We investigate topological phenomena in a spatially modulated Dirac-$\delta$ lattice, where the scattering potential varies periodically in space. Changing the potential modulation frequency leads to Hofstadter's butterfly-like energy spectrum and enables the emergence of topological transport regimes characterized by nontrivial Chern numbers. We show how the considered modulated system is connected to the Hofstadter model via the Harper equation. By adiabatically varying spatial modulation parameters, we demonstrate controllable quantum transport and verify the topological nature of these effects through Wannier center displacement and bulk invariant calculations. We also propose an experimentally feasible realization of such a system using optically controlled three-level atoms. Our findings showcase spatially engineered Kronig–Penney-type systems as versatile platforms for investigating and exploiting different topological quantum transport regimes.

We report the experimental observation of two distinct Berry phases $(+\frac{2\pi }{3}$ and $-\frac{2\pi }{3})$ generated on the surface of a Möbius cavity resonator at microwave frequencies supporting the ${\mathrm{TE}}_{1,0,n}$ mode family. This resonator consists of a twisted, mirror-asymmetric prism with a cross section of the triangular $D_3$ symmetry group, bent around on itself to form a ring. This geometric class supports resonant modes with nonzero electromagnetic helicity (i.e., a nonzero $\vec{E}·\vec{B}$ product) at microwave frequencies. There exist modes with three-fold rotational symmetry as well as those that exhibit no rotational symmetry. The latter result in an accumulated Berry phase while the former do not, which is determined from the measured frequency shift of the modes when compared to a mirror-symmetric resonator of otherwise equivalent geometry.

Dicke quantum batteries harness the collective effects of high-power charging, yet whether robust, near-perfect superabsorption can persist under mesoscopic disorder and limited global control in realistic solid-state platforms remains unclear. Using reinforcement learning (RL), we identify a time-dependent cavity frequency that enables efficient and robust charging of disordered Tavis-Cummings (Dicke-type) batteries, yielding an ensemble-averaged normalized ergotropy exceeding 0.97 with fluctuations below 0.02. We demonstrate superlinear scaling of the charging power for charging times scaling as $T\propto N^{-1/2}$. Notably, the learned controls are well described by simple few-segment, single-sweep ramps reminiscent of multilevel Landau-Zener driving, under which the dynamics remains concentrated in the collective bright manifold. Exploiting this feature, we introduce a truncated total-spin ($J$-irreducible-representation) description that faithfully captures the charging dynamics for system sizes up to $N\sim 30$. These findings demonstrate a scalable framework for designing high-performance Dicke batteries and highlight the versatility of RL in disordered many-body systems.

In this paper, we investigate biphoton generation via spontaneous four-wave mixing in Doppler-broadened atomic vapors. Thermal motion breaks the zero-temperature symmetry between copropagating and counterpropagating geometries and introduces competition between the biphoton generation rate and propagation loss. A temperature-dependent pump-detuning scheme restores an approximate mirror symmetry between the frequency-resolved responses of the two geometries, yielding nearly identical biphoton generation rates and correlation strengths over a broad temperature range. We find that increasing atomic thermal velocity drives the temporal correlation function from damped oscillations at low temperatures to a purely exponential decay at higher temperatures by enhancing decoherence and pushing the effective Rabi frequency into the imaginary regime. This behavior is captured analytically through the temperature-dependent modification of the poles governing the oscillatory correlations. The generated biphotons retain strong nonclassical character, accompanied by spectral narrowing and temperature-induced temporal broadening. Numerical simulations incorporating Doppler averaging validate these predictions, quantify the model's accuracy, and confirm that the same qualitative trends persist at higher optical depths. These results identify a simple and tunable route to narrowband biphoton generation in warm vapors, with direct relevance to quantum memories and long-distance quantum communication.

A concept for the analysis of quantum light at its most fundamental timescale is provided, namely, at the oscillation cycle of a mode or the inverse frequency of an involved photon. We demonstrate theoretically how local quantum measurements allow one to reconstruct and visualize a quantum field under study at subcycle scales, even when its temporal-mode structure is a priori unknown. Generalized subcycle-resolved field quadratures for each value of the time delay with respect to a sampling pulse and for each value of the carrier-envelope phase of the sampled field are introduced. Generation and tomography of ultrabroadband squeezed states as well as photon-subtracted states derived from them are described, incorporating also single-photon states. We decompose the fields into separable modes that are localized in time and illustrate the dynamics of their contributions in the detection process. Since the temporal sampling requires millions of repetitions for each time delay, an efficient reconstruction scheme is developed based on the Gram-Charlier expansion, allowing us to minimize the amount of different phase values required for a desired precision. Our results represent a cornerstone in the emerging chapter of quantum physics termed time-domain quantum optics.

This paper presents a comprehensive and versatile methodology for generating arbitrary coherent superpositions of excited states in multiladder quantum systems. The proposed scheme is distinguished by its applicability to systems with an arbitrary number of parallel ladders and an arbitrary number of excited states within each ladder. The complexity inherent to such multistate systems is addressed by employing the Morris-Shore (MS) transformation, which unitarily decouples the original system into a set of independent, simpler ladder subsystems. Within this simplified MS basis, we design and implement three distinct quantum control strategies. First, an analytic pulse-area method, enhanced by an imposed SU(2) dynamical symmetry, enables exact state preparation. Second, composite-pulse sequences are constructed to provide robust high-fidelity transfer resilient to systematic pulse-area errors. Third, adiabatic passage techniques, exemplified by chained stimulated Raman adiabatic passage (STIRAP), ensure robustness against variations in experimental parameters. Detailed numerical simulations confirm the effectiveness of each technique in creating target superpositions, such as equal distributions among all states or specific subsets of excited states.

We propose the realization of a spin-2 Floquet spinor Bose-Einstein condensate via Floquet engineering of the quadratic Zeeman energy. In the Floquet system, the coupling strengths of all angular-momentum-conserving spin-flip processes are renormalized by driving-parameter-dependent Bessel functions. Such Floquet-engineered interactions significantly enrich such possible states in homogeneous gases. We search for these possible states by using the variational method. Ground-state phase diagrams, which map the distributions of these possible states, are presented in the space of the driving parameters.

Electromagnetic waves propagating through vacuum can polarize virtual electron-positron pairs; this polarization in turn nonlinearly modifies their propagation. A semiclassical nonlinear wave equation describing the propagation is derived from the Euler-Heisenberg Lagrangian density, which captures vacuum polarization effects up to the one-loop level. Here we present a reduced-action-integral approach that enables rapid modeling of nonlinear phenomena arising from the Euler-Heisenberg Lagrangian. Application of the variational principle to the reduced action provides equations of motion for familiar light-pulse parameters, such as spot size, phase, polarization, and phase-front curvature, without requiring full-field simulations. Three examples demonstrate the utility of the approach: phase modulation, birefringence, and frequency mixing.

Spectrum tomography for the energy ($E$) of a ring-shaped Bose-Hubbard circuit is illustrated. There is an interparticle interaction $U$ that controls superfluidity (SF) and the transition to the Mott insulator (MI) regime. The circuit is coupled to an electromagnetic cavity mode of frequency ${\omega }_0$, and the coupling is characterized by a generalized fine-structure constant $\alpha$ that controls the emergence of superconductivity (SC). The $(U,\alpha ,{\omega }_0,E)$ diagram features SF and SC regions, a vast region of fragmented possibly chaotic states, and an MI regime for large $U$. The mesoscopic version of the Meissner effect and the Anderson-Higgs mechanism are discussed.

Multiple-scattering processes can be modified when the energy of the incident particle coincides with the energy of a resonant state of the target scatterers. Of particular interest, “proximity” resonances are known to occur for the case of $s$-wave resonances having scattering lengths larger than the distance between scatterers. We investigate the resonant scattering between an incident charged particle and ensembles of $N$ atoms. The equations for the partial-wave multiple-scattering amplitudes are derived and then solved for the case $N=2$. We consider $s$-, $p$-, and $d$-wave resonances and examine how the scattering cross section depends on the distance between and the orientation of the two target atoms. Numerical calculations of the scattering by ensembles of $N=250$ atoms reveal a marked suppression of the scattering cross section at resonance as well as the persistence of multiple scattering at low number densities.

Rotational optical tweezers are used to probe the mechanical properties of unknown microsystems. Quantifying the angular trap stiffness is essential for interpreting the rotational dynamics of probe particles. While methods to determine trap stiffness are well established for translational degrees of freedom, angular trapping is often treated analogously even though rotational and translational motions are sensitive to distinct experimental parameters and offer separate insights. This work details passive analysis techniques for determining the angular trap stiffness within the linear restoring torque model and examines the influence of several factors unique to rotational optical tweezers. We show that the parameters of an ancillary measurement beam can be tuned to minimize its influence on angular trapping dynamics, providing necessary improvements for nanoparticle-scale analysis. We also explore the combined effects of shape-induced and material birefringence in spheroidal vaterite probes, and present a framework for assessing hydrodynamic and inertial contributions.

Systems of individual electrons electrostatically trapped on condensed-noble-gas surfaces have recently attracted considerable interest as potential platforms for quantum computing. The electrons serve as charge qubits in the system, and the purity of the noble-gas surface protects the relevant quantum properties of each electron. Previous work has indicated that manipulation of a confining double-well potential for electrons on superfluid helium can generate entanglement suitable for two-qubit gate operations. In this work, we incorporate a time-dependent tuning of the potential shape to further explore operation of two-qubit gates with the superfluid helium system. Through numerical time evolution of the closed system (without decoherence), we show that control-induced errors can be minimized to allow for fast high-fidelity two-qubit gates. In particular, we simulate operation of the $\sqrt{i\mathrm{SWAP}}$ and cz gates and obtain estimated fidelities of 0.999 and 0.996 with execution times of 2.9 and 9.4 ns, respectively. Furthermore, we examine the stability of these gate fidelities under nonideal execution conditions, which reveals new properties to consider in the device design. Finally, we reflect on the impact of screening and decoherence on our results. The methodology presented here enables future efforts to isolate control-induced effects from environmental noise, which is an important step towards the realization of high-fidelity two-qubit gates with electrons on helium.

We report measurements of the astrophysically predominant $K\alpha$ emission from $\mathrm{Ca}$ XIX (He-like ion) and its satellite lines resonantly excited by dielectronic recombination (DR). We achieve an electron-energy resolution of 8 eV in a cryogenic electron beam ion trap, and determine the energies of the exciting electrons and the emitted photons up to the KLN ($n\le 8$) manifold with 0.05 and 0.1% respective uncertainties. For the KLL satellites, energies agree very well with our predictions using the Flexible Atomic Code (fac) and previous state-of-the-art calculations. Our calculations also agree with our experimental direct excitation cross sections for $K\alpha$ within their 10% uncertainty. We extract DR coefficient rates from our fac calculations and find good agreement with values tabulated in the OPEN-ADAS database. Moreover, we compare fac synthetic spectra, incorporating the DR atomic data benchmarked in the present work, with recent x-ray imaging and spectroscopy mission observations. This comparison highlights the contributions of DR satellites to the $\mathrm{Ca}$ xix main lines.

A theoretical study of the dissociative recombination of ${\mathrm{NeH}}^{+}$ with low-energy electrons is presented. The calculations are performed using the Multichannel Quantum Defect Theory, adapted to the relatively rare case where the driving force of the process is the nonadiabatic interaction between molecular states rather than the Rydberg-valence electronic one. The resulting cross sections show good shape agreement with the available experimental results and fill the lack of missing theoretical data at low energy, below 2 eV, where no detailed quantum calculations are currently available. The implications of our results for fusion edge plasmas and, possibly, for astrochemistry, are discussed.

In this study, the multiconfiguration Dirac-Hartree-Fock method was employed to calculate the Landé $g$-factors and electric-quadrupole moments of the $5d6s{}^3{D}_{1,2,3}$ and ${}^1{D}_2$ states, as well as the quadratic Zeeman shift coefficients of the ${}^3{D}_{1,2}$ clock states in ${\mathrm{Lu}}^{+}$. The electron correlation, particularly higher-order electron correlation effects, along with the Breit interaction and quantum electrodynamics corrections, are systematically taken into account. Our calculated Landé $g$-factors, electric-quadrupole moments, and quadratic Zeeman shift coefficients for the $5d6s{}^3{D}_{1,2}$ clock states are in agreement with experimental measurements within $1\%$. The influence of hyperfine interactions and perturbing states on these parameters was also analyzed. Our results demonstrate that both the magnetic-dipole and the electric-quadrupole hyperfine interactions contribute significantly to the hyperfine-induced Landé $g$-factors and electric-quadrupole moments of the ${}^3{D}_{1,2}$ states. In particular, for ${}^{176}\mathrm{Lu}^{+}$, the electric-quadrupole hyperfine interaction accounts for approximately $15\%$ of the total hyperfine-induced electric-quadrupole moment of the ${}^3{D}_{1,2}$ states. This study provides valuable theoretical support for evaluating frequency shifts in the ${\mathrm{Lu}}^{+}$ optical clock.

Unidirectional reflection amplification is a desirable functionality for nonreciprocal photonic components. We propose an inversionless route to this effect in a homogeneous ensemble of a three-level $\mathrm{\Lambda }$-type atomic system using a spatially graded coupling field. The linear position dependence of the coupling strength breaks the spatial symmetry of the optical susceptibility, enabling strong reflection from one side while suppressing the counterpropagating reflection channel. With the assistance of spontaneously generated coherence and a weak incoherent pumping field, the medium provides coherent gain without population inversion, resulting in essentially unidirectional reflection amplification, including the emergence of dual amplification bands. Moreover, both the magnitude and spectral location of the amplification can be reconfigured by tuning the relative phase, detuning, and intensity of the coupling field, offering flexible control over the nonreciprocal response. These results establish a simple, nonmagnetic, and highly tunable platform for realizing reflection-type nonreciprocal amplifiers and related functionalities, with potential applications in quantum optics and integrated photonic devices.

The Goos-Hänchen (GH) effect, a fundamental beam displacement phenomenon occurring during total internal reflection, has been challenging to utilize directly in high-sensitivity applications due to its inherently small, subwavelength scale. The weak measurements have been widely used to amplify this effect involving abstruse quantum interpretations. In this work, we present a clear and insightful classical wave-optics derivation and visualization of the weak-measurement amplification process for the GH effect from the perspective of optical differential interference. By leveraging the polarization-dependent lateral displacements induced by the GH effect, we demonstrate that the reflected field can be decomposed into a fundamental Gaussian component and a first-order differential component. Through coherent superposition using a simple polarizer, we establish an interferometric setup that directly converts the nanometer-scale GH shift into a micrometer-scale displacement of the output field centroid. Our model, derived entirely from classical electromagnetic theory and Fourier optics, provides a clear physical picture of the amplification mechanism. This work offers a versatile and powerful framework for precision measurement, nanophotonics, and interferometric sensing.

Inferring causal models from observed correlations is a challenging task, crucial to many areas of science. In order to alleviate the computational effort when sifting through possible causal explanations for some given observations, it is important to know whether symmetries in the observations correspond to symmetries in the underlying realization so that one can quickly discard impossible explanations. Via an explicit example, we demonstrate that, in general, symmetries cannot be exploited to reduce the hypothesis space. We use a tripartite probability distribution over binary events that is realized by using three (different) independent sources of classical randomness. We prove that even removing the condition that the sources distribute systems described by classical physics, the requirements that (i) the sources distribute the same physical systems, (ii) these physical systems respect relativistic causality, and (iii) the correlations are the observed ones are incompatible.