We investigate theoretically nonlinear Thomson scattering by multiple electrons ionized from individual atoms during a short high-intensity laser pulse. The emitted light is influenced by the distance that the electrons move apart from each other during the passage of the pulse, owing to coherence effects. We examine trajectories of electrons born from the same atom via successive ionizations as the laser pulse ramps up. While the overall trajectory of an individual electron is influenced by the ponderomotive force, we find that the separation between electrons arises mostly from stronger and differing initial drift velocities associated with the moment of ionization in the laser field. In the case of helium, we find that the separation between its two ionized electrons becomes appreciable (compared to emitted wavelengths) primarily along the dimension of laser linear polarization. This distorts the angular emission patterns of nonlinear Thomson scattering in comparison with emission from individual free electrons. Radiation scattered perpendicular to the laser polarization tends to add constructively, while radiation scattered along the direction of linear laser polarization tends to add incoherently. This effect becomes more pronounced for atoms with higher numbers of ionized electrons. The effect influences primarily the lower harmonic orders.

We propose several schemes with diverse optional types of couplings to accelerate adiabatic topological quantum state transfers (QSTs) in optomechanical lattices (OMLs) by utilizing shortcut engineering, which hold significant promise for constructing topologically protected quantum networks. We first investigate the counterdiabatic (CD) schemes of shortcut to adiabatic topological QST in a small OML consisting of five sites. We show that a perfect QST can be achieved with either the full or the partial-CD terms, and the QST duration can be reduced to about one-tenth of that in the adiabatic version. To circumvent the next-nearest-neighbor couplings in the CD shortcut schemes, the detuning-induced transitionless-quantum-driving scheme is proposed and improved by incorporating the technique of soft quantum control, for which the auxiliary shortcut coupling becomes a nearest-neighbor hopping when considering a cyclic OML. We find that all shortcut schemes can maintain robustness of the adiabatic topological QST against disorders to a great extent, and more importantly, due to the much shorter duration, the shortcut schemes can guarantee a higher efficiency of QST when decay of the OML is taken into account. Finally, we expand the size of OML and propose a more general method by engineering an optimal coupling parameter to speed up the adiabatic topological QSTs, which can significantly shorten the QST duration, especially in large-scale OMLs.

Chromatic aberration occurs when high-order harmonic generation (HHG) is focused far away from its generation medium, exhibiting as the focal point being shifted with the harmonic order, which becomes an obstacle for HHG being a powerful table-top light source. Here, we demonstrate through simulations that chromatic aberration can be efficiently corrected by using a flat-top beam as the driving laser, which is induced by the plasma when the laser beam propagates in a gas medium. We show that the elimination of HHG chromatic aberration with the flat-top beam is robust in a gas-filled waveguide under proper gas pressure and waveguide length. We reveal that formation of the flat-top beam and reduction of HHG chromatic aberration in a gas cell is strongly dependent on its position with respect to the laser focus. HHG chromatic aberration can only be effectively reduced with the flat-top beam when the gas cell is located at the laser focus, which is attributed to the geometric phase of the driving laser. We also analyze that high harmonics are generated under the phase-matching conditions simultaneously when chromatic aberration is reduced. Finally, we show the limitation of a single-layer model by comparing its predictions with macroscopically propagated results in the gas medium. We expect that our method can be useful for focusing high harmonics in the extreme ultraviolet and soft x rays for various applications.

We investigate nonequilibrium energy transport and nonclassical correlations of photons in a nonequilibrium two-mode circuit quantum electrodynamic system with noncommutative qubit-photon couplings. Our calculations include a quantum master equation, which can handle ultrastrong qubit-photon interactions. For weak qubit-photon couplings, the nonmonotonic behavior of nonequilibrium energy flow is explored for large temperature bias, stemming from the breakdown of microscopic energy transition cycles. Two distinct scaling features of the coupling parameters in energy currents are analytically shown. Moreover, the longitudinal qubit-photon coupling shows a dramatic contribution to the exhibition of antibunching behaviors of autocorrelation and violation of the Cauchy-Schwarz inequality of the cross correlation of photons. These results may provide insight into quantum energy modulation devices and photon correlation manipulation in circuit quantum electrodynamics.

We study the scaling of the ground-state entanglement entropy of various free fermionic models on one-dimensional lattices, where the hopping and pairing terms decay as a power law. We seek to understand the scaling of entanglement entropy in generic models as the exponent of the power law $\alpha$ is varied. We ask if a common ${\alpha }_c$ exists across different systems governing the transition to area-law scaling found in local systems. We explore several examples numerically and argue that when applicable, the scaling of entanglement entropy in long-range models is constrained by predictions from the low-energy theory. In contrast, disordered models and models without a continuum limit show fractal scaling of entanglement approaching volume-law behavior as $\alpha$ approaches zero. These general features are expected to persist on turning on interactions.

Quantum convolutional neural network (QCNN), an early application for quantum computers in the noisy intermediate-scale quantum era, has been consistently proven successful as a machine learning (ML) algorithm for several tasks with significant accuracy. Derived from its classical counterpart, QCNN is prone to overfitting. Overfitting is a typical shortcoming of ML models that are trained too closely to the availed training dataset and perform relatively poorly on unseen datasets for a similar problem. In this work we study post-training approaches for mitigating overfitting in QCNNs. We find that a straightforward adaptation of a classical post-training method, known as neuron dropout, to the quantum setting leads to a significant and undesirable consequence: a substantial decrease in success probability of the QCNN. We argue that this effect exposes the crucial role of entanglement in QCNNs and the vulnerability of QCNNs to entanglement loss. Hence, we propose a parameter adaptation method as an alternative method. Our method is computationally efficient and is found to successfully handle overfitting in the test cases.

Out-of-time-order correlators (OTOCs) serve as a proxy for quantum information scrambling, which refers to the process where information stored locally disperses across the many-body degrees of freedom in a quantum system, rendering it inaccessible to local probes. Most experimental implementations of OTOCs to probe information scrambling rely on indirect measurements based on global observables, using techniques such as Loschmidt echoes andmultiple quantum coherences, via time-reversal evolutions. In this article, we establish a direct connection between OTOCs with global and local observables in the context of NMR experiments, where the observable is the total magnetization of the system. We conduct a numerical analysis to quantify the differences in the evolution of both magnitudes, evaluating the excitation dynamics in spin ring systems with 8 to 16 spins, using a many-body Hamiltonian and long-range interactions. Our analysis decomposes the global echo into a sum of local echoes and cross-contributions, leading to local and global OTOCs. The results indicate that, after an initial transient period, local OTOCs determine the global ones. We observe that the difference between the average of local OTOCs and the global one, as well as their fluctuations, becomes negligible as the system size increases. Thus, for large homogeneous systems, global and local OTOCs become equivalent. This behavior aligns with that observed in highly interacting or chaotic systems in several experiments.

Higher-order interactions in spin-based Hamiltonians are crucial in addressing numerous fundamentally significant physical problems. In this work, Rydberg-atom graph gadgets are introduced to effectively program $K\mathrm{th}$-order interactions within a Rydberg-atom system. This approach facilitates the determination of the ground states of an Ising-type Hamiltonian, encoded to solve higher-order unconstrained optimization problems. A favorable scaling behavior, $O\left(N^K\right)$, is expected in terms of the number of atoms required for $N$-vertex hypergraph optimization problems.

We present a quantum-classical algorithm called LAS-QKSD for multireference systems, by combining a classical localized active space (LAS) fragment-based multireference algorithm with the quantum Krylov subspace diagonalization (QKSD) method for quantum computers. The algorithm uses wave-function information from a LAS self-consistent field (LASSCF) calculation to prepare an initial state with better overlap with the target ground state than the Hartree-Fock state. This is coupled with the use of QKSD to ultimately converge to the exact energy, providing faster convergence than starting from the Hartree-Fock state. Fragmentation has the twofold benefit of fewer configurations on the classical side of the algorithm as well as fewer state preparation gates on the quantum side. First, we compare the LAS-QKSD method to the classical LASSCF method and to QKSD with a Hartree-Fock initial state. We then examine ways to load the LASSCF wave function using direct initialization and a QKSD-motivated spectral filtering approach. Finally, using a bimetallic complex, we show that the LAS-QKSD method is an efficient alternative to highly expensive complete active space self-consistent field calculations on strongly correlated systems.

We present a deperturbation analysis of the spin-orbit-coupled $A{}^1{\mathrm{\Sigma }}^{+}$ and $b{}^3{\mathrm{\Pi }}_{0^{+}}$ states of LiRb based on the rovibrational energy levels observed previously by photoassociation spectroscopy in the bosonic ${}^7\mathrm{Li}{}^{85}\mathrm{Rb}$ molecule. Using the genetic algorithm (GA), we optimize the pointwise ab initio potential energy curves of the $A{}^1{\mathrm{\Sigma }}^{+}$ state and the $b{}^3\mathrm{\Pi }$ state based on spectroscopic data. Then we fit the pointwise potentials obtained by the GA and the ab initio spin-orbit-coupling term with the expanded Morse oscillator (EMO) function. Finally, we optimize the parameters of the EMO function using the sequential least-squares quadratic programming algorithm with the guidance of the experimental data and pointwise potentials obtained by the GA. From the fitted results, we calculate the transition dipole moment matrix elements for transitions from the rovibrational levels of the coupled $A{}^1{\mathrm{\Sigma }}^{+}\text{–}b{}^3{\mathrm{\Pi }}_{0^{+}}$ state to the Feshbach state and the absolute rovibrational ground state for the fermionic ${}^6\mathrm{Li}{}^{87}\mathrm{Rb}$ molecule. Based on the calculated transition dipole moment matrix elements, several levels of the coupled $A{}^1{\mathrm{\Sigma }}^{+}\text{–}b{}^3{\mathrm{\Pi }}_{0^{+}}$ state are predicted to be suitable as the intermediate state for stimulated Raman adiabatic passage transfer from the Feshbach state to the absolute rovibrational ground state. In addition, we also provide a similar estimation for the $B{}^1\mathrm{\Pi }\text{–}c{}^3{\mathrm{\Sigma }}_1^{+}\text{–}b{}^3{\mathrm{\Pi }}_1$ state based on available ab initio interaction potentials.

We analyze the Casimir-Lifshitz effect associated with the electromagnetic field in the presence of a rectangular waveguide consisting of two distinct dielectric materials in $(3+1)$-dimensional space-time. We employ the surface-mode technique to derive a generalized Lifshitz formula for this specific geometry. Our formulation accounts for the unique dielectric properties of the materials composing the waveguide, leading to a precise calculation of the Casimir-Lifshitz energy. In the asymptotic limit, our results recover the classical expressions for perfectly reflecting boundaries. This work extends the applicability of the Lifshitz formula to more complex systems and provides valuable insights into the influence of dielectric materials on the electromagnetic Casimir effect.

In an earlier paper, we explored how quantum vacuum torque can arise: a body or nanoparticle that is out of thermal equilibrium with its environment experiences a spontaneous torque. But this requires that the body be composed of nonreciprocal material, which seems to necessitate the presence of an external influence, such as a magnetic field. Then the electric polarizability of the particle has a real part that is nonsymmetric. This effect occurs to first order in the polarizability. To that order, no self-propulsive force can arise. Here, we consider second-order effects, and show that spontaneous forces can arise in vacuum, without requiring exotic electromagnetic properties. Thermal nonequilibrium is still necessary, but the electric susceptibility of the body need only be inhomogeneous. We investigate four examples of such a body: a needle composed of distinct halves; a sphere and a ball, each hemisphere being made of a different substance; and a thin slab, each face of which is different. The results found are consistent with previous numerical investigations. Here, we take into account the skin depth of metal surfaces. We also consider the frictional forces that would cause the body to acquire a terminal velocity, which might be observable. More likely to be important is relaxation to thermal equilibrium, which can still lead to a terminal velocity that might be experimentally verifiable. A general treatment of such forces on a moving body, expressed in momentum space, is provided, which incorporates both propulsive and frictional forces. The source of the propulsive force is the nonsymmetric pattern of radiation from different parts of the body, the higher reflectivity of the metal portion playing a crucial role.

We present measurements of the hyperfine structure (HFS) of the $n{P}_J$ Rydberg states for a large range of principal quantum number $n$ $(n=41–55)$ employing microwave spectroscopy in an ultracold cesium Rydberg ensemble. A microwave field with $30\text{−}\mu \mathrm{s}$ duration couples the $nS\to nP$ transition, yielding a narrow linewidth spectroscopy that approaches the Fourier limit, which allows us to resolve the hyperfine structure of $n{P}_J$ states. By analyzing the hyperfine splittings of $n{P}_J$ states, we determine the magnetic dipole HFS coupling constant ${\overline{A}}_{\mathrm{HFS},P_{1/2}}=3.760\left(26\right)$ GHz for the $P_{1/2}$ state, and ${\overline{A}}_{\mathrm{HFS},P_{3/2}}=0.718\left(27\right)$ GHz and ${\overline{B}}_{\mathrm{HFS},P_{3/2}}=-0.084\left(102\right)$ GHz for the $P_{3/2}$ state. Systematic uncertainties caused by stray electromagnetic field, microwave field power, and Rydberg interaction are analyzed. This measurement is significant for the investigation of Rydberg electrometry and quantum simulation with dipole interaction involving the $n{P}_J$ state.

We have developed a method to simulate quantum spin models with the Dzyaloshinskii-Moriya interaction (DMI) using Rydberg atom quantum simulators. Our approach involves a two-photon Raman transition and a transformation to the spin-rotating frame, both of which are feasible with current experimental techniques. As a model that can be simulated in our setup but not in solid-state systems, we consider an $S=\frac{1}{2}$ spin chain with a Hamiltonian consisting of the DMI and Zeeman energy. We study the magnetization curve in the ground state of this model and quench dynamics. Further, we show the existence of quantum many-body scar states and asymptotic quantum many-body scar states. The observed nonergodicity in this model demonstrates the importance of the highly tunable DMI that can be realized by the proposed quantum simulator.

We investigate the Uhlmann phase, a generalization of the celebrated Berry phase, for Bose-Einstein condensates (BECs) at finite temperature. The Uhlmann phase characterizes topological properties of mixed states, in contrast to the Berry phase, which is defined for pure states at zero temperature. Using the $\text{SU}(1,1)$ symmetry of the Bogoliubov Hamiltonian, we derive a general formula for the Uhlmann phase of BECs. Numerical calculations reveal that the Uhlmann phase can differ from the Berry phase in the zero-temperature limit, contrary to previous studies. As the temperature increases, the Uhlmann phase exhibits a winding behavior, and we relate the total winding degree to the Berry phase. This winding indicates that the Uhlmann phase takes values on a Riemann surface. Furthermore, we propose an experimental scheme to measure the Uhlmann phase of BECs by purifying the density matrix using an atomic interferometer.

Nonlinear Landau-Zener tunneling is an important nonlinear phenomenon. We propose to stimulate the nonlinear tunneling in a spin-orbit-coupled spinor Bose-Einstein condensate. The system provides an experimentally tunable nonlinearity as well as multiple avoided crossings with tunable gap size in nonlinear dispersion relations. The nonlinearity generates tilted cusp and loop structures around the avoided crossings, and the physical consequence of these nonlinear structures is the nonlinear Landau-Zener tunneling. The spin-momentum locking induced by the spin-orbit coupling leads to a time-resolved observation of the nonlinear tunneling by measuring atom populations.