We introduce a variational quantum computing approach for quantum state reconstruction within a discretized logical framework, using experimental measurement data as input. By mapping the reconstruction cost function onto an Ising model, the problem can be solved using a variational eigensolver on present-day quantum hardware identifying dominant logical elements of the density matrix. As a proof of concept, we demonstrate the method on quantum structured light, in particular, entangled photons carrying orbital angular momentum, and show that the reconstruction procedure can yield reliable performance even on noisy devices. Our results highlight the potential of variational algorithms as a complementary approach to quantum state tomography, particularly for high-dimensional structured light, where classical approaches can face bottlenecks.

The interaction between intense laser pulses and atoms or molecules is a central topic in strong-field physics. By numerically solving the time-dependent Schrödinger equation (TDSE) under the single-active-electron approximation, we investigate the ultrafast electronic dynamics of an aligned ${\mathrm{N}}_2$ molecule at different internuclear distances driven by a circularly polarized extreme-ultraviolet laser. The results show that the photoelectron momentum distributions (PMDs) exhibit a four-lobe structure which can be explained by the attosecond perturbation theory. As the internuclear distance increases, the intensities of PMDs, ionization probabilities, electron currents, and ultrafast magnetic-field strengths all decrease, primarily attributed to the reduced coupling strength between the initial state and the continuum state. Notably, the electron current generated from the highest occupied molecular orbital (HOMO) is 1 order of magnitude stronger than that of HOMO−1. The ultrafast magnetic field oscillates along the axial direction when the initial state is the HOMO, whereas it rotates in the plane when the initial state is the HOMO−1. Furthermore, we employ a deep-learning technique to predict the ultrafast magnetic field, achieving more than $98\%$ agreement between model predictions and TDSE results.

We theoretically investigate a non-Hermitian optical dimer whose parameters are renormalized by dispersive and dissipative backaction from the coupling of the passive cavity with a ring-trapped Bose-Einstein condensate. The passive cavity is driven by a two-tone control laser, where each tone is in a coherent superposition of Laguerre-Gaussian beams carrying orbital angular momenta $\pm \ell \hslash$. This imprints an optical lattice on the ring trap, leading to Bragg-diffracted sidemode excitations. Using an exact Schur-complement reduction of the full light-matter dynamics, we derive a frequency-dependent self-energy and identify a static regime in which the atomic response produces a complex shift of the passive optical mode. This renormalized dimer supports a tunable exceptional point, enabling spectroscopic signatures in the optical transmission due to a probe field, which can in turn be utilized for estimating the winding number of the persistent current. Exploiting the associated half-integer topological charge, we propose a digital exceptional-point-based sensing scheme based on eigenmode permutation, providing a noise-resilient method to sense superfluid rotation without relying on fragile eigenvalue splittings. Importantly, the sensing proposals are intrinsically nondestructive, preserving the coherence of the atomic superfluid.

We theoretically investigate a quadratic optomechanical system comprising a single-mode optical cavity linearly coupled to one mechanical resonator and quadratically coupled to a second resonator. By tuning the cavity detuning and optomechanical coupling strengths, we demonstrate the transition from optical bistability to multistability with up to seven steady-state solutions. Notably, simultaneous ground-state cooling of both mechanical resonators occurs on the dynamically stable branch of the nonlinear steady-state solutions, offering new opportunities for combined nonlinear optical and quantum cooling functionalities. Beyond the multistable regime, we systematically study dual-mode ground-state cooling and find that robust simultaneous cooling can be achieved over a broad parameter range, except when the linear and quadratic couplings become comparable, where a dark-mode effect arises. In this case tuning the second-order optomechanical-induced frequency shifts effectively suppresses dark-mode interference, enabling controllable and simultaneous ground-state cooling. Our results provide a versatile framework for engineering multimode quantum states in optomechanical systems and open new avenues for the development of multifunctional quantum devices, including ultrasensitive sensors, scalable quantum memories, and integrated quantum networks.

Phase-gradient metasurfaces (PGMs) empowered by the parity-dependent diffraction effect provide new degrees of freedom for the arbitrary manipulation of electromagnetic waves. However, the specific roles and mechanisms of the phase gradient ξ and reciprocal-lattice vector $G$ in wave-front reshaping require clarification for further development and applications. In this work, we propose and demonstrate a theoretical framework to elucidate how the interplay between the phase gradient ξ, reciprocal-lattice vector $G$, and unit-cell number $m$ governs the diffraction behavior of PGMs. A general reversal diffraction phenomenon and a diffraction cycle are demonstrated, with the typical parity-dependent diffraction in conventional PGMs with $\xi =G$ as a special case. In addition, we clarify the roles of the reciprocal-lattice vector $G$ and the phase gradient ξ in PGM diffractions, where the reciprocal-lattice vector $G$ determines the number of allowable diffraction orders in far-field diffraction, whereas the phase gradient ξ determines the selection rule for the diffraction order. This study provides not only unique physical insights into the diffraction mechanisms of PGMs but also a complete theory for wave-front manipulation based on PGMs.

The Fabry-Pérot resonator is a cornerstone of photonics and wave physics, providing a universal mechanism for spectral confinement and resonant enhancement of wave-matter interactions. In this work, we establish an analytically tractable class of Fabry-Pérot metacavities in which the reflecting elements are realized by single-layer periodic arrays of circular dielectric cylinders acting as metamirrors. Both the reflection efficiency and reflection phase of such metamirrors are obtained in closed form and shown to be widely and independently tunable, encompassing ideal electric and magnetic mirror limits with unit reflectivity. Building on these results, we derive explicit analytical expressions that fully describe the optical responses of Fabry-Pérot cavities composed of two such parallel metamirrors with negligible near-field couplings. Our combined analytical and numerical investigations reveal that these metamirrors provide exceptional flexibility for tailoring Fabry-Pérot resonances across a broad spectral range, enabling precise control over resonance positions and quality factors. In particular, the framework naturally predicts the emergence of Fabry-Pérot bound states in the continuum with formally infinite $Q$ factors. These results establish dielectric-metamirror-based Fabry-Pérot cavities as a versatile and fundamentally transparent platform for engineering high-$Q$ optical resonances.

A projective measurement cannot decrease the von Neumann entropy if the outcome is ignored. However, under certain sound assumptions and using the quantum violation of Leggett-Garg inequalities, we have previously demonstrated that this property is not inherited by a classical simulation of such a measurement process. In the simulation, a measurement erases prior information by partially resetting the system, suggesting that the quantum-state update following a measurement cannot be entirely epistemic. The erasure of information has been proved by assuming that the maximally mixed quantum state corresponds to maximal ignorance of the classical state. A more intricate proof employed the weaker hypothesis that the entropy is finite at some stage of the simulation. In this paper, we focus on the quantum-foundational implications of this theorem. We first provide a simple proof by directly using the second hypothesis. Second, we identify information erasure as the mechanism breaking the time symmetry in ontological theories. This symmetry break has been previously proved by Pusey and Leifer. Third, we show that information erasure, and thus symmetry break, can be avoided by employing a branching similar to many-worlds theory. The information flow and the time asymmetry are transferred to the measurement devices and the subsequent comparison of results, which inherently involve time-asymmetric processes. Thus, causality and the absence of information erasure suggest that measurements have multiple actual outcomes. Similarly, Deutsch and Hayden argued that Bell's theorem leads to the same conclusion if locality is given for granted. We conclude by showing that the problem of the clumsiness loophole in an experimental Leggett-Garg test of macrorealism is mitigated by the information-erasure theorem.

We demonstrate a hardware-agnostic protocol for realizing fast, high-fidelity gates through dynamical decoupling (DD) pulse sequences applied to a central qubit coupled to target qubits. The target qubits are controlled by leveraging their intrinsic interaction with the central qubit, eliminating the need for slow, error-prone direct control. We develop and implement the DD-gate protocol within two distinct frameworks: a general model with minimal assumptions, benchmarked on a gate-based digital quantum simulator given by the IBM quantum (IBMQ) platforms; and an experimentally realistic case with a ${}^{15}\mathrm{N}$-vacancy center (${}^{15}\mathrm{NV}$) in diamond. Using IBMQ, we are able to elucidate the underlying quantum dynamics of the DD gates and test them, independently of experimental constraints. For ${}^{15}\mathrm{NV}$, we realize the protocol considering system-specific properties, which could represent a significant reduction in gate duration and improved technological scalability compared with current dynamical-decoupling-based control. We also propose a simple application for high-efficiency polarization of the ${}^{15}\mathrm{N}$ nuclear spin that could potentially be less technically demanding than current methods. Altogether, this work provides a robust strategy for quantum control that can be implemented in arbitrary systems fitting the central-target qubit architecture. Beyond these results, our open-source simulations and implementations for both platforms provide a practical framework for simulating time-dependent qubit dynamics on noisy intermediate-scale quantum era gate-based quantum processors.

We propose a parallel protocol for implementing distributed nonlocal quantum gates between spatially separated stationary qubits encoded in dual-species quantum emitters (i.e., color-center and superconducting qubits). By utilizing entangled photon pairs with distinct frequencies as a quantum data bus, our approach connects spatially separated devices without requiring quantum frequency conversion or preshared entanglement, while maintaining an always-ready and resource-efficient property for distributed quantum computing and networks. Furthermore, we demonstrate the feasibility of implementing parallel distributed nonlocal quantum gates on multiple pairs of spatially separated qubits using a single high-dimensional entangled photon pair, which directly benefits from the enhanced quantum capacity provided by optical qudit encoding. Our protocol establishes a scalable and practically implementable framework for distributed quantum networks, potentially enabling the development of future large-scale quantum computing architectures.

Using a linearly polarized fundamental pulse and an elliptically polarized second-harmonic pulse, we have recently shown that both all-optical one-dimensional orientation of linear molecules which belong to the point group $C_{\infty \mathrm{v}}$ [Hossain and Sakai, J. Chem. Phys. 153, 104102 (2020)] and three-dimensional orientation of asymmetric-top molecules which belong to the point group $C_2\mathrm{v}$ [Hossain, Esaki, and Sakai, Phys. Rev. A 108, 063109 (2023)] can be achieved. Here, we show that our method can further be extended to realize enantioselective three-dimensional orientation of chiral molecules which belong to the point group $C_1$. Using alanine molecules as a sample, we show that three-dimensional molecular orientation is realized when the relative phase difference between the two wavelengths is not an integer multiple of $\pi /2$. Moreover, the two enantiomers of the alanine molecule are oriented in the opposite directions along the laser propagation direction, indicating that the realized three-dimensional orientation is enantioselective. The advantages of our method, such as robustness and controllability, are discussed.

The three-body fragmentation dynamics of propene trications $\left[{({\mathrm{H}}_3\mathrm{C}–\mathrm{CH}={\mathrm{CH}}_2)}^{3+}\right]$ induced by 1-MeV/u ${\mathrm{C}}^{4+}$ ion impact is investigated using cold-target recoil-ion momentum spectroscopy. Complete kinematical information is obtained for four three-body fragmentation channels: ${\mathrm{H}}^{+}+{\mathrm{CH}}_2{}^{+}+{\mathrm{C}}_2{\mathrm{H}}_3{}^{+}, {\mathrm{H}}^{+}+{\mathrm{CH}}_3{}^{+}+{\mathrm{C}}_2{\mathrm{H}}_2{}^{+}$, and the channels involving ${\mathrm{H}}_2{}^{+}$ production, i.e., ${\mathrm{H}}^{+}+{\mathrm{H}}_2{}^{+}+{\mathrm{C}}_3{\mathrm{H}}_3{}^{+}$ and ${\mathrm{H}}_2{}^{+}+{\mathrm{CH}}_2{}^{+}+{\mathrm{C}}_2{\mathrm{H}}_2{}^{+}$. By analyzing the fragment momentum correlations visualized in Dalitz plots, Newton diagrams, and native frames, each channel exhibits two distinct sequential processes, with a concerted process uniquely identified in the ${\mathrm{H}}_2{}^{+}+{\mathrm{CH}}_2{}^{+}+{\mathrm{C}}_2{\mathrm{H}}_2{}^{+}$ channel. Relative contributions of competing fragmentation pathways are determined. The sequential pathways with CH breakage prior to CC breakage predominate in the channels ${\mathrm{H}}^{+}+{\mathrm{CH}}_2{}^{+}+{\mathrm{C}}_2{\mathrm{H}}_3{}^{+}$ and ${\mathrm{H}}^{+}+{\mathrm{CH}}_3{}^{+}+{\mathrm{C}}_2{\mathrm{H}}_2{}^{+}$. Due to comparable timescales, the ${\mathrm{H}}_2$ formation kinetically competes with the molecular fragmentation. This competition enables fragmentation pathways featuring initial ${\mathrm{H}}_2{}^{+}$ emission, which contribute to two ${\mathrm{H}}_2{}^{+}$ production channels. This study provides valuable insights into understanding the dissociation behaviors of asymmetric hydrocarbon chains under several-MeV/u ion impact, revealing the role of ${\mathrm{H}}_2$ formation kinetics in molecular fragmentation dynamics.

Coherent perfect absorption (CPA) is the time-reversed process of laser emission, enabling complete light capture via the destructive interference of coherent light waves. Its tunable characteristics open avenues for next-generation optical modulators. However, the existing research predominantly focuses on isotropic materials, leaving a significant gap in theoretical studies addressing anisotropic systems. To fill this gap, this study investigates the CPA effect in composite media composed of anisotropic substrates combined with metallic nanoparticles as fillers. Employing effective medium theory, we calculate the effective permittivity and permeability of the composite, and derive the reflection and transmission coefficients for the resulting effective medium. The CPA condition is quantitatively characterized by the logarithm of scattered light intensity. By strategically coordinating the parameters such as nanoparticle size, anisotropic substrate permittivity, and composite layer thickness, while accounting for the inherent nonlocal effect, we achieve precise control over CPA properties. Notably, two distinct CPA variation trends are observed when tuning the substrate permittivity. Furthermore, through careful parameter optimization, we identify and demonstrate that the composite medium can achieve CPA in two separate wavelength bands under identical volume fractions. This dual-band CPA phenomenon represents a capability in anisotropic composite systems. This work provides a foundational theoretical framework for developing advanced nanocomposite-based optical absorbers utilizing anisotropic media, opening the possibilities for tunable optical device design.

Optical skyrmions are topological structures of light whose defining property, the skyrmion number, is robust against perturbations. This makes them attractive for applications in quantum information storage, where resilience to decoherence is paramount. However, their preservation during coherent storage remains unexplored. We report the first experimental demonstration of storing and retrieving optical skyrmions in a cold ${}^{87}\mathrm{Rb}$ vapor using a dual-path electromagnetically induced transparency memory. Crucially, we show that the skyrmion number remains invariant for storage times up to several microseconds, even when subjected to imbalanced loss between the two paths and substantial perturbations in control beam power. Our work demonstrates the survival of a nontrivial topological invariant in a quantum memory, marking a significant step towards topologically protected photonic technologies.

Fault-tolerant quantum computing (FTQC) requires fast and accurate decoding of quantum error correction syndromes. However, in large-scale systems, the number of available decoders is much smaller than the number of logical qubits, leading to a fundamental resource shortage. To address this limitation, virtualized quantum decoder (VQD) architectures have been proposed to share a limited pool of decoders across multiple qubits. While the minimize longest undecoded sequence heuristic has been introduced as an effective scheduling policy within the VQD framework, its locally greedy decision-making structure limits its ability to consider global circuit structure, causing inefficiencies in resource balancing and limited scalability. In this work, we propose constraint-optimal driven allocation (CODA), an optimization-based scheduling algorithm that leverages global circuit structure to minimize the longest undecoded sequence length. Across 19 benchmark circuits, CODA achieves an average 74% reduction in the longest undecoded sequence length. Crucially, while the theoretical search space scales exponentially with circuit size, CODA effectively bypasses this combinatorial explosion. Our evaluation confirms that the scheduling time scales linearly with the number of qubits, determined by physical resource constraints rather than the combinatorial search space, ensuring robust scalability for large-scale FTQC systems. These results demonstrate that CODA provides a global optimization-based, scalable scheduling solution that enables efficient decoder virtualization in large-scale FTQC systems.

Multiensemble superradiance extends Dicke superradiance to multiple ensembles and supports dark states whose properties depend on the initial state. In the large-$N$ limit, we derive analytical covariance matrices for these dark states, revealing interensemble entanglement that enhances quantum metrology. The minimum eigenvalue, determined by the curvature of the superradiance potential, corresponds to the optimal multiparameter spin-squeezing coefficient, which is given by the Rayleigh quotient of the spin-squeezing matrix, linking metrological sensitivity to the geometric structure of the underlying dynamics. The multiparameter squeezing coefficient provides a variational framework for optimizing metrological performance. These results enable optimal estimation of arbitrary linear combinations of multiple parameters, offering a concrete protocol for distributed quantum sensing and a promising route toward multimode quantum interferometry.

We present a scheme for coherent manipulation of populations and robust creation of arbitrary coherent superposition of metastable states of a quantum system with lambda configuration of operating energy levels using laser pulses with a quadratically chirped carrier frequency. The case of a single “broadband” laser pulse is considered, when the width of the laser pulse frequency spectrum (without chirp) exceeds the frequency distance between the metastable energy levels of the system. The results of the dressed state analysis and numerical simulation are presented, demonstrating reliable, robust, and rapid creation of an arbitrary coherent superposition of metastable states by changing the parameters of the laser pulses.

We present a canonical formalism for computing quantum fluctuations of certain discrete degrees of freedom in systems governed by integrable partial differential equations with known Hamiltonian structure, provided these models are classical-field approximations of underlying many-body quantum systems. We then apply the formalism to both the two-soliton and three-soliton breather solutions of the nonlinear Schrödinger equation, assuming the breathers are created from an initial elementary soliton by quenching the coupling constant. In particular, we compute the immediate postquench quantum fluctuations in the positions, velocities, norms, and phases of the constituent solitons. For each case, we consider both the white-noise and correlated-noise models for the fluctuation vacuum state. Unlike previous treatments of the problem, our method allows for analytic solutions. Additionally, in the correlated-noise case, we consider the particle-number-conserving [also called $\text{U}\left(1\right)$-symmetry-conserving] Bogoliubov modes, i.e., modes with the proper correction to preserve the total particle number. We find that in most (but not all) cases, these corrections do not change the final result.

We demonstrate that Liouvillian exceptional points (LEPs), previously explored only in continuous Lindbladian dynamics, also emerge in discrete brickwork completely positive trace-preserving (CPTP) circuits. By analytically solving a minimal two-qubit brickwork model, we identify the conditions under which discrete-time LEPs arise and show that they retain the hallmark square-root eigenvalue splitting and linear-in-time sensitivity enhancement. These results establish a direct bridge between continuous non-Hermitian physics and discrete quantum-circuit architectures, opening a path toward the realization of exceptional-point-based sensing on near-term quantum processors.