In this paper, we introduce a physically realizable model which contains three externally controllable agents and study how variations of these agents can lead to generation of high-intensity superradiant bursts. To this end, a chain of closely packed quantum emitters (two-level atoms) is placed inside an optical cavity, filled by a second-order nonlinear material. The closeness of the emitters in the chain calls for emitter-emitter interaction, which we explicitly take into account. The model presented here therefore includes three effects: atom-photon coupling, atomic spacing in the chain, and nonlinearity strength, making the model the most realistic among those previously reported. The main aim of the present work is to study the interplay of these three. We pursue this aim by numerically calculating the time evolution of the density operator that describes the whole system, thereby obtaining relevant mean values. To simulate the system, the latest versions of the QuantumOptics.jl and CollectiveSpins.jl packages, within the julia programming language, are employed. For tangible results, the emission rates as a function of time arising from such a system are presented. To verify the correctness of our computational method and for later comparison we also include a reproduction of the previously reported models. From this comparison we conclude that the total Hamiltonian which contains all three controlling parameters can produce much more intense superradiation bursts. Moreover, contour diagrams of the maximum values of the emission rates, in terms of these parameters, are also presented. From such diagrams one can readily read the parametric values for which the superradiation mode is enhanced. A detailed physical discussion of the latter is also presented. It is thus feasible that the results of the present work will help engineer an effective system which exhibits intensive superradiation.

Strong and controllable tripartite interactions play a pivotal role in quantum information and nonlinear quantum optics, yet are challenging to realize. In this work, we propose a hybrid system consisting of a nitrogen-vacancy (NV) center coupled to Kerr magnons (magnons with Kerr nonlinearity) in two yttrium-iron-garnet spheres. By adiabatically eliminating the ground state of the NV qutrit in the dispersive regime, an effective tripartite interaction among magnons and an NV qubit encoded in its excited states is obtained. In the strong driving limit, Kerr magnons can be linearized and give rise to degenerate parametric amplification for squeezing magnons. As a result, both the tripartite interaction and cooperativity are exponentially enhanced twice, which is about $exp\left(\xi \right)$ times more than schemes only involving single-squeezing. Hence, our proposal is more experimentally feasible because a modest squeezing parameter is sufficient. With this amplified tripartite coupling strength, the system dynamics are greatly accelerated, leading to fast generation of tripartite entanglement. In addition, a noise-resilient perfect magnon blockade can be achieved, well predicted by both the analytical approach and numerical simulation with the quantum master equation. Our results suggest that the NV center represents a promising interface for engineering many-body interactions in quantum magnonics, offering a versatile platform for exploring fundamental quantum phenomena such as entanglement and correlations.

We introduce path-entangled quantum states, spatially correlated systems governed via production angles, to achieve geometric control of entanglement beyond spin or polarization constraints. By driving the system through cyclic adiabatic evolution of an external parameter in Mach-Zehnder interferometers, we demonstrate that Berry phases and production angles become additional degrees of freedom for Bell correlations. We identify an approximate critical angle $24.{97}^{\circ }$ that geometrically manifests the Bell limit for certain measurement settings, delineating boundaries between local hidden-variable theories and quantum mechanics. This framework simplifies state preparation while enabling geometry-driven entanglement control, thus providing distinct experimental advantages.

Quantum resource theories provide a structured and elegant framework for quantifying quantum resources. While state-based resource theories have been extensively studied, the measurement-based resource theories remain relatively underexplored. In practical scenarios where a quantum state or a set of measurements is only partially known, conventional resource measures often fall short in capturing the resource content. In such cases, $\epsilon$-measures offer a robust alternative, making them particularly valuable. In this work, we investigate the quantification of measurement-based resources using distance-based measures, followed by a detailed analysis of the mathematical properties of $\epsilon$-measures. We also extend our analysis by exploring the connections between $\epsilon$-measures and some key quantities relevant to resource manipulation tasks. Importantly, the analysis of resources based on sets of measurements is tedious compared to that of single measurements, as the former allows more general transformations such as controlled implementation. Yet our framework applies not only to resources associated with individual measurements but also to those arising from sets of measurements. In short, our analysis is applicable to existing resource theories of measurements, and it has the potential to be useful for all resource theories of measurements that are yet to be developed.

In the quantum state discrimination game, there exist phenomena suggesting a complex, and as yet not well-explored, relation of the resource of entanglement in sets of quantum states with their local distinguishability, and in this work, we take a step towards filling this gap within the minimum-error quantum state discrimination protocol. We derive upper and lower bounds on the optimal discrimination probability of $k$-inseparable or $n$-entangled quantum states in an arbitrary multipartite ensemble in terms of the robustness of entanglement and the optimal discrimination probability of the corresponding closest $k$-separable or $n$-unentangled states. Our bounds hold regardless of the dimension of the constituent systems, the number of parties involved, the size of the ensemble, and whether the measurement strategies are local or global. Additionally, we determine further lower bounds on the difference between the success probability of discriminating $k$-inseparable (or $n$-entangled) states and that of discriminating the corresponding $k$-separable or $n$-unentangled states by analyzing two special cases of two-state multiparty ensembles: one where both states have equal entanglement and another where one state is $k$-separable or $n$-unentangled. The case of equal entanglement reveals that it is always easier to discriminate between entangled states than those in the corresponding closest separable ensemble, a phenomenon we refer to as “more indistinguishability with less entanglement.” The second case, however, does not assure the same. Further, for two-element ensembles, where the state with lesser entanglement is also more probable, we find that this entanglement must exceed a certain threshold for the ensemble to exhibit “more indistinguishability with less entanglement.”

This study explores the use of equivariant quantum neural networks (QNN) for generating molecular force fields, focusing on the rMD17 dataset. We consider a QNN architecture based on previous research and point out shortcomings in the parametrization of the atomic environments. These shortcomings limit its expressivity as an interatomic potential and precludes transferability between molecules. We propose a revised QNN architecture that addresses these shortcomings. While both QNNs show promise in force prediction, with the revised architecture showing improved accuracy, they struggle with energy prediction. Further, both QNNs architectures fail to demonstrate a meaningful scaling law of decreasing errors with increasing training data. These findings highlight the challenges of scaling QNNs for complex molecular systems and emphasize the need for improved encoding strategies, regularization techniques, and hybrid quantum-classical approaches.

The barren plateaus phenomenon, where the gradients of parametrized quantum circuits become vanishingly small, poses a significant challenge in quantum machine learning. Foundational analyses of barren plateaus often use methods like the Weingarten formula, which, under the global Haar random assumption, effectively links the gradient variance to the Hilbert space dimension. However, many practical circuits, such as Hardware-Efficient Ansätze, utilize a different, non-Haar random model based on the sampling of parameters for single-qubit rotation gates. This study introduces a direct computational method for the gradient expectation and variance, specifically for parametrized quantum circuits constructed from single-qubit rotation gates. Our analysis reveals a direct, quantitative relationship between the gradient variance, circuit depth, and number of effective parameters. Numerical simulations further confirm the validity of our theoretical results. Our findings highlight the role of effective parameters as a key indicator for gradient trainability and provide a precise framework for analyzing barren plateaus in these specific non-Haar random circuits.

We investigate early-stage ultrafast nuclear dynamics of ${\mathrm{O}}_2^{+}$ in the electronically highly excited states by ionizing ${\mathrm{O}}_2$ with high-order harmonics of a few-cycle near-infrared (NIR) laser pulse $(\sim 5.8\mathrm{fs})$ in the extreme ultraviolet (XUV) region and by probing the nuclear dynamics of ${\mathrm{O}}_2^{+}$ with the few-cycle NIR laser pulse. The kinetic energy distribution of the yield of ${\mathrm{O}}^{+}$ fragments recorded as a function of time delay of the NIR pulse with respect to the XUV pulse until 100 fs exhibit a periodic parabola-like structure in the low-kinetic-energy region below 0.3 eV and a curvilinear structure in the higher-kinetic-energy region (0.2–1.5 eV). By numerical simulations of a vibrational wave packet in the optically coupled electronic states of ${\mathrm{O}}_2^{+}$, we reveal that the periodic parabola-like structure represents a vibrational motion of a wave packet in the $\mathrm{a}{}^4\mathrm{\Pi }_u$ state of ${\mathrm{O}}_2^{+}$, which appears only when the probe pulse is sufficiently short, and that the curvilinear structure represents dissociating and oscillating components of a vibrational wave packet in the $3{}^2\mathrm{\Pi }_u$ state of ${\mathrm{O}}_2^{+}$, which is predominantly excited to the $8{}^2\mathrm{\Pi }_g$ and $9{}^2\mathrm{\Pi }_g$ states by the probe pulse.

Recent advancements in optical tweezers enable the trapping of arbitrary numbers of neutral atoms and molecules, and even arrays of tweezers with variable geometry can be realized. These fascinating breakthroughs require novel full-dimensional beyond mean-field treatments for systems with more than two confined particles spread over traps that are arranged arbitrarily in space. In this work, the suitability of a quantum-chemistry inspired approach adopting Cartesian Gaussians as basis functions is investigated. For this purpose, the six-dimensional integrals associated with a realistic atom-atom interaction described by a Morse model potential were implemented. The performance, correctness, and efficiency of the implementation is assessed by comparing full configuration-interaction calculations (exact diagonalizations) for two atoms in an isotropic harmonic trap with quasiexact reference results.

We investigate the influence of a constant and time-dependent linear gravitational-like potential on one-dimensional quantum droplets, governed by an extended Gross-Pitaevskii equation incorporating a repulsive cubic effective mean-field (EMF) term and an attractive quadratic beyond-mean-field (BMF) correction. By constructing a tailored external confinement, we derive an exact analytical family of droplet wave functions and explicitly characterize the effective interaction contributions. A key result is that the droplet's c.m. follows a strictly Newtonian equation of the form ${\ddot{X}}_{\mathrm{c}.\mathrm{m}.}\left(t\right)=-a\left(t\right)$, demonstrating that its falling trajectory depends solely on the applied linear potential. Notably, this motion is completely independent of atom number and of the EMF-BMF balance, providing a clean classical baseline for dynamics in linear-potential environments. Temporal modulation of the potential induces clear deviations from static trajectories and leaves measurable imprints on internal coherence, captured through the Shannon entropy and the Wigner phase-space distribution, with the modulation strength controlling phase-space localization, deformation, and coherence degradation. Numerical simulations substantiate the stability of the analytical solutions, demonstrating their robustness. These findings suggest promising implications for quantum sensing and metrological applications using ultradilute quantum fluids.

We theoretically investigate a quantum spin Hall topological laser formed by an array of dielectric ring resonators endowed of saturable gain. The system preserves time-reversal symmetry, the clockwise and counterclockwise modes in each ring resonator acting as two pseudospin states that experience opposite synthetic magnetic fields. We consider ring resonators featuring an internal S-shaped waveguide asymmetrically coupling the two pseudospin states. In spite of the nonmagnetic nature of the configuration, we show that an effective breaking of reciprocity is induced by the interplay of spatial parity breaking with saturable gain and a Kerr optical nonlinearity. This enables robust single-mode topological lasing even in the presence of realistic levels of backscattering.

We demonstrate substantial field enhancement in plasma nanoshells through high-order Mie resonances using combined Mie theory and particle-in-cell simulations. Optimal shell geometries yield approximately threefold electric field enhancement for 800-nm irradiation, with transient buildup times of tens of femtoseconds before plasma expansion disrupts resonance. Few-cycle pulses produce reduced enhancement due to insufficient resonance establishment. These findings enable optimized laser-plasma interactions for applications including diagnostics of laser-cluster interaction and energetic ion production from engineered core-shell targets, highlighting the critical role of temporal dynamics in nanoplasma resonances.

We reveal a sharp form of complementarity in the Janus state: a coherent superposition of two squeezed vacua. Using a compact generating-function formalism, we derive closed-form expressions for $g^{\left(k\right)}$ at all orders and introduce squeezing polynomials $P_k\left(z\right)$ that factorize the moments and expose phase-dependent scaling. The Janus state operates as a phase-tunable quantum switch for multiphoton correlations: when which-path or which-state information is available (trivial interference), the higher-order coherences $g^{\left(k\right)}$ reproduce the single-squeezed limit and diverge; when path information is erased and genuine interference from coherent superposition is enabled, multiphoton events are extinguished, driving $g^{\left(k\right)}\to 0$ for all $k\ge 3$. We obtain a hierarchy of suppression with exponents set by the interference phase and the parity of $k$, and we visualize the mechanism via pronounced Wigner-function negativity. The framework is general and experimentally accessible, enabling engineered nonclassical light with tailored higher-order statistics.

Third quantization is used in open quantum systems to construct a superoperator basis in which quadratic Lindbladians can be turned into a normal form. From it follows the spectral properties of the Lindbladian, including eigenvalues and eigenvectors. However, the connection between third quantization and the semiclassical representations usually employed to obtain the dynamics of open quantum systems remains opaque. We introduce an alternative basis for third quantization that bridges this gap between third quantization and the $Q$ representation by projecting the master equation onto a superoperator coherent-state basis. The equation of motion reduces to a multidimensional complex Ornstein-Uhlenbeck process.

Spectroscopic methods play a vital role in quantum sensing, which uses the quantized nature of atoms or molecules to reach astonishing precision for sensing of, e.g., electric or magnetic fields. In the theoretical treatment, one typically invokes semiclassical methods to describe the light-matter interaction between quantum emitters, e.g., atoms or molecules, and a strong coherent laser field. However, these semiclassical approaches struggle to predict the stochastic measurement fluctuations beyond the mean value, necessary to estimate the sensitivity of spectroscopic quantum sensing protocols. Here, we develop a theoretical framework based on the recently developed photon-resolved Floquet theory (PRFT), which is capable of predicting the measurement statistics describing higher-order cumulants of coherent quantum states of light. The PRFT constructs flow equations for the cumulants of the photonic measurement statistics utilizing only the semiclassical dynamics of the matter system. We apply the PRFT to spectroscopic quantum sensing using dissipative two-level and four-level systems (describing electric-field sensing with Rydberg atoms), and demonstrate how to calculate the Fisher information of the measurement statistics with respect to various system parameters. In doing so, we demonstrate that the PRFT is a flexible tool allowing for improving the sensitivity of spectroscopic quantum sensing devices by several orders of magnitude.

The common approach for producing Rydberg atoms in warm vapor cells is with lasers arranged in a counterpropagating, collinear configuration. Doppler effects in these configurations reduce the efficiency of excitation to the Rydberg state while also producing broadened spectral features. In this work, we demonstrate a three-laser Doppler-free excitation using laser beams whose $k$ vectors sum to 0, resulting in an enhancement in the Rydberg density and narrowed spectral features. A three-times enhancement to Rydberg density along with a near four-times reduction in spectroscopic linewidths are observed compared to a collinear configuration. This Doppler-free configuration could prove beneficial to Rydberg atomic technologies, such as electric field sensing with small volumes or deterministic photon sources.