We propose a variational alternative to the Trotter-Suzuki decomposition that provides greater control over errors while preserving the unitary structure of time evolution. The variational parameters in our ansatz are derived from a global action principle, where Euler-Lagrange equations govern their optimal dynamics. Unlike conventional wavefunction-based variational methods, our approach specifically targets the time evolution operation, and this allows a single set of optimized parameters to be applied to any initial state for a fixed Hamiltonian avoiding costly optimization procedures. Our method outperforms the standard Trotter-Suzuki formulas, typically achieving higher accuracy than higher-order Suzuki schemes. This translates directly to quantum computing applications, where it enables the design of quantum circuits with fewer gates, which reduces noise and improves precision. Although we focus on quantum dynamics, the method is broadly applicable to problems involving general time-evolution operators. Applied to various model Hamiltonians, our approach reduces errors by factors of 2 to 5 compared to Trotter-Suzuki decompositions, demonstrating its promise for accurate quantum simulation with improved efficiency. In certain cases, the variational ansatz achieves higher accuracy than more complex higher-order Suzuki formulas while reducing the gate count by nearly half within a single circuit layer. Furthermore, we derive approximate analytical expressions for the variational parameters up to cubic order in time, valid for generic Hamiltonians. These approximations enable long-time quantum simulations with improved accuracy over equivalent Suzuki decompositions, providing ready-to-use evolution formulas that match Suzuki's gate complexity while delivering better performance.

Cloaking for bending waves propagating in thin elastic plates is known to present numerous obstacles to its experimental realization, particularly due to the complexity of the effective medium obtained by coordinate transformation. Here we demonstrate the possibility of cloaking in a unidirectional manner using affine transformations. This class of transformations maps the Kirchhoff-Love plate equation to its anisotropic extension. Unlike most publications in the literature, the resulting cloak is easily manufacturable using composite materials. In addition to numerical demonstrations of the unidirectional cloaking phenomenon, we present here a design method of an effective cloaking device, as well as the experimental validation of the manufactured cloak.

Repeated local measurements typically have adversarial effects on entangling unitary dynamics, as local measurements usually degrade entanglement. However, recent works on measurement-only dynamics have shown that strongly entangled states can be generated solely through noncommuting random multisite and multispin projective measurements. In this work, we explore a generalized measurement setup in a system without intrinsic unitary dynamics and show that volume-law entangled states can be generated through local, nonrandom, yet noncommuting measurements. Specifically, we construct a one-dimensional model comprising a main fermionic chain and an auxiliary (ancilla) chain, where generalized measurements are performed by locally coupling the system to detector qubits. Our results demonstrate that long-time states with volume-law entanglement or mutual information are generated between different parts of the main chain purely through nonunitary measurement dynamics. Remarkably, we find that such large-entanglement generation can be achieved using only the measurements of one-body operators. Moreover, we show that measurements of nonlocal higher-body operators can be used to control and reduce entanglement generation by introducing kinetic constraints to the dynamics. We discuss the statistics of entanglement measures along the quantum trajectories, the approach to stationary distributions of entanglement or long-time steady states, and the associated notions of limited ergodicity in the measurement-only dynamics. Our findings highlight the potential of nonrandom measurement protocols for controlled entanglement generation and the study of nonunitary many-body dynamics.

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.

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.

Chiral phonons have been identified in several chiral crystals, primarily in crystals with trigonal symmetry and threefold screw axes. In this study, chiral phonons in ${\mathrm{CrSi}}_2$, a chiral crystal with a sixfold helical structure, were investigated. Circularly polarized Raman spectroscopy revealed a subtle splitting of doubly degenerate $E_2$ phonon modes between cross-circular polarization configurations. These observations, supported by first-principles phonon calculations, indicate the presence of chiral phonons in ${\mathrm{CrSi}}_2$, expanding the scope of materials that exhibit chiral vibrational modes beyond the conventional trigonal class.

The projected ensemble, an ensemble of pure states on a subsystem conditioned on projective measurement outcomes on its complement, provides a finer probe of ergodicity and information structure than the reduced density matrix of the subsystem in bipartite quantum states. This framework can be generalized to partial projected ensembles in tripartite settings, where outcomes from part of the measured subsystem are discarded, leading to ensembles of mixed states. We show that information measures defined for such ensembles, in particular the Holevo information, yield a more detailed characterization of how quantum information is distributed between subsystems compared to conventional entanglement measures. Using exact analytical results supported by numerical results, we uncover a qualitative change in the scaling of the Holevo information with system size in partial projected ensembles generated by Haar-random states, as the relative sizes of the subsystem are varied. In one phase, the Holevo information decays exponentially with system size, while in the other it grows linearly, thereby defining distinct information phases separated by sharp transitions signaled by nonanalyticities in the Holevo information. The exponentially decaying phase rigorously establishes the existence of a measurement-invisible quantum-correlated phase, a manifestation of many-body information scrambling with no bipartite analog. We contrast this information-phase diagram with the entanglement-phase structure of tripartite Haar-random states obtained from logarithmic negativity, and show that the Holevo information reveals additional fine structure beyond conventional entanglement measures. Finally, we show that these information phases, as characterized by the Holevo information, emerge in the dynamics of chaotic quantum circuits and discuss the associated timescales.

Understanding electrical resistivity in metals remains a central challenge in quantifying charge transport at finite temperature. Current first-principles calculations based on the Boltzmann transport equation often match experiments, yet they almost always neglect the effect of thermal expansion and phonon anharmonicity. We show that both effects exert an opposite impact on electron-phonon coupling and on electrical resistivity. Thermal expansion enhances the coupling and leads to overestimation of resistivity, whereas anharmonic effects reduce it. By explicitly incorporating both effects, we establish a more complete description of resistivity in elemental metals, demonstrated here for Pb, Nb, and Al.

We present a first-principles machine-learning computational framework to investigate anharmonic effects in polarization-orientation (PO) Raman spectra of molecular crystals, focusing on anthracene and naphthalene. By combining machine learning models for interatomic potentials and polarizability tensors, we enable efficient, large-scale simulations that capture temperature-dependent vibrational dynamics beyond the harmonic approximation. Our approach reproduces key qualitative features observed experimentally. We show, systematically, what are the fingerprints of anharmonic lattice dynamics, thermal expansion, and Raman tensor symmetries on PO-Raman intensities. However, we find that the simulated polarization dependence of Raman intensities shows only subtle deviations from quasiharmonic predictions, failing to capture the pronounced temperature-dependent changes that have been reported experimentally in anthracene. We propose that part of these inconsistencies stem from the impossibility to deconvolute certain vibrational peaks when only experimental data is available. This work therefore provides a foundation to improve the interpretation of PO-Raman experiments in complex molecular crystals with the aid of theoretical simulations.

Measurements can detect errors in a decohered quantum memory allowing active error correction to increase the memory time. Previous understanding of this mechanism has focused on evaluating the performance of error-correction algorithms based on measurement results. In this work, we instead intrinsically characterize the information dynamics in a quantum memory under repeated measurements, using coherent information and relative entropy. We consider the dynamics of a $d$-dimensional stabilizer code subject to Pauli errors and noisy stabilizer measurements and develop a $(d+1)$-dimensional statistical mechanics model for the information-theoretic diagnostics. Our model is dual to the model previously obtained for the optimal decoding algorithm, and the potential decoding transition in the quantum memory again manifests as a thermal phase transition in the statistical mechanics model. We explicitly derive the model and study the phase transition in information encoding in three examples: surface codes, repetition codes, and the XZZX code.

We present direct observation of the ringdown dynamics in the rotating frame of a resonantly driven single-mode nonlinear nanomechanical resonator. An additional close-to-resonance harmonic force excites nonlinear oscillations about the fixed point in the rotating frame. When the secondary drive is removed, we measure decay of the in-phase and quadrature components toward this fixed point. We show that the decay of the in-phase signal is nonexponential, even though the vibration amplitude decays exponentially if both forces are switched off, revealing an unexpected interplay between conservative nonlinearity and dissipation. A minimalistic model captures these dynamics as well as the spectrum of the vibrations excited by the additional force, relating them to the dissipation-induced symmetry breaking of the dynamics in the rotating frame.

We systematically investigate whether classical hydrodynamic field theories can predict the long-time dynamics of many-particle quantum systems. We study both numerically and analytically the time evolution of a chain of spins (or qubits) subjected to stroboscopic dynamics. The time evolution is implemented by a sequence of local and nearest-neighbor gates that conserve the total magnetization. The long-time dynamics of such a system is believed to be describable by a hydrodynamic field theory, which, importantly, includes the effect of noise. Based on a field theoretical analysis and symmetry arguments, we map each operator in the spin model to the corresponding fields in hydrodynamics. This allows us to predict which expectation values decay exponentially and which decay with a hydrodynamic long-time tail. We illustrate these findings by studying the time evolution of all 255 Hermitian operators that can be defined on four neighboring sites. All operators not protected by hydrodynamics decay exponentially, while the others show a slow hydrodynamic decay. While most hydrodynamic power laws seem to follow the analytical predictions, we also discuss cases where there is an apparent discrepancy between analytics and the finite-size numerical data.

The interplay between electronic and lattice degrees of freedom in the insulator-to-metal transition (IMT) in rare-earth nickelates is a long-standing question. In the present work, broadband ultrafast transient reflectivity (TR) spectroscopy is applied to study the photoinduced IMT in ${\mathrm{NdNiO}}_3$. Both coherent and incoherent terms of the TR signal show discontinuous behavior around the same pump fluence value. A drastic drop in the sample reflectivity appearing at $\sim 100$ fs timescale in the high excitation density regime indicates the closing of the gap across the IMT. In this regime, coherent phonons associated with the low-temperature crystal phase are not observed even at early time delays, indicating an ultrafast transformation of the lattice potential. A detailed analysis of the coherent phonons indicates a strong coupling between some phonon modes, electronic excitations, and possibly the magnetic order. In this study, we provide insights into the ultrafast dynamics of rare-earth nickelates.

Understanding heat conduction in defective silicon is crucial for electronics and thermoelectrics. Conventional understanding relies on the phonon gas picture, treating defects as scattering centers that reduce phonon lifetimes without altering frequencies and group velocities. We go beyond the phonon gas picture by employing the Wigner transport equation to investigate heat conduction in vacancy-defective silicon. Our findings reveal that, while thermal conduction in pristine silicon stems mainly from particlelike propagation of vibrational modes, wavelike tunneling becomes increasingly important in the presence of vacancies. Contrary to the conventional belief that defects only perturb mode lifetimes, we demonstrate that vacancies also diminish velocity operators—a dominant factor in thermal conductivity reduction, surpassing even the effect of lifetime shortening. Furthermore, incorporating anharmonic frequencies and anharmonicity-renormalized interatomic force constants shows that, while anharmonicity suppresses thermal conductivity in pristine silicon, this effect weakens with vacancy concentration and reverses to enhance conductivity. These findings challenge conventional knowledge and provide insights into thermal conduction in defective materials.

A fundamental characteristic of non-Hermitian quantum systems is the nonunitary time evolution under a Euclidean inner product. Until now, a consensus on the mathematical formulation for the dynamics of non-Hermitian quantum systems has been lacking. Here, we focus on the quantum metric framework, in which the state normalization is preserved through a fundamental redefinition of the inner product of the Hilbert space. We generalize this framework to the $\mathcal{PT}$-broken region and provide a method to construct the metric operator of a general two-band non-Hermitian model. Then we redefine the Loschmidt echo and investigate the nonadiabatic dynamics in non-Hermitian quantum systems. Taking the $\mathcal{PT}$-symmetric staggered tight-binding chain model as an example, we demonstrate that the long-time average of the rate function can be used to characterize the $\mathcal{PT}$ phase transitions. In the non-Hermitian SSH model, we investigate the dynamical quantum phase transitions (DQPTs) and show that this framework can remove the accidental DQPTs and thus build a bridge between DQPTs and equilibrium topological phase transitions.

Inspired by “fracton hydrodynamic” universality classes of dynamics with unusual conservation laws, we present a dynamical universality class that arises out of local area-preserving dynamics in the noncommutative plane. On this symplectic manifold, the area-preserving spatial symmetry group $\mathrm{SL}(2,\mathbb{R})⋊{\mathbb{R}}^2$ is a symmetry group compatible with nontrivial many-body dynamics. The conservation laws associated with this symmetry group correspond to the dipole and quadrupole moments of the particles. We study the unusual dynamics of a crystal lattice subject to such symmetries, and argue that the hydrodynamic description of lattice dynamics breaks down due to relevant nonlinearities. Numerical simulations of classical Hamiltonian dynamical systems with this symmetry are largely consistent with a tree-level effective field theory estimate for the endpoint of this instability.

Non-Hermitian (NH) systems exhibit intricate spectral topology and feature biorthogonal left and right eigenstates, resulting in distinct NH phenomena, such as the NH skin effect and ultraspectral sensitivity. However, conventional techniques fall short in directly measuring complex-valued spectra and biorthogonal eigenstates, particularly in higher-dimensional NH systems where geometry and boundary conditions become pivotal. Here, we present a Green's function-based method, which enables direct measurement of these fundamental quantities in arbitrary wave-based NH lattices. By capturing amplitude and phase responses for all pump-probe configurations experimentally, we acquire the full Green's function in two-dimensional non-Hermitian acoustic crystals. Utilizing this technique, we first demonstrate eigenstate skewness and spectral collapse in nonreciprocal lattices. Additionally, by controlling geometry and boundary conditions in reciprocal lattices, we observe spectral transitions—a unique hallmark of higher-dimensional NH systems. Our results not only render complex spectral topology and left eigenstates experimentally accessible and practically meaningful, but also establish a universal and versatile framework for exploring complex spectral features and NH dynamics across diverse physical systems.

Temporal interfaces enable dynamic wave control but face a fundamental bottleneck: temporal reflection and refraction amplitudes depend on incident momentum because the projection coefficients between pre and postquench eigenstates—defined by biorthogonal overlaps in the non-Hermitian case—are generically momentum dependent, which distorts broadband wave packets. We develop a projection-operator framework for temporal scattering and show that when a $\mathcal{PT}$-symmetry phase transition occurs across the interface, the final eigenbasis satisfies a phase-transition-induced orthogonality condition, which renders the overlap magnitudes momentum independent and thus yields broadband-stable temporal splitting. Experiments in an electrically reconfigurable acoustic-cavity lattice confirm momentum-robust scattering over a finite bandwidth, and our analysis further reveals nonorthogonality-induced amplification/attenuation and a null-excitation criterion for stable temporal boundary states. These results position geometric projection as a unifying mechanism for non-Hermitian temporal physics and a practical route to versatile time-domain devices.

${\mathrm{CuGeO}}_3$ has long been studied as a prototypical example of the spin-Peierls transition in a $S=1/2$ Heisenberg chain. Despite intensive investigation of this quasi-one-dimensional material, systematic measurements and calculations of the phonon excitations in the dimerized phase have not to date been possible, leaving certain aspects of the spin-Peierls phenomenon unresolved. We perform state-of-the-art density functional theory (DFT) calculations to compute the electronic structure and phonon dynamics in the low-temperature dimerized phase. We also perform high-resolution neutron spectroscopy to measure the full phonon spectrum over multiple Brillouin zones. We find excellent agreement between our numerical and experimental results that extend to all measurement temperatures. Notable features of our phonon spectra include a number of steeply dispersive modes, nonmonotonic dispersion features, and specific phonon anticrossings, which we relate to the mode eigenvectors. By calculating the magnetic interactions within DFT and studying the effects of different phonon modes on the superexchange paths, we discuss the possibility of observing spin-phonon hybridization effects in experiments performed both in and out of equilibrium.

In this work, we investigate the softening of the in-plane transverse acoustic phonon driven by electronic nematicity in ${\mathrm{BaFe}}_2{\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right)}_2$ using inelastic x-ray scattering, with a focus on the optimally doped sample ($x=0.31$) sample—a system exhibiting signatures of a putative nematic quantum critical point and minimal disorder among iron pnictides. We observe only a modest softening of the phonon frequency and no evidence of critical damping, suggesting that the nematic quantum critical fluctuations couple only weakly to the lattice from our quantum critical model. Given the close proximity of the structural and magnetic transition temperatures in the underdoped sample—which implies that spin-nematic fluctuations couple strongly to the lattice—we conjecture that the quantum critical nematic fluctuations are predominantly orbital in origin.