In the study of optical properties of large atomic systems, a weak laser driving is often assumed to simplify the system dynamics by linearly coupled equations. Here we investigate the light-scattering properties of atomic ensembles beyond weak-field excitation through the cumulant expansion method. By progressively incorporating higher-order correlations into the steady-state equations, an enhanced accuracy can be achieved in comparison to the exact solutions obtained by solving a full density matrix. Our analysis reveals that, in the regime of weak dipole-dipole interaction, the first-order expansion yields satisfactory predictions for optical depth, while denser atomic configurations necessitate consideration of higher-order correlations. As the intensity of incident light increases, atom saturation effects become noticeable, giving rise to significant changes in light transparency, energy shift, and decay rate. This saturation phenomenon extends to subradiant atom arrays even under weak driving conditions, leading to substantial deviations from the linear model. Our findings demonstrate that the mean-field model is a good extension to linear models as it balances both accuracy and computational complexity. However, the crucial role of higher-order cumulants in large and dense atom systems remains unclear, since it is challenging theoretically owing to the exponentially increasing Hilbert space in such light-matter interacting systems.

Spin twisting is generally not incompatible with steady-state atomic coherence because spin twisting as a representative kind of parametric processes is based on far-off-resonant atom-field interactions while the steady-state atomic coherence is achieved mainly by resonant interactions. Here we propose a resonant plus far-off-resonant scheme to generate the compatibility. The resonant part uses coherent population trapping (CPT) for steady-state maximal atomic coherence, while the far-off-resonant part utilizes Stark shift for a nonlinear response in analog with the motion of a cavity mirror under radiation pressure. The compatible combination CPT with the spin optodymanics analog to cavity optomechanics leads to the spin one- or two-axis-twisting together with steady-state maximal atomic coherence. The general conditions and the parameter regimes for the compatibility are analyzed. This scheme illustrates that the combination of CPT with Stark shift provides an efficient way for the compatibility of the spin twisting squeezing with steady-state maximal atomic coherence.

Entanglement formation between the magnons as the internal degrees of freedom and the center-of-mass motion (CM) as the external degrees of freedom of a levitated yttrium iron garnet (YIG) sphere in a cavity-magnomechanical system is studied. Here, we propose a scheme for generating magnon-CM entanglement independent from the mass and size of the sphere in the hybrid magnonic system by driving the magnon with the parametric amplification. First, we show that the power and frequency of the driving field significantly affect this entanglement, since the driving field increases effective magnon-CM coupling. But, by increasing the magnon damping rate, this entanglement considerably decreases. Moreover, in the next step, we demonstrate the manipulation and enhancement of this entanglement by driving the magnon into the squeezed state. Our results present an approach for preparing quantum states and may find promising applications in the quantum metrology and sensing.

We study the hybrid system of a high-quality yttrium-iron-garnet (YIG) sphere magnetically coupled to a superconducting flux qubit via the magnetic fields created by two persistent-current quantum states. When the flux qubit is operated away from its sweet point, a coherent nonlinear two-magnon interaction can be predicted; that is, the qubit is excited by absorbing a pair of magnons. We show that the spontaneous emission of the qubit can be exploited to steer the magnon mode of the YIG sphere into a Schrödinger cat state with high fidelity. Our scheme has practical advantages in that it eliminates the need for precise control of the evolution time and the projective measurement, and is also insensitive to the qubit's pure dephasing.

Quantum convolutional neural networks (QCNNs) have gathered attention as one of the most promising algorithms for quantum machine learning. Reduction in the cost of training as well as improvement in performance are required for practical implementation of these models. In this study, we propose a channel attention mechanism for QCNNs and show the effectiveness of this approach for quantum phase classification problems. Our attention mechanism creates multiple channels of output state based on measurement of quantum bits. This simple approach improves the performance of QCNNs and outperforms a conventional approach using feed-forward neural networks as the additional postprocessing.

Quantum entanglement plays a pivotal role in various quantum information processing tasks. However, a universal and effective way to detect entanglement structures is still lacking, especially for high-dimensional and multipartite quantum systems. Noticing the mathematical similarities between the common representations of many-body quantum states and the data structures of images, we are inspired to employ advanced computer vision technologies for data analysis. In this work, we propose a hybrid convolutional neural network–transformer model for both the classification of Greenberger-Horne-Zeilinger and $W$ states and the detection of various entanglement structures. By leveraging the feature-extraction capabilities of convolutional neural networks and the powerful modeling abilities of transformers, we not only can effectively reduce the time and computational resources required for the training process but can also obtain high detection accuracies. Through numerical simulation and physical verification, it is confirmed that our hybrid model is more effective than traditional techniques and thus offers a powerful tool for characterizing multipartite entanglement structures.

Interferometric phase estimation is an essential tool for precise measurements of quantities such as displacement, velocity, and material properties. The lower bound on measurement uncertainty achievable with classical resources is set by the shot-noise limit (SNL) that scales asymptotically as $1/\sqrt{N}$, where $N$ is the number of resources used. The experiment of Daryanoosh et al. [Nat. Commun. 9, 4606 (2018)] showed how to achieve the ultimate precision limit, the exact Heisenberg limit (HL), in ab initio phase estimation with $N=3$ photon-passes, using an entangled biphoton state in combination with particular measurement techniques. The advantage of the HL over the SNL increases with the number of resources used. Here we present, and implement experimentally, a scheme for generation of the optimal $N=7$ triphoton state. We study experimentally and theoretically the generated state quality and its potential for phase estimation. We show that the expected usefulness of the prepared triphoton state for HL phase estimation is significantly degraded by even quite small experimental imperfections, such as optical mode mismatch and unwanted higher-order multiphoton terms in the states produced in parametric downconversion.

Sampling from a quantum distribution can be exponentially hard for classical computers and yet could be performed efficiently by a noisy intermediate-scale quantum device. A prime example of a distribution that is hard to sample is given by the output states of a linear interferometer traversed by $N$ identical boson particles. Here, we propose a scheme to implement such a boson-sampling machine with ultracold atoms in a polarization-synthesized optical lattice. We experimentally demonstrate the basic building block of such a machine by revealing the Hong-Ou-Mandel interference of two bosonic atoms in a four-mode interferometer. To estimate the sampling rate for large $N$, we develop a theoretical model based on a master equation that accounts for particle losses, but does not include technical errors. Our results show that atomic samplers have the potential to achieve a quantum advantage over today's best supercomputers with $N\gtrsim 40$.

The calculation of higher-order binding corrections to bound systems is a fundamental problem of theoretical physics. For any nonrelativistic expansion, one needs the Foldy-Wouthuysen transformation, which disentangles the particle and the antiparticle degrees of freedom. This transformation is carried out here to eighth order in the momenta or to eighth order in the momentum operators, which is equivalent to the eighth order of the fine-structure constant. Matrix elements of the eighth-order terms are evaluated for $F_{5/2}$ and $F_{7/2}$ states in hydrogenlike ions and compared with the Dirac-Coulomb energy levels.

We theoretically reveal that plasmonic nanocavities with 1 nm gaps between the nanosphere and gold film support multiple resonances possessing ultrahigh-field confinement and enhancements for both the electric and magnetic fields. Here, a circulated electric line supported by the plasmonic mode implies that a magnetic dipole is introduced in the center of the gap region. A high magnetic enhancement factor of up to 100 is observed, and multipolar expansion demonstrates that symmetry breaking suppresses the radiation of the dipole mode. The surface charges accumulate in the gap area and form the electric-quadrupole ($\mathbf{Qe}$) mode and magnetic-dipole ($\mathbf{m}$) modes in the gap region, and then the interference between the dominant $\mathbf{Qe}$ and $\mathbf{m}$ modes gives rise to a pronounced magnetic-based Fano resonance in the scattering spectrum, thus boosting the magnetic response in the scattering valley. Next, we introduce a magnetic dipole in the gap region, where a significant radiative magnetic Purcell factor of up to 200 is observed, following the Fano valley of the scattering spectrum. Furthermore, the ultrasmall magnetic mode volume and high magnetic field at the Fano valley frequencies demonstrate this phenomenon. This study unlocks new possibilities for single-photon emission from the magnetic-dipole transition in the nanoparticle-on-mirror configuration.

We demonstrate the inadequacy of mean-field theory by exploring the effects of initial-state correlations on the dynamics of continuous time crystals, motivating the need for higher-order cumulant expansions. We exemplify this using cat states for which the mean field fails to predict a phase transition but the second-order cumulant-expansion theory captures it. Motivated by the symmetries of the system, we choose a truncation of the cumulant expansion at the second order and demonstrate that it is sufficient to accurately capture the dynamical features overlooked by the mean field.

The aging transition refers to the shift from an oscillatory state to a globally stopped state due to some forms of deterioration in classical physics. Similar behavior has also been observed in quantum oscillators. Although it has received extensive attention in coupled-oscillator systems, it has not yet been studied in coupled qubits. In this paper, we explore the aging transition in a network of coupled qubits. Our model describes numerous qubits driven by a laser, with both dissipative and coherent qubit-qubit couplings. The ratio of inactive qubits to total qubits and the population in the excited state of the qubits are employed to characterize the aging transition. We find a transition where the population in the excited states suddenly drops when the ratio exceeds a threshold. This behavior is intriguing and contrasts with coupled oscillators, for which no sudden drop is observed. Additionally, we demonstrate how the couplings and driving laser influence the threshold. The underlying physics of the sudden drop is elucidated. The region where the aging transition occurs is determined based on stability-analysis theory.

The preparation of an equilibrium thermal state of a quantum many-body system on noisy intermediate-scale quantum (NISQ) devices is an important task in order to extend the range of applications of quantum computation. Faithful Gibbs state preparation would pave the way to investigate protocols such as thermalization and out-of-equilibrium thermodynamics and provide useful resources for quantum algorithms, where sampling from Gibbs states constitutes a key subroutine. We propose a variational quantum algorithm (VQA) to prepare Gibbs states of a quantum many-body system. The novelty of our VQA consists of implementing a parameterized quantum circuit acting on two distinct, yet connected (via cnot gates), quantum registers. The VQA evaluates the Helmholtz free energy, where the von Neumann entropy is obtained via postprocessing of computational-basis measurements on one register, while the Gibbs state is prepared on the other register via a unitary rotation in the energy basis. Finally, we benchmark our VQA by preparing Gibbs states of the transverse-field Ising and Heisenberg XXZ models and achieve remarkably high fidelities across a broad range of temperatures in state-vector simulations. We also assess the performance of the VQA on IBM quantum computers, showcasing its feasibility on current NISQ devices.

We examine the emergence of dynamical memory effects in quantum processes having indefinite time direction and causal order. In particular, we focus on the class of phase-covariant qubit channels, which encompasses some of the most significant paradigmatic open quantum system models. In order to assess the memory in the time evolution of the system, we utilize the trace distance and the entanglement based measures of non-Markovianity. While the indefinite time direction is obtained through the quantum time flip operation that realizes a coherent superposition of forward and backward processes, the indefinite causal order is achieved via the quantum switch map, which implements two quantum processes in a coherent superposition of their two possible orders. Considering various different families of phase-covariant qubit channels, we demonstrate that, when implemented on memoryless quantum processes, both the quantum time flip and the quantum switch operations can generate memory effects in the dynamics according to the trace distance based measure under certain conditions. On the other hand, with respect to the entanglement based measure, we show that neither the quantum time flip nor the quantum switch could induce dynamical memory for any of the considered phase-covariant channels.

We investigate the use of a single generalized dephasing qubit for sensing a quantum reservoir, where the antisymmetry of the coupling between the qubit and its reservoir is broken. Our results indicate that, in addition to the decay factor encoding channel, the antisymmetry breaking coupling introduces another phase factor encoding channel. We propose an optimal measurement strategy for the generalized dephasing qubit, which enables the practical measurement precision to reach the theoretical ultimate precision quantified by the quantum signal-to-noise ratio (QSNR). As an application example, the generalized dephasing qubit is employed to estimate the $s$-wave scattering length of an atomic Bose-Einstein condensate. It is found that the QSNR contributed by the antisymmetry breaking coupling is at least two orders of magnitude higher than the QSNR contributed by the antisymmetry coupling at the millisecond timescale, and the optimal relative error can achieve a scaling $\propto 1/\left(\chi t\right)$ in long-term encoding, where $\chi$ represents the relative driving strength and $t$ is the encoding time. Our work opens a way for supersensitive sensing of quantum reservoirs.

Over the last decade, strong-field nanoplasmonics has emerged as a field for which subwavelength localization of light plays a crucial role. We present universal scaling laws for ion cutoff and anisotropy in laser-nano-sphere ionization and experimentally demonstrate that the wavelength dependence of the local nanoscale field enhancement can be deduced by measuring the ion anisotropy. The results open new directions in studies of nanotargets by detecting angular-dependent charge particle distribution.

Two-component systems consisting of mutually interacting particles can demonstrate both intracomponent transport effects and intercomponent entrainment (or drag) effects. In the presence of superfluidity, the intracomponent transport is characterized by dissipative conductivity and superfluid weight in the framework of two-fluid model, and intercomponent entrainment gives rise to normal and nondissipative drag effects. We present unified treatment of all these effects for spatially homogeneous two-component atomic Bose-Einstein condensates based on the Bogoliubov theory, focusing specifically on the drag effects. Calculating finite-frequency intra- and intercomponent conductivities with taking into account quasiparticle damping, we derive and numerically check analytical Drude-like approximations applicable at low frequencies and Lorentz-like approximations applicable at higher frequencies in the vicinity of the resonant energy of spin-to-density Bogoliubov quasiparticle conversion. As possible physical realizations of two-component atomic systems, we consider three-dimensional Bose-Bose mixtures and closely spaced two-layered systems of magnetic dipolar atoms.