For electronic devices based on two-dimensional semiconductors (2DSs), few-layer transition-metal dichalcogenides offer enhanced electrical performance over their monolayer counterparts. Metal-semiconductor junctions (MSJs), which lie at the heart of such devices, incorporate not only a metal-semiconductor (M-S) interface but also multiple van der Waals semiconductor-semiconductor (S-S) interfaces when few-layer 2DSs are employed. However, the role of S-S interfaces within MSJs—particularly their interplay with the M-S interface and their potential device applications—remains poorly understood. Here, based on density-functional theory calculations, we conduct a systematic investigation of MSJs based on few-layer 2DSs ($\mathrm{Mo}{\mathrm{S}}_2$ and InSe) contacted with both two- and three-dimensional metals. We find layer-dependent band offsets between adjacent 2DS layers, which can change sign depending on the proximity to the metal contact. This spatial variation directly results in quantized band bending that deviates from classical models. These phenomena are attributed to the interplay and competition among three effects: (1) the decay of interface dipole with distance from the M-S interface, leading to layer-resolved band bending; (2) the interlayer quasibonding mixes up the energy levels between different 2DS layers, thereby affecting the band bending; and (3) screening by partially metallized 2DS layers within the junction, which can alter the direction of band bending. Leveraging these insights, we provide an understanding of the density-functional theory calculated Schottky barrier height values for few-layer 2DSs in MSJs. Additionally, we demonstrate that the interlayer band offset can suppress direct tunneling in vertical field-effect transistors based on 2DSs.

The evaluation of the Jones polynomial at roots of unity is a paradigmatic problem for quantum computers. In this work, we present experimental results obtained from existing noisy quantum computers for special cases of this problem, where it is classically tractable. Our approach relies on the reduction of the problem of evaluating the Jones polynomial of a knot at lattice roots of unity to the problem of computing quantum amplitudes of qudit stabilizer circuits, which are classically efficiently simulatable. More specifically, we focus on evaluation at the fourth root of unity, which is a lattice root of unity, where the problem reduces to evaluating amplitudes of qubit stabilizer circuits. To estimate the real and imaginary parts of the amplitudes up to additive error we use the Hadamard test, yielding non-Clifford circuits that nevertheless we can always efficiently compute the correct output of. Hence, we further argue that this setup defines a standard benchmark for near-term noisy quantum processors. Additionally, we study the benefit of performing quantum error mitigation with the method of zero-noise extrapolation.

Time-reversal symmetry breaking is an important facet of controlling nonreciprocal responses. Here, we propose a method of photocontrol over superconducting-diode-like nonreciprocities, where time-reversal symmetry breaking is achieved via photon exchange with chiral cavity modes. We reveal the origin of the nonreciprocal superconducting response as the embedding of chirality in a many-body ground state through photon-induced orbital magnetization. With twisted bilayer graphene as an example, we demonstrate the general principles of photocontrol of diode responses, which are valid for a wide range of superconductors and cavity designs. The cavity control of superconducting nonreciprocities, particularly in the microwave regime, offers a noninvasive means of exploring functionalities in quantum circuits with ultrafast switching and on-chip integration. This control method can serve as an important contribution to the toolbox for nonreciprocal models in circuit quantum electrodynamics, primed to be harnessed for scalable and modular quantum devices.

The recent discovery of inverse current in coupled transport (ICC) in classical systems [Wang et al., Phys. Rev. Lett. 124, 110607 (2020)]—where an induced current flows opposite to two mutually parallel thermodynamic forces, yet remains consistent with the second law of thermodynamics—reveals a striking and counterintuitive transport phenomenon. Using an exactly solvable model of strongly coupled quantum dots, we develop a thermodynamic framework to describe the ICC phenomenon at the quantum level. By systematically connecting the microscopic and macroscopic formulations of the entropy production rate in terms of appropriate entropic biases and entropic fluxes, our analysis identifies the conditions under which a genuine ICC effect can arise in quantum thermal transport and highlights potential applications in autonomous quantum engines and refrigerators.

Symbolization is a well-established tool to transform a time series into a sequence of symbols. Here, we compare two symbolization approaches: recurrence microstate (RM), based on recurrence patterns in phase space, and symbolic dynamics (SD), derived from ordinal binary sequences. Specifically, we compare the performance of two sets of features, based on RM and SD, for parameter estimation and classification. For parameter estimation, we generate synthetic data simulated from chaotic dynamical systems with a parameter that varies in a range that is divided into ten equal bins, and for classification, we use freely available electrocardiogram (ECG) recordings from patients with four different cardiac pathologies. We train a Random Forest (RF) classifier to infer to which parameter bin a synthetic time series belongs and to which pathology an ECG recording corresponds. We find that using RM features to train the RF algorithm consistently provides higher accuracy; however, SD features encapsulate complementary information that can improve the performance. Combining the two sets of features generally enhances the performance.

We carried out nonresonant inelastic x-ray scattering measurements at the V $L_1$ edge to probe the local V $3d$ charge density in ${\mathrm{VO}}_2$ across the metal-insulator transition (MIT). Without relying on spectral calculations, we were able to integrate directly from the orientational dependence an image of the occupied $3d$ shell and extract quantitatively the orbital occupations of the V ions. We found that in the low-temperature insulating $M_1$ phase the V is highly polarized toward the $\sigma$ configuration. The orbital occupation undergoes significant redistribution upon transition into the metallic $R$ phase, leading to a more isotropic charge density. The massiveness of the orbital switching promotes the scenario in which electron correlations and lattice degrees of freedom are strongly coupled, so that the contribution of the lattice and the electronic part to the energetics must be treated on equal footing to model the MIT quantitatively.

Optical orientation enables the injection of spin-polarized electron and hole populations in III-V and group-IV semiconductors. In particular, absorption of circularly polarized light near the direct gap generates spin-oriented electrons in the conduction band with polarization up to 50%, while hole polarization, opposite to that of electrons, can reach 83%. In recent years, increasing attention has focused on the orbital angular momentum, as charge-to-orbital conversion in semiconductors has been shown to be significantly more efficient than charge-to-spin. This has opened possibilities for exerting torques on ferromagnets in a semiconductive platform and is motivating research on the generation of carrier populations with net orbital angular momentum. In this framework, here, we exploit a “full-zone” $\mathbf{k}·\mathbf{p}$ model to theoretically investigate the injection of orbital angular momentum in bulk Ge and GaAs by means of the absorption of circularly polarized photons. We show that the orbital polarization of holes considerably exceeds 100% for photon energies close to the direct gap of the semiconductors. These results offer a route for generating high orbital polarizations and suggest that semiconductors are a convenient platform for future development of orbitronic and opto-orbitronic devices.

We study time-optimal state-to-state control for two- and multiqubit operations motivated by neutral-atom quantum processors within the Rydberg blockade regime. Block diagonalization of the Hamiltonian simplifies the dynamics and enables the application of a semianalytic approach to the Pontryagin   maximum principle   (PMP) to derive optimal laser controls. We provide a general formalism for $N$ qubits. For $N=2$ qubits, we classify normal and abnormal extremals, showcasing examples where abnormal solutions are either absent or suboptimal. For normal extremals, we establish a correspondence between the laser detuning from atomic transitions and the motion of a classical particle in a quartic potential, yielding a reduced, semianalytic formulation of the control problem. Combining PMP-based insights with numerical optimization, our approach bridges analytic and computational methods for high-fidelity, time-optimal control.

We propose the quasi Monte Carlo method for linear combination of unitaries via classical postprocessing (LCU-CPP) on quantum applications. The LCU-CPP framework has been proposed as an approach to reduce hardware resources, expressing a general target operator $F\left(A\right)$ as $F\left(A\right)={\int }_{V}f\left(\mathit{t}\right)G(A,\mathit{t})d\mathit{t}$, where each $G(A,\mathit{t})$ is proportional to a unitary operator. On a quantum device, $\mathrm{Re}\left[\mathrm{Tr}\right(G(A,\mathit{t})\rho \left)\right]$ can be estimated using the Hadamard test and then combined through classical integration, allowing for the realization of nonunitary functions with reduced circuit depth. While previous studies have employed the Monte Carlo method or the trapezoid rule to evaluate the integral in LCU-CPP, we show that the quasi Monte Carlo method can achieve even lower errors. In two numerical experiments, ground-state property estimation and Green’s function estimation, the quasi Monte Carlo method achieves the lowest errors with a number of Hadamard test shots per unitary that is practical for real hardware implementations. These results indicate that quasi Monte Carlo is an effective integration strategy within the LCU-CPP framework.

Combinatorial optimization problems have wide-ranging applications in industry and academia. Quantum computers may help solve them by sampling from appropriately prepared Ansatz quantum circuits. However, current quantum computers are limited by their qubit count, connectivity, and noise. This is particularly restrictive when considering optimization problems beyond the quadratic order. Here, we introduce Ansätze based on an approximate quadratization of high-order Hamiltonians that do not incur a qubit overhead. The price paid is a loss in the quality of the noiseless solution. Crucially, these approximations yield shallower Ansätze that are more robust to noise than the standard Quantum Approximate Optimization Algorithm one. We show this through simulations with variable noise strengths. Furthermore, we also propose a noise-aware Ansatz design method for quadratic optimization problems. This method implements only a portion of the target Hamiltonian by limiting the number of layers of swap gates in the Ansatz. We find that for both problem types, under noise, our approximate implementation of the full problem structure can significantly enhance the solution quality. Our work opens a path to enhance the solution quality that approximate quantum optimization achieves on noisy hardware.

The continuous-variable (CV) Gaussian no-go theorem fundamentally limits the suppression of Gaussian displacement errors using only Gaussian gates and states. Prior studies have employed Gottesman-Kitaev-Preskill (GKP) states as ancillary qumodes to suppress small Gaussian displacement errors. However, when the displacement magnitude becomes large, inevitable lattice-crossing errors arise beyond the correctable range of the GKP state. To address this issue, we concatenate the Gaussian-noise-suppression circuit with an outer analog Steane code that corrects such occasional lattice-crossing events as well as other abrupt displacement errors. Contrary to conventional concatenation, which primarily aims to reduce logical error rates, the Steane-GKP duality in encoding provides complementary protection against displacement errors at different scales: The inner GKP layer employs non-Gaussian resources to suppress continuous Gaussian noise and reduce residual variance, while the outer analog Steane code corrects discrete lattice-crossing events that exceed the GKP correctable range. It is precisely this separation of error-mitigation roles that enables CV error correction. In contrast to prior work on concatenating GKP and repetition codes to establish error correction for discrete qubit/qudit encoding, we provide correction in the continuous encoding space. Analytical studies show that, under infinite squeezing, the concatenated code suppresses the variance of Gaussian displacement errors acting on all qumodes by up to 50%, while enabling unbiased correction of lattice-crossing errors with a success probability determined by the ratio between the residual Gaussian error standard deviation and the lattice-crossing magnitude. Even with finite squeezing, the proposed architecture still provides Gaussian-error suppression and lattice-crossing correction. Moreover, the presence of the outer analog Steane code relaxes the squeezing requirement of the inner GKP states, indicating near-term experimental feasibility. This work establishes a viable route toward fault-tolerant continuous-variable quantum computation and provides insight into the design of concatenated CV error-correcting architectures.

We present the phenomenology of the one-dimensional nonreciprocal Cahn-Hilliard model for varying nonreciprocity $\left(\alpha \right)$ and different boundary conditions. At small $\alpha$, a perturbed uniform state evolves to a defect-laden configuration that lacks global polar order. Defects are the sources and sinks of traveling waves. For a given $\alpha$, defects with a unique wave number that increases monotonically with $\alpha$ are selected. A critical threshold ${\alpha }_c$ marks the onset of a transition to states with finite global polar order. For periodic boundary conditions, above ${\alpha }_c$, the system shows traveling waves that are completely ordered. In contrast, traveling waves are incompatible with the Neumann and Dirichlet boundary conditions. Instead, for $\alpha \gtrsim {\alpha }_c$, we find fluctuating domains that show intermittent polar order, and at large $\alpha$, the system partitions into two domains with opposite polar order.

Many driven systems alternate between bursts of activity and quiescence and can become trapped in an absorbing state, such as complete inactivity in reaction-diffusion processes or extinction in predator-prey dynamics. It is generally assumed that, conditioned on survival, their long-lived (quasistationary) behavior is unique and independent of the initial condition. We show that this need not hold, even for memoryless Markov dynamics. When the configuration space fractures into multiple macroscopic communicating classes, where configurations can be reached from one another within a class but not across classes, the system retains a measurable memory of its preparation, which can directly affect the critical exponents near absorbing transitions. Using a minimal birth-death-diffusion model, we demonstrate that the quasistationary state is unique when birth processes are present, but becomes nonunique and initial-condition dependent when they are suppressed. This mechanism, arising from vanishing of interclass escape-rate ratios in thermodynamic limit, challenges the conventional universality hypothesis and suggests possibility of history-dependent critical scaling in controlled lattice or colloidal systems with tunable particle number.

Recently, there is interest in coincident “sharpening” and “learnability” transitions in monitored quantum systems. In the latter, an outside observer's ability to infer properties of a quantum system from measurements undergoes a phase transition. Such transitions appear to be related to the decodability transition in quantum error correction, but the precise connection is not clear. Here, we study these problems under one framework we call the general quantum inference problem. In cases as above where the system has a Markov structure, we say that the inference is on a quantum hidden Markov model. We show a formal connection to classical hidden Markov models and that they coincide for certain setups. For example, we prove this for those involving Haar-random unitaries and measurements. We introduce the notion of Bayes nonoptimality, where parameters used for inference differs from true ones. This allows us to expand the phase diagrams of above models. At Bayes optimality, we obtain an explicit relation between “sharpening” and “learnability” order parameters, explicitly showing that the two transitions coincide. Next, we study concrete examples. We review quantum error correction on the toric and repetition code and their mapping to two-dimensional (2D) random-bond Ising model (RBIM) through our framework. We study the Haar-random $\mathrm{U}\left(1\right)$-symmetric monitored quantum circuit and tree, mapping each to inference models that we call the planted simple symmetric exclusion process and planted xor, respectively, and expanding the phase diagram to Bayes nonoptimality. For the circuit, we deduce the phase boundary numerically and analytically argue that it is of a single universality class. For the tree, we present an exact solution of the entire phase boundary, which displays re-entrance as does the 2D RBIM. We discuss these phase diagrams, with their interpretations for quantum inference problems and rigorous arguments on their shapes.

We present a density-functional-theory (DFT) study of the effects of oxygen vacancies on the structural and electronic properties of vanadium dioxide $\left({\mathrm{VO}}_2\right)$. Our motivation is the reported suppression of the metal-insulator transition by oxygen vacancies and the lack of a clear consensus on its origin. We use the $\mathrm{DFT}+V$ method with a static intersite vanadium-vanadium interaction term, $V$, to calculate the properties of the oxygen-deficient metallic rutile and insulating monoclinic M1 phases of ${\mathrm{VO}}_2$ on the same footing. We find that oxygen vacancies induce local distortions in the M1 phase, but do not destroy the dimerization usually associated with the insulating behavior. In spite of this, we find that the M1 phase becomes metallic as a result of the partial filling of the conduction band due to a rigid-band-like doping effect.

We investigate one- and two-mode variants of the ${\mathbb{Z}}_3$-symmetric quantum Rabi model, which describe the interaction of a qutrit with one or two bosonic modes and are directly relevant for circuit-QED and spin-qudit platforms. We find a canonical transformation that allows one to obtain the spectrum of the ${\mathbb{Z}}_3$ Rabi model using perturbation theory in a magnetic field. We show that in a certain parameter regime (deep-strong coupling and a small magnetic field), the three lowest eigenstates become ${\mathbb{Z}}_3$ qutrit-boson cat states. In order to characterize these states, we introduce a joint qutrit-boson Wigner function and derive its closed-form expression for the qutrit-boson cat states. Numerical calculations across a wide range of coupling strengths show that the proposed Wigner function is a useful tool that allows one to unambiguously identify the states of the ${\mathbb{Z}}_3$ qutrit-boson cat states.

We report on a quantum state characterization method, which we call Rabi-based quantum state tomography (RQST), that we have validated on single-qubit quantum states, in particular on the electron and nuclear spins of a single nitrogen-vacancy (NV) center in diamond, demonstrating high fidelities. The difference of RQST with conventional tomography methods is in the implementation of rotation operators and construction of the density matrix from the measured datasets. We demonstrate efficient quantum state control of the electron spin at room temperature with an average fidelity of 0.995 over more than 40 measurements on different states on the Bloch sphere. Also, we apply the methodology to the dark NV nuclear spin state. The state is read via the electron spin using the C-NOT two-qubit entanglement gate and demonstrates fidelities of the same order.