Ultrafast strong-field laser-plasma physics is shown to offer a promising framework for relativistic nonlinear quantum electrodynamics (QED). As one of its key advantages, this approach to relativistic nonlinear QED does not require an external beam of relativistic particles. Instead, high-energy electrons are produced in this setting as a part of ultrafast strong-field laser-plasma interactions. An intense ultrashort laser pulse generates and accelerates dense electron bunches to relativistic energies, giving rise to photon-pair emission confined to the nanometer scale in space and the attosecond scale in time. As a lowest-order nonlinear QED process, relativistic electrons in laser-driven plasmas are shown to give rise to attosecond bursts of two-photon emission, providing an ultrabroadband source of correlated photon pairs. As a physically insightful estimate, the rate of this two-photon emission is expressed via a product ${\alpha }^2\gamma {\omega }_{\text{turn}}$, where $\alpha$ is the fine-structure constant, $\gamma$ is the Lorentz factor, and ${\omega }_{\text{turn}}$ is the local relativistic curvature frequency. Photon pairs with strongest correlations, providing a resource for photon entanglement, are emitted at a much lower rate, estimated as ${\alpha }^2{\gamma }^2{\omega }_{\text{turn}}{E}_{\perp }/E_S$, where $E_{\perp }$ is the laser electromagnetic field, determining the transverse Lorentz force, and $E_S$ is the Schwinger critical field. Our study offers a clear framework for isolating quantum aspects of laser-driven relativistic plasma electrodynamics from their classical counterparts, enabling a physically justifiable approach to the analysis of nonlinear QED phenomena in complex laser-plasma interactions driven by ultrashort high-intensity laser pulses. With purely quantum-field effects isolated via ${\alpha }^2$ and $E_{\perp }/E_S$ factors, the role of classical plasma analysis in this framework is to simulate spatially and temporally resolved electron trajectories, thus providing direct in situ probes for local $\gamma$ and ${\omega }_{\text{turn}}$ factors of plasma electrons. At laser intensities corresponding to relativistic laser amplitudes $a_0\sim 20–200$, accessible at multipetawatt laser facilities, laser-solid interactions drive electrons to $\gamma \sim 100$, and their correlated attosecond photon-pair emission emerges as a previously unexplored quantum light source competing with classical coherent synchrotron emission.

While machine learning holds great promise for quantum technologies, most current methods focus on predicting or controlling a specific quantum system. Metalearning approaches, however, can adapt to new systems for which little data are available, by leveraging knowledge obtained from previous data associated with similar systems. In this paper, we metalearn dynamics and characteristics of closed and open two-level systems, as well as the Heisenberg model. Based on experimental data of a Loss-DiVincenzo spin qubit hosted in a Ge/Si core/shell nanowire for different gate voltage configurations, we predict qubit characteristics, i.e., $g$ factor and Rabi frequency using metalearning. The algorithm we introduce improves upon previous state-of-the-art metalearning methods for physics-based systems by introducing techniques such as adaptive learning rates and a global optimizer for improved robustness and increased computational efficiency. We benchmark our method against other metalearning methods, a vanilla transformer, and a multilayer perceptron, and demonstrate improved performance.

Spin squeezing in atomic ensembles enables atom interferometry with sensitivities below the shot-noise limit, but the associated entanglement is highly susceptible to loss, making imperfections in atom optics a central limitation. Bragg diffraction is an established technique for driving transitions between atomic momentum states and enables large-momentum transfer through higher-order diffraction while preserving the internal state. However, it is intrinsically limited by two competing mechanisms: Short light pulses induce parasitic diffraction into off-resonant orders beyond an effective two-level description, while long pulses face velocity selectivity. We derive analytical expressions in a second-quantized framework for the atom optics and phase uncertainty of a Mach-Zehnder interferometer including these effects. We demonstrate that sub-shot-noise scaling is achieved only in a regime of intermediate pulse duration. Furthermore, we show that deleterious effects of higher-order diffraction are partially mitigated by optimizing the input quantum state.

Inorganic structural chemistry leads naturally to a theory for crystal structure prediction formalized by a generalized disjunctive programming, which is formulated using continuous and Boolean variables to involve the algebraic equations, disjunctions, and logic propositions. Since the feasibilities of continuous variables change drastically depending on Boolean variables and vice versa, iterative optimization of continuous and Boolean variables efficiently transforms a randomly generated initial structure into an optimal solution. Boolean variables are introduced to elucidate the “combinatorial backbone” of the original continuous optimization problem, which corresponds to the graphs describing crystal structures with clear chemical meanings. Since many subgraphs are discovered in numerous graphs generated through structural optimizations, on-the-fly learning of the feasibilities of them accelerates the discovery of the optimal solutions. This theory enables designing a wide variety of crystal structures with small computations based on only the atomic radii and feasible coordination numbers of each atom.

We propose a hybrid quantum computing architecture composed of alternating fluxonium and transmon qubits, which are coupled via transmon tunable couplers. We show that this system offers excellent scaling properties, characterized by engineered zero $ZZ$ crosstalk in the idle regime, a substantial reduction of level-crowding challenges through the alternating arrangement of different qubit types within the lattice, and parameter regimes that circumvent the capacitive loading problem commonly associated with fluxoniums. In numerical simulations, we show a parametrically driven CZ gate that achieves a closed-system infidelity that is orders of magnitude below the coherence limit for gate durations $\gtrsim 30\mathrm{ns}$ using a two-tone flux pulse on the tunable coupler. Furthermore, we show that this gate scheme retains its fidelity in the presence of multiple spectator qubits, making it a scalable solution for large lattices. Moreover, for the implementation of error correcting codes, our approach can leverage the long coherence times and large nonlinearities of fluxoniums as data qubits, while fixed-frequency transmons with established readout techniques can serve as measurement ancillae.

Counterdiabatic (CD) driving provides a powerful route to fast and robust state preparation by suppressing diabatic excitations during finite-time evolution. Yet, deriving analytical CD protocols for complex systems remains challenging, motivating the development of variational approaches. These methods typically rely on minimizing trace-based functionals to construct approximate control Hamiltonians. However, in unbounded systems, such functionals can become ill-defined because of the unbounded bosonic Hilbert space, leading to divergent cost functions and unphysical variational coefficients. Here, we introduce a variational optimization framework equipped with physically motivated renormalization schemes that regularize the trace-based metric by restricting it to relevant displaced and low-energy subspaces. As a paradigmatic example, we apply our method to the quantum Rabi model beyond the dispersive approximation and identify two distinct CD contributions that couple the atomic degree of freedom to the position and momentum quadratures of the field. These terms suppress diabatic excitations across coupling regimes ranging from strong to deep-strong light-matter interaction. We further formulate a fidelity-based quantum optimal-control strategy that bypasses the limitations of trace-based variational methods. Finally, we show that the resulting CD terms can be implemented via Floquet engineering through parametric modulation of the native Hamiltonian. Our results demonstrate that CD driving can be consistently extended to continuous-variable systems with unbounded Hilbert spaces, providing a controlled and scalable framework for quantum control in strongly interacting light-matter platforms.

Entanglement entropy is a fundamental measure of quantum correlations and a key resource underpinning advances in quantum information and many-body physics. We uncover a universal relationship between bipartite entanglement entropy and particle number after the barrier in a one-dimensional Fermi-Hubbard system with an external asymmetric potential. Decomposing the von Neumann entropy into number entropy $S_n$ and configurational entropy $S_c$, we show that in the barrier-dominated tunneling regime both components are individually well-defined functions of the postbarrier particle density $n_A$, even though $S_c$ encodes off-diagonal coherences that are not directly accessible from density measurements alone. Using Kolmogorov-Arnold networks—a novel machine learning architecture—we learn the relationship for entropy and its components across a broad range of interaction strengths and barrier heights with high predictive accuracy. Furthermore, we propose a simple analytical binary-entropy-like expression that quantitatively captures the observed correlation for fixed parameters. Our findings open avenues for characterizing quantum correlations in transport phenomena and provide a powerful framework for estimating the full von Neumann entropy—including its configurational component—from a single transport observable.

We propose concrete protocols to realize quantum criticality due to excited-state quantum phase transitions (ESQPTs) experimentally in presumably the simplest and most resilient system involving a single trapped ion oscillating in a radio-frequency Paul trap. We identify a specific class of excited states of the Extended Rabi Model (ERM) Hamiltonian, which occur between two critical ESQPT energies of the model in its (anti-)Jaynes-Cummings superradiant phase. Properties of these states motivate the definition of several ESQPT witness observables. We study their critical scaling behaviors as well as various distinct state evolutions by driving the system across the quantum criticalities by changing the qubit-phonon coupling strength linearly in time at different finite rates. A mapping of the theoretical control parameters of the ERM to the experimental parameters of a trapped-ion setup is provided, and simulations are performed for values referencing existing state-of-the-art setups, addressing both unitary state evolutions and relevant open-system corrections.

General spin-$\frac{1}{2}$ chains with symmetric nearest-neighbor interaction are studied. We rigorously prove that all spin models in this class, except for known integrable systems, are nonintegrable in the sense that they possess no nontrivial local conserved quantities. This result confirms that there are no missing integrable systems; i.e., integrable systems in this class are exactly those that are already known. In addition, this result excludes the possibility of intermediate systems that have a finite number of nontrivial local conserved quantities. Our findings support the expectation that integrable systems are exceptional in quantum many-body systems and most systems are nonintegrable.

Strong, symmetrically focusing plasma lenses are promising for accommodating the small beams associated with plasma-based accelerators and collider final foci. However, while focusing with active and passive plasma lenses has been experimentally demonstrated, compatibility with high-brightness beams relevant for applications has not. In this work, we show experimentally that passive plasma lenses can preserve free-electron-laser-quality slice emittance while focusing two orders of magnitude more strongly than quadrupole magnets, and that the focal parameters can be controlled.

Heat dissipation capability of laser gain media is critical for ensuring the performance and stability of high-power laser systems. However, the thermal transport mechanisms in these materials, particularly in rare-earth ion-doped laser crystals under high-power heating conditions, remain largely elusive. In this study, we employ molecular dynamics simulations combined with a neuroevolution machine learning potential to investigate the effects of temperature and Ce doping on thermal transport in alumina. Our simulations reveal a significant reduction in thermal conductivity under uniform elevated temperatures and finite Ce doping concentrations, providing quantitative insight into thermal transport in Ce-doped ${\mathrm{Al}}_2{\mathrm{O}}_3$ under conditions relevant to laser operation. Simulation results demonstrate that elevated temperature causes a broad-spectrum suppression of thermal transport, while Ce doping predominantly affects specific low-frequency acoustic phonon modes and a few optical modes. Further analysis uncovers a mode-selective phonon-dopant scattering mechanism, where resonant and polarization-dependent coupling leads to the selective suppression of distinct phonon modes. This mechanism is in sharp contrast with the conventional understanding of phonon-dopant scattering observed in typical crystals. These findings provide valuable insights into the atomic-level mechanisms governing heat dissipation in alumina laser crystal, thus paving the way for the design of laser materials with optimized thermal properties for high-power applications.

Nanoelectromechanical resonators provide an ideal platform for investigating the interplay between electron transport and nonlinear mechanical motion. Externally driven suspended carbon nanotubes, containing an electrostatically defined quantum dot, are especially promising. These devices possess two main sources of nonlinearity: the electromechanical coupling and the intrinsic contributions of the resonator that induce a Duffing-like nonlinear behavior. In this work, we observe the interplay between the two sources across different driving regimes. The main nonlinear feature we observe is the emergence of archlike resonances in the electronic transport when the resonator is strongly driven. We show that our model is in good agreement with our experimental electron transport measurements on a suspended carbon nanotube. This characterization paves the way for the exploration of nonlinear phenomena using mesoscopic electromechanical resonators.