Large, three-dimensional (3D) trapped-ion crystals offer improved sensitivity in quantum sensing protocols, and are expected to be implemented as platforms in near-future experiments. However, numerical techniques used to study the laser cooling of such crystals are inefficient as the number of ions, N, in the crystal increases. Here we develop a powerful numerical framework to simulate laser cooling of up to 105 ions stored in a Penning trap. We apply this framework to characterize and optimize the cooling of ellipsoidal 3D crystals. We document pathways to enhanced cooling based on the addition of an axial component to the potential-energy-dominated E & times; B modes. Furthermore, we observe greatly enhanced cooling of the perpendicular kinetic energy to below 1 mK in prolate ion crystals, enabling a simplified cooling beam setup for such crystals. We propose specific values of trap and laser beam parameters which lead to optimal cooling in a variety of examples. This work illustrates the feasibility of preparing large 3D crystals for high-sensitivity quantum science protocols, motivating their use in future experiments.
The vibrational excitation of molecular hydrogen (H2) by low-energy electrons is one of the most fundamental reactions in atomic and molecular physics, playing a key role in the chemistry of the interstellar medium and studies of fusion plasmas. Crossed-beam experiments and electron swarm measurements have long been used to determine scattering cross sections for vibrational excitation, but the two methods produce conflicting results, a disagreement that has persisted for more than five decades and remains unsolved. Theoretical treatments have historically relied on approximations tailored to specific processes, such as separating the direct and resonant pathways or employing model polarization potentials, with no single method accurately describing all vibrational transitions. The absence of benchmark-quality calculations, combined with decades-long disagreements between experimental methods, means that even for this simple problem, there are no definitive cross-section data, leaving the reliability of theoretical approaches applied to more complex molecules uncertain and without quantified error bounds. Here we present ab initio calculations of low-energy vibrational excitation in H2 that treat electronic and vibrational motion on equal footing, providing a unified description of direct and resonant scattering across all vibrational transitions. The results are in stark contrast with previous theoretical estimates for many vibrational transitions, revealing systematic differences in both magnitude and energy dependence. At the same time, they provide strong confirmation of the swarm-derived cross sections, unlike previously accepted calculations that favored the beam data. This reversal of the long-standing situation shifts attention away from the swarm data and back onto the interpretation of the crossed-beam measurements. By providing accurate and self-consistent cross sections for vibrational excitation of H2, the present calculations will enable more reliable modeling of low-temperature plasmas and interstellar clouds.
The anomalies in light scattering associated with singularities are of great importance in both basic physics research and engineering applications. In this work, we focus on the coherent perfect absorption (CPA) singularities in an anti-parity-time (anti-PT) symmetric system and explore the coherent absorption characteristics of the system under various damping conditions. We theoretically and experimentally demonstrated CPA in both anti-PT symmetric phase and anti-PT symmetry broken phase, where dissipation plays a crucial role in the absorption efficiency of the system. Additionally, we theoretically observe pairs of CPA singularities with opposite topological charges, indicating the topological nature of the CPA in the anti-PT symmetric phase. We depict the movement of the zeros of the scattering matrix in the complex-frequency plane and the generation and annihilation processes of topological charges under different dissipative scenarios. Our work offers a promising platform for investigating non-Hermitian physics in open systems and provides valuable insights into the manipulation of electromagnetic waves in photonic systems.
The quasi-continuous-wave background (qCWB) is a common phenomenon in the nonlinear dynamics of ultrafast fiber lasers, yet its role in soliton-state transitions remains insufficiently understood. Here, we confirm the presence of qCWB by combining soliton filtering with short-time Fourier transform measurements. Mutual information and Pearson correlation analyses reveal that qCWB plays a decisive role in nonequilibrium switching between soliton molecules and pulsating soliton molecules. We further identify a bidirectional feedback mechanism, in which qCWB induces soliton-state switching via positive feedback, while the resulting soliton states exert negative feedback on qCWB, thereby maintaining dynamic balance. This mechanism enables a qCWB-based optical encoding platform for active control of ultrafast laser states.
We present a theoretical study of enhanced lateral and rotary photon drag within a four-level quantum-dot setup under the effect of dual interdot tunneling. Using the density-matrix formalism, we derive analytical expressions for the susceptibility, group index, and phase index under weak-field conditions. By introducing dual interdot tunneling in a four-level quantum-dot molecule, we demonstrate a mechanism for tunable, slowlight-enhanced photon drag, where the interaction between tunneling couplings and control fields allows precise manipulation of dispersion and light dragging. Our results reveal that rotary and lateral photon drag depend on the difference between the group and phase indices, leading to significant phase and frequency shifts even at relatively low translational and rotational velocities. The study highlights the potential of tunneling-coupled quantum-dot molecules as versatile platforms for slow-light-enhanced photon drag, with promising applications in precision metrology and nanoscale photonic devices.
Quantum machine learning integrates quantum features such as superposition and entanglement into learning processes, offering the potential to outperform classical methods in certain tasks. Among these approaches, quantum reservoir computing (QRC) has attracted considerable attention due to its simple structure and strong performance. However, existing QRC models remain limited in high-dimensional nonlinear representation and scalability, thereby constraining their computational performance in practical applications. In this work, we propose a quantum reservoir architecture that employs hyperentangled states to expand the effective dimensionality of the reservoir, enabling high-dimensional quantum information to be encoded within a small number of physical carriers. Based on this architecture, we design an optical scheme that serves as a physically realizable reservoir layer for QRC. Numerical simulations on high-dimensional nonlinear regression tasks, including the nonlinear autoregressive moving average of order 30 and Mackey-Glass tasks, demonstrate that the proposed architecture achieves high prediction accuracy and strong nonlinear mapping capability, thus paving a way toward scalable, high-dimensional, and efficient QRC.
Motivated by the realistic scenario in which impurity incorporation can induce lattice deformation, we investigate the cooperative control of solid-state high-harmonic generation (HHG) by external strain and donor-like local potential modulation within a one-dimensional time-dependent Schr & ouml;dinger equation (TDSE) framework. Strain reshapes the electronic structure by modifying the band gap, effective mass, and transition dipole moment, leading to substantial changes in HHG intensity and chirp. To provide qualitative comparison with realistic materials, strain effects are further analyzed using first-principles-based semiconductor Bloch equation calculations. We identify two distinct strain classes: tensile strain narrows the band gap in wurtzite ZnO, whereas compressive strain narrows it in monolayer black phosphorus, resulting in opposite strain dependences of the HHG response. Resolving the spectrum relative to the minimum band gap reveals three regimes: intraband-dominated subgap harmonics, dephasing-sensitive band-edge harmonics, and strain-enhanced above-gap emission. The dephasing time T2 primarily affects the band-edge region, while strain robustly governs the plateau intensity. Impurity-induced local potential modulation introduces additional excitation channels that cooperate with strain, particularly enhancing higher-order harmonic emission. These results clarify the complementary roles of strain, impurity-induced local potential modulation, and dephasing in tailoring strong-field emission in solids.
The amplitude encoding of an arbitrary n-qubit state vector requires Omega(2(n)) gate operations, owing to the exponential dimension of the Hilbert space. We can, however, form dimensionality-reduced representations of quantum states using matrix product states (MPSs). In this article, we illustrate that MPS techniques enable the preparation of quantum states representative of functions with complexity up to low-degree piecewise polynomials via shallow-depth quantum circuits with accuracy exceeding 99.99%. We extend these results to the approximate amplitude encoding of pixel values. We showcase this approach by efficiently preparing a 128 & times; 128 ChestMNIST medical image on 14 qubits with fidelity exceeding 99.2% on a circuit with a total depth of just 425 single-qubit rotation and CNOT gates.
We present a low-cost implementation of lambda-enhanced gray molasses cooling in a nonstandard beam geometry and with an inexpensive laser locking setup that only provides limited coherence of the Raman laser beams. In contrast to the established use of resource-intensive phase-locking methods, our laser system uses two independent lasers, frequency- locked to a spectral feature produced by an electromagnetically induced transparency (EIT) resonance. We show that this approach achieves sufficient coherence to enable effective gray molasses cooling without the need for costly GHz electronics, significantly reducing the complexity and cost of experimental setups and represents a step toward more accessible cold atom technologies. Furthermore, the cooling remains efficient even with a nonoptimal beam geometry, typical of cold-atoms quantum computing platforms based on optical tweezers. A wave-function Monte Carlo analysis supports the experimental findings, offering insight into the cooling dynamics of this unconventional scheme.
Coupling between quantum and classical systems is consistent, provided the evolution is linear in the state space, preserves the split of systems into quantum and classical degrees of freedom, and preserves probabilities. The evolution law must be a completely positive and norm preserving map. We prove that if the dynamics is memoryless there are two classes of these dynamics, one which features finite jumps in the classical phase space and one which is continuous. We derive the most general form of the dynamics. This is achieved by applying the complete positivity conditions using a Cauchy-Schwarz inequality applicable to classical-quantum systems. The key technical result is a generalization of the Pawula theorem.
We report a joint experimental and theoretical study of K -shell photoionization of S and F atoms in sulfur hexafluoride, SF 6 . In our experiment, we employed tunable synchrotron radiation to generate photoelectrons emitted from the 1 s shells of the S and F atoms with different kinetic energies. Coincident electron- and fragment-ion detection, carried out using cold target recoil-ion momentum spectroscopy, enabled access to polarization-averaged molecular-frame photoelectron angular distributions. In addition, we performed electronic structure calculations within the relaxed-core Hartree-Fock approximation by using a stationary single-center method. The calculations are in good agreement with the experiment. Our results support previous findings on the feasibility of imaging three-dimensional atomic arrangements in molecules using polarization-averaged molecular-frame angular distributions. In particular, we demonstrate that such imaging is possible even at intermediate photoelectron kinetic energies and even for six experimentally undistinguished F 1 s emitters.
Angular distributions of K-shell photoelectrons from CO molecules ionized by 20 keV photons are studied experimentally and theoretically in the molecular frame of reference. At this high photon energy, nondipole contributions to the light-matter interaction induce a strong forward-directed emission of the photoelectrons, which is only slightly modulated by scattering from the molecular potential. In addition, we observe a recently predicted asymmetry in the photoelectron emission distributions with respect to the polarization direction [Phys. Rev. Lett. 123, 243201 (2019)], which is caused by a rotational recoil of the nuclei imposed by the fast photoelectrons and high-energy photons. By addressing the C and O atoms separately in CO, we are able to resolve the molecular site that experiences this rotational kick, which was not possible in the previous study of N2 molecules. Our experimental results are explained by ab initio electronic structure calculations augmented by a semiclassical modeling of the rotational-recoil effect.
The transport of intensity equation (TIE) enables deterministic phase retrieval in optical microscopy, yet its application to heterogeneous media exhibiting both refractive-index variations and attenuation remains an open problem. We derive a coupled TIE-TPE (transport of phase equation) framework from the paraxial wave equation with a complex optical potential, in which the refractive index field is decomposed into a spatially uniform mean field and a local fluctuation field through Reynolds decomposition. This formulation yields a nondivergent system of transport equations that simultaneously reconstructs refractive-index fluctuations and attenuation coefficients within the condition |Delta n| < 1, without requiring linearization or weak-absorption approximations. We establish explicit validity bounds that define the measurable parameter space for a given experimental configuration, emerging from both photon counting statistics and the diffraction limit. Experimental validation using microlens arrays and HeLa cells demonstrates robust recovery of optical properties, including in the transparent-limit regime where attenuation signals approach detection thresholds. We further verify that the attenuation asymmetry parameter remains consistent with zero across three orders of magnitude in optical depth, indicating preservation of optical reciprocity in structurally heterogeneous biological media.
Spectral engineering has enabled the design of electromagnetic structures with enhanced local density of states (LDOS), a key ingredient for wave-mediated technologies. Recent studies motivated by parity-time (PT)-symmetric systems with reactive coupling have suggested that exceptional point (EP) degeneracy may provide a universal route to LDOS enhancement. In this study, we demonstrate that the presence of an EP alone does not guarantee LDOS enhancement. Instead, we rigorously show that EP-induced LDOS enhancement is critically governed by the phase correlation between the coalescing eigenvectors, as revealed by a Green's function formalism combined with Jordan chain analysis. As a result, the maximum LDOS can emerge away from the EP, even in passive systems, in sharp contrast to reactively coupled configurations where the EP coincides with maximal enhancement. Full wave simulations of coupled meta-atoms serve as a representative example that confirms the theoretical predictions and demonstrates experimentally accessible implementations.
A quantum battery is considered as a one-dimensional spin-1/2 chain, which can be described by the XY Heisenberg Hamiltonian. An external magnetic field is applied to charge the quantum battery. The results demonstrate that the counterrotating-wave terms play an important role in the performance of the quantum battery, including enhancing the maximum stored energy and charging power, and improving the charging precision. Some quasimomentum canonical modes play an important role in the charging process. The analytical expressions of the maximum stored energy, charging power, and charging precision are derived. Especially, for the pure counterrotating-wave terms, the "behavior" expressions can be employed to obtain the optimal values of the parameters. This study provides a potential way to design high stored energy and fast charging quantum batteries.
Solving combinatorial optimization problems using variational quantum algorithms (VQAs) might be a promising application in the NISQ era. However, the limited trainability of VQAs could hinder their scalability to large problem sizes. In this paper we improve the trainability of variational quantum eigensolver (VQE) by utilizing convex interpolation to solve portfolio optimization. Based on convex interpolation, the location of the ground state can be evaluated by learning the property of a small subset of basis states in the Hilbert space. This enlightens naturally the proposals of the strategies of close-to-solution initialization, regular cost function landscape, and recursive ansatz equilibrium partition. The successfully implementation of a 40-qubit demonstration using only 10 superconducting qubits demonstrates the effectiveness of our proposals. Furthermore, the quantum inspiration has also spurred the development of a prototype greedy algorithm. Extensive numerical simulations indicate that the hybridization of VQE and greedy algorithms achieves a mutual complementarity, combining the advantages of both global and local optimization methods. Our proposals can be extended to improve the trainability for solving other large-scale combinatorial optimization problems that are widely used in real applications, paving the way to unleash quantum advantages of NISQ computers in the near future.
In the field of transverse modes, the intrinsic relationship between orbital angular momentum (OAM, e), which encodes azimuthal twisting, and the radial quantum number p, which governs the transverse energy-radius distribution, has long lacked a unified, coordinate-free characterization. In the fixed-order degenerate subspace, it remains an open theoretical question how these degrees of freedom are jointly constrained by a common structure, and how one can distinguish and read out (p, |e|) with minimal information in practice. Starting from symmetry, we prove that while second-order intensity moments are strictly degenerate, fourth-order moments are the first and minimal tier that lifts the (p, |e|) degeneracy. This yields a basis-invariant dimensionless observer Q which turns a structural statement into a single-frame, calibration-free readout. On the engineering side, we exploit the intrinsic order-charge coupling to construct Euler-Hermite-Gaussian (EHG) modes: equal-amplitude, staircase-phase superpositions on the fixed-order ladder whose astigmatic evolution realizes a Dirichlet kernel with strict notches. In a single-parameter "m measurement," the EHG selector implements a low-crosstalk, near-identity transfer in the m basis; the observer Q then provides a basis-agnostic verification of (p, |e|). This coupling-guided construction yields zero sampling leakage at the designed lattice sites, supports cross-platform, reproducible thresholds under aberrations and apertures, and forms an end-to-end standard operating procedure for state preparation, demultiplexing, and quality control in high-dimensional photonic and quantum links.
Charge exchange (CX) x-ray spectroscopy is a vital diagnostic tool for astrophysical and fusion plasmas, yet its accuracy is limited by uncertainties in the orbital angular momentum (l) distribution of captured electrons. This study presents a systematic evaluation of prevalent l-distribution models through laboratory measurements of L-shell x-ray emission from Ar16+ colliding with He at energies of 1.2-10 keV/u. The measured x-ray spectra and relative line ratios are compared with theoretical results that are calculated with the use of n-resolved CX cross sections derived from the multichannel Landau-Zener method combined with various analytical l-distribution models and cascade deexcitation branching ratios. A systematic comparison reveals that the statistical l-distribution provides the best approximation to the measured spectra across the energy range considered. This work provides reliable atomic data for spectral models that can be used in astrophysical and fusion plasma diagnostics.
Quantum key distribution (QKD) has emerged as a promising solution to protect current cryptographic systems against the threat of quantum computers. As QKD transitions from laboratories to real-world applications, its implementation under various environmental conditions has become a pressing challenge. Major obstacles to practical QKD implementation are the loss of photons in the transmission media and the presence of extreme noise, which can severely limit long-range transmission. In this paper, we investigate the impact of extreme noise on QKD system parameters, including timing jitter, rate-dependent timing shifts, changes in effective detector dead time, and rate-dependent detection efficiency. Contrary to manufacturers' specifications, which assume these parameters to be constant, we demonstrate that these parameters can exhibit significant variations in extreme noise conditions. We show that changes in these parameters play a key role in determining system performance in noisy environments. To address these nonidealities, we develop a model that adapts to detectordependent timing distortions and recovery effects. In particular, our model is independent of source parameters and can be implemented using data from the detection unit. Our results show that the model enables reliable characterization and optimization of QKD performance under strong noise.
Clock atom interferometry is an emerging technique in precision measurements that is particularly well suited for sensitivity enhancement through large momentum transfer (LMT). While current systems have demonstrated momentum separations of several hundreds of photon momenta, next-generation quantum sensors are targeting an LMT enhancement factor beyond 104. However, the viability of LMT clock interferometers has recently come into question due to the potential impact of laser frequency noise. Here, we resolve this concern by analyzing the cumulative fidelity of sequential state inversions in an LMT atom interferometer. We show that the population error from n pulses applied from alternating directions scales linearly with n. This is a significant advantage over the n2 scaling that occurs when probing a two-level system n times from the same direction. We further show that contributions to the interferometer signal from parasitic paths generated by imperfect pulses are negligible, for any loss mechanism. These results establish that laser frequency noise is not a practical limitation for the development of high-fidelity LMT clock atom interferometers.
