Modular architectures offer a scalable path toward fault-tolerant quantum computing by interconnecting smaller quantum processing units provided that high-rate, fault-tolerant interfaces can be realized across modules. We present a comprehensive analysis and comparison of known methods for establishing such interfaces, including lattice surgery, transversal gates, and grow-and-distill protocols based on code growing and logical distillation. Using the surface code, we identify optimal interface strategies across a wide range of hardware parameters, such as gate fidelities, entangling rates, and memory resources, and estimate the requirements to achieve logical error rates of ${10}^{-6}$ and ${10}^{-12}$. Our results establish when the interface becomes a bottleneck in the computation and provide guidance for experimental implementations with superconducting, atomic, and solid-state hardware.

We explore how the quantum geometric properties of the Bloch wave function, characterized by the Hilbert-Schmidt quantum distance, impact magnetic phases in solid-state systems. To this end, we investigate the spin susceptibility within the random phase approximation, considering the on-site Coulomb interaction. We demonstrate that spin susceptibility can be decomposed into a trivial part, dependent solely on the band dispersion, and a geometric part, where the quantum distance plays a crucial role. Focusing on a model of a quadratic band-touching semimetal, we show that a magnetic phase transition between ferromagnetic and antiferromagnetic orders can be induced solely by tuning the wave function geometry, even while the energy spectrum is held constant. This highlights the versatility of quantum geometry as a mechanism for tuning magnetic properties independent of the energy spectrum. Applying our framework to the Fe pnictide and kagome lattice models, we further show that the geometric contribution is decisive in stabilizing their known antiferromagnetic and ferromagnetic states, respectively. Our work sheds light on the hidden quantum geometric aspects necessary for understanding and engineering magnetic order in quantum materials.

We present an algorithm for extracting a smooth, positive-definite function $\psi \left(x\right)$ encoded in a quantum state vector of size $2^n$, while avoiding exponentially suppressed subnormalization of a naive extraction step. The method addresses a known limitation in quantum algorithms for differential equations, namely, the efficient retrieval of classical information from quantum-encoded solutions. It applies to settings where the target solution is smooth and positive definite, such as certain instances of the heat equation and related diffusive models arising in fluid dynamics and finance.

Analog quantum simulators provide access to many-body dynamics beyond the reach of classical computation. However, extracting physical insights from experimental data is often hindered by measurement noise, limited observables, and incomplete knowledge of the underlying microscopic model. Here, we develop a machine learning approach based on a variational autoencoder (VAE) to analyze interference measurements of tunnel-coupled one-dimensional Bose gases, which realize the sine-Gordon quantum field theory. Trained in an unsupervised manner, the VAE learns a minimal latent representation that strongly correlates with the equilibrium control parameter of the system. Applied to nonequilibrium protocols, the latent space uncovers signatures of frozen-in solitons following rapid cooling, and reveals anomalous postquench dynamics not captured by conventional correlation-based methods. These results demonstrate that generative models can extract physically interpretable variables directly from noisy and sparse experimental data, providing complementary probes of equilibrium and nonequilibrium physics in quantum simulators. More broadly, our work highlights how machine learning can supplement established field-theoretical techniques, paving the way for scalable, data-driven discovery in quantum many-body systems.

Driven-dissipative bosonic systems exhibit a rich variety of nonequilibrium steady states (NESS) due to the interplay between coherent drives, interactions, and dissipation. In the semiclassical limit, the flow topology of phase-space dynamics governs the stability and structure of these dynamical phases. Topological transitions occur when the number, chirality, and connectivity of NESS changes, reflecting a global reorganization of the dynamical phase-space landscape. Here, we study the impact of these topological signatures in a quantum driven-dissipative Kerr oscillator. Employing a Lindblad master equation and quantum-trajectory methods, we reveal that quantum dynamics retain key topological features of the underlying semiclassical flows, with clear signatures accessible via quantum state tomography and linear response. In this manner, we predict flow-topological phases whose signatures arise from a reorganization of fluctuation modes and are not captured by Liouvillian-gap closing alone. We, thus, generalize the conventional criteria for diagnosing phase transitions. Our findings position phase-space flow topology as a powerful tool to identify and control robust quantum phases, enabling advances in error correction and sensing.

We demonstrate an experimental approach to determine magneto-optical effects that combines x-ray magnetic circular dichroism (XMCD) with x-ray interferometry, based on the concepts of Young's canonical double slit. By covering one of two slits with a magnetic thin film and employing XMCD, we show that it is possible to determine both the real and the imaginary parts of the complex refractive index by measuring the fringe shifts that occur due to a change in the sample magnetization. Our hybrid spectroscopic-interferometric methodology provides a means to probe changes in the magnetic refractive index in terms of the electron spin moment.

Solving the many-electron problem, even approximately, is one of the most challenging and simultaneously most important problems in contemporary condensed matter physics with various connections to other fields. The standard approach is to follow a divide and conquer strategy that combines various numerical and analytical techniques. A crucial step in this strategy is the derivation of an effective model for a subset of degrees of freedom by a procedure called downfolding, which often corresponds to integrating out energy scales far away from the Fermi level. In this work, we present a rigorous formulation of this downfolding procedure, which complements the renormalization group picture put forward by Honerkamp [Phys. Rev. B 85, 195129 (2012)]. We derive an exact effective model in an arbitrarily chosen target space (e.g., low-energy degrees of freedom) by explicitly integrating out the rest space (e.g., high-energy degrees of freedom). Within this formalism, we state conditions that justify a perturbative truncation of the downfolded effective interactions to just a few low-order terms. Furthermore, we utilize the exact formalism to formally derive the widely used constrained random phase approximation, uncovering underlying approximations and highlighting relevant corrections in the process. Finally, we detail different contributions in the material examples of fcc nickel and the infinite-layer cuprate ${\mathrm{SrCuO}}_2$. Our results open up a pathway to obtain effective models in a controlled fashion and to judge whether a chosen target space is suitable.

We analyze a system of dipolar atoms confined in geometries of quasi low dimensionality. Due to the long-range and anisotropic nature of dipolar interactions, the system supports both stable solitons and quantum droplets. In quasi-one-dimensional geometries, the transition between these states is known to manifest either as a first-order phase transition, associated with bistability, or as a smooth crossover. We investigate this transition by calculating the structure factor and showing that the response of the breathing mode provides an experimentally accessible probe. In addition, we identify regions of both bistability and smooth crossover in quasi-two-dimensional geometries. Finally, we connect our findings to previous experimental results and delineate the conditions under which two-dimensional dipolar bright solitons can be realized.

Understanding the response of diamond abrasive grains to mechanical loading remains crucial for optimizing their performance in precision manufacturing. In that context, the role of the initial residual stress and crystallographic orientation of the grains is poorly understood. We investigate synthetic diamond grains in two different grit sizes (D126, D252) subjected to directional loading up to 100 N using x-ray diffraction and spatially resolved Raman spectroscopy. Small grains with preferential (111) orientation show an unexpected stress evolution under contact pressures up to 16 GPa, with Raman shifts increasing from 1331.3 to $1333.0{\mathrm{cm}}^{-1}$, indicating an enhanced local compressive stress state at the probed surface regions. Conversely, large (311)-oriented grains exhibit a heterogeneous stress development with Raman shifts varying from 1331.5 to $1332.6{\mathrm{cm}}^{-1}$, including regions that become more tensile (or less compressive) relative to their initial state. The relationship between the relative Raman shift and a spatially weighted contact pressure follows an empirical power law $\delta E\propto p_{\text{c}}^{\prime \alpha }$ with opposite, orientation-dependent exponents: $\alpha \approx +0.25$ for (111) grains showing progressive compression enhancement and $\alpha \approx -0.35$ for (311) grains exhibiting a trend toward tensile stress components with increasing load. Molecular dynamics simulations reveal that the Schmid factor governs this orientation-dependent response: The low Schmid factor (0.27) for [111] loading restricts dislocation glide, leading to stress retention with high elastic recovery, while the high Schmid factor (0.45) for [311] loading facilitates plastic flow and stress relaxation despite lower peak stress. These findings demonstrate that stress evolution in diamond under directional loading is controlled by the geometric relationship between loading direction and slip systems, providing mechanistic insights for diamond tool design.

We present a comprehensive first-principles investigation of carbon self-interstitial defects in diamond, ranging from mono- to hexa-interstitial complexes. By density functional theory, empowered by interatomic potential models, we efficiently sample the complex configurational landscape and identify both known and previously unreported defect geometries. Our results reveal a pronounced energetic driving force for aggregation: The formation energy per interstitial decreases systematically from isolated split interstitials to compact multi-interstitial clusters, with the tetra-interstitial platelet emerging as a particularly stable structural motif. Additionally, charge analysis indicates that the predominantly covalent bonding in diamond becomes more polar within the defect centers. Analysis of defect energy levels shows that only the investigated mono-, di-, penta-, and hexa-interstitial complexes introduce in-gap electronic states, whereas the tri- and tetra-interstitial clusters are electronically inert. Vibrational spectroscopies further reveal that self-interstitials generate characteristic signatures. Short carbon-carbon bonds inside the defect cores give rise to high-frequency vibrational modes between 1375 and ${1925\mathrm{cm}}^{-1}$, which are strongly infrared active but exhibit weak Raman activity. Through a systematic analysis of metastable configurations, we identify the 3H defect center as a neutral di-interstitial defect. Based on this identification, we further suggest that the TR12 center may arise from a 3H-containing defect like a metastable hexa-interstitial configuration. Taken together, these findings provide a coherent picture of the structural, electronic, and vibrational characteristics of carbon self-interstitials and establish a robust framework for their experimental identification.

The ability to tune interparticle interactions is one of the main advantages of using ultracold quantum gases for quantum simulation of many-body physics. Current experiments with ultracold polar molecules employ shielding with microwave or static electric fields to prevent destructive collisional losses. The interaction potential of microwave-shielded molecules can be tuned by using microwaves of two different polarizations, while for static-field-shielded molecules the tunability of interactions is more limited and depends on the particular species. In this work, we propose a general method to tune the interactions between static-field-shielded molecules by applying a microwave field. We carry out coupled-channel scattering calculations in a field-dressed basis set to determine loss rate coefficients and scattering lengths. We find that both the s-wave scattering length and the dipole length can be widely tuned by changing the parameters of the microwave field, while maintaining strong suppression of lossy collisions.

The dead zones of atomic magnetometers bring great challenges for the mobile applications in high-sensitivity magnetic field measurement. To solve this fundamental problem, we present a dead-zone-free orientation-based atomic magnetometer employing free-induction-decay (FID) scheme. A modulation scheme combining synchronous optical pumping and radio frequency pulse manipulation is utilized to achieve the free precession of spin polarization in all magnetic field orientations, and the FID signals are detected in two directions with an orthogonal pump-probe light configuration. We perform a comprehensive theoretical analysis of spin polarization evolution for arbitrary magnetic field orientations, and both simulation and experimental results demonstrate that the fusion of information from the pump and probe beams enables the dead-zone-free measurement of the magnetometer, with the sensor exhibiting a sensitivity in the range of $0.6–1.4\mathrm{pT}/{\mathrm{Hz}}^{1/2}$ across all directions. The proposed scheme is of great significance for the advancement of pulsed atomic magnetometers and their various applications in mobile platforms.

Adaptive control in biological systems, such as intestinal immunity, remains poorly understood despite detailed knowledge of underlying regulatory networks. We propose an alternative framework based on stochastic martingale turnover, in which cells proliferate through mutual competition and decay without cell-type-specific regulation. Through stochastic simulations and mathematical analysis, we show that this process autonomously generates balanced population compositions associated with low-decay probabilities. The compositional dynamics can be described as a random walk whose step lengths decrease in low-decay regions. Reduced decay leads to larger total population sizes and an increase in the number of compatible microscopic states, which in turn shapes the distribution of compositions under fluctuating conditions. More generally, the dynamics follow a modified Langevin equation, in which constant mass is replaced by a fitness-dependent effective mass proportional to the total population size. Thus, biological systems regulate resistance to change, not merely direction, in shaping their macroscopic behavior.

Quantum information scrambling (QIS) describes the rapid spread of initially localized information across an entire quantum many-body system through entanglement generation. Once scrambled, the original local information becomes encoded globally, inaccessible from any single subsystem. In this work, we introduce a circuit-based decoding protocol. By utilizing the concept of postselected closed timelike curves (PCTCs), we demonstrate how postselection allows us to interpret an ordinary quantum experiment as an example of a paradox-free trajectory, simulating a consistent time loop and reliable information recovery. Specifically, when conditioned on a final postselected outcome, this experiment can be interpreted as decoding the scrambled information even before the original information is generated. Furthermore, the success probability of the PCTC is governed by out-of-time-ordered correlations, which is a standard measure of QIS. We experimentally implement our protocol on cloud-based Quantinuum and IBM quantum processors. Our approach illuminates a unique quantum task under postselection: the causally consistent simulation of future-to-past scrambled information retrieval.

In this work, we propose performing key operations in quantum computation and communication using room-temperature atoms moving across a grid of high-quality-factor, small-mode-volume cavities. These cavities enable high-cooperativity interactions with single atoms to be achieved with a characteristic timescale much shorter than the atomic transit time, allowing multiple coherent operations to take place. We study scenarios where we can drive a Raman transition to generate photons with specific temporal shapes and to absorb, and hence detect, single photons. The strong atom-cavity interaction can also be used to implement the atom-photon controlled-phase gate, which can then be used to construct photon-photon gates, create photonic cluster states, and perform nondemolition detection of single photons. We provide numerics validating our methods and discuss the implications of our results for several applications.

The electromagnetic response of matter is governed by three fundamental multipole families: electric, magnetic, and toroidal. While the electric and magnetic are cornerstones of physics, the toroidal dipole (TD) has eluded direct, quantitative measurement for over 60 yr. Its far-field signature is masked by the electric dipole, and its behavior in the quantum regime remains largely unexplored. We address this long-standing problem by presenting a complete quantum-mechanical formalism for the TD in a nanostructure and proposing a spectroscopic method for its direct measurement. Focusing on a particle confined to a toroidal manifold—a geometry physically realized in systems such as carbon nanotube tori and topological insulator nanorings—we analyze the spectral response to an external toroidal current. We demonstrate that the resulting Aharonov-Bohm-like energy shifts are directly proportional to the expectation value of the TD operator. The transition energies exhibit a linear dependence on this current, with a quantized slope that directly reveals the change in the TD quantum number between eigenstates. This provides a clear experimental blueprint to unveil, measure, and characterize this elusive third multipole moment and its quantum nature, opening avenues in quantum metamaterials, nanoscience, and the study of fundamental symmetries.

Propagation of intense laser pulses in underdense plasma always involves nonlinear evolution of the laser envelope, which leads to large deviations from the initial Gaussian intensity profile. The modulations in the intensity envelope have strong influence not only on the formation of the wakefield, but also in the process of electron injection; thus, it defines the final spectral and angular distribution of the accelerated electrons. Better understanding of the laser pulse evolution is crucial for the correct interpretation of the experimental results, especially in the highly relativistic regime. We developed a simple numerical tool, which adequately models the evolution of the laser envelope up to the point when the spectral broadening becomes significant. Our numerical modeling sheds light on the asymmetric wakefield generation and consequent off-axis injection of electrons, observed in experiments and particle-in-cell simulations.

The intermediate valence of Ce in ${\mathrm{CeCo}}_5$ challenges standard density functional theory (DFT) and static $\mathrm{DFT}+U$ approaches, which fail to capture its magnetic properties. By combining $\mathrm{DFT}+U$ with exact diagonalization of the Anderson impurity model for the Ce $4f$ shell, we find a substantial reduction of Ce spin and orbital moments, consistent with $\mathrm{DFT}+\mathrm{DMFT}$ (dynamical mean-field theory), arising from ${\mathrm{Ce}}^{4+}-{\mathrm{Ce}}^{3+}$ valence fluctuations. The total magnetic moment of 6.70 ${\mu }_B$ agrees with experiment, and the calculated $4f$ density of states reproduces photoemission and bremsstrahlung isochromat spectra. The uniaxial magnetic anisotropy energy reaches 4.8 meV/f.u. when Coulomb correlations on both Ce $4f$ and Co $3d$ shells are included, in very good agreement with experimental data. These results highlight the importance of dynamical correlations and provide guidance for exploring high-performance, low-rare-earth-content permanent magnets.