Bound states in the continuum (BICs) have attracted intense interest, yet their many-particle counterparts remain largely unexplored in waveguide quantum electrodynamics. We identify and characterize a bound state embedded in the doublon continuum (BIDC) that emerges when four atoms couple to a coupled-resonator waveguide with strong on-site interaction. Exploiting this interaction-enabled BIDC, we show that (i) a distant, four-atom entangled state can be prepared with high fidelity, and (ii) quantum entangled states can be coherently transferred between spatially separated nodes. Our results establish a scalable mechanism for multiparticle state generation and routing in waveguide platforms, opening a route to interaction-protected quantum communication with many-particle BICs.

We experimentally investigate the dynamics of ergotropy in an open quantum system subject to dissipation by using a linear-optical setup. The total ergotropy, together with its coherent and incoherent components, is extracted at distinct stages of the system's evolution. Our findings reveal that the incoherent ergotropy undergoes sudden vanishing within a finite time, a process that accelerates with increasing environmental temperature. In contrast, the coherent ergotropy exhibits stronger robustness against dissipation. In the non-Markovian regime, the total ergotropy displays oscillatory revivals, with the incoherent component reemerging after a period of complete disappearance. Furthermore, by tracking the passive energy of the system, we demonstrate the evolutionary interplay between useful and useless forms of energy.

Quantum adiabaticity is the evolution of a quantum system that remains close to an instantaneous eigenstate of a time-dependent Hamiltonian. Using Floquet formalism, we derive a rigorous sufficient condition for adiabaticity in closed, finite-dimensional periodically driven systems that is valid for arbitrarily many driving periods. The condition is stroboscopic and geometric, depending only on single-cycle information: the Fubini-Study length of the instantaneous eigenray and a quasienergy-separation measure extracted from the Floquet operator. We also formulate a state-targeted refinement that reduces conservativeness when only one adiabatic branch is relevant. Rather than synthesizing control pulses, the result provides a certification criterion for a given periodic protocol. We illustrate the criterion and contrast it with conventional instantaneous-gap conditions in three representative examples.

Quantum key distribution (QKD) leverages the principles of quantum mechanics to exchange a secret key between two parties. Unlike classical cryptographic systems, the security of QKD is not reliant on computational assumptions but is instead rooted in the fundamental laws of physics. In a QKD protocol, any attempt by an eavesdropper to intercept the key is detectable: this provides an unprecedented level of security, making QKD an attractive solution for secure communication in an era increasingly threatened by the advent of quantum computers and their potential to break classical cryptographic systems. However, QKD also faces several practical challenges such as transmission loss and noise in quantum channels, finite key size effects, and implementation flaws in QKD devices. Addressing these issues is crucial for the large-scale deployment of QKD and the realization of a global quantum Internet. A whole body of research is dedicated to the hacking of the quantum state source, for example using Trojan-horse attacks (THAs), where the eavesdropper injects light into the system and analyzes the back-reflected signal. In this paper, we study the vulnerabilities against THAs of the iPOGNAC encoder, first introduced in [M. Avesani et al., Opt. Lett. 45, 4706 (2020)], to propose adapted countermeasures that can mitigate such attacks.

Quantum advantage arises from quantum states that cannot be efficiently simulated on a classical computer. Such states are characterized by a property known as nonstabilizerness. In this work, we investigate whether nonstabilizerness can be generated by placing an initially stabilizer state in contact with a heat bath. Under minimal thermodynamic assumptions, we derive a necessary and sufficient condition for when this is possible. This yields an analytic characterization of all nonstabilizer qubit states reachable through such thermal processes, together with explicit bounds on their nonstabilizerness. This, in turn, allows us to identify optimal regimes for generating this resource, including the Hamiltonians that maximize nonstabilizerness and the critical temperatures at which it emerges. Beyond the qubit case, we establish a general trade-off between the nonstabilizerness attainable under thermal operations and the initial nonequilibrium free energy of the system.

Quantum key distribution (QKD) provides an information-theoretic secure method for communication, guaranteeing long-term security against future computational advances. However, as the implementation evolves towards multi-user networks, a significant operational challenge arises: actively compensating for the continuous reference frame drifts between distant nodes. Luckily, reference-frame-independent QKD (RFI-QKD) can circumvent this requirement, offering a practical pathway to scalable quantum communication. Here, we demonstrate a three-node RFI-QKD network built around a central, frequency-configurable, directly-modulated source. The long-term stability of the RFI QKD system is confirmed over a 53 km installed metropolitan fiber link, achieving 24 hours of continuous, recalibration-free operation at a 625 MHz repetition rate. We further show its network reconfigurability by demonstrating on-demand switching between the 52 km user and a second user on a 100 km link, applying user-specific repetition rates to highlight the source's flexibility in a multi-user environment. Our results establish the synergy between the RFI protocol and cost-effective direct-modulation sources as a robust architecture for building low-maintenance, scalable quantum access networks.

Virtual quantum maps simulate nonphysical transformations using physical operations and classical postprocessing, but known constructions of virtual broadcasting are operationally inefficient. Motivated by the expectation that relaxing symmetry requirements may improve operational performance, we study virtual quantum broadcasting for qubits under phase covariance. We show that phase covariance, flip covariance, permutation invariance, and classical consistency already fix the structure of the virtual map and imply broadcasting on equatorial states, without assuming broadcasting explicitly. Within this class, we identify a unique map of minimal simulation cost. Remarkably, and in direct analogy with the fully unitarily covariant case, the positive part of this optimal map coincides with the optimal symmetric phase-covariant cloning channel, showing that optimal cloning again emerges as the physically realizable map closest to virtual broadcasting, despite the reduced symmetry. Although phase covariance lowers both the simulation cost and this distance compared with the unitarily covariant case, the resulting virtual map remains sample inefficient.

Spatial correlations of photon pairs from spontaneous parametric down-conversion (SPDC) underpin quantum imaging and entanglement certification. We present the first systematic study of spectral filter bandwidth effects on transverse spatial correlations in bulk Type-I BBO for degenerate and nondegenerate configurations. In the far field, the degenerate conditional momentum width is pump-limited and filter-invariant, while nondegenerate configurations exhibit monotonic growth in both marginal and conditional momentum widths—with the walk-off axis approximately 100 times more sensitive than the non-walk-off axis. In the near field, we identify a previously unreported flat-dip-rise profile: the conditional position width narrows by approximately 10% at an optimal bandwidth ${\mathrm{\Delta }}_{\mathrm{dip}}\approx 1.35\mathrm{\Delta }{\lambda }_{\mathrm{SPDC}}$ before rising due to geometric displacement. When the filter is placed on the idler arm, the dip shifts by the exact factor ${({\lambda }_i/{\lambda }_s)}^2$. Both results are universal for any nondegenerate SPDC source, requiring only a finite crystal length, $d\theta /d\lambda \ne 0$, and incoherent spectral averaging. The Reid Einstein-Podolsky-Rosen uncertainty product is consistently smaller on the walk-off axis—a structural advantage of bulk birefringent geometry absent in quasiphase-matched sources. The optimal filter bandwidth ${\mathrm{\Delta }}_F={\mathrm{\Delta }}_{\mathrm{dip}}$ is determined entirely by the intrinsic phase-matching bandwidth of the crystal and is directly readable from the X-entanglement spectral width of the source.

We present low-energy positron scattering calculations by a range of oxygen-containing molecules, including water, formaldehyde, and carbon dioxide, using the Schwinger multichannel method, with both ab initio and model potential approaches. We employ our recently improved version of the Schwinger multichannel method, including $f$-type Gaussian functions [Phys. Rev. A 112, 042803 (2025)] for the ab initio calculations, providing more accurate treatment of polarization effects through virtual single excitations of the target. We also extend the application of a model potential approximation, recently implemented in the Schwinger multichannel method [Phys. Rev. A 110, 022805 (2024)], to oxygen-containing molecules. The semiempirical parameters for the oxygen atom in its different hybridizations are optimized to reproduce the current ab initio calculations for water and carbon dioxide. They are then employed to predict cross sections for other targets such as methanol, ethanol, formic acid, acetone, and parabenzoquinone. This work demonstrates the transferability of the model to systems other than the hydrocarbons previously studied and provides a computationally efficient strategy for studying positron scattering from increasingly complex molecules.

Accurate knowledge about electron dynamics in strong-field phenomena is crucial for understanding ultrafast processes at the attosecond scale. In this work, we improve the time-retrieval method for extracting electron ionization and return times from experimental observables in high-order harmonic generation. We address an existing approximation for the condition to maximize the harmonic intensity, and we introduce an accurate alternative equation. Furthermore, we introduce a numerical approach to incorporate Coulomb corrections into the retrieval procedure by calculating the Coulomb-induced shift of the electron velocity via integration of the Coulomb force, and the Coulomb action via integration of the Coulomb potential, both along the electron trajectories. Our refined method significantly improves the precision and reliability of the retrieved electron dynamics, establishing a robust foundation for future attosecond-scale investigations.

We theoretically investigate resonant x-ray scattering from two noninteracting ${\mathrm{Ne}}^{+}$ ions driven by an intense attosecond pulse using a nonrelativistic, QED-based time-dependent framework. Our model includes Rabi oscillations, photoionization, Auger decay, and quantum interference among elastic scattering and resonance fluorescence pathways. We analyze how the total scattering signal depends on pulse intensity, atomic configuration, and initial electronic state. We find that the total resonant scattering yield exceeds its nonresonant counterpart; the angular dependence of the signal qualitatively resembles a two-atom structure factor; and the visibility of interference fringes is sensitive to pulse area and the initial electronic state. Only a subset of final states reached via resonance fluorescence exhibits interference, determined by the indistinguishability of photon emission pathways. Fringe visibility is maximized in the linear scattering regime, where ionization is minimal and resonance fluorescence pathways can be largely indistinguishable. These results highlight optimal conditions for applying ultrafast resonant x-ray scattering to single-particle imaging.

X-ray characteristic spectra constitute one of the most sensitive probes for testing relativistic quantum mechanics and quantum electrodynamics, playing a key role in both atomic physics research and industrial applications. Advanced relativistic theory has, however, been unable to predict such spectra, calling several foundational ideas into question. A core problem lies in the process to convergence and the lack of sensitive metrics for convergence of many-body systems. We present a convergence metric for many-body systems: the radiative-lifetime-invariance metric ($\mathrm{\Sigma }$ metric) which can be used to identify cases of false convergence that go undetected using previous criteria. This new metric suggests that the radiative lifetime of all upper states of the $K\alpha$ transition are the same to within 0.347% under appropriate circumstances. We develop a suite of convergence criteria innovating best practice in many-body theory. We apply the suite to develop a theoretical model for the copper $K\alpha$ satellite spectrum which, despite being the best-tested system experimentally and theoretically in x-ray physics, is yet to be adequately explained theoretically. We predict the dominant $\left[1s\right]\to \left[2p\right]$ transition spectrum, and all major satellite transition spectra. The resulting wave functions are highly convergent, according to the range of convergence metrics developed herein. The theoretical Cu $K\alpha$ spectrum is in strong agreement with the world-leading experiments. We confirm the necessity and accuracy of the diagram spectrum and the $\left[4s\right]$ satellite spectrum; define and demonstrate the necessity of a convergent $\left[3d\right]$ satellite spectrum; and calculate and highlight the importance of the $\left[3p\right]$, $\left[3s\right]$, $\left[3d4s\right]$, and $\left[3d^2\right]$ shakeoff satellite processes. All are real and observed in the experimental spectra.

Bose-Einstein condensates of ${}^{87}\mathrm{Rb}$ atoms with a hyperfine spin of 2 are open quantum systems, where the atoms are lost through two-body inelastic collisions. In this dissipation process, a collision channel with total spin of 4 is forbidden by angular momentum conservation, which results in magnetization of the atoms remaining in the condensate. Here, we investigate the quantum many-body properties of spin-2 bosons that undergo two-body atomic loss. We show that the system finally reaches a steady state, which is a mixture of the states with maximum total spins. In addition, we find that a nonclassical steady state can be obtained by applying and quenching the quadratic Zeeman field.

Atoms coupled to optical cavities provide a novel platform for understanding high-orbital exotic phenomena in strongly correlated materials. In this study, we investigate strongly correlated ultracold bosonic gases that are coupled to two orthogonally arranged optical cavities and driven by a blue-detuned running-wave laser field. In contrast, with the single-cavity configuration, which couples atoms to $d$ and $p$ orbitals, our results demonstrate that atoms initially in the $s$-orbital state can be scattered into both $p_x$- and $p_y$-orbital states. This process can occur in either a symmetric or asymmetric manner, depending on the frequencies of the two cavities. For the symmetric configuration, we observe that atoms are scattered into the $p_x$ and $p_y$ orbitals equally. In the asymmetric case, photons emitted into one cavity mode suppress the scattering into the orthogonal mode. Notably, the coupling of atoms with multiple cavity modes leads to the emergence of $p$-orbital self-organized phases, accompanied by the stripe and checkerboard orbital-density waves that break different symmetries.

We propose a method to control the energy distribution in multimode mechanical systems using a single nonlinear feedback loop. We demonstrate that this feedback mechanism simultaneously amplifies the fundamental vibrational mode while suppressing all higher-order modes, effectively channeling energy into the lowest-frequency mode. This process mimics the energy redistribution of Fröhlich condensation but is achieved here through a designed feedback force that combines a low-pass gain and a high-pass loss. In the feedback-induced steady state, the fundamental mode exhibits a phase-space distribution similar to that of a phonon laser, characterized by a ring shape and amplitude squeezing. Additionally, we show that the linewidth of the fundamental mode is narrowed by an order of magnitude, corresponding to a significant enhancement in phase coherence. This scheme offers a robust approach to generating coherent mechanical states and phonon lasing without the need for optical gain media or intrinsic material nonlinearities.

We demonstrate that vector (two-component) breathers of the Manakov equations can be excited through a simple initial modulation of the plane wave imposed only on a single component. We investigate two types of breathers—vector rogue waves and superregular breathers—that can be triggered by such “nonideal” localized perturbations. Both fundamental and complex higher-order breathers can be generated using such initial modulations. The characteristic time of evolution when the maximum broadening of the spectra is achieved is calculated for several values of modulation parameters. There are two different characteristic times (one for each of the wave components) in the case of excitation of higher-order breathers. The difference between them also depends on modulation parameters. Our numerical simulations are compared with exact analytical results.

The generation of terahertz (THz) radiation through two-color laser-induced plasma filamentation in air has been extensively investigated due to the broadband and relatively high yield of the THz radiation. The angular distribution of the generated THz radiation has a distinct dip along the propagation axis, which limits the applications of the THz radiation. Here, we employ an interference model of electric-field elements with an asymmetric current pulse to investigate the cone angle dependency on the electron temperature and laser pulse duration. The leading edge ${\tau }_1$ and the trailing edge ${\tau }_2$ of this asymmetric current pulse play significant roles, which are determined by the laser duration and electron collision, respectively. With this model, our simulations obtain the THz cone angles agreeing with the ones observed in previous experiments. We also find that the THz intensity increases with ${\tau }_1$ and decays with ${\tau }_2$. The THz cone angle first increases and then decreases with the growth of ${\tau }_2$. Particularly, the electron temperature affects the ${\tau }_2$, which could provide a potential diagnostic method to determine electron temperature by measuring THz angular distribution.

We present a path-integral Monte Carlo estimator for calculating the dipole polarizability of interacting Coulomb plasma in the long-wavelength limit, i.e., the optical region. We present comprehensive details and method validation studies for our approach based on both collective and one-particle dipole autocorrelation functions in imaginary time. The simulation of thermal equilibrium in imaginary time has exact Coulomb interactions and Boltzmann quantum statistics. For reference, we use an analytically continued Drude model as the long-wavelength limit of the Lindhard response. Our collective response shows a perfect match to the analytical reference. The one-particle response is used in systematic studies of physical and numerical parameters and to discuss the phenomenological Drude scattering model.

Various light-matter interactions lead to diverse phase diagram structures in superradiant phase transition (SPT) studies. Such systems consist of multiqubit and multimode with anisotropic couplings, one- and two-photon interactions, Stark shifts, intercavity hoppings, qubit-qubit interactions, and so on. We find a general phase diagram feature that the origin is in normal phase and SPT happens only once along the radial direction of a chosen coupling parameter vector with the mean-field method at finite temperature. We can calculate the phase boundary and SPT properties by a concise method. We illustrate it with specific models and find SPT can be achieved in the strong coupling regime by means of multimode collective behavior. We also find the disorder will shift the phase boundary. These general features facilitate SPT studies and their diverse applications.