The Tavis-Cummings (TC) model, which serves as a natural physical realization of a quantum battery, comprises $N_b$ atoms as battery cells that collectively interact with a shared photon field, functioning as the charger, initially containing $n_0$ photons. In this paper, we introduce the invariant subspace method to effectively represent the quantum dynamics of the TC battery. Our findings indicate that in the limiting case of $n_0\gg N_b$ or $N_b\gg n_0$, a distinct SU(2) symmetry emerges in the dynamics, thereby ensuring the realization of optimal energy storage. We also establish a negative relationship between the battery-charger entanglement and the energy storage capacity. As a result, we demonstrate that asymptotically optimal energy storage can be achieved in the scenario where $N_b=n_0\gg 1$. Our approach not only enhances our comprehension of the algebraic structure inherent in the TC model but also contributes to the broader theoretical framework of quantum batteries. Furthermore, it provides crucial insights into the relation between energy transfer and quantum correlations.

Supervised quantum learning is an emergent multidisciplinary domain bridging between variational quantum algorithms and classical machine learning. Here, we study experimentally a hybrid classifier model using quantum hardware simulator (a linear array of four superconducting transmon artificial atoms) trained to solve multilabel classification and image recognition problems. We train a quantum circuit on simple binary and multilabel tasks, achieving classification accuracy around 95%, and a hybrid quantum model with data reuploading with accuracy around 90% when recognizing handwritten decimal digits. Finally, we analyze the inference time in experimental conditions and compare the performance of the studied quantum model with known classical solutions.

Full counting statistics (FCS) provides a powerful framework to access the statistical information of a system from the characteristic function. However, applications of FCS for generic interacting quantum systems are often hindered by the intrinsic difficulty of classical simulation of quantum many-body problems. Here we propose a quantum algorithm for FCS that can obtain both the particle distribution and cumulants of interacting systems. The algorithm evaluates the characteristic functions by quantum computing and then extracts the distribution and cumulants with classical postprocessing. With digital signal processing theory, we analyze the dependence of accuracy with the number of sampling points for the characteristic functions. We show that the desired number of sampling points for accurate FCS can be reduced by filtering some components of the quantum state that are not of interest. By numeral simulation, we demonstrate the FCS of domain walls for several representative models. Remarkably, we show that the accuracy of our algorithm does not significantly change with the system size for a given number of measurements. The algorithm suggests an avenue for studying full counting statistics on quantum computers.

Position error is treated as the leading obstacle that prevents Rydberg antiblockade gates from being experimentally realizable, because of the inevitable fluctuations in the relative motion between two atoms, invalidating the antiblockade condition. In this work we report progress towards a high-tolerance antiblockade-based Rydberg swap gate enabled by the use of a modified antiblockade condition combined with carefully optimized laser pulses. Depending on the optimization of diverse pulse shapes, our protocol shows that the amount of time spent in the double Rydberg state can be shortened by more than $70\%$ with respect to the case using the perfect antiblockade condition, which significantly reduces this position error. Moreover, we benchmark the robustness of the gate by taking into account the technical noises, such as the Doppler dephasing due to atomic thermal motion, the fluctuations in laser intensity and laser phase, and the intensity inhomogeneity. Compared to other existing antiblockade-gate schemes, we are able to maintain a predicted gate fidelity above 0.91 after a very conservative estimation of various experimental imperfections, especially considered for a realistic interaction deviation of $\delta V/V\approx 5.92\%$ at $T\sim 20$ $\mu \mathrm{K}$. Our work creates an opportunity for the experimental demonstration of Rydberg antiblockade gates in the near future.

The Quantum Projection Operator-Based Newton Method for Trajectory Optimization (q-pronto) is a numerical method for solving quantum optimal control problems. This paper significantly improves prior versions of the quantum projection operator by introducing a regulator that stabilizes the solution estimate at every iteration. This modification is shown to not only improve the convergence rate of the algorithm, but also steer the solver towards better local minima compared to the unregulated case. Numerical examples showcase how q-pronto can be used to solve multi-input quantum optimal control problems featuring time-varying costs and undesirable populations that ought to be avoided during the transient.

We present all-electron close-coupling calculations of cross sections for electron capture and target excitation occurring in 0.25–200 keV/u ${\mathrm{He}}^{+}\text{−}\mathrm{H}$ collisions. We show overall good agreements with available experimental data for total electron capture, $n=2$ target excitation, and related ${\mathrm{H}}_{\alpha }$ emission. We especially focus on $n\ell$-selective capture and excitation processes and critically compare our results with those from previous theoretical investigations. Our results confirm the oscillatory structures observed in the spin-averaged $\mathrm{H}\left(2s\right)$ and $\mathrm{H}\left(2p\right)$ excitation cross sections for impact energies below 50 keV/u. Furthermore, we interpret these oscillations as the consequence of interferences between excitation to $\mathrm{H}\left(2s\right)$ and $\mathrm{H}\left(2p_0\right)$ in both spin-singlet and spin-triplet symmetries, as well as between electron capture to $\mathrm{He}(1s2p{}^{1}P,$ $|M_L|=1)$ and excitation to $\mathrm{H}\left(2p_1\right)$ in spin-singlet symmetry.

We investigate signatures of anisotropy on the dynamics of time-resolved near-threshold molecular photoemission through simulations on a one-dimensional asymmetric model molecule. More precisely, we study the relationship between the fundamental Wigner delays that fully characterizes the dynamics of one-photon ionization and the delays inferred from two-color interferometric RABBIT measurements. Our results highlight two different properties pertaining to each of these delays. The first one is related to the inherent necessity to set an arbitrary electron position origin to define and compute the Wigner delay. The second one is the dependency of the RABBIT delay on the frequency of the probe laser. Our results show that the angular variations of both delays converge for a specific choice of the position origin and in the limit of a vanishing infrared probe frequency.

We examine the quench dynamics across quantum phase transitions from a Mott insulator (MI) to a superfluid (SF) phase in a two-component bosonic mixture in an optical lattice. We show that two-component Bose mixtures exhibit qualitatively different quantum dynamics than one-component Bose gas. In addition to second-order MI-SF transitions, we investigate quench dynamics across a first-order MI-SF phase boundary. The Bose mixtures show the critical slowing down of dynamics near the critical transition point, as proposed by the Kibble-Zurek mechanism. For MI-SF transitions with homogeneous lattice-site distributions in the MI phase, the dynamical critical exponents extracted by the power-law scaling of the proposed quantities obtained via numerical simulations are in very close agreement with the mean-field predictions.

We study the dynamical quantum phase transition (DQPT) of the Bose-Hubbard model utilizing the recently developed Loschmidt cumulants method. We determine the complex Loschmidt zeros of the Loschmidt amplitude analogous to the Lee-Yang zeros of the thermal partition function. We obtain the DQPT critical points through identifying the crossing points with the imaginary axis. The critical points show high accuracy when compared to those obtained using the matrix product states method. In addition, we show how the critical points of DQPT can be determined by analyzing the energy fluctuation of the initial state, which makes it a valuable tool for future studies in this area. Finally, DQPT in the extended Bose-Hubbard model is also investigated.

We consider a Bose-Einstein condensate inside an optical cavity and two crossed coherent pump fields. We determine perturbatively the phase boundary separating the normal superfluid phase and the superradiance phase. In the regime of negative cavity detuning, we map out the phase diagrams for both an attractive and a repulsive optical lattice. It turns out that the situation is quite different in the two cases. Specifically, in the case of an attractive lattice, if a system is in the superradiant phase with one pump laser, adding another pump does not drive the system out of the superradiance phase, while for the repulsive lattice, increasing another pump potential has suppressive effects on the superradiance. We also find that, in the case of an attractive lattice, equally increasing two pump lattice potentials can induce a transition from the normal phase to the superradiance phase. In stark contrast, for the repulsive lattice, the system will remain in the normal phase as the pump depths are tuned within a wide range, independently of the cavity detuning and the decay rate.

Strong coupling at the subwavelength scale has attracted extensive attention due to the interesting physical phenomena brought about by the enhanced light-matter interaction. Here we propose a borophene-based hybrid plasmonic structure, which facilitates the strong coupling between borophene surface plasmon (BSP) and magnetic polariton (MP) modes. The dispersive propagating BSPs can strongly interact with the nondispersive localized MPs by tuning the borophene electron density, resulting in two hybrid polariton bands featuring remarkable Rabi splitting (with 65 meV). The interaction behavior exhibits unique features of energy transfer, dual-band light trapping, and mode splitting characteristics, which can be described by the classical coupled oscillator model. The coupling strength exhibits a nearly linear relationship with spacer permittivity, increasing as the spacer permittivity decreases, and can reach a maximum of 200 meV. In addition, the anisotropic BSP-MP coupling can be achieved under different polarized light, which is attributed to the anisotropic properties of the borophene monolayer. This work may not only provide a platform for the study of light-matter interaction, but also has the potential to construct easy-to-fabricate borophene-based plasmonic devices.

We study theoretically quantum statistical and spectral properties of the resonance fluorescence of a single atom or system with angular momentum $J=1/2-J^{\prime }=1/2$ driven by a monochromatic linearly polarized laser field, due to quantum interference among its two antiparallel $\pi$ transitions. A magnetic field parallel to the laser polarization is applied to break the degeneracy (Zeeman effect). In the nondegenerate case, the $\pi$ transitions evolve at different generalized Rabi frequencies, producing quantum beats in the intensity and the dipole-dipole, intensity-intensity, and quadrature-intensity correlations. For a strong laser and large Zeeman splitting the beats have mean and modulation frequencies given by the average and difference, respectively, of the Rabi frequencies, unlike the beats studied in many spectroscopic systems, characterized by a modulated exponential-like decay. Further, the Rabi frequencies are those of the pairs of sidebands of the Mollow-like spectrum of the system. In the two-time correlations, the cross contributions, i.e., those with products of probability amplitudes of the two $\pi$ transitions, have a lesser role than those from the interference of the probability densities. In contrast, there are no cross terms in the total intensity. We also consider nonclassical and non-Gaussian properties of the phase-dependent fluorescence for the cases of weak to moderate excitation and in the regime of beats. The fluorescence in the beats regime is nonclassical, mainly from third-order dipole fluctuations, which reveal them to be also strongly non-Gaussian, and their quadrature spectra show complex features around the Rabi frequencies. For small laser and Zeeman detunings, a weak to moderate laser field pumps the system partially to one of the ground states, showing slow decay in the two-time correlations and a narrow peak in the quadrature spectra.

We investigate optical sideband generation via the nonlinear magnon in a hybrid cavity-optomagnonical system consisting of a yttrium iron garnet sphere, a microwave copper cavity, and an optical cavity. By using a perturbation method to solve the nonlinear Heisenberg-Langevin equations, we obtain the analytical formulation of the nonlinear-magnon-induced optical second-order sideband (OSS) generation. It is found that the number of peaks in the OSS spectrum can change from 2 to 4 with the increase of the effective optical cavity-magnon coupling strength (EOCCS) within the experimental parameter range, which is caused by the gradual increase in overlap between the first- and second-order spectra. Moreover, the results show that the maximum value of the OSS will appear with an increase of the EOCCS, and the microwave cavity-magnon coupling strength (MCCS) will regulate the size and location of the maximum value. Subsequently, the output spectrum of the optical high-order sidebands is investigated using numerical simulations to demonstrate the modulation behavior of the OSS by the EOCCS and MCCS. In addition to providing insight into the interaction of the magnon with a microwave and light, our results may offer a new perspective for the development and design of microwave-controlled optical frequency combs.

We consider a quantum Otto cycle with an interacting Bose-Einstein condensate at finite temperature. We present a procedure to evolve this system in time in three spatial dimensions, in which closed (adiabatic) strokes are described by the Gross-Pitaevskii equation and open (isochoric) strokes are modeled using a stochastic Ginzburg-Landau equation. We analyze the effect on the thermodynamic efficiency of the strength of interactions, the frequency of the harmonic trap, and the temperatures of the reservoirs. The efficiency has little sensitivity to changes in the temperatures but decreases as interactions increase. However, stronger interactions allow for faster cycles and for substantial increases in power.

We propose a convenient and easily computable nonclassicality quantifier for bosonic field states based on the Wigner-Yanase skew information. The proposed nonclassicality quantifier is reflected by the quantum interaction between the maximum phase angle of the homodyne rotated quadrature operator and the bosonic field states. If the value of the nonclassical quantifier is greater than one-half, the state is nonclassical, and the quantifier is one-half for the pure classical. It is worth mentioning that an increase in the strength of nonclassicality inducing operations, such as squeezing and photon addition, leads to an enhancement of the nonclassicality in the quantum state. By computing some well-known nonclassical states and summarizing their existence features, we have confirmed the validity of our proposed nonclassicality quantifier. We have shown that in a range of values of the nonclassicality of the Gaussian state, revealing the sufficiency of the quantifier. We also compare it with two common nonclassical quantifiers to illustrate its advantages.

Quantum memories for quantum communication need to be able to store photons for an extended time and then to release them on demand. This can be achieved in atomic frequency comb ensemble-based quantum memories by control pulses that transfer the excitation to and from long-lived spin states. However, such pulses can give rise to coherent and incoherent noise due to their interaction with the memory ensemble. In this paper, we experimentally demonstrate the ability to switch off the coherent noise from such control pulses during the echo emission in a spin-wave quantum memory, using the linear Stark effect in rare-earth-ion doped crystals. By applying an electric-field pulse, the echo emission was coherently switched off prior to the first spin transfer pulse, and the stored data pulse was restored only when both an optical recall pulse and a rephasing electrical pulse were applied, giving a high degree of control of both desired and undesired emissions. We estimate the effectiveness of this technique by turning off the free-induction decay of a narrow ensemble of ions. This technique can thus improve the noise performance of spin-wave storage at the single-photon level by quenching coherent optical radiation created by strong control pulses. The method demonstrated here represents a proof of principle that the spin-wave storage scheme can be combined with Stark control. The combined scheme serves as an addition to the toolbox of techniques that can be used to realize a full version of a quantum repeater.

Recent experiments have employed rapidly expanding toroidal Bose-Einstein condensates (BECs) to mimic the inflationary expansion in the early universe. One expected signature of the expansion in such experiments is spontaneous particle creation (of phonons) which is observable in density-density correlations. We study entanglement of these particles, which are known to result in a two-mode squeezed state. Using techniques for Gaussian states of continuous variable systems, we quantify the entanglement generated in this system, including effects such as decoherence and the use of an initially squeezed state, which can suppress and enhance entanglement, respectively. We also describe a protocol to experimentally measure the correlations entering the covariance matrix, allowing an experimental quantification of the entanglement properties of the inflationary BEC.