Simulation of quantum many-body systems is one of the most promising applications of quantum computers. It is crucial to efficiently implement the time-evolution operator as a quantum circuit to execute such simulations on near-term quantum computing devices with limited computational resources. However, standard approaches such as Trotterization sometimes require a deep quantum circuit, which is hard to implement on near-term quantum computers. Here, we propose a hybrid quantum-classical algorithm, called local subspace variational quantum compilation (LSVQC), for compiling the time-evolution operator of quantum many-body systems. The LSVQC performs a variational optimization to reproduce the action of the target time-evolution operator within a physically reasonable subspace. The optimization is performed for small local subsystems based on the Lieb-Robinson bound, which allows us to execute the cost function evaluation using small-scale quantum devices and/or classical computers. We demonstrate the validity of the LSVQC algorithm through numerical simulations of a simple spin-lattice model and an effective model of a parent compound of cuprate superconductors, ${\mathrm{Sr}}_2{\mathrm{CuO}}_3$, constructed by the ab initio downfolding method. It is shown that the LSVQC achieves a 95% reduction of the circuit depth for simulating quantum many-body dynamics compared to the Trotterization at best while maintaining the same computational accuracy. We also demonstrate that the restriction to a subspace leads to a substantial reduction of required resources and improved accuracy compared to the case of considering the entire Hilbert space. Furthermore, we estimate the gate count needed to execute the quantum simulations using the LSVQC on near-term quantum computing architectures in the noisy intermediate-scale or early fault-tolerant quantum computing era. Our estimation suggests that the acceptable physical gate error rate for the LSVQC can be about one order of magnitude larger than that for the Trotterization.

Entanglement fluctuations associated with the Schrödinger evolution of wave functions offer a unique perspective on various fundamental issues ranging from quantum thermalization to state preparation in quantum devices. Very recently, a subset of the present authors showed that, in a class of free-fermion lattice models and interacting spin chains, entanglement dynamics enters into a new regime at long time, with entanglement probes displaying persistent temporal fluctuations, whose statistics falls into the seemingly disparate paradigm of mesoscopic fluctuations in condensed-matter physics. This motivates us to revisit here entanglement dynamics of a canonical bosonic model in many-body physics, i.e., a coupled harmonic oscillator chain. We find that when the system is driven out of equilibrium, the long-time entanglement dynamics exhibits strictly the same statistical behaviors as those of free-fermion models. Specifically, irrespective of entanglement probes and microscopic parameters, the statistical distribution of entanglement fluctuations is flanked by asymmetric tails: sub-Gaussian for upward fluctuations and sub-Gamma for downward; moreover, the variance exhibits a crossover from the scaling $\sim 1/L$ to $\sim L_A^3/L^2$ as the subsystem size $L_A$ increases ($L$ the total system size). This insensitivity to the particle statistics, dubbed boson-fermion universality, is contrary to the common wisdom that statistical phenomena of a many-body nature depend strongly on particle statistics. Together with our previous work, the present study indicates rich fluctuation phenomena in long-time entanglement dynamics that await in-depth exploration.

Recent studies have highlighted the potential for achieving room-temperature superconductivity in the clathrate hydride prototype $A{B}_2{\mathrm{H}}_{24}$ with the $P6/mmm$ space group symmetry. To investigate the underlying superconducting mechanism for this class of frameworks, we performed comprehensive first-principles calculations by systematically substituting metal elements from groups I to IV of the periodic table. The results identify 21 dynamically stable compounds at 300 GPa, exhibiting an extraordinary variation in superconducting critical temperatures ($T_{\mathrm{c}}$), ranging from 276 K in $\mathrm{AcS}{\mathrm{c}}_2{\mathrm{H}}_{24}$ to 2 K in ${\mathrm{ThCe}}_2{\mathrm{H}}_{24}$. Through detailed structural analysis, we proposed two key model parameters, $S_{\text{couple}}$ and coupling strength (CS), to elucidate the strong correlation with superconducting properties (showing correlation coefficients of 0.87 and 0.93 with respect to $T_{\mathrm{c}}$). These model parameters are further demonstrated to be extendable for other clathrate-type hydrides, such as $\mathrm{L}{\mathrm{i}}_{2}M{\mathrm{H}}_{16/17}$ and binary superhydrides ${M\mathrm{H}}_6$, ${M\mathrm{H}}_9$ and ${M\mathrm{H}}_{10}$. The proposed CS shows strong predictive capability, achieving an overall correlation coefficient of 0.85 across different clathrate hydride systems. These findings provide insights into the fundamental mechanisms governing the superconducting properties of clathrate hydrides, offering useful guidance for future superconductor design.

We present a detailed quantum theory and simulations of few-photon Fock state pulses interacting with a two-level system (TLS) in an open waveguide-QED system. For a rectangular pulse shape, we present an exact temporal scattering theory to derive analytical expressions for the TLS population, using one-photon and two-photon pulses, for both chiral and symmetric emitters. We also derive the stationary (long-time) and time-dependent spectra for one-photon excitation, and show how these differ at a fundamental level when considering TLS population effects. Numerically, we also present matrix product state (MPS) simulations, which allow us to compute more general photon correlation functions for arbitrary quantum pulses, and we use this approach to also show results for Gaussian quantum pulses and to confirm the accuracy of our analytical solutions. In the case of a chiral TLS, we show how a one-photon pulse, of any temporal shape, yields a transmitted long-time spectrum that is identical to the input pulse, despite significant TLS population effects. However, the dynamical spectra and spectral intensity show rich population effects, allowing for their detection in spectroscopic experiments. We also show the striking differences between one-photon and two-photon excitation, where the latter shows clear signatures of nonlinear saturation effects. In addition, we demonstrate how significant population TLS dynamics also occur for pulses that are relatively long compared to the radiative decay time (showing that a weak excitation approximation, which neglects finite TLS population effects, cannot be made) and investigate the population signatures, nonlinear features and dynamical behavior as a function of quantum pulse length.

We have investigated the image potential states (IPSs) for the unreconstructed $\mathrm{Ir}\left(001\right)\text{−}(1\times 1)$ and the reconstructed $\mathrm{Ir}\left(001\right)\text{−}(5\times 1)$ surfaces by using two-photon photoemission spectroscopy. We reveal that the $\mathrm{Ir}\left(001\right)\text{−}(5\times 1)$ reconstructed surface covered by Xe adatoms exhibits a band gap of 100 meV, which arises from the new periodic potential created by the reconstruction, whereas no band gap was observed on the $\mathrm{Ir}\left(001\right)\text{−}(1\times 1)$ surface within the measured momentum range. The experimental results are compared with the theoretical ones obtained within density functional theory for both the clean and Xe-covered $\mathrm{Ir}\left(001\right)\text{−}(5\times 1)$ reconstructed surfaces. The planar averaged charge density distributions of the IPS for both the surfaces show that the Xe adsorption does not significantly alter the positions of charge density maxima, which rationalizes why the band gap arising from the substrate superlattice potential does not change significantly upon Xe adsorption. The agreement between the experimentally observed band gap for the IPS on the Xe-covered surface and the theoretical calculations highlights the robustness of IPS behavior under Xe adsorption.

Measurements can qualitatively alter correlations and entanglement emerging in gapless quantum matter. We show how a single round of measurements on gapless quantum systems can, upon rotating the measurement basis, induce nontrivial transitions separating regimes displaying universal characteristics governed by distinct boundary conformal field theories. We develop the theory of such “measurement-induced boundary transitions” by investigating a gapless parent of the one-dimensional cluster state, obtained by appropriately symmetrizing a commuting projector Hamiltonian for the latter. Projective measurements on the cluster state are known to convert the wave function, after post-selection or decoding, into a long-range-ordered Greenberger-Horne-Zeilinger (GHZ) state. Similar measurements applied to the gapless parent (i) generate long-range order coexisting with power-law correlations when post-selecting for uniform outcomes, and (ii) yield power-law correlations distinct from those in the pre-measurement state upon decoding. In the post-selection scenario, rotating the measurement basis preserves long-range order up until a critical tilt angle marking a measurement-induced boundary transition to a power-law-ordered regime. Such a transition—which does not exist in the descendant cluster state—establishes new connections between measurement effects on many-body states and nontrivial renormalization-group flows. We extend our analysis to tricritical Ising and three-state Potts critical theories, which also display measurement-induced boundary transitions, and propose general criteria for their existence in other settings.

We theoretically analyze the quasiparticle properties of stationary light polariton (SLP) modes in atoms via perturbative approaches of effective atom-light interaction Hamiltonian. First, we explicitly show the five SLP eigenmodes of the Hilbert space of the SLP system. Then, the quasiparticle properties, such as group velocities and effective masses of SLPs, are investigated under various parametric conditions, which include the emergence of negative effective mass and effective mass singularity. Finally, we derive the intrinsic lifetimes of each SLP mode, which features a complementary relation among the lifetimes of SLP modes. Our results offer quantitative analysis of the quasiparticle properties in Hilbert space spanned by the SLP modes, and it is expected to contribute to SLP-based atom-light interaction applications, such as photonic quantum gates, optical simulation of many-body physics, and exotic dispersion engineering.

We demonstrate that ultrasound diffusion—typically associated with the transport of average wave energy and the breaking of time-reversal symmetry—can nonetheless be revealed through a time-reversal experiment. This is achieved using an unprecedented configuration: A single piezoelectric transducer, acting as a time-reversal mirror (TRM), is buried deep inside a strongly scattering medium (a dense granular suspension), while an array of transducers is positioned at a distance, outside the scattering region. A short pulse is emitted by a single array element and the TRM records the resulting ultrasonic field, composed of a coherent ballistic wave followed by a diffuse coda wave. When the entire coda is time-reversed and re-emitted from the TRM, the wave refocuses at the original source with a focal spot size that decreases with the inverse of the TRM depth, consistent with diffusive transport. By time-reversing short coda segments at increasing times $t$, we observe a focal spot size scaling as $1/\sqrt{Dt}$, where $D$ is the ultrasound diffusion coefficient. Fitting this evolution with a microscopic diffusion model allows us to extract $D$. Remarkably, this measurement does not require ensemble averaging, because of the inherent stability of time-reversal against statistical fluctuations.

Quantum reservoir computing (QRC) harnesses quantum systems to tackle intricate computational problems with exceptional efficiency and minimized energy usage. This paper presents a QRC framework that utilizes a minimalistic quantum reservoir, consisting of only a few two-level atoms within an optical cavity. The system is inherently scalable, as newly added atoms automatically couple with the existing ones through the shared cavity field. We demonstrate that the quantum reservoir outperforms traditional classical reservoir computing in both memory retention and nonlinear data processing through two tasks, namely, the prediction of time-series data using the Mackey-Glass task and the classification of sine-square waveforms. Our results show significant performance improvements with an increasing number of atoms, facilitated by nondestructive, continuous quantum measurements and polynomial regression techniques. These findings confirm the potential of QRC as a practical and efficient solution to addressing complex computational challenges in quantum machine learning.

We report on the emergence of chimera states in extended systems characterized by the coexistence of more than two spatiotemporal solutions. Specifically, we show that three-domain chimeras may generically appear in reaction-diffusion systems under three conditions, with a crucial role played by the existence of a Maxwell point. We verify the universality of our findings by analyzing five different models, and demonstrate that such new chimeras are independent of the boundary conditions imposed.

The layered nature and simple structure of FeSe reveal this iron-based superconductor as a unique building block for the design of artificial heterostructure materials. While superconductivity develops in ultrathin films of FeSe on $\mathrm{SrTi}{\mathrm{O}}_3$ substrates, it remains unclear whether it can be developed on more chemically inert, layered materials such as graphene. Here, we report on the characterization of the structural, chemical, and electronic properties of few-layer and multilayer FeSe on bilayer graphene grown on SiC(0001) using low-temperature scanning tunneling microscopy/spectroscopy (STM/STS) and x-ray photoelectron spectroscopy (XPS). STM imaging of our FeSe films with thicknesses up to three layers exhibits the tetragonal crystal structure of bulk FeSe, which is supported by XPS spectra consistent with the FeSe bulk counterpart. While our STS measurements at 340 mK reveal a metallic character for few-layer FeSe on BLG/SiC(0001), they show an absence of superconductivity, as the low-lying electronic structure exhibits a spatially anisotropic dip-like feature robust against magnetic fields. However, we find that superconductivity in FeSe/BLG/SiC(0001) starts to emerge from a thickness of five layers. Cooper pairing strength increases with film thickness, reaching critical temperatures up to 6 K above 20 layers. Our results underscore the significance of the substrate and thickness as key factors driving the suppression of superconductivity in FeSe in the 2D limit.

Nonreciprocal interaction between two spatially separated subsystems plays a crucial role in signal processing and quantum networks. Here, we propose an efficient scheme to achieve nonreciprocal interaction and entanglement between two qubits by combining coherent and dissipative couplings in a superconducting platform, where two coherently coupled transmon qubits simultaneously interact with a transmission line waveguide. The coherent interaction between the transmon qubits can be achieved via capacitive coupling or via an intermediary cavity mode, while the dissipative interaction is induced by the transmission line via reservoir engineering. With high tunability of superconducting qubits, their positions along the transmission line can be adjusted to tune the dissipative coupling, enabling to tailor reciprocal and nonreciprocal interactions between the qubits. A unidirectional interaction can be achieved when the separation between the two qubits is $(4n+3){\lambda }_0/4$, where $n$ is an integer and ${\lambda }_0$ is the photon wavelength. This nonreciprocal interaction enables the generation of nonreciprocal entanglement between the two transmon qubits. Furthermore, applying a drive field to one of the qubits can stabilize the system into a nonreciprocal steady-state entangled state. Remarkably, the nonreciprocal interaction in this work does not rely on the presence of nonlinearity or any explicitly nonreciprocal elements, which has more potential applications in designing nonreciprocal quantum devices, processing quantum information, and building quantum networks.

We present a scheme for modeling quantum plasmas in the warm dense matter regime via a hybrid smoothed particle hydrodynamic–molecular dynamic treatment, here referred to as “Bohm SPH.” This treatment is founded upon Bohm's interpretation of quantum mechanics for partially degenerate fluids, does not apply the Born-Oppenheimer approximation, and is computationally tractable, capable of modeling dynamics over ionic timescales at electronic time resolution. Bohm SPH is also capable of modeling non-Gaussian electron wave functions. We present an overview of our methodology, validation tests of the single particle case including the hydrogen 1s wave function, and comparisons to simulations of a warm dense hydrogen system performed with wave packet molecular dynamics.