We numerically demonstrate a novel regime for generation of coherent pulse trains in a normal-dispersion photonic crystal microresonator. The results show that, if one pumps not a split mode but the neighboring one, it is possible to excite either a single wide triangular-shaped pulse or a periodic sequence of them, depending on the spectral interval between pumped and split modes, with a low pump threshold and high pump-to-comb conversion efficiency. Properties of the generated signals are studied, and excitation conditions are revealed. It is found that nonlinear coupling of counterpropagating waves significantly affects the conditions and dynamics of the generation process.

Heterostructures introduce additional degrees of freedom to two-dimensional materials, providing an alternative way to manipulate solid high-order harmonic generation (HHG). However, the impact of interlayer coupling on the HHG of heterostructures remains unexplored. Taking the phosphorene/graphene heterostructure as an example, we theoretically investigate the interlayer coupling effect in the HHG of heterostructures by solving the semiconductor Bloch equations with the accelerated Bloch-like basis in the Wannier gauge. It is found that there are obvious differences between the high-order harmonics generated by the phosphorene/graphene heterostructure, its monolayers, and the phosphorene/graphene heterostructure without any interlayer couplings. Further study reveals that the lattice strain affects the harmonic yield of the phosphorene/graphene heterostructure, but does not alter the structure of the ellipticity dependence of the harmonic yield. Conversely, the interlayer coupling significantly changes the ellipticity and orientation dependences of the harmonic yield by allowing the transitions between the energy bands of different layers. Moreover, we find that the nearest-neighbor couplings between the specific carbon and phosphorus atoms dominate the overall interlayer coupling. Finally, an all-optical method for retrieving the dephasing time is proposed by means of a one-to-one mapping relationship between the dephasing time and the ellipticity in which the seventh-harmonic yield reaches the maximum. This work provides deeper insight into the HHG mechanism in heterostructures.

Subwavelength resonant metasurfaces empowered by bound states in the continuum (BICs) have recently emerged as promising platforms for boosting optical nonlinearities as they support high-quality ($Q$) quasi-BIC resonances with strong electromagnetic-field confinement. To date, most such studies have focused on optical resonances, termed symmetry-protected quasi-BICs, because they can be readily accessible in broken-symmetry structures. However, for traditional designs of which meta-atoms are placed on low-refractive-index substrates, their smallest achievable geometric symmetries impose a limitation on the realization of ultrahigh-$Q$ resonances and thereby on the harmonic generation efficiency. Here we demonstrate a strategy that allows us to improve the $Q$ factors of quasi-BICs over a broad range of geometrical asymmetry parameter values. Specifically, using asymmetric dielectric metasurfaces placed above a perfectly reflecting mirror, we construct merging BICs in the geometrical parameter space, which pushes the square asymptotic behavior of the $Q$ factors of traditional configurations into the power of 4 or even 6. We demonstrate that merging-BIC designs can remarkably enhance the second-harmonic generation efficiency. This work suggests a route to suppress out-of-plane radiation losses of metasurfaces with potential applications to nonlinear optics, lasing, and sensing.

Topological edge pumping, as an efficient form of energy transfer protected by nontrivial topology, connects the topological characters of a modulated system with its transport behavior. It provides a powerful means to achieve robust wave manipulation between distant nodes and has been intensively investigated in quantum and optical systems. Here we propose to implement multiform energy transfers through the topological defect state in photonic waveguide arrays emulating the extended Fock state lattice (FSL). The FSL supports a topological defect state which enables efficient edge transfer and more complex transmission behavior due to phase transitions induced by on-site energies. By cross-linking two identical FSLs and modulating the linking strength, we construct a fast and robust topological beam splitter with tunable output ratios. When on-site energies are introduced on the occupied odd sites, the cross-linking FSL undergoes a series of phase transitions, after which the original defect channel state becomes a topological channel to transfers excitations from central site to different sites. In addition, we confirm that the multifunctional topological devices based on the defect state in the extended FSL chain manifests strong robustness against mild perturbations in coupling strengths and can be mapped to spatially modulated photonic waveguide arrays. This work verifies a promising route towards robust and multiform photonic excitation transports which may find applications in wave manipulation in integrated optics.

We explore the Majorana stellar representation on the Riemann sphere for optical scalar beams and vector fields, utilizing the spin and orbital angular momentum of light. In this framework, star constellations for a scalar Laguerre-Gaussian beam basis appear at the poles of the sphere, while those for vector fields correspond to well-defined circular polarizations. Leveraging the $\text{su}\left(2\right)$ symmetry of angular momentum, we populate the sphere through generalized unitary rotations of basis elements, producing optical analogs of Bloch and generalized $\text{su}\left(2\right)$ coherent states for both scalar beams and vector fields. These rotations position constellations for scalar Hermite-Gaussian beams and vector Hermite-Gaussian partial Poincaré fields, restricted to linear polarization, along the equator of the sphere. We also address the inverse problem, reconstructing optical scalar beams and vector fields from their stellar constellations. Additionally, we explore scalar and vector analogs of cat codes and kings of quantumness, contributing to the visualization and characterization of complex optical beams and fields. These insights suggest potential applications in optical communications.

Atom-field interactions, induced by the vacuum of the electromagnetic field, exhibit a variety of fundamental phenomena and effects. In this paper, we study the electromagnetically induced atomic grating due to the vacuum state of the radiation field. Using an ensemble of cold atoms, strongly coupled to an optical cavity, we show that a probe field, propagating through the atomic medium, diffracts to zeroth- and first-order diffraction peaks with few photons and even by the electromagnetic vacuum field of the cavity mode. As the number of photons in the cavity increases, the intensity of the first-order diffraction peak initially rises and then exhibits a decreasing trend. Furthermore, we observe that the first-order peak intensity reaches its maximum at resonance for both the vacuum and single-photon cavity state. However, as the number of photons increases further, this peak at resonance transforms into a dip, accompanied by two side peaks at off-resonance positions. This transition from a peak to a dip may potentially be used to distinguish the quantum state of the cavity.

The proof that neutron interference is possible using split-crystal interferometers opens the way to extended arm separation and length and to new experiments exploring quantum mechanics and gravity. Therefore, this paper reexamines the effect of gravitational and Coriolis forces on the Laue diffraction of neutrons by perfect crystals and the operation of crystal interferometers. We give in analytical form the transfer matrices for the propagation of the neutron quantum state, either pure or mixed and subjected to gravity and Coriolis force, in free space and perfect crystals. They are used to study the effect of interferometer aberrations on the quantum-mechanical phase due to the Earth's gravity. We also give an alternative way to understand the impact of gravitational and Coriolis forces in terms of the crystal displacements and tilts perceived by the neutron.

Previous studies have proven that neutron interference using split-crystal interferometers is possible. This proof paves the way for extended arm separation and length, opening the doors to new experiments exploring quantum mechanics and gravity. In a previous publication, we took a closer look at how gravitational and Coriolis forces affect dynamical diffraction in crystal interferometry. This paper uses the formalism developed, which allows for extended capabilities in studying the interferometer operation, to investigate numerically the contributions to the neutron phase of geometrical aberrations and dynamical diffraction. In addition, this work explores an alternative geometry, with the interferometer operated vertically, to determine the gravitationally induced quantum-mechanical phase independently of the dynamical diffraction and self-weight bending of the interferometer.

Based on the favorable properties of previously used one-dimensional (1D) atomic model potentials, we introduce an upgraded 1D atomic model potential for the 1D simulation of the quantum dynamics of a single active electron atom driven by a strong, linearly polarized near-infrared laser pulse. By comparing numerical simulation results of typical strong-field physics scenarios in 1D and 3D, we show that this upgraded 1D model potential gives single-atom high-order-harmonic-generation spectra with impressively increased accuracy for the most frequently used driving laser pulse parameters.

We investigate in this paper the macroscopic quantum states of multiple atoms in an optical cavity with non-Hermitian coupling motivated by recent experiments. The tunable atom-photon nonlinear interaction is included to demonstrate the variety of quantum phases in the realistic realization of Dicke phase transition with cold atoms. The energies of macroscopic quantum states are obtained in terms of the spin-coherent-state variational method, in which the nonunitary operator of the similarity transformation is used to diagonalize the effective non-Hermitian Hamiltonian, which is pseudo-Hermitian below the exceptional point (EP). The superradiant phase (SP) and related quantum phase transition (QPT) disappear completely for the non-Hermitian Dicke model (DM) because of imaginary coupling, which leads to instability of states with nonzero photons. The situation is dramatically changed by including the nonlinear atom-photon interaction generated by the pump field. It is found that the second EP emerges at a higher value of coupling. The spectrum is complex between the two EPs and real otherwise. Phase diagrams are obtained for red and blue cavity pump field detunings, showing explicitly the revival of the SP and QPT, respectively. However, the QPT from the SP to the normal phase with the increase of coupling is opposite to the transition in the normal DM. The critical property of the Berry phase indicates the first-order nature of the QPT, which again is different from the DM, where the QPT is known as the second order.

The generation of robust entanglement in quantum system arrays is a crucial aspect of the realization of efficient quantum information processing. Recently, the field of quantum magnonics has garnered significant attention as a promising platform for advancing in this direction. In our proposed scheme, we utilize a one-dimensional array of coupled cavities, with each cavity housing a single yttrium iron garnet (YIG) sphere coupled to the cavity mode through magnetic dipole interaction. To induce entanglement between YIGs, we employ a local squeezed drive, which provides the necessary nonlinearity for entanglement generation. Our results demonstrate the successful generation of bipartite and tripartite entanglement between distant magnon modes, all achieved through a single quantum drive. Furthermore, the steady-state entanglement between magnon modes is robust against magnon dissipation rates and environment temperature. Our results may lead to applications of cavity-magnon arrays in quantum information processing and quantum communication systems.

Considering the extreme sensitivity of a non-Hermitian system to boundary conditions, we introduce a single impurity as an effective boundary and investigate its impact on the system. In the absence of impurities, we observe the non-Hermitian skin effect and topologically nontrivial phase, characterized by the spectral and non-Bloch winding numbers, respectively. After introducing a single impurity, we investigate multiple transitions between $\mathcal{PT}$-symmetry-broken and -unbroken phases from both numerical and analytical perspectives. Moreover, we reveal that the introduction of a single impurity disrupts the translational invariance of the system, resulting in the unidirectional accumulation of bulk states around the impurity and the formation of bound states with purely imaginary eigenvalues localized at the impurity. Meanwhile, the directions of unidirectional accumulation of bulk states and the ranges of existence of bound states are both regulated by the intracell hopping strength and on-site potential strength. Our work demonstrates the sensitivity of non-Hermitian systems to a single impurity and promotes the investigation of non-Hermitian systems with impurities.

Measurement-based quantum computation (MBQC) offers a fundamentally unique paradigm to design quantum algorithms. Indeed, due to the inherent randomness of quantum measurements, the natural operations in MBQC are not deterministic and unitary, but are rather augmented with probabilistic by-products. Yet, the main algorithmic use of MBQC so far has been to completely counteract this probabilistic nature in order to simulate unitary computations expressed in the circuit model. In this work, we propose designing MBQC algorithms that embrace this inherent randomness and treat the random by-products in MBQC as a resource for computation. As a natural application where randomness can be beneficial, we consider generative modeling, a task in machine learning centered around generating complex probability distributions. To address this task, we propose a variational MBQC algorithm equipped with control parameters that allow one to directly adjust the degree of randomness to be admitted in the computation. Our algebraic and numerical findings indicate that this additional randomness can lead to significant gains in expressivity and learning performance for certain generative modeling tasks, respectively. These results highlight the potential advantages in exploiting the inherent randomness of MBQC and motivate further research into MBQC-based algorithms.

Light-pulse atom gravimetry has emerged as a powerful technique for precisely measuring absolute gravitational acceleration. Raman lasers are commonly used in this technique to coherently manipulate the atomic wave packet, of which the effective frequency is chirped continuously to compensate for the Doppler shift. In this study we investigate an additional phase shift in the Raman lasers that arises due to the frequency chirp. We directly measure this phase shift by recording the beat signal of the Raman lasers with a high-speed oscilloscope, providing an independent evaluation of the resultant error in atom gravimeters caused by this phase shift. Concurrently, we detect the influence of the additional phase shift on gravity measurements using our atom gravimeter, and the results are in good agreement with the independent evaluation. Notably, there may still be a residual error at the microgal level when performing differential measurements by reversing the direction of the effective wave vector of the Raman lasers. We propose that this residual error can in principle be eliminated by selecting the same chirp frequency range for both directions in the differential measurement.

In this work, we present a configuration-interaction method based on the variational Monte Carlo approach, implemented within the multiconfigurational Dirac-Hartree-Fock framework, to address the challenges of large basis sets in high-precision calculations for multielectron atoms. We validate its effectiveness on small basis set atomic systems, demonstrating strong convergence consistency. Extending the method to larger basis set atomic systems, we compute energy levels and transition parameters, showing that the variational Monte Carlo configuration-interaction method reduces the number of configurations by an average of 94% while maintaining comparable accuracy in energy level structure and transition parameters. This reduction in computational cost enables precise atomic structure calculations for systems with prohibitively large basis sets.

We theoretically study controlling the generation of an attosecond pulse with specific helicity and arbitrary ellipticity from an oriented CO molecule using the ionization-gating technique. The results show that driven by a one-color linearly polarized laser field, the high-order harmonics generated in adjacent half optical cycles possess opposite helicities, resulting in the helicities of the attosecond pulses alternating from pulse to pulse. The introduction of a weak second harmonic field to create the ionization gating can effectively control high-order harmonic generation to filter out attosecond pulses with a given helicity by suppressing the ionization of the electrons every other half optical cycle. Then attosecond pulses with the same helicity are obtained, and the desired helicity can be selectively controlled by adjusting the relative phase of the laser field. Moreover, we show that controlling the ellipticity and helicity of a 155-as pulse is feasible by tuning the molecular orientation in the ionization-gating laser field.

Over the past two decades, a chip-based atom double well has matured as a powerful art with many applications in modern science. However, its molecular counterpart, which holds potential for higher precision and broader science, has been little studied. In this paper, we propose a scheme for chip-based electrostatic double-well potential for polar molecules. The structure consists of two square wires and three square electrodes deposited on an insulating chip. By adjusting the applied voltages, versatile operations on the double well can be easily realized, such as precisely changing the depth and height of the double well, dynamically deforming the double-well potential to a single well and vice versa, and adjusting the double well from symmetric to asymmetric. We have developed numerical and analytical methods to verify the feasibility of our scheme with both light (${}^{15}\mathrm{ND}_3$) and heavy (${}^{202}\mathrm{Hg}{}^{19}\mathrm{F}$) polar molecules. The microscopic integrated double-well potential presents a promising platform for confined molecule interferometry. Also, it provides new perspectives for the study of cold collisions, sympathetic cooling, quantum information processing, and quantum simulation.