

Polaron picture of the two-photon quantum Rabi model

We employ a polaron picture to investigate the properties of the two-photon quantum Rabi model (QRM), which describes a two-level or spin-half system coupled with a single bosonic mode by a two-photon process. In the polaron picture, the coupling in the two-photon process leads to spin-related asymmetry so that the original single bosonic mode splits into two separated frequency modes for the opposite spins, which correspond to two \textit{bare} polarons. Furthermore, the tunneling causes these two bare polarons to exchange their components with each other, thus leading to additional \textit{induced} polarons. According to this picture, the variational ground-state wave function of the two-photon QRM can be correctly constructed, with the ground-state energy and other physical observables in good agreement with the exact numerics in all the coupling regimes. Furthermore, generalization to multiple induced polarons involving higher orders in the tunneling effect provides a systematic way to yield a rapid convergence in accuracy even around the difficult spectral collapse point. In addition, the polaron picture provides a distinctive understanding of the spectral collapse behavior, that is about the existence of discrete energy levels apart from the collapsed spectrum at the spectral collapse point. This work illustrates that the polaron picture is helpful to capture the key physics in this nonlinear light-matter interaction model and indicates that this method can be applicable to more complicated QRM-related models.

quant-ph↗

arXiv2018

$\mathcal{Z}_2$ classification for a novel antiferromagnetic topological insulating phase in three-dimensional topological Kondo insulator

Antiferromagnetic topological insulator (AFTI) is a topological matter that breaks time-reversal symmetry. Since its proposal, explorations of AFTI in strong-correlated systems are still lacking. In this paper, we show for the first time that a novel AFTI phase can be realized in three-dimensional topological Kondo insulator (TKI). In a wide parameter region, the ground states of TKI undergo a second-order transition to antiferromagnetic insulating phases which conserve a combined symmetry of time reversal and a lattice translation, allowing us to derive a $\mathcal{Z}_2$-classification formula for these states. By calculating the $\mathcal{Z}_2$ index, the antiferromagnetic insulating states are classified into (AFTI) or non-topological antiferromagnetic insulator (nAFI) in different parameter regions. On the antiferromagnetic surfaces in AFTI, we find topologically protected gapless Dirac cones inside the bulk gap, leading to metallic Fermi rings exhibiting helical spin texture with weak spin-momentum locking. Depending on model parameters, the magnetic transitions take place either between AFTI and strong topological insulator, or between nAFI and weak topological insulator. By varying some model parameters, we find a topological transition between AFTI and nAFI, driving by closing of bulk gap. Our work may account for the pressure-induced magnetism in TKI compound SmB$_6$, and helps to explore richer AFTI phases in heavy-fermion systems as well as in other strong-correlated systems.

cond-mat.str-el↗

arXiv2018

Benchmarking the simplest slave-particle theory with Hubbard dimer

Slave-particle method is a powerful tool to tackle the correlation effect in quantum many-body physics. Although it has been successfully used to comprehend various intriguing problems, such as Mott metal-insulator transition and Kondo effect, there is still no convincing theory so far on the availability and limitation of this method. The abuse of slave-particle method may lead to wrong physics. As the simplest slave-particle method, $\mathbb{Z}_2$ slave spin, which is widely applied to many strongly correlated problems, is highly accessible and researchable. In this work, we will uncover the nature of $\mathbb{Z}_2$ slave-spin method by studying a two-site Hubbard model. After exploring some properties of this toy model, we make a comparative analysis of the results obtained by three methods: (i) slave-spin method on mean-field level, (ii) slave-spin method with gauge constraint and (iii) the exact solution as a benchmark. We find that, protected by particle-hole symmetry, the slave-spin mean-field method can recover the static properties of ground state exactly at half filling. Furthermore, in the parameter space where both $U$ and $T$ are small enough, slave-spin mean-field method is also reliable in calculating dynamic and thermal dynamic properties. However, when $U$ or $T$ is considerably large, the mean-field approximation gives ill-defined behavior, which results from the unphysical states in enlarged Hilbert space. These findings lead to our conclusion that the accuracy of slave particle can be guaranteed if we can exclude all unphysical states by enforcing gauge constraints.Our work demonstrates the promising prospect of slave-particle method in studying complex strongly correlated models with specific symmetry or in certain parameter space.

cond-mat.str-el↗

arXiv2018

Quantum criticality and state engineering in the simulated anisotropic quantum Rabi model

Promising applications of the anisotropic quantum Rabi model (AQRM) in broad parameter ranges are explored, which is realized with superconducting flux qubits simultaneously driven by two-tone time-dependent magnetic fields. Regarding the quantum phase transitions (QPTs), with assistant of fidelity susceptibility, we extract the scaling functions and the critical exponents, with which the universal scaling of the cumulant ratio is captured with rescaling of the parameters due to the anisotropy. Moreover, a fixed point of the cumulant ratio is predicted at the critical point of the AQRM. In respect to quantum information tasks, the generation of the macroscopic Schrödinger cat states and quantum controlled phase gates are investigated in the degenerate case of the AQRM, whose performance is also investigated by numerical calculation with practical parameters. Therefore, our results pave a way to explore distinct features of the AQRM in circuit QED systems for QPTs, quantum simulations and quantum information processings.

quant-ph↗