Epitaxial strain has been shown to drive structural phase transitions along with novel functionalities in perovskite-based thin films. Aliovalent doping at the $A$ site can drive an insulator-to-metal and magnetic transitions in perovskites along with a variety of interesting structural and electronic phenomena. Using first-principles calculations, we predict the formation of a multiferroic half-metallic phase with a large magnetic moment in the double perovskite, ${\mathrm{Bi}}_2{\mathrm{FeCrO}}_6$, by coupling epitaxial strain with $A$-site hole doping. We also demonstrate that epitaxial strain can be used to manipulate the hole states created by doping to induce half-metal to insulator, antipolar to polar, antiferromagnetic to ferromagnetic, orbital ordering and charge ordering transitions. Our work also suggests that hole doping under strain could lead to mitigation of issues related to antisite defects and lowered magnetization in thin films of the material.

We demonstrate time-resolved nonlinear extreme-ultraviolet absorption spectroscopy on multiply charged ions, here applied to the doubly charged neon ion, driven by a phase-locked sequence of two intense free-electron laser pulses. Absorption signatures of resonance lines due to $2p–3d$ bound–bound transitions between the spin-orbit multiplets ${{}^{3}P}_{0,1,2}$ and ${{}^{3}D}_{1,2,3}$ of the transiently produced doubly charged ${\mathrm{Ne}}^{2+}$ ion are revealed, with time-dependent spectral changes over a time-delay range of $(2.4\pm 0.3)\mathrm{fs}$. Furthermore, we observe 10-meV-scale spectral shifts of these resonances owing to the ac Stark effect. We use a time-dependent quantum model to explain the observations by an enhanced coupling of the ionic quantum states with the partially coherent free-electron laser radiation when the phase-locked pump and probe pulses precisely overlap in time.

Motivated by the novel insulating state of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ from strong spin-orbit coupling (SOC), we investigate, by means of angle resolved photoemission, the metal-insulator transition (MIT) mechanism in ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{RhO}}_4$ whose mother compound is isovalent and isostructural but has smaller SOC strength compared to ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$. Transport and angle resolved photoemission results from single crystalline ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{RhO}}_4$ revealed that the MIT occurs coincidentally with a multi- to single-band transition (Lifshitz transition) at $x=0.4$. Starting from $x=0.4$, there is a gradual but anomalous enhancement in the band gap size with additional electron doping, suggesting that the insulating phase in ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{RhO}}_4$ is a new type which has been rarely investigated. These results suggest that the insulating phase in ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{RhO}}_4$ is likely induced by the moderate SOC strength and electron doping effect from the La. Our findings not only elucidate the MIT mechanism in ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{RhO}}_4$, but may also open new avenues for novel MIT research in moderate SOC regimes.

We consider granular quantum matter defined by Sachdev-Ye-Kitaev dots coupled via random one-body hopping. Within the framework of Schwarzian field theory, we identify a zero-temperature quantum phase transition between an insulating phase at weak and a metallic phase at strong hopping. The critical hopping strength scales inversely with the number of degrees of freedom on the dots. The increase of temperature out of either phase induces a crossover into a regime of strange metallic behavior.

In the absence of time-reversal symmetry, viscous electron flow hosts a number of interesting phenomena, of which we focus here on the Hall viscosity. Taking a step beyond the hydrodynamic definition of the Hall viscosity, we derive a generalized relation between the Hall viscosity and the transverse electric field using a kinetic equation approach. We explore two different geometries where the Hall viscosity is accessible to measurement. For hydrodynamic flow of electrons in a narrow channel, we find that the viscosity may be measured by a local probe of the transverse electric field near the center of the channel. Ballistic flow, on the other hand, is dominated by boundary effects. In a Corbino geometry, viscous effects arise not from boundary friction but from the circular flow pattern of the Hall current. In this geometry, we introduce a viscous Hall angle that remains well defined throughout the crossover from ballistic to hydrodynamic flow and captures the bulk viscous response of the fluid.

Macromolecules contain molecular units as the coding information for their correlated structures in physical dimensions. The relationship between these two features is governed by the interaction energies of the involved molecular units and their encoded sequences. We present a neural network algorithm that treats molecular units themselves as neural networks, which has the flexibility to allow each unit to respond to its own environment and to influence others in the system. Through a deep neural network and a self-consistent procedure, molecular units in the network establish a strong correlation to produce the desirable features in the physical world. The proposed framework is applied to the HP model. Both the forward problem of predicting folded structures from given sequences and the inverse problem of predicting required sequences for a given structure are examined.

We study the emergence of critical dynamics in the steady shear rheology of fluidized soft glassy materials. Within a mesoscale elastoplastic model accounting for a shear band instability, we show how additional noise can induce a transition from a phase separated to homogeneous flow, accompanied by critical-like fluctuations of the macroscopic shear rate. Both macroscopic quantities and fluctuations exhibit power law behaviors in the vicinity of this transition, consistent with previous experimental findings on vibrated granular media. Altogether, our results suggest a generic scenario for the emergence of criticality when shear weakening mechanisms compete with a fluidizing noise.

Nonadiabatic holonomic quantum computation (NHQC) has been developed to shorten the construction times of geometric quantum gates. However, previous NHQC gates require the driving Hamiltonian to satisfy a set of rather restrictive conditions, reducing the robustness of the resulting geometric gates against control errors. Here we show that nonadiabatic geometric gates can be constructed in an extensible way, called $\mathrm{NHQC}+$, for maintaining both flexibility and robustness against certain types of noises. Consequently, this approach makes it possible to incorporate most of the existing optimal control methods, such as dynamical decoupling, composite pulses, and a shortcut to adiabaticity, into the construction of single-looped geometric gates. Furthermore, this extensible approach of geometric quantum computation can be applied to various physical platforms such as superconducting qubits and nitrogen-vacancy centers. Specifically, we performed numerical simulation to show how the noise robustness in recent experimental implementations [Phys. Rev. Lett. 119, 140503 (2017); Nat. Photonics 11, 309 (2017)] can be significantly improved by our $\mathrm{NHQC}+$.approach. These results cover a large class of new techniques combing the noise robustness of both geometric phase and optimal control theory.

Charge and energy transfer in biological and synthetic organic materials are strongly influenced by the coupling of electronic states to a highly structured dissipative environment. Nonperturbative simulations of these systems require a substantial computational effort, and current methods can only be applied to large systems if environmental structures are severely coarse grained. Time evolution methods based on tensor networks are fundamentally limited by the times that can be reached due to the buildup of entanglement in time, which quickly increases the size of the tensor representation, i.e., the bond dimension. In this Letter, we introduce a dissipation-assisted matrix product factorization (DAMPF) method that combines a tensor network representation of the vibronic state within a pseudomode description of the environment where a continuous bosonic environment is mapped into a few harmonic oscillators under Lindblad damping. This framework is particularly suitable for a tensor network representation, since damping suppresses the entanglement growth among oscillators and significantly reduces the bond dimension required to achieve a desired accuracy. We show that dissipation removes the “time-wall” limitation of existing methods, enabling the long-time simulation of large vibronic systems consisting of 10–50 sites coupled to 100–1000 underdamped modes in total and for a wide range of parameter regimes. For these reasons, we believe that our formalism will facilitate the investigation of spatially extended systems with applications to quantum biology, organic photovoltaics, and quantum thermodynamics.

Quantum computing has seen tremendous progress in past years. Due to implementation complexity and cost, the future path of quantum computation is strongly believed to delegate computational tasks to powerful quantum servers on the cloud. Universal blind quantum computing (UBQC) provides the protocol for the secure delegation of arbitrary quantum computations, and it has received significant attention. However, a great challenge in UBQC is how to transmit a quantum state over a long distance securely and reliably. Here, we solve this challenge by proposing a resource-efficient remote blind qubit preparation (RBQP) protocol, with weak coherent pulses for the client to produce, using a compact and low-cost laser. We experimentally verify a key step of RBQP—quantum nondemolition measurement—in the field test over 100 km of fiber. Our experiment uses a quantum teleportation setup in the telecom wavelength and generates 1000 secure qubits with an average fidelity of $(86.9\pm 1.5)\%$, which exceeds the quantum no-cloning fidelity of equatorial qubit states. The results prove the feasibility of UBQC over long distances, and thus serves as a key milestone towards secure cloud quantum computing.

We study the effect of a linear tunneling coupling between two-dimensional systems, each separately exhibiting the topological Berezinskii-Kosterlitz-Thouless (BKT) transition. In the uncoupled limit, there are two phases: one where the one-body correlation functions are algebraically decaying and the other with exponential decay. When the linear coupling is turned on, a third BKT-paired phase emerges, in which one-body correlations are exponentially decaying, while two-body correlation functions exhibit power-law decay. We perform numerical simulations in the paradigmatic case of two coupled $XY$ models at finite temperature, finding evidences that for any finite value of the interlayer coupling, the BKT-paired phase is present. We provide a picture of the phase diagram using a renormalization group approach.

Cold samples of calcium atoms are prepared in the metastable ${}^3{P}_1$ state inside an optical cavity resonant with the narrow band (375 Hz) ${{}^{1}S}_0\to {{}^{3}P}_1$ intercombination line at 657 nm. We observe a superradiant emission of hyperbolic secant shaped pulses into the cavity with an intensity proportional to the square of the atom number, a duration much shorter than the natural lifetime of the ${}^3{P}_1$ state, and a delay time fluctuating from shot to shot in excellent agreement with theoretical predictions. Our incoherent pumping scheme to produce inversion on the ${{}^{1}S}_0\to {{}^{3}P}_1$ transition should be extendable to allow for continuous wave laser operation.

This Letter proposes a realistic implementation of the curved relativistic mirror concept to reach unprecedented light intensities in experiments. The scheme is based on relativistic plasma mirrors that are optically curved by laser radiation pressure. Its validity is supported by cutting-edge three-dimensional particle-in-cell simulations and a theoretical model, which show that intensities above ${10}^{25}\mathrm{W}{\mathrm{cm}}^{-2}$ could be reached with a 3 PetaWatt (PW) laser. Its very high robustness to laser and plasma imperfections is shown to surpass all previous schemes and should enable its implementation on existing PW laser facilities.

Recent theoretical investigations [A. B. Belonoshko et al. Nat. Geosci. 10, 312 (2017)] revealed the occurrence of the concerted migration of several atoms in bcc Fe at inner-core temperatures and pressures. Here, we combine first-principles and semiempirical atomistic simulations to show that a diffusion mechanism analogous to the one predicted for bcc iron at extreme conditions is also operative and of relevance for the high-temperature bcc phase of pure Ti at ambient pressure. The mechanism entails a rapid collective movement of numerous (from two to dozens) neighbors along tangled closed-loop paths in defect-free crystal regions. We argue that this phenomenon closely resembles the diffusion behavior of superionics and liquid metals. Furthermore, we suggest that concerted migration is the atomistic manifestation of vanishingly small $\omega$-mode phonon frequencies previously detected via neutron scattering and the mechanism underlying anomalously large and markedly non-Arrhenius self-diffusivities characteristic of bcc Ti.

State-to-state scattering dynamics of ${\mathrm{H}}_2\mathrm{O}$ from Cu(111) has been studied by a fully coupled quantum mechanical model which explicitly accounts for the most important molecular degrees of freedom, based on a first-principles determined potential energy surface. When ${\mathrm{H}}_2\mathrm{O}$ in its antisymmetric O-H stretching vibration (${\nu }_3$) collides with the surface, we find that the intramolecular vibrational energy redistribution (IVR) is predominant from ${\nu }_3$ to the symmetric stretching mode (${\nu }_1$), while rather inefficient to the bending mode (${\nu }_2$). This mode-specific IVR results from the strong couplings between stretching modes, which equally dispose the initial energy in ${\nu }_3$ into local stretching modes differing by a phase factor. Given the number of stretching modes in ${\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{CH}}_4$, this mechanism naturally explains why the product ${\nu }_1/{\nu }_3$ ratio calculated here is $\sim 3$ times over that recently measured for ${\mathrm{CH}}_4$ scattering on Ni(111), suggesting that the nonstatistical IVR is more general than expected in polyatomic molecules.