Recently, Nathan and Rudner derived a Gorini-Kossakowski-Sudarshan-Lindblad master equation from the Redfield equation. The claim is that the level of approximation is equal to that of the Redfield equation. Here we benchmark the Nathan-Rudner equation (NRE) against the exact solution of a damped harmonic oscillator, and we compare its performance to that of the time-dependent Redfield equation (RE). We find that which of the equations performs better depends on the regime considered. It turns out that the short-time dynamics is generally much better captured by the RE, whereas the NRE delivers results comparable to those of the rotating-wave approximation. For the steady state, in the high-temperature limit the RE again performs better and its solution approaches the exact result for ultrahigh temperatures. Nevertheless, here also the NRE constitutes a good approximation. In the low-temperature limit, in turn, the NRE provides a better approximation than the RE. For too strong coupling, here the RE might even fail completely by predicting unphysical behavior.

We report an intermediate regime in the quench time, ${\tau }_q$, separating the usual validity of the Kibble-Zurek mechanism (KZM) and its breakdown for rapid quenches in open systems under finite quench protocols. It manifests in the power-law scaling of the transition time with ${\tau }_q$ as the system appears to enter the adiabatic regime, even though the ramp is already terminated and the final quench value is held constant. This intermediate regime, which we dub the delayed KZM, emerges due to the dissipation, preventing the system from freezing in the impulse regime. This results in a large delay between the actual time the system undergoes a phase transition and the time inferred from a threshold-based criterion for the order parameter, as done in most experiments. We demonstrate using the open Dicke model and its one-dimensional lattice version that this phenomenon is a generic feature of open systems that can be mapped onto an effective coupled oscillator model. We also show that the phenomenon becomes more prominent near criticality, and its effects on the transition time measurement can be further exacerbated by large threshold values for an order parameter. Due to this, we propose an alternative method for threshold-based criterion which uses the spatiotemporal information, such as the system's defect number, for identifying the transition time.

We consider the optimal control of switching on a coupling term between two quantum many-body systems. Specifically, we (i) quantify the energetic cost of establishing a weak junction between two quantum spin-$1/2$ chains in finite time $\tau$ and (ii) identify the energetically optimal protocol to realize it. For linear driving protocols, we find that for long times the excess (irreversible) work scales as ${\tau }^{-\eta }$, where $\eta =1,2$ or a nonuniversal number depending on the phase of the chains. Interestingly, increasing a $J_z$ anisotropy in the chains suppresses the excess work, thus promoting quasiadiabaticity. The general optimal control problem is solved, employing a Chebyshev Ansatz. We find that the optimal control protocol is intimately sensitive to the chain phases.

Here we report on an elastic and inelastic neutron scattering study addressing the effect of electric fields on quantum paraelectric ${\mathrm{SrTiO}}_3$. Our elastic scattering results find small changes as a function of field in a superlattice reflection that sample the octahedral rotations, which is indicative of only weak coupling of octahedral rotation and electric polarization. Using inelastic neutron scattering, we measured the dispersion of the transverse acoustic and transverse optic phonon in ${\mathrm{SrTiO}}_3$ as a function of applied electric fields. By collecting not only the change in gap, but also of the dispersion, we can better quantify the changes in the lattice dynamics. The findings are put in context to recent DFT calculations predicting the E-field effect on the atomic motions of the lowest lying transverse optical (soft) mode. We find hints of nonlinear coupling to the acoustic mode and to the phonon with polarization perpendicular to the E field, which shows the nonlinearity in the chemical potential that is also relevant when ${\mathrm{SrTiO}}_3$ is excited by strong E-field (THz) pulses.

We present a novel and simple picture of the generation dynamics of coherent longitudinal optical (LO) phonons and LO-phonon-plasmon-coupled (LOPC) modes by the ultrafast infrared pump-pulses in gallium arsenide (GaAs) employing the low-temperature approximation. LO phonons exhibit a pronounced coupling with plasmons formed by the optically excited electrons in the excited states of GaAs. This coupling results in the coherent oscillation of the LOPC modes in the excited states. The pump pulse also induces stimulated Raman scattering, which generates the coherent LO-phonon oscillation in the ground state. This picture is incorporated into a simplified model, and the time evolution of the density operator is calculated using the Lindblad-type quantum master equation. The theoretical results explain well the reported experimental results on the coherent oscillation of LO phonons and LOPC modes observed through transient reflection measurements. Above all, our model provides a natural reason for the simultaneous manifestation of the LO phonons and the LOPC modes.

Tellurium (Te) has recently attracted research interest for its outstanding electronic and optoelectronic properties and interesting topological physical related to topological Weyl cones in its band structure. To explore its high field transport and optoelectronic properties, the study of the ultrafast dynamics of photoexcited carriers plays a pivotal role. In this work, we study the ultrafast dynamics of photoexcited carriers in Te at the vicinity of Weyl nodes using transient reflection spectroscopy with 2-µm pump and 4-µm probe. We find that the relaxation process in a 100-nm Te flake is dominated by a 30-ps Auger recombination at high carrier density and a 200-ps multiphonon emission process at low carrier density. In contrast, the relaxation process in a 20-nm flake is ten times faster than the 100-nm flake due to the assistance of surface defects. The reflection modulation depth of the 20-nm sample is up to 60%, promising great potential for fast optical switch and optical modulator applications in the midinfrared region.

When subjected to quasiperiodic driving protocols, superconducting systems have been found to harbor robust time-quasiperiodic Majorana modes, extending the concept beyond static and Floquet systems. However, the presence of incommensurate driving frequencies results in dense energy spectra, rendering conventional methods of defining topological invariants based on band structure inadequate. In this work, we introduce a real-space topological invariant capable of identifying time-quasiperiodic Majorana modes by leveraging the system's spectral localizer, which integrates information from both Hamiltonian and position operators. Drawing insights from non-Hermitian physics, we establish criteria for constructing the localizer and elucidate the robustness of this invariant in the presence of dense spectra. Our numerical simulations, focusing on a Kitaev chain driven by two incommensurate frequencies, validate the efficacy of our approach.

Onsager's regression hypothesis connects the temporal relaxation of close-to-equilibrium systems with their dynamical correlation functions at thermal equilibrium. While the hypothesis is provably correct in classical systems, it is known to fail in the quantum regime. Here, we derive a suitably adjusted quantum version of Onsager's original hypothesis. Rigorous analytical results are complemented by a variety of numerical examples.

The linear cross-entropy (LXE) has been recently proposed as a scalable probe of the measurement-driven phase transition between volume- and area-law-entangled phases of pure-state trajectories in certain monitored quantum circuits. Here, we demonstrate that the LXE can distinguish distinct area-law-entangled phases of monitored circuits with symmetries, and extract universal behavior at the critical points separating these phases. We focus on (1+1)-dimensional monitored circuits with an onsite ${\mathbb{Z}}_2$ symmetry. For an appropriate choice of initial states, the LXE distinguishes the area-law-entangled spin-glass and paramagnetic phases of the monitored trajectories. At the critical point, described by two-dimensional percolation, the LXE exhibits universal behavior which depends sensitively on boundary conditions, and the choice of initial state. With open boundary conditions, we show that the LXE relates to crossing probabilities in critical percolation, and is thus given by a known universal function of the aspect ratio of the dynamics, which quantitatively agrees with numerical studies of the LXE at criticality. The LXE probes correlations of other operators in percolation with periodic boundary conditions. We show that the LXE is sensitive to the richer phase diagram of the circuit model in the presence of symmetric unitary gates. Last, we consider the effect of noise during the circuit evolution, and propose potential solutions to counter it.

We study the performances of an imperfect quantum many-body Otto engine based on free fermion systems. Starting from the thermodynamic definitions of heat and work along ideal isothermal, adiabatic, and isochoric transformations, we generalize these expressions in the case when the hypotheses of ideality are relaxed (i.e., nonperfect thermalization with the external baths, as well as nonperfect quantum adiabaticity in the unitary dynamic protocols). These results are used to evaluate the work and the power delivered by an imperfect quantum many-body heat engine in a finite time, whose working substance is constituted by a quantum Ising chain in a transverse field. We discuss the emerging optimal working points as functions of the various model parameters.

We study the possible signatures of prethermal strong Hilbert space fragmentation (HSF) for one-dimensional (1D) fermions subjected to a periodic drive. We extend the results of Ghosh et al. [Phys. Rev. Lett. 130, 120401 (2023)] to show the possibility of such fragmentation for a large class of experimentally relevant drive protocols. Moreover, we demonstrate the persistence of HSF when the fermion chain is taken away from half filling. Both analyses indicate the robustness of the fragmentation phenomenon reported earlier. We also provide an alternate derivation of the Floquet Hamiltonian of the driven chain which yields insight into the generic nested commutator structure of its higher order terms. Finally, we study the density-density out-of-time-correlators (OTOC) of the driven chain both away and near the special drive frequencies at which its first-order Floquet Hamiltonian exhibits fragmentation. We show that these OTOCs, for a chain with open boundary conditions, exhibit a distinct periodic unscrambling of information at special drive frequencies; such unscrambling can therefore serve as a marker of prethermal HSF. We provide an approximate analytic explanation of the role of HSF behind such periodic unscrambling and discuss experiments which can detect signatures of strong HSF in such driven chains.

By strongly driving a transmon-resonator system, the transmon qubit may eventually escape from its cosine-shaped potential. This process is called transmon ionization (TI) and known to be detrimental to the qubit coherence and operation. In this work we investigate the onset of TI in an irreversible, parametrically driven, frequency conversion process in a system consisting of a superconducting three-dimensional cavity coupled to a fixed-frequency transmon qubit. Above a critical pump power we find a sudden increase in the transmon population. Using Rényi entropy, Floquet modes, and Husimi Q functions, we infer that this abrupt change can be attributed to a quantum-to-classical phase transition. Furthermore, in the context of the single-photon detection, we measure a TI-uncorrected detection efficiency of up to 86% and estimate a TI-corrected value of up to $78\%$ by exploiting the irreversible frequency conversion. Our numerical simulations suggest that increasing the detuning between the pump and qubit frequencies and increasing the qubit anharmonicity can suppress the TI impact. Our findings highlight the general importance of the TI process when operating coupled qubit-cavity systems.

This paper presents the first sound speed and Grüneisen parameter data for fluid iron compressed to 3 TPa (30 million atmospheres) and 20 $\mathrm{g}/{\mathrm{cm}}^3$ on the Hugoniot. Both the sound speed and Grüneisen parameter are derivatives of the equation of state (EOS) and thus tightly constrain the contours of the EOS surface. The sound speed data are systematically lower than expected from a simple extrapolation of previous data. The Grüneisen parameter shows a 30% drop at pressures and temperatures above the melt transition. Furthermore, while some models compare well with either the sound speed or Grüneisen parameter, none of today's state-of-the-art models can explain both sets of data. Thus these new data will provide pivotal benchmarks for both future theoretical EOSs of warm dense iron and modeling planetary states and processes.

We study entanglement transitions in Clifford (stabilizer) random tensor networks (RTNs) and monitored quantum circuits, by introducing an exact mapping onto a (replica) statistical mechanics model. For RTNs and monitored quantum circuits with random Haar unitary gates, entanglement properties can be computed using statistical mechanics models whose fundamental degrees of freedom (“spins”) are permutations, because all operators commuting with the action of the unitaries on a tensor product Hilbert space are (linear combinations of) permutations of the tensor factors (“Schur-Weyl duality”). When the unitary gates are restricted to the smaller group of Clifford unitaries, the set of all operators commuting with this action, called the commutant, will be larger, and no longer form a group. We use the recent full characterization of this commutant by Gross et al. [Commun. Math. Phys. 385, 1325 (2021)] to construct statistical mechanics models for both Clifford RTNs and monitored quantum circuits, for on-site Hilbert-space dimensions which are powers of a prime number $p$. The elements of the commutant form the spin degrees of freedom of these statistical mechanics models, and we show that the Boltzmann weights are invariant under a symmetry group involving orthogonal matrices with entries in the finite number field ${\mathbf{F}}_p$ (“Galois field”) with $p$ elements. This implies that the symmetry group and consequently all universal properties of entanglement transitions in Clifford circuits and RTNs will, respectively, in general depend on and only on the prime $p$. We show that Clifford monitored circuits with on-site Hilbert-space dimension $d=p^M$ are described by percolation in the limits $d\to \infty$ at (a) $p=$ fixed but $M\to \infty$, and at (b) $M=1$ but $p\to \infty$. In the limit (a) we calculate the effective central charge, and in the limit (b) we derive the following universal minimal cut entanglement entropy $S_A=(\sqrt{3}/\pi )lnpln{L}_A$ for $d=p$ large at the transition. We verify those predictions numerically, and present extensive numerical results for critical exponents at the transition in monitored Clifford circuits for prime number on-site Hilbert-space dimension $d=p$ for a variety of different values of $p$, finding that projective and forced measurement schemes yield the same critical exponents and that they approach percolation values at large $p$. We clearly establish multifractal scaling of the purity, reflected in a continuous spectrum of critical exponents, while the typical exponent is the prefactor of the logarithm in the entanglement entropy. As a technical result, we generalize the notion of the Weingarten function, previously known for averages involving the Haar measure, to averages over the Clifford group.

We report anomalous circularly polarized Raman spectra of phonons in right- and left-handed quartzes. The phonon branches splitting from the $E$-mode at a finite wave number were found to be chiral with mutually opposite angular momenta. Our analysis reveals the Raman selection rules for chiral phonons. We also find that the conservation of angular momentum should be satisfied between two photons and one chiral phonon in the Raman process. Furthermore, we experimentally proved that the helicity of phonons with a certain frequency and wave vector should be opposite in the other enantiomer.

From a systematic study of thermal and charge transport in various single crystals of compensated topological insulators we identify the evolution of a large low-temperature thermal Hall effect as a characteristic common feature. In order to separate phononic and electronic contributions in the measured longitudinal and transverse thermal conductivity, the electronic contributions are estimated from corresponding electrical resistivity and Hall effect measurements on the same samples by using the Wiedemann-Franz law. As may be expected for charge-compensated topological insulators the longitudinal thermal conductivity is phonon dominated in all samples. However, we also find a pronounced field-linear thermal Hall effect that becomes most pronounced in the low-temperature range, where all samples are good electrical insulators. This indicates an underlying phononic mechanism of the thermal Hall effect and in this respect the topological insulators resemble other, mainly ionic, insulators, which have been reported to show a phonon-induced thermal Hall effect, but its underlying phononic mechanism remains to be identified. Our observation of a comparable thermal Hall ratio in topological insulators supports a theoretical scenario that explains a thermal Hall effect through skew scattering on charged impurities.

We show that the polarizations of sidebands emitted from bulk gallium arsenide driven by a strong terahertz (THz) field while probed with a weak near-infrared laser can be viewed as interferograms from a Michelson-like interferometer for Bloch waves. The fringe contrast of an interferogram is a measure of the nonequilibrium dephasing of Bloch waves in this strongly driven system. An interferogram's oscillation frequency is determined by, among other factors, the THz field strength $F_{\mathrm{THz}}$ and the reduced masses of electron-hole pairs. A simple analytic model brings all measured and calculated spectra into agreement with adjustment of only a single dephasing constant, and predicts the scaling of interferograms with $F_{\mathrm{THz}}$.

Density functional perturbation theory (DFPT) is a well-established method to study responses of molecules and solids, especially responses to atomic displacements or to different perturbing fields (electric, magnetic). Like for density functional theory (DFT), the treatment of metals is delicate, due to the Fermi-Dirac (FD) statistics and electronic bands crossing the Fermi energy. At zero temperature, there is an abrupt transition from occupied states to unoccupied ones, usually addressed with smearing schemes. Also, at finite temperature, fractional occupations are present, and the occupation numbers may vary in response to the perturbation. In this paper, we establish the characteristics of DFPT stemming from the underlying variational principle, in the case of metals. After briefly reviewing variational DFT for metals, the convexity of the entropy function of the occupation number is analyzed, and at finite temperature, the benefit of resmearing the FD broadening with the Methfessel-Paxton one is highlighted. Then the variational expressions for the second-order derivative of the free energy are detailed, exposing the different possible gauge choices. The influence of the inaccuracies in the unperturbed wave functions from the prior DFT calculation is studied. The whole formalism is implemented in the abinit software package.

We consider flux equilibrium in dissipative nonlinear wave systems subject to external energy pumping. In such systems, the elementary excitations, or quasiparticles, can create a Bose-Einstein condensate. We develop a theory on the Bose-Einstein condensation of quasiparticles for various regimes of external excitation, ranging from weak and stationary to ultrastrong pumping, enabling us to determine the number of quasiparticles near the bottom of the energy spectrum and their distribution along wave vectors. We identify physical phenomena leading to condensation in each of the regimes. For weak stationary pumping, where the distribution of quasiparticles deviates only slightly from thermodynamic equilibrium, we define a range of pumping parameters where the condensation occurs and estimate the density of the condensate and the fraction of the condensed quasiparticles. As the pumping amplitude increases, a powerful influx of injected quasiparticles is created by the Kolmogorov-Zakharov scattering cascade, leading to their Bose-Einstein condensation. With even stronger pumping, kinetic instability may occur, resulting in a direct transfer of injected quasiparticles to the bottom of the spectrum. For the case of ultrastrong parametric pumping, we have developed a stationary nonlinear theory of kinetic instability. The theory agrees qualitatively with experimental data obtained using Brillouin light scattering spectroscopy during parametric pumping of magnons in room-temperature films of yttrium-iron garnet.

Non-Hermitian topological systems have attracted much interest due to their unique topological properties when the non-Hermitian skin effect (NHSE) appears. However, the experimental realization of NHSE conventionally requires nonreciprocal couplings, which are compatible with limited systems. Here, we propose a mechanism of loss-induced Floquet NHSE, where the loss provides the basic source of non-Hermicity and the Floquet engineering brings about Floquet-induced complex next-nearest-neighbor couplings. We also extend the generalized Brillouin zone theory to nonequilibrium systems to describe the Floquet NHSE. Furthermore, we show that this mechanism can realize the second-order NHSE when generalized to two-dimensional systems. Our proposal can be realized in photonic lattices with helical waveguides and other related systems, which opens the door for the study of topological phases in Floquet non-Hermitian systems.