We present a comprehensive investigation of entanglement dynamics in multi-level V-type atomic systems embedded within photonic crystals (PCs). We mainly focus on the synergistic roles of resonant dipole-dipole interactions (RDDIs) and quantum interference through analytical modeling and numerical simulations using the Schrödinger equation. Key findings reveal that resonant interaction dominates when the interatomic distance is comparable with the localization length of photon-atom bound states lying in the band gap region. For atoms with antiparallel dipole orientations, both initially entangled and separable states exhibit robust entanglement preservation due to strong collective interactions. Conversely, when dipoles are oriented orthogonally, initially entangled states exhibit unique oscillatory patterns in their entanglement dynamics. This effect arises from the formation of dark states due to destructive interference within the structured photonic environment, with RDDIs sustaining non-Markovian dynamics. We further demonstrate that positioning the atomic excited states deeper within the photonic band gap accelerates the decay of entanglement oscillations due to the exponential suppression of resonant energy exchange mediated by evanescent modes. Our analysis establishes RDDIs and quantum interference as potential tools for tailoring entanglement dynamics, paving the way for controlled quantum coherence in PC platforms.

Quantum asymmetry and coherence are genuinely quantum resources that are essential to realize quantum advantage in information technologies. However, all quantum processes are fundamentally constrained by quantum speed limits, which raises the question of the corresponding bounds on the rate of consumption of asymmetry and coherence. In the present work, we derive a formulation of the quantum speed limit for observables in terms of the trace-norm asymmetry of the time-dependent quantum state relative to the observable. This quantum speed limit can be directly observed in experiment through weak-value measurement and provides a lower bound to the quantum Fisher information about the parameter conjugate to the observable. It can be further related to quantum coherence relative to the eigenbasis of the observable. We obtain a complementary relation for the speeds of three mutually unbiased observables for a single qubit. As an application, we derive a notion of a quantum thermodynamic speed limit.

Flat bands exhibit high degeneracy and intrinsic localization, offering a promising platform for enhanced light-matter interactions. Here, we investigate the resonant interaction between a two-level emitter and a chiral flat band hosted by a photonic lattice. In the weak-coupling regime, the emitter undergoes Rabi oscillations with a lifted photonic mode whose spatial structure reflects the nature of compact localized states and the onset of Anderson localization. We show that weak hopping disorder induces a delocalization of the lifted mode, whereas the effective emitter–field coupling strength, and the associated mode volume experienced by the emitter, remains protected against structural fluctuations. We illustrate our approach using selected flat-band lattices. Our findings provide a route to flat-band-state preparation via quench dynamics and robust cavity-QED control.

Quantum measurement backaction can inject energy into a system, enabling work extraction at a single temperature without feedback. Treating measurement as a finite-time physical interaction, we identify a finite-time dynamical constraint on measurement-driven quantum engines. Positive work extraction is guaranteed when the measurement-induced dynamics is bistochastic. While short-time measurements satisfy this condition, increasing the measurement duration generically breaks bistochasticity, driving the working substance toward detailed-balance thermalization or dynamical freezing. Finite-measurement time thus sets a generic dynamical limitation within the class of single-temperature measurement-driven quantum engines.

We build on the foundational work of Deffner and Zurek [New J. Phys. 18, 063013 (2016)] to show how central equilibrium structures of statistical mechanics can be understood within standard quantum mechanics using the concept of envariance (environment-assisted invariance). In particular, we show how the Binomial, Poisson, and Gaussian distributions naturally emerge from entangled system-environment states. We revisit the Gibbs paradox from a quantum-information perspective, demonstrating that the standard Sackur-Tetrode entropy and its $1/N!$ factor arise from indistinguishability enforced through entanglement with an environment, without introducing additional thermodynamic corrections. Within the same framework, we analyze ionization equilibrium and show how the classical Saha equation is recovered, while clarifying how indistinguishability enters through an entanglement-induced reduction of permutation redundancy. Assuming the standard exchange symmetries of identical quantum particles, we further show how the Bose-Einstein and Fermi-Dirac distributions follow as the equilibrium weighting of symmetry-allowed occupation configurations. Overall, our results support the view that equilibrium statistical mechanics can be consistently interpreted as an emergent consequence of quantum information-theoretic structure and symmetry, rather than as a collection of independent phenomenological postulates.

Accurate identification of Hamiltonian parameters is essential for modeling and controlling open quantum systems. In this work, we demonstrate that the multichannel Hankel alternative view of the Koopman ($m\mathrm{HAVOK}$) algorithm is a robust and reliable spectral data-driven method for retrieving Hamiltonian parameters from the evolution of first-moment observables in open quantum systems. The method relies on the discrete spectrum of the Koopman operator to obtain these parameters, which are computed using the $m\mathrm{HAVOK}$ algorithm; a theoretical justification for this approach is presented. The method is tested on noiseless quadratures of an open two-dimensional quantum harmonic oscillator and shown to retrieve oscillation frequencies, damping rates, nonlinear Kerr shifts, the qubit-photon coupling strength of a Jaynes-Cummings interaction, and the modulated frequency of a time-dependent Hamiltonian. The majority of the recovered parameters remained within 5% of their actual values. Compared with Fourier- and matrix-pencil estimators, our approach yields lower errors for dynamics with strong dissipation. Overall, these findings suggest that Koopman operator theory provides a practical framework for studying quantum dynamical systems.

Squeezed coherent quantum light is described by the complex squeeze parameter $\zeta =s{e}^{i\vartheta }$ and the coherent-state displacement parameter $\beta =\left|\beta \right|e^{i{\varphi }_{\beta }}$. Our study of strong-field ionization [D. Habibović and D. B. Milošević, Phys. Rev. A 112, L051103 (2025)], which focused on bright squeezed vacuum quantum light with $\beta =0$ and $s\gg 1$, is extended to ionization by squeezed coherent light. It is shown that the amplitudes $s$ and $\left|\beta \right|$ strongly influence the ionization process, whereas phases $\vartheta$ and ${\varphi }_{\beta }$ play a less significant role. The present work provides a unified treatment of coherent, squeezed vacuum, and squeezed coherent light within a single theoretical framework. The quantum nature of the driving field is incorporated by averaging the strong-field ionization probability over the Husimi-Kano phase-space distribution. Using neon as a representative target, we analyze how the parameters characterizing squeezed coherent light modify photoelectron spectra and intensity-dependent enhancements. We show that nonclassical photon statistics can lead to a pronounced steepening of the rescattering plateau and, at low mean intensities, to an effective suppression of the nominal high-energy cutoff. These effects originate from the reduced weight of high-intensity field components inherent to squeezed states and have no analog in strong-field ionization driven by a coherent light. Our results highlight the potential for quantum-statistical control of strong-field ionization and provide a foundation for exploring laser-matter interactions driven by quantum light.

Multielectron tunneling ionization creates ionic coherence crucial for lasing and driving electron motion in molecules. While tunneling is well understood as a single active electron process, less emphasis has been placed on theoretical descriptions of bound electrons during tunneling. This work systematically investigates multielectron tunneling ionization based on the strong field approximation, establishing a theoretical foundation and demonstrating the equivalence of wave function and density matrix approaches for subcycle ionic dynamics. An accurate subcycle nonadiabatic ionization rate is also derived and incorporated into the theory to improve its quantitative accuracy. Applying the theory to ${\mathrm{N}}_2$ and ${\mathrm{CO}}_2$, this work showcases how an intense laser field can induce ionic coherence in molecules as observed in previous experiments. These findings encourage future investigations into multielectron tunneling ionization and its applications in lasing and in controlling chemical reactions.

Non-Hermiticity can do more than create isolated bound states in the continuum: under suitable conditions it can generate entire families of bound states and even yield anomalous extended states within an otherwise localized continuum. Here, we show that a second-order non-Hermitian skin effect (NHSE) can produce corner-localized modes whose eigenvalues remain embedded in the continuum of extended bulk bands. The key mechanism is an internal subspace structure revealed by an appropriate basis transformation, which decomposes the Hilbert space into two orthogonal subspaces: one supports second-order skin-induced corner modes, while the other hosts topologically trivial continuum bands with extended bulk states. This orthogonality suppresses hybridization between the corner modes and the continuum even when they overlap in energy. Moreover, the resulting corner-localized modes are robust against disorder, highlighting their topological protection. Our results establish a route to realizing a second-order NHSE embedded in the continuum and broaden the landscape of non-Hermitian topological phases.

We theoretically investigate spontaneous photon-pair emission from a pair of excited giant atoms coupled to a waveguide. We analyze three distinct coupling configurations: separate, braided, and nested. The system decays via two independent channels, and the resulting two-photon wave function is governed by their quantum interference, which is controllable by tuning the self- and cross-interference phases. We show that these phases provide enhanced control over the directionality of the two-photon emission in the braided configuration. Furthermore, they facilitate a transition from photon bunching to antibunching for counterpropagating photons. Notably, the braided and nested configurations exhibit oscillatory behavior with an increased correlation length. The nested configuration is unique in that it can generate antibunched photons even when they are copropagating. Our results illustrate that giant atoms offer advantageous control over photon-pair properties—including emission directionality, quantum statistics, and correlation length—compared to conventional small atoms.

We report a family of quantum speed limits (QSLs) that give evolution time lower bounds between an initial and a final state whose separation is described by a certain representation-basis-dependent norm derived from the weighted ${\ell }_w^p$-seminorm. These QSLs are applicable to open, closed, time-dependent, or time-independent systems in finite-dimensional Hilbert spaces whose density matrices are piecewise time differentiable. They can be extended to systems over separable Hilbert spaces as well. Crucially, these QSLs are valid for arbitrary operators, not just density matrices, provided that a modest technical condition is fulfilled. When compared to the existing QSLs applied to pure state time-independent Hamiltonian evolution, qubit spontaneous emission, high-fidelity gate implementation, coherent state photon loss, and operator coherence or dephasing, ours consistently show improved sharpness in most cases, along with greater universality and still retaining computational efficiency.

Topological phase transitions are central to quantum physics due to their robust edge states, offering profound applications in quantum computing and material science. We investigate reentrant topological phase transitions in the Su-Schrieffer-Heeger model, where periodic modulation of the hopping amplitude drives multiple cycles of closing and reopening for the zero-energy edge states. These transitions are evidenced by quantized changes in the winding number and the spectrum of Lyapunov exponents. We find that the modulation phase shift governs both the critical points and the number of reentrances. Furthermore, we confirm the existence of edge states utilizing equivalent electrical circuit lattices, providing compelling evidence for the feasibility of the model. Our work establishes periodic modulation as a powerful and versatile tool for engineering reentrant topological phenomena, highlighting the delicate interplay between driving parameters and topological invariants.

Whether the operational quantum description of reality is complete can already be asked at the level of preparations: Can the empirically accessible properties of a finite preparation set be reproduced exactly by an ontological or hidden-variable model, or must every such completion contain an additional hidden structure that is not operationally accessible? We formalize this question through epistemic completeness, a preparation-side notion of classicality requiring exact preservation of empirical preparation properties, defined here as optimal one-way communication-task payoffs, by the corresponding not-fine-tuned ontic quantities obtained by conditioning on the ontic state and allowing all response schemes compatible only with positivity and normalization. For the canonical family of set-distinguishability tasks, we prove a universal equality: in every epistemically complete theory and for every finite preparation set, the average pairwise distinguishability equals the average set distinguishability. Any nonzero deviation therefore certifies epistemic incompleteness and lower-bounds the excess ontic communication power that every ontic completion must conceal. Since unrestricted classical communication models, and more generally finite-dimensional commuting quantum theories, are epistemically complete, every nonzero deviation yields a quantum advantage in a one-way communication task under a natural benchmark-task constraint. The same deviations also furnish dimension-independent witnesses of coherence of the preparation set and of measurement incompatibility in any realization attaining the certifying task values. To characterize the extremal quantum deviations, we formulate dimension-independent semidefinite-programming relaxations and dimension-dependent seesaw lower bounds and show that quantum theory violates the equality in both directions. In particular, the trine and tetrahedral qubit ensembles are numerically certified dimension-independent maximizers of the positive deviation for the $n=3$ and $n=4$ equalities, respectively. These positive-direction violations persist for arbitrarily low positive visibility and arbitrarily large leakage short of complete disclosure in the corresponding visibility and leakage families. We further show that the Kochen–Specker $\psi$-epistemic model exactly saturates the hidden ontic excess implied by the maximal positive violations for $n=3$ and $n=4$, making the witness quantitatively sharp. Finally, our numerical results indicate that the maximal positive deviation increases with the number of preparations, motivating the question of the absolute maximal epistemic incompleteness of quantum theory.

In this work, we introduce the quantum-classical hybrid physics-informed neural network with multiplicative and additive couplings (QPINN-MAC): a novel hybrid architecture that integrates the framework of physics-informed neural networks (PINNs) with that of quantum neural networks (QNNs). Specifically, we prove that through strategic couplings between classical and quantum components, the QPINN-MAC retains the universal approximation property, ensuring its theoretical capacity to represent complex solutions of ordinary differential equations (ODEs). Simultaneously, we demonstrate that the hybrid QPINN-MAC architecture actively mitigates the barren-plateau problem, regions in parameter space where cost-function gradients decay exponentially with circuit depth, a fundamental obstacle in QNNs that hinders optimization during training. Furthermore, we prove that these couplings prevent gradient collapse, ensuring trainability even in high-dimensional regimes. We empirically validate these theoretical findings through computational simulations of a coupled resistor-inductor-capacitor circuit, demonstrating the superior accuracy and convergence of our hybrid architecture (QPINN-MAC) when compared with both classical (PINN) and purely quantum (standard QPINN) approaches. Thus, our results establish a new pathway for constructing quantum-classical hybrid models with theoretical convergence guarantees, which are essential for the practical application of QPINNs.

We consider hybrid qubit-oscillator systems together with Gaussian, multiqubit, as well as qubit-controlled Gaussian unitaries. We propose implementations of logical gates for approximate Gottesman-Kitaev-Preskill codes in this model using two oscillators and three qubits. We show that these gate implementations become exact in the limit of large squeezing: The logical gate error is upper-bounded by a linear function of the squeezing parameter and depends polynomially on the number of encoded qubits. For a subset of Clifford gates, our constructions overcome a fundamental drawback of well-known Gaussian implementations, which—when executed without subsequent error correction—suffer from a constant logical gate error. In contrast, our fully unitary circuit construction achieves an asymptotically vanishing logical gate error rate even without applying a recovery map, i.e., performing syndrome measurements and adaptive corrections.

Entanglement for pure bipartite states is most commonly quantified on a state-by-state basis by assigning to each pure state a scalar value, such as the von Neumann entropy of a reduced density matrix. This provides a precise local characterization of “how entangled” a given state is. At the same time, this local description naturally invites a set of complementary, more global questions about the structure of the space of pure states: How abundant are the states with a given amount of entanglement within the full state space? Do the manifolds of constant entanglement exhibit distinct geometric regimes or “phases” when viewed in the natural geometry of projective Hilbert space? These questions shift the focus from assigning an entanglement value to a single state to understanding the global organization and geometry of entanglement across the entire manifold of pure states. In this work, we develop a geometric framework in which these questions become natural. We regard the projective Hilbert space of pure states, endowed with the Fubini-Study metric, as a Riemannian manifold and promote bipartite entanglement to a macroscopic functional on this manifold. Its level sets stratify the space of pure states into hypersurfaces of constant entanglement, and we define a geometric entanglement entropy as the log-volume of these hypersurfaces, weighted by the Fubini-Study gradient of entanglement. This quantity plays the role of a microcanonical entropy in “entanglement space”: it measures the degeneracy of a given entanglement value in the natural quantum geometry. The framework is illustrated first in the simplest case of a single spin-$1/2$ (where the Fubini-Study metric reduces to the Bloch sphere metric), and then for bipartite entanglement of spin systems, including a two-qubit example where explicit calculations can be carried out, along with a sketch of the extension to spin chains.

We ask whether the optimal probe is entangled, and, if so, what is its character and amount for estimating the noise parameter of a large class of local quantum encoding processes that we refer to as vector encoding, examples of which include the local depolarizing and bit-flip channels. We first establish that vector encoding is invariably “continuously commutative” for optimal probes. We utilize this result to deal with the queries about entanglement in the optimal probe. We show that, for estimating the extent of noise of the two-party arbitrary-dimensional local depolarizing channel, there is a descending staircase of optimal-probe entanglement for increasing depolarizing strength. For the multiqubit case, the analysis again leads to a staircase, but which can now be monotonic or not, depending on the multiparty entanglement measure used. We also find that when sufficiently high depolarizing noise is to be estimated, fully product multiparty states are the only choice for being optimal probes. In many cases, for even moderately high depolarizing noise, fully product states are optimal. For two-qubit local bit-flip channels, the continuous commutativity of the channel and optimal probe imply that a product state suffices for obtaining the optimal precision. Furthermore, for the simultaneous estimation of two noise parameters of local bit-flip and local depolarizing channels, entanglement is not necessary; the product probe exhibits the best precision.

Nonsemisimple extensions of the Ising anyon model developed in our previous work Iulianelli et al., [Nat. Commun. 16, 6408 (2025)] enable universal topological quantum computation via braiding alone, overcoming the Clifford-only limitation of semisimple theories. The nonsemisimple theory provides new anyon types indexed by a real parameter $\alpha$, the neglecton. Braiding acts unitarily with respect to an indefinite Hermitian form, while the computational subspace sits in a positive-definite sector. We demonstrate that this universality is robust, persisting over an open interval of the neglecton parameter $\alpha$ where the computational subspace remains positive-definite. We identify special values of $\alpha$ where the physical subspace decouples exactly from negative-norm components, ensuring fully unitary evolution and suppressed leakage. We further present an alternative encoding supporting exact single-qubit Clifford gates alongside a non-Clifford phase gate. We show that high-precision tuning of $\alpha$ is not required for efficient gate compilation, significantly enhancing the physical plausibility of nonsemisimple anyonic architectures.