Although entanglement is considered as a crucial resource for quantum information processing, it is still unknown whether the presence of quantum entanglement in the working medium can enhance the charging performance of a quantum battery (QB). Here, we address this problem by considering a QB with working medium consisted of an entangled coupled-cavity array. We show that the optimal charging performance of the QB is achieved in the two-cavity case when cavities are initially in a maximum entangled state without cavity couplings. We then extend our discussion to multicavity case. It is demonstrated that the charging power of the QB under an entangled coupled-cavity array can be further enhanced by increasing the number of cavities. Our findings reveal the advantage of an entangled working medium in enhancing the charging performance of a QB, and therefore, contribute to the experimental realization of quantum batteries with excellent performance.

In recent experimental investigations of unitary Fermi gases, it has been observed that the thermal conductivity $\kappa$ approaches the quantum limit under the phase transition temperature $T_c$. While relevant theoretical calculations have been conducted for this phenomenon, the interactions among particles within the heat current were neglected, violating conservation laws and revealing a significant disparity between computed results and experimental data. This paper addresses this issue by incorporating particle interactions within the heat current in Feynman diagrams, based on the Ward identity. We derive a modified Kubo-based expression for $\kappa$ that accounts for the interaction. We also consider the anomalous thermal conductivity from current transfer via Cooper pairs for the theory completeness. Our computations reveal that near $T_c$, the fluctuations in the $t$-matrix paradigm are characterized by the pseudogap order and bosonic excitations contribute significantly to $\kappa$, leading to a conspicuous peak in addition to the critical universal scaling laws. Our results align well with the experiment near $T_c$.

Nonlinear optical effect of spontaneous parametric down-conversion (SPDC) is widely used to produce entangled photon pairs for quantum information research and applications. We report on SPDC in 4-N, N-dimethylamino-4'-N'-methyl-stilbazolium tosylate (DAST) crystals. While being an early-phase exploration, the experimentally measured yield of the down-converted photons at the optical telecom wavelength is already comparable to that in the state-of-the-art periodically poled potassium titanyl phosphate (KTP) crystal. It hints at a greater potential being available, especially in its crystal orientation with an order of magnitude higher nonlinear coefficient, which is unfortunately not accessible in the present setup. The findings make the DAST system a promising candidate for entangled photon sources at telecom wavelengths.

We propose a quantum beam splitter (QBS) with tunable reflection and transmission coefficients. More importantly, our device based on a Hermitian parity-time $\left(\mathcal{PT}\right)$ symmetric system enables the generation and manipulation of asymmetric quantum coherence of the output photons. For the interference of two weak coherent-state inputs, our QBS can produce antibunched photons from one output port and bunched photons from the other, showcasing high parity asymmetry and strong coherence control capabilities. Beyond the Hong-Ou-Mandel effect, perfect photon blockade with vanishing $g^{\left(2\right)}\left(0\right)$ is achievable in two-photon interference. These striking effects of the QBS fundamentally arise from the parity-symmetry-breaking interaction and the quantum interference between the photon scattering channels. Our results could inspire novel applications and the development of innovative photonic devices for the manipulation of weak quantum light.

Variational quantum approaches have shown great promise in finding near-optimal solutions to computationally challenging tasks, including solving optimization problems. Nonetheless, optimization problems with constraints may not have been handled in a disciplined fashion thus far. To address this gap, this work proposes a hybrid quantum-classical algorithmic paradigm termed the variational quantum eigensolver with constraints (VQEC) that extends the celebrated VQE to handle optimization with constraints. As with the standard VQE, the vector of optimization variables is captured by the state of a variational quantum circuit (VQC). To deal with constraints, VQEC optimizes a Lagrangian function classically over both the VQC parameters as well as the dual variables associated with constraints. To comply with the quantum setup, variables are updated via a perturbed primal-dual method leveraging the parameter shift rule. Among a wide gamut of potential applications, we showcase how VQEC can approximately solve quadratically constrained binary optimization problems, find stochastic binary policies satisfying quadratic constraints on the average and in probability, and solve large-scale linear programs over the probability simplex. Under an assumption on the error for the VQC to approximate an arbitrary probability mass function, we provide bounds on the optimality gap attained by a VQC. Numerical tests on a quantum simulator investigate the effect of various parameters and corroborate that VQEC can generate high-quality solutions.

The cluster state acquired by evolving the nearest-neighbor (NN) Ising model from a completely separable state is the resource for measurement-based quantum computation. Instead of an NN system, a variable-range power-law interacting Ising model can generate a weighted graph state (WGS) whose genuine multipartite entanglement may reveal intrinsic characteristics of the evolving Hamiltonian. We establish that the generalized geometric measure (GGM) in the time evolved state with an arbitrary number of qubits is sensitive to falloff rates and the range of interactions of the evolving Hamiltonian. We report that the time-derivative and time-averaged GGM at a particular time can detect the transition points present in the falloff rates of the interaction strength, separating different regions, namely, long-range, quasilocal, and local ones in one- and two-dimensional lattices with deformation. Moreover, we illustrate that in the quasilocal and local regimes there exists a minimum coordination number in the evolving Ising model for a fixed total number of qubits which can mimic the GGM of the long-range model. In order to achieve a finite-size subsystem from the entire system, we design a local measurement strategy that allows a WGS of an arbitrary number of qubits to be reduced to a local unitarily equivalent WGS having fewer qubits with modified weights.

This paper presents measurements of scattering of positrons from ethane. Grand total, total elastic, total positronium formation, total ionization, and angular differential elastic-scattering cross sections have been observed and are shown along with comparisons to previous experimental and theoretical values. There are a number of discrepancies with previous data and some discussion of these and possible explanations are presented. This wider range of scattering measurements provides a more comprehensive picture of positron scattering from this target and then allows us to suggest pathways to resolution of the existing disagreements. However, it is clear that there is significant further work to be done before we can consider that we have a full understanding of the scattering process from this relatively simple target species.

We derive a model to explain the observed suppression of optically induced loss in a weakly interacting Fermi gas as the $s$-wave scattering length is increased [C. A. Royse et al., Phys. Rev. Lett. 133, 083404 (2024)]. We incorporate spin-dependent loss into a quasiclassical collective spin vector model to show that loss suppression occurs via a transition to a magnetized dynamical state, where two-body $s$-wave scattering is inhibited via the Pauli principle. By comparing measurements in mixtures and coherently prepared samples, we show that the data are quantitatively explained by the model, which is applicable to the optical control of energy-space lattices for new quantum simulators.

Kerr instability amplification can amplify over an octave of spectrum, a broad bandwidth supporting few-cycle pulses. However, dispersion management in this regime is crucial, and we explore the parameters required to maintain the ultrashort pulse undergoing amplification. At low pump intensities, we experimentally observe an interference between the strongly chirped supercontinuum seed and weakly amplified pulse, reproduced in our model by manipulating the seed dispersion. Extending our model to cases of high gain, our simulation predicts the dispersion is near zero, and the phase can be compensated preamplification to generate near-transform-limited amplified few-cycle pulses. We discuss chirping the seed pulse to avoid saturation, a route for generating sub-mJ few-cycle pulses in a single stage from Kerr instability chirped-pulse amplification.

Using the pseudomode model to simulate open quantum systems is important in quantum physics. Here we report a systematic and efficient pseudomode model based on dissipative, noninteracting harmonic oscillators, capable of accurately simulating any type of environmental spectrum. The dynamical evolution is reduced to a Lindblad master equation with only one dissipation term. A simple procedure to determine the parameters of the pseudomodes is proposed. We evaluate its accuracy by some spin-boson models under different environmental spectra. Our findings consistently exhibit high accuracy. Finally, we extend this pseudomode method to larger systems with more complicated interactions. Our method may have potential applications in quantum information.

From the perspective of quantum information theory, the effect of Unruh radiation on a two-level accelerated detector can be modeled as a quantum channel. In this work we employ the tools of channel-position finding to locate Unruh channels. The signal-idler and idler-free protocols are explored to determine the position of the target Unruh channel within a sequence of background channels. We derive the fidelity-based bounds for the ultimate error probability of each strategy and obtain the conditions where the signal-idler protocol is superior to the protocol involving idler-free states. It is found that the lower bound of the error probability for the signal-idler scheme exhibits clear advantages in all cases, while the idler-free scheme can only be implemented when the temperatures of the two channels are very close and the number of initial states is insufficient. Interestingly, it is shown that the optimal detection protocol relies on the residual correlations shared between the emitted probe state and the retained idler modes.

Small interconnected quantum processors can collaborate to tackle quantum computational problems that typically demand more capable devices. These linked processors, referred to as quantum nodes, can use shared entangled states to execute nonlocal operations. As a consequence, understanding how to distribute entangled states among nodes is essential for developing hardware and software. We analyze a protocol where entanglement is continuously distributed among nodes that are physically arranged in a regular pattern: a chain, a honeycomb lattice, a square grid, and a triangular lattice. These regular patterns allow for the modular expansion of networks for large-scale distributed quantum computing. Within the distribution protocol, we investigate how nodes can optimize the frequency of attempting entanglement swaps, trading off multiple entangled states shared with neighboring nodes for fewer states shared with non-neighboring nodes. We evaluate the protocol's performance using the virtual neighborhood size—a metric indicating the number of other nodes with which a given node shares entangled states. Employing numerical methods, we find that nodes must perform more swaps to maximize the virtual neighborhood size when coherence times are short. In a chain network, the virtual neighborhood size's dependence on swap attempt frequency differs for each node based on its distance from the end of the chain. Conversely, all nodes in the square grid exhibit a qualitatively similar dependence of the virtual neighborhood size on the swap frequency.

Non-Gaussian quantum gates are essential components for optical quantum information processing. However, the efficient implementation of practically important multimode higher-order non-Gaussian gates has not been comprehensively studied. We propose a measurement-based method to directly implement general, multimode, and higher-order non-Gaussian gates using only fixed non-Gaussian ancillary states and adaptive linear optics. Compared to existing methods, our method allows for a more resource-efficient and experimentally feasible implementation of multimode gates that are important for various applications in optical quantum technology, such as the two-mode cubic quantum nondemolition gate or the three-mode continuous-variable Toffoli gate, and their higher-order extensions. Our results will expedite the progress toward fault-tolerant universal quantum computing with light.

Microwave-dressed Rydberg atoms in pairs of states that are optically accessible and engineered so that the frequency of the transition between them is tunable while exhibiting a low sensitivity to stray electric fields have been interfaced with a superconducting coplanar waveguide (CPW) microwave resonator. This was achieved with helium atoms in triplet Rydberg states with principal quantum numbers $n=55$ and 56. The atoms were dressed with two strong microwave fields—a control field and a differential polarizability nulling field. The control field allowed the atoms to be coupled to the resonator using the two-color two-photon transition between the equal parity $1s55s{}^{3}S_1\equiv |55s\rangle$ and $1s56s{}^{3}S_1\equiv |56s\rangle$ Rydberg states, with one photon supplied by it and the other provided by the resonator. The nulling field was detuned from the field-free $1s55s{}^{3}S_1\to 1s55p{}^{3}P_J\equiv |55p\rangle$ transition to admix $|55p\rangle$ character into the $|55s\rangle$ state and eliminate the differential static electric dipole polarizability of the $|55s\rangle$ and $|56s\rangle$ states. The experiments were performed with atoms located $\sim 300\mu \mathrm{m}$ above a $\lambda /4$ niobium nitride superconducting CPW resonator operated at temperatures between 3.59 and $3.73\mathrm{K}$. The measured ac and dc Stark shifts of the Rydberg states at this atom-circuit interface in the presence of the dressing fields are in good quantitative agreement with the results of Floquet calculations. The differential polarizability nulling fields used in the experiments allowed the difference in the Stark shifts of the $|55s\rangle$ and $|56s\rangle$ states over the range of dc fields offset from the residual stray field by $\pm 65$ mV/cm to be reduced from $2\pi \times 2.1$ MHz to $2\pi \times 125$ kHz.

We propose the implementation of a rapid adiabatic passage (RAP) scheme to generate entanglement in Rydberg atom-array systems. This method transforms a product state in a multiqubit system into an entangled state with high fidelity and robustness. By employing global and continuous driving laser fields, we demonstrate the generation of a two-qubit Bell state and a three-qubit $W$ state, via sequential RAP pulses within the Rydberg blockade regime. As an illustrative example, applying this technique to alkali-metal atoms, we predict fidelities exceeding 0.9995 for the two-qubit Bell and three-qubit $W$ state, along with excellent robustness. Furthermore, our scheme can be extended to generate entanglement between weakly coupled atoms and to create four-qubit Greenberger-Horne-Zeilinger states through spatial correlations. Our approach holds the potential for extension to larger atomic arrays, offering a straightforward and efficient method to generate high-fidelity entangled states in neutral atom systems.

Topological phases associated with non-Abelian charges can exhibit a distinguished bulk-edge correspondence compared with Abelian phases, although elucidating this relationship remains challenging in traditional solid-state systems. In this paper, we propose a theoretical framework for synthesizing non-Abelian quaternion charges in ultracold atomic gases. By designing artificial spin-orbit coupling patterns, the topological edge modes demonstrate a clear correspondence with the band topology determined by various quaternion charges. This paves the way for observing the interface modes whose existence is attributed to the nonconservation multiplication relation, which is fundamental to non-Abelian charges. This scheme can be readily implemented using current ultracold atom techniques, offering a promising approach to explore the intriguing non-Abelian characteristics of the system.

We investigate the coherent interactions mediated by the coupled resonator waveguide between two types of giant atoms. We find that the effective coupling and collective dissipation can be controlled on demand by adjusting the configuration of the giant atoms. As a result, the external driving gives birth to a substantial steady-state entanglement between two giant atoms, which exhibits a Rabi splitting character. In the three-giant-atom setup, we find that the nonzero next neighbor atomic entanglement can surpass the neighbor ones and is able to be adjusted by tuning the driving phase, which serves as an artificial magnetic field. The enhancement of next neighbor atomic entanglement cannot be realized in the small-atom setup. We hope these controllable interactions in giant-atom arrays are useful applications in the quantum information process.

A set of multipartite orthogonal states is genuinely nonlocal if it is locally indistinguishable in every bipartition of the subsystems. If the set is locally reducible, we say it has genuine nonlocality of type I. Otherwise, we say it has genuine nonlocality of type II. Due to the complexity of the problem, the construction of genuinely nonlocal sets in general multipartite systems has not been completely solved so far. In this paper, we first provide a nonlocal set of product states in bipartite systems. We obtain a genuinely nonlocal set of type I without entanglement in general $n$-partite systems ${\otimes }_{i=1}^n{\mathbb{C}}^{d_i}$ $[3\le (d_1-1)\le d_2\le \cdots \le d_n,n\ge 3]$. Then we present two constructions with genuine nonlocality of type II in ${\mathbb{C}}^{d_1}\otimes {\mathbb{C}}^{d_2}\otimes {\mathbb{C}}^{d_3}$ $(3\le d_1\le d_2\le d_3)$ and ${\otimes }_{i=1}^n{\mathbb{C}}^{d_i}$ $(3\le d_1\le d_2\le \cdots \le d_n,n\ge 4)$. Our results further positively answer the open problem that there does exist a genuinely nonlocal set of type II in multipartite systems [M. S. Li, Y. L. Wang, F. Shi, and M. H. Yung, J. Phys. A: Math. Theor. 54, 445301 (2021)] and highlight its related applications in quantum information processing.

A spectral minimum in the high-order harmonic generation (HHG) is of great interest, usually generated by specific atoms or molecules. Here we show through detailed simulations that the minimum in the HHG spectrum can be created and tuned by varying the phase-matching conditions when a gas medium is located at the focus of a long-wavelength laser. We reveal that the minimum can be shifted spatially depending on the quantum orbit due to its sensitivity to the phase-matching condition. We provide a formula to relate the position of the minimum to the coherence length of HHG. We also show that the position(s) and the number of (multiple) minimum(s) can be changed by varying the beam waist and the intensity of the driving laser. In addition, we establish the scaling relations with the laser wavelength to maintain the spectral position of the minimum. We expect that this method can be effectively implemented to shape the attosecond pulse in the extreme ultraviolet and soft x rays.