Absence of thermalization after a global quantum quench is a well-established numerical observation in integrable many-body systems, and can be empirically related to a violation of the eigenstate thermalization hypothesis (ETH) in such models. Still, in many of those examples, a weaker version of the conventional ETH (wETH) has been numerically reported or even rigorously proven. In this paper, we show analytically and illustrate numerically that the absence of thermalization is already possible after a local quench. A closely related finding is a strong violation of the ETH, meaning that not even the wETH is fulfilled anymore. In our analytical explorations, we focus on XX-spin-chain models with open boundary conditions, where the local quench is generated by initiating the system in thermal equilibrium and then suddenly switching on (or slightly changing) a single-spin impurity either at the end or in the center of the chain. Numerically, we observe qualitatively similar phenomena also for more general XXZ models in the case of an end-impurity, but not in the case of a central impurity.

We compute the degeneracy of energy levels in the Kitaev quantum double model for any discrete group $G$ on any planar graph forming the skeleton of a closed orientable surface of arbitrary genus. The derivation is based on the fusion rules of the properly identified vertex and plaquette excitations, which are selected among the anyons, i.e., the simple objects of the Drinfeld center $\mathcal{Z}\left({\mathrm{Vec}}_G\right)$. These degeneracies are given in terms of the quantum dimensions of the anyons and allow one to obtain the exact finite-temperature partition function of the model, valid for any finite-size system.

We develop a symmetry indicator framework to efficiently predict the topology of superlattice-induced minibands with spin-orbit coupling. Our algorithm requires input only from the parent material before the superlattice is applied. The simplification arises by assuming a perturbatively weak superlattice potential; however, our results extend beyond the perturbative regime as long as the superlattice-induced gaps remain open. We first consider a time-reversal- and inversion-symmetric system subject to a weak superlattice potential and derive a compact formula for the ${\mathbb{Z}}_2$ invariant of the lowest miniband. We then extend to time-reversal breaking systems and compute the Chern number. We apply our theory to selected transition-metal dichalcogenides, HgTe/CdTe quantum wells, and thin films of three-dimensional topological insulators and Dirac semimetals. We find topological superlattice bands can arise even from nontopological materials, broadening the pool of candidates for realizing topological flat bands. Our theory predicts which geometry and periodicity of superlattice will yield topological bands for a given material, providing a clear guiding principle for designing topological superlattice heterostructures.

Non-Hermitian multichannel Kondo problems host both non-Fermi-liquid and non-Hermitian physics, which provide a prototypical model to explore exotic collective quantum phenomena driven by the two different ingredients. Here, we first propose an experimental setup that realizes this model with exact channel symmetry as well as a controllable $\mathcal{PT}$ symmetry. Then, we perform a multimethod study of this model, focusing on the low-energy spectrum, the thermodynamic quantities, and the transport properties associated with different fixed points. Using the Bethe ansatz approach, we identify the existence of the Yu-Shiba-Rusinov–type state previously found in the non-Hermitian single-channel Kondo model. Then, based on non-Hermitian numerical renormalization group calculations, we reveal clear numerical signatures of the Yu-Shiba-Rusinov state emerging in the relatively strong non-Hermiticity regime of the $\mathcal{PT}$-asymmetric model. Furthermore, our boundary conformal field theory, which is found to be applicable for the $\mathcal{PT}$-symmetric model, uncovers an anomalous temperature dependence of the Kondo conductance, which is beyond conventional Hermitian Kondo systems.

Mechanical strain can be used to control physical properties in materials. The experimental investigation of strain-induced effects at the nanoscale is of importance not only for its fundamental aspects but also for the development of device applications. Transmission x-ray microscopy is a particularly well-suited technique for nanoscale imaging of magnetic materials, but its compatibility with in situ mechanical straining of samples is limited. In this work, we present a setup for applying tailored in situ mechanical strains to freestanding thin films by means of a microelectromechanical system (MEMS) actuator. We then present a proof-of-concept experiment in which a freestanding 80-nm-thick (001) ${\mathrm{BiFeO}}_3$ multiferroic thin film is strained with the MEMS device, allowing us to control the coupled ferroelectric/spin cycloidal configuration.

Optically active solid-state systems such as self-assembled quantum dots, rare-earth ions, and color centers in diamond and SiC are promising candidates for quantum network, computing, and sensing applications. Although the nuclei in these systems naturally lead to electron spin decoherence, they can be repurposed if they are controllable, as long-lived quantum memories. Prior work showed that a metric known as the one-tangling power can be used to quantify the entanglement dynamics of sparse systems of spin-$1/2$ nuclei coupled to color centers in diamond and SiC. Here we generalize these findings to a wide range of electron-nuclear central-spin systems, including those with spin $>1/2$ nuclei, such as in III–V quantum dots (QDs), rare-earth ions, and some color centers. Focusing on the example of an (In)GaAs QD, we offer a procedure for pinpointing physically realistic parameter regimes that yield maximal entanglement between the central electron and surrounding nuclei. We further harness knowledge of naturally occurring degeneracies and the tunability of the system to generate maximal entanglement between target subsets of spins when the QD electron is subject to dynamical decoupling. We also leverage the one-tangling power as an exact and immediate method for computing QD electron spin dephasing times with and without the application of spin echo sequences and use our analysis to identify coherence-sustaining conditions within the system.

Artificial spin systems, sometimes referred to as artificial spin ices, are arrays of coupled nanoscale magnets that order according to the lattice geometry, nanomagnet shape and magnetic anisotropy. Here, we characterize a family of artificial spin systems that are formed by placing arrays of out-of-plane nanomagnets on the vertices of the Archimedean lattices. On demagnetizing these lithographically fabricated nanomagnet arrays using a magnetic field protocol and subsequently imaging the magnetic configuration using magnetic force microscopy, we observe experimentally how these systems order. We compare our experimental results with those predicted by Monte Carlo simulations to assign an effective temperature to each lattice. We find that, for all of the lattices, the assigned effective temperature is above the transition temperature. This reflects the difficulty of obtaining system-spanning order in lattices with out-of-plane nanomagnets. We consider to what extent further-neighbor interactions affect the thermal properties and spin-spin correlations in each lattice, illustrating our results with four example lattices. We can divide the lattices into three main groups: bipartite lattices that admit a perfect antiferromagnetic ground state, frustrated lattices where ordering proceeds via a single step, and frustrated lattices with two-step ordering. Our work highlights the diversity of magnetic ordering that can be hosted in two-dimensional artificial spin systems with out-of-plane nanomagnets, and demonstrates the importance of including longer-range dipolar interactions to explain the magnetic ordering.

We investigate the microscopic origin of size-dependent magnetization suppression in Ni nanocrystals using photoemission measurements with variable probing depth. While the interior retains bulklike electronic states, the near-surface region exhibits enhanced electronic correlation, leading to conduction-electron localization and the emergence of a magnetic dead layer. This intrinsic modification proves the validity of the dead-layer model quantitatively describing the size dependence of the saturation magnetization. These findings establish a direct link between surface electronic correlation and magnetism at reduced dimensions.

We introduce a synthetic Mach-Zehnder interferometer for digitized quantum computing devices to probe fractional exchange statistics of anyonic excitations that appear in quantum spin liquids. Employing an IonQ quantum computer, we apply this scheme to the toric ladder, a quasi-one-dimensional version of the toric code. We observe interference patterns resulting from the movement of electric excitations in the presence and absence of magnetic ones. We model the noise in IonQ via depolarizing Lindbladian dynamics, and find quantitative agreement with the measurements obtained from the quantum device. The synthetic Mach-Zehnder interferometer can thus also serve as an effective means to probe the coherence length and time scales of multiqubit noisy quantum devices.

We examine the validity of a potential extension of the adiabatic theorem to quantum quenches, i.e., nonadiabatic changes. In particular, the transverse field Ising model (TFIM) and the axial next nearest neighbor Ising (ANNNI) model are studied. The proposed extension of the adiabatic theorem is stated as follows: Consider the overlap between the initial ground state and the postquench Hamiltonian eigenstates for quenches within the same phase. This overlap is largest for the postquench ground state. In the case of the TFIM, this conjecture is confirmed for both the paramagnetic and ferromagnetic phases numerically and analytically. In the ANNNI model, the conjecture could be analytically proven for a special case. Numerical methods are employed to investigate the conjecture's validity beyond this special case.

A codimension-two spiral spin liquid is a correlated paramagnetic state with one-dimensional ground state degeneracy hosted within a three-dimensional lattice. Here, via neutron scattering experiments and numerical simulations, we establish the existence of a codimension-two spiral spin liquid in the effective honeycomb-lattice compound ${\mathrm{Cs}}_3{\mathrm{Fe}}_2{\mathrm{Cl}}_9$, which demonstrates an alternate path to spiral spin liquids by overcoming the long-standing impediment of weak further-neighbor interactions. In the long-range ordered regime, competing spiral and spin density wave orders emerge as a function of applied magnetic field, among which a possible order-by-disorder transition is identified.

We investigate the fate of Laughlin's wave function in ideal Chern bands which can be mapped to generalized zero Landau levels in spatially dependent magnetic fields. By exploiting its exact mapping onto a classical Coulomb gas and leveraging previous results of one-component plasmas in nonuniform neutralizing backgrounds, we demonstrate that the ideal Laughlin wave function undergoes a phase transition from its well-known fully gapped topologically ordered plasma state into a power-law correlated dielectric state even for the fixed filling of $1/3$, as the magnetic field becomes increasingly more inhomogeneous. This dielectric state is gapless even though it does not spontaneously break translational symmetry. Remarkably, for a fixed filling $\nu =1/m$, the exponent governing density correlations in this state changes continuously as a function of the degree of spatial inhomogeneity of the magnetic field, and can range from 4 near a Berezinskii-Kosterlitz-Thouless transition to the plasma state, up to $2m$ in the limit of fields generated by point solenoids.

In the present work we report the observations of two-dimensional plasma resonance with linear dispersion in superconducting NbN films fabricated by atomic layer deposition on sapphire and high-resistive silicon substrates. We have detected several consecutive resonance dips in terahertz transmission of the sample processed into the disk geometry. Increasing temperature results both in the pronounced shifts of their resonant frequencies and their broadening. Above the critical temperature of superconducting phase transition these features vanish. Both the linear dispersion of these features and their temperature evolution allows us to attribute these resonances to the collective plasma modes in a two-component electron system. Dispersion of these plasmonic excitations showed linear dependence, similar to the theoretical prediction calculated for the two-component system consisting of Cooper pairs and normal electrons.

Using high-accuracy many-body quantum Monte Carlo (QMC) methods, we study the effect of interlayer coupling on the properties of two-dimensional freestanding bilayers (BLs) of ${\mathrm{MoS}}_2$ and phosphorene. The properties of the two BL-materials are very different and largely determined by the interlayer interaction, which is purely van der Waals in ${\mathrm{MoS}}_2$ and partially electronic/chemical in phosphorene, resulting in a modest layer-dependent property modulation in ${\mathrm{MoS}}_2$ and strong modulation in phosphorene. Multireference and symmetry considerations are used to construct state-of-the-art accuracy QMC trial wave functions. We determine the quasiparticle band gaps for both materials, ${\mathrm{\Delta }}_{\mathrm{\Gamma }\to \mathrm{K}}^{\text{qp}}=2.45\pm 0.05\mathrm{eV}$ in BL-${\mathrm{MoS}}_2$ and ${\mathrm{\Delta }}_{\mathrm{\Gamma }\to \mathrm{\Gamma }}^{\text{qp}}$=1.59 $\pm 0.1\mathrm{eV}$ in BL-phosphorene. These benchmark band-gap values make it possible to consolidate the interpretation of the widely scattered experimental and theory data.

We report resistivity and Hall effect measurements in the chiral antiferromagnet EuPtSi. Depending on the magnetic field orientation with respect to the crystallographic axes, EuPtSi presents different topological magnetic phases below the Néel temperature $T_N=4.05\mathrm{K}$. In particular, for a field $H\parallel$ [111], it exhibits the well-known skyrmion lattice A phase inside the conical phase between $T=0.45\mathrm{K}$ and $T_N$ in the field range 0.8–1.4 T. Further outstanding features of EuPtSi are the nanoscale size of its skyrmions (placing EuPtSi in the less common regime of nonadiabatic electron-skyrmion scattering) and its substantial topological Hall effect. Remarkably, the skyrmion phase can be extended down to very low temperatures (lower than 0.1 K) through field-cooling regardless of the cooling rate and of the magnetic history. Similarly the metastability of the A' and B phases ($H\parallel$ [100]) at low temperature is evidenced by our measurements. These results suggest that EuPtSi is a peculiar example where the competition between topological stability and thermal agitation can lead to a metastable quantum skyrmion state.

We investigate the coupling of a multimode metal-insulator-metal cavity to a two-dimensional electron gas (2DEG) in a quantum well in the presence of a strong magnetic field. The TM cavity mode is strongly hybridized with an intersubband transition of the 2DEG, forming a polaritonic mode in the ultrastrong coupling regime, while the TE mode remains an almost purely cavity mode. The magnetoplasmon excitation emerging from the presence of the magnetic field couples with both TM and TE modes, exhibiting different coupling strengths and levels of spatial field inhomogeneity. While the strong homogeneity of the bare TE mode gives rise to the standard anticrossing of strong coupling, the inhomogeneous polaritonic TM mode is shown to activate an observable Coulombic effect in the spectral response, often referred to as nonlocality. This experiment demonstrates a cavity-induced modification of the 2DEG response and offers a route to probing the effect of Coulomb interactions in ultrastrongly coupled systems via reshaping of their cavity mode profiles.

We summarize the key ingredients required for universal topological quantum computation using Majorana zero modes in networks of topological superconductor nanowires. Particular emphasis is placed on the use of both sparse and dense logical qubit encodings, and on the transitions between them via projective parity measurements. Combined with hybridization, these operations extend the computational capabilities beyond braiding alone and enable universal gate sets. In addition to outlining the theoretical foundations—including the algebra of Majorana operators, along with the stabilizer formalism—we introduce an efficient numerical method for simulating the time-dependent dynamics of such systems. This method, based on the time-dependent Pfaffian formalism, allows for the classical simulation of realistic device architectures that incorporate braiding, projective measurements, and disorder. The result is a semipedagogical overview and computational toolbox designed to support further exploration of topological quantum computing platforms.

The potential of solid-state quantum emitters for applications critically depends on several key figures of merit. One of the most important is the coherence time of the emitted single photons, which can be compromised by fast dephasing and spectral diffusion. In hexagonal boron nitride (hBN), blue-emitting color centers (or B centers) are seen as favorable in this regard, in the light of prior studies mainly based on resonant excitation. Yet, their time-coherence properties in the more accessible regime of nonresonant excitation (or photoluminescence) have not been extensively characterized. Here, we investigate the coherence and spectral diffusion of the photoluminescence from a B center in the continuous-wave regime using photon correlation Fourier spectroscopy. We determine that the emission line shape consists of a homogeneous contribution, whose linewidth increases with the laser power and which is broadened by spectral diffusion at a timescale of 10 to 100 $\mu \mathrm{s}$. At low power and short time, the emission line is only a factor $\approx 2$ above the Fourier limit, while at long times, the inhomogeneous linewidth increases up to more than a gigahertz. Our work deepens the understanding of decoherence processes of this preeminent family of quantum emitters in hBN.

Hybrid platforms that couple microwave photons to collective spin excitations offer promising routes for coherent information processing, yet conventional magnets face inherent trade-offs among coupling strength, coherence, and tunability. We demonstrate that bilayer cuprate antiferromagnets—exemplified by ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$—provide an alternative approach enabled by their unique magnon spectrum. Using a neutron-constrained bilayer spin model, we obtain the complete $\mathrm{\Gamma }$-point spectrum and identify an in-plane acoustic $\alpha$ mode that remains gapless and Zeeman linear, alongside an in-plane optical $\beta$ mode stabilized by weak anisotropy whose frequency can be tuned from GHz to THz ranges. When coupled to a single-mode microwave cavity, these modes create two distinct channels with a magnetically tunable $\alpha$-photon coupling where $|{\mathcal{G}}_{\alpha }|\propto B^{-1/2}$ and a nearly field-independent $\beta$0photon coupling. This asymmetric behavior enables continuous, single-parameter control spanning from dispersive to strong coupling regimes. In the dispersive limit, Schrieffer-Wolff analysis reveals cavity-mediated magnon-magnon coupling $g_{\alpha \beta }^{\mathrm{eff}}\propto g_{\alpha }{g}_{\beta }/({\omega }_m-{\omega }_c)$, while near triple resonance ($f_c=f_{\alpha }=f_{\beta }$) the normal modes reorganize into bright and dark superpositions governed by a single collective energy scale. The calculated transmission exhibits vacuum Rabi splittings, dispersive shifts, and Fano-like lineshapes that provide concrete experimental benchmarks and suggest the potential for programmable filtering and coherent state transfer across the GHz-THz frequency range if realized experimentally with suitable interfaces.

We investigate many-body effects on the spin-split electron bands in altermagnets by computing the electron self-energy arising from interactions with magnons, phonons, and hybridized magnon-phonon modes. These interactions lead to band broadening, which can obscure the intrinsic spin-splitting in spectroscopic measurements. We consider a $d$-wave Lieb lattice altermagnet as a representative example. Our results reveal that the spin-splitting remains spectroscopically resolvable and provide theoretical estimates of lifetime effects relevant for experimental detection. For electron-magnon coupling, we find a distinct difference between spectral function broadening for up and down spins close to the Fermi surface, which is not present in the case of electron-phonon coupling. We relate it to the spin splitting of the magnon modes in altermagnets. The results, including magnetoelastic coupling, are very similar to the pure magnon case. This provides insights into quasiparticle dynamics in altermagnets and contributes to the broader understanding of many-body interactions in spin-split systems. By including the temperature dependence of the self-energies, we also quantify how thermal fluctuations influence the broadening of the electronic states.