We study bound states in the continuum (BICs) in a system of two identical two-level giant atoms coupled to a one-dimensional waveguide. By deriving dark-state conditions, we clarify how coupling configurations and atomic parameters influence decay suppression. Through analysis of the long-time dynamical behavior of atoms and bound photons, we carry out a detailed classification of bound states and explore the connections between these dynamical behaviors and the system’s intrinsic light-matter interactions. The system supports static bound states with persistent atomic excitations, and oscillating bound states (OBSs) with periodic atom-photon or atom-atom excitation exchange. Under certain conditions, OBSs can contain more harmonic components owing to the emergence of additional quasidark modes, rendering them promising platforms for high-capacity quantum information processing. These findings advance the understanding of BICs in waveguide quantum electrodynamics with multiple giant atoms and reveal their prospective applications in quantum technologies.

We develop a nonequilibrium quantum field theory of the free-electron laser (FEL) based on the Preparata model, using the real-time Keldysh formalism. Starting from a microscopic Lagrangian for a relativistic electron beam coupled to a single radiation mode, we construct a Keldysh functional integral, perform the large-$N$ rescaling, and integrate out the electronic degrees of freedom. This yields an effective action for the FEL mode in which dispersion, gain, and noise are all generated by a single electronic self-energy built from the current correlations of the beam. For a stationary Gaussian beam, we obtain closed analytic expressions for the retarded and Keldysh components of the self-energy, which directly encode frequency pulling, gain reduction due to energy spread, and the noise spectrum experienced by the field. At low frequency, the theory reduces to a Landau-Ginzburg-Keldysh description of a single complex mode with a mass, growth rate, nonlinearity, and noise strength fully determined by beam current, energy spread, and detuning. In this framework, the FEL threshold appears as a continuous nonequilibrium phase transition in the laser universality class: The coherent field amplitude plays the role of an order parameter, while the amplitude of critical fluctuations is fixed by the microscopic noise kernel. The result is a minimal open quantum field theory analog of the Vlasov-Maxwell FEL theory, in which gain, dispersion, and noise arise from a unified self-energy framework rather than from separate phenomenological ingredients.

Building upon the recently introduced particle interpretation of the double-slit experiment [C. J. Villas-Boas et al., Phys. Rev. Lett. 134, 133603 (2025)] which attributes interference phenomena to detector-coupled (bright) and detector-uncoupled (dark) states of light, we develop a continuous-mode extension of the bright- and dark-state framework. This extension addresses a conceptual distinction between interference and diffraction, that is, the transition from a finite set of discrete paths to a continuum of modes. Through the construction of a complete detector-oriented basis for single-slit diffraction, we demonstrate that the observed diffraction pattern arises from projection of the photon state onto a single bright mode by identifying the detectable and undetectable modes, with photons detected at intensity minima having zero probability, as they reside in modes spanning an infinite-dimensional dark subspace. Our approach thus provides a unified particle-based explanation of diffraction that connects quantum and classical wave optics, and reveals distinctive quantum signatures in higher-order correlations.

We present a general algorithm, based on machine learning, which can create optimal unitary operators to implement quantum teleportation in any system with a well-defined set of measurements in a relevant entangled basis. We illustrate it with a collective spin model and demonstrate its versatility by applying it to teleportation of single and multiple qubit states, coherent and Dicke states, and for systems with prior distributions and unequal dimensions. All cases display significant regimes of quantum advantage over corresponding classical schemes with no entanglement. The algorithm offers the flexibility to choose a balance between target fidelity and computational cost.

In this exploratory numerical study, we assess the suitability of quantum linear solvers (QLSs) toward providing a quantum advantage for networks-based linear system problems (NLSPs). NLSPs naturally arise from graphs and are of importance as they are connected to real-world applications. The achievable advantage with a QLS for an NLSP depends on the interplay between the scaling of condition number and sparsity of matrices associated with the graph family. We analyze 50 graph families and identify that within the scope of our study, only 21 of them exhibit prospects for an exponential advantage with the Harrow-Hassidim-Lloyd (HHL) algorithm relative to an efficient classical solver. We call graph families that offer advantage with HHL as good graph families. We also compare the performance of the considered 50 graph families with 7 other QLSs. Furthermore, we report that some graph families graduate from offering no advantage with HHL to promising an exponential advantage with improved algorithms, such as the Childs-Kothari-Somma algorithm. We also introduce a unified graph superfamily and show the existence of infinite good graph families in it. Since the runtime expressions for linear solvers involve condition number, which in itself is not easy to compute, ascertaining advantage prospects with quantum linear solvers itself is not an easy problem. Thus, we conjecture the conditions under which one may visually examine a graph family and guess the prospects for an advantage. Finally, we very briefly touch upon some practical issues that may arise even if the aforementioned graph theoretic requirements are satisfied, including quantum hardware challenges.

In this work, we present a quantum Markov chain algorithm for many-body systems that utilizes a special phase of matter known as the Many-Body Localized (MBL) phase. We show how the properties of the MBL phase enable one to address the conditions for ergodicity and sampling from distributions of quantum states. We demonstrate how to exploit the thermalized-to-localized transition to tune the acceptance rate of the Markov chain and apply the algorithm to solve a range of combinatorial optimization problems of quadratic order and higher. The algorithm can be implemented on any quantum processing unit capable of simulating the Floquet dynamics of a one-dimensional Ising chain with nearest-neighbor interactions, providing a practical way of sampling from thermal distributions of Hamiltonians that cannot be natively implemented on the quantum hardware.

Tomographic measurements are the standard tool for characterizing quantum states, yet they are usually regarded only as means for state reconstruction or fidelity measurement. Here, we show that the same Pauli-basis measurements ($X, Y, Z$) can be directly employed for the certification of nonlocality at no additional experimental cost. Our framework allows any tomographic data—including archival data sets—to be reinterpreted in terms of fundamental nonlocality tests. We introduce a generic, constructive method to generate tailored Bell inequalities and showcase their applicability to certify the nonlocality of states in realistic experimental scenarios. Recognizing the stabilizer nature of the considered operators, we analyze our inequalities in the context of witnessing quantum magic—a crucial resource for quantum computing. Our approach requires Pauli measurements only and tests the quantum magic solely through the resources present in the state. Our results establish a universal standard that unifies state tomography with nonlocality certification and its application to quantum magic witnessing, thereby streamlining both fundamental studies and practical applications.

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.