Inference is the task of drawing conclusions about unobserved variables given observations of related variables. Applications range from identifying diseases from symptoms to classifying economic regimes from price movements. Unfortunately, performing exact inference is intractable in general. One alternative is variational inference, where a candidate probability distribution is optimized to approximate the posterior distribution over unobserved variables. For good approximations, a flexible and highly expressive candidate distribution is desirable. In this work, we use quantum Born machines as variational distributions over discrete variables. We apply the framework of operator variational inference to achieve this goal. In particular, we adopt two specific realizations: one with an adversarial objective and one based on the kernelized Stein discrepancy. We demonstrate the approach numerically using examples of Bayesian networks, and implement an experiment on an IBM quantum computer. Our techniques enable efficient variational inference with distributions beyond those that are efficiently representable on a classical computer.

We report the realization of fully compensated synthetic antiferromagnets (SAFs) in $\mathrm{Pt}/\mathrm{Co}/\mathrm{Ru}/{\mathrm{Fe}}_{1-x}{\mathrm{Tb}}_x$ multilayers with perpendicular magnetic anisotropy, pronounced anomalous Hall resistances and magneto-optical responses. In particular, the properties of SAFs are systematically investigated through optimizing the thicknesses of the $\mathrm{Co}$ layer, the $\mathrm{Ru}$ spacer, and the concentration (x) of the ferrimagnet (FIM) ${\mathrm{Fe}}_{1-x}{\mathrm{Tb}}_x$ layers. The incorporation of the FIM ${\mathrm{Fe}}_{1-x}{\mathrm{Tb}}_x$ films into SAFs accelerates the search for a magnetic multilayer with fully compensated magnetism and exhibiting pronounced anomalous Hall resistance and magneto-optical signals. These advantages enable spin-orbit-torque switching and magneto-optical Kerr effect imaging experiments on the fully compensated SAFs to be readily investigated. Through incorporating FIMs into SAFs, we believe that the present material system could provide a promising platform not only for adequately addressing the fully compensated spin dynamics of SAFs at room temperature, but also for developing intriguing SAF-based spin-orbitronic devices.

We use balanced optical low-coherence interferometry to measure the ultralow light power back-reflected and/or back-scattered by optical interfaces. Indeed, backscattered light from optical surfaces can be a critical source of noise in interferometric gravitational-wave detectors, such as the Laser Interferometer Gravitational-Wave Observatory (LIGO), Virgo, or the Laser Interferometer Space Antenna (LISA), and therefore, it is required to characterize, with high sensitivity, the scattering properties of elementary components involved in their optical design. The use of a broadband light source makes an easy identification of the scattering interface possible, while the implementation of balanced interferometric detection allows very low light levels to be detected, thanks to the simultaneous use of coherent gain and source intensity noise suppression. Experimental results are reported, first on bare glass optical windows (N-BK7 and S-LAH66) and then on a silver-coated mirror, that demonstrate the ability of this setup to quantify specular- or diffuse-reflectance coefficients as low as ${10}^{-11}$.

It is demonstrated that a system of two interfacing hyperbolic metamaterials may be used for direct subwavelength imaging in the visible range. The proposed configuration consists of two anisotropic photonic crystals: a multilayered metal-dielectric structure and an adjoined wired structure. The results of analytical calculations and numerical modeling show that the light emitted by a dipole placed on the top of the multilayered structure propagates within a narrow cone along the direction $\mathbf{n}$, where $\mathrm{Re}\epsilon (\omega ,\mathbf{n})=0$. Owing to impedance matching and negative refraction at the interface, a bright image with a maximum transverse size of approximately $50$ nm is focused in the wired structure at the wavelength $\lambda =670$ nm.

Emergent behaviors in antiferroelectric thin films due to a coupling between surface electrochemistry and intrinsic polar instabilities are explored within the framework of the modified 2-4-6 Landau-Ginzburg-Devonshire (LGD) thermodynamic approach. By using phenomenological parameters of the LGD potential for a bulk antiferroelectric and a Stephenson-Highland (SH) approach, we study the role of surface ions with a charge density proportional to the relative partial oxygen pressure on the dipole states and their reversal mechanisms in antiferroelectric thin films. The combined LGDSH approach allows the boundaries of antiferroelectric, ferroelectriclike antiferroionic, and electretlike paraelectric states as a function of temperature, oxygen pressure, surface-ion formation energy and concentration, and film thickness to be delineated. This approach also allows the characterization of the polar and antipolar orderings dependence on the voltage applied to the antiferroelectric film, as well as the analysis of their static and dynamic hysteresis loops. The applications of the antiferroelectric films covered with a surface-ion layer for energy and information storage are explored and discussed.

The stability of high-end piezoacoustic drop-on-demand (DOD) inkjet printing is sometimes compromised by the entrainment of an air bubble inside the ink channel. Here, bubble pinch-off from an oscillating meniscus is studied in an optically transparent DOD printhead as a function of the driving waveform. We show that bubble pinch-off follows from low-amplitude high-frequency meniscus oscillations on top of the global high-amplitude low-frequency meniscus motion that drives droplet formation. In a certain window of control parameters, phase inversion between the low- and high-frequency components leads to the enclosure of an air cavity and bubble pinch-off. Although phenomenologically similar, bubble pinch-off is not a result of capillary-wave interaction such as observed in drop impact on a liquid pool. Instead, we reveal geometrical-flow focusing as the mechanism through which, at first, an outward jet is formed on the retracted concave meniscus. The subsequent high-frequency velocity oscillation acts on the now toroidal-shaped meniscus and it accelerates the toroidal ring outward, resulting in the formation of an air cavity that can pinch off. Through incompressible boundary-integral simulations, we reveal that bubble pinch-off requires an unbalance between the capillary and inertial time scales and that it does not require acoustics. The critical control parameters for pinch-off are the pulse timing and amplitude. To cure the bubble entrainment problem, the threshold for bubble pinch-off can be increased by suppressing the high-frequency driving through appropriate waveform design. The present work therefore aids the improvement of the stability of inkjet printers through a physical understanding of meniscus instabilities.

Microwave delivery to samples in a cryogenic environment can pose experimental challenges such as restricting optical access, space constraints, and heat generation. Moreover, existing solutions that overcome various experimental restrictions do not necessarily provide a large homogeneous oscillating magnetic field over macroscopic length scales, which is required for control of spin ensembles or fast gate operations in scaled-up quantum computing implementations. Here, we show fast and coherent control of a negatively charged nitrogen-vacancy spin ensemble by taking advantage of the high permittivity of a ${\mathrm{K}\mathrm{Ta}\mathrm{O}}_3$ dielectric resonator at cryogenic temperatures. We achieve Rabi frequencies of up to 48 MHz, with a total power-to-field conversion factor $C_{\mathrm{P}}=9.66\mathrm{mT}/\sqrt{\mathrm{W}}$ (approximately $191{\mathrm{MHz}}_{\mathrm{Rabi}}/\sqrt{\mathrm{W}}$). We use the nitrogen-vacancy-center spin ensemble to probe the quality factor, the coherent enhancement, and the spatial distribution of the magnetic field inside the diamond sample. The key advantages of the dielectric resonator utilized in this work are ease of assembly, in situ tunability, a high magnetic field conversion efficiency, a low-volume footprint, and optical transparency. This makes ${\mathrm{K}\mathrm{Ta}\mathrm{O}}_3$ dielectric resonators a promising platform for the delivery of microwave fields for the control of spins in various materials at cryogenic temperatures.

A continual change in resistance plays an important role in simulating the biological synapses and an abrupt switching mode helps to store information with fast speed and low-power operation. The manipulation of dual-mode switching and comprehension of switching mechanisms, which are the key to developing multifunctional resistance random access memory devices, has made little progress due to the complexity of the influencing factors. We observe that both gradual and abrupt reset behaviors exist in the $\mathrm{Ta}/{\mathrm{Ta}}_2{\mathrm{O}}_5/\mathrm{In}\text{−}\mathrm{Sn}\text{−}\mathrm{O}$ (ITO) structure, and the utilization of various compliance currents ($I_{\mathrm{CC}}$) can control the reset processes of the device. The difference in switching behavior is a result of the different compositions of the conductive filaments (CFs). $\mathrm{Ta}$ and oxygen-vacancy dual filaments contribute to resistive switching at low $I_{\mathrm{CC}}$, whereas the change in valence states of ${\mathrm{Ta}\mathrm{O}}_x$ dominates the formation and rupture of CFs at high $I_{\mathrm{CC}}$. The introduction of a thin ${\mathrm{Mo}\mathrm{S}}_2$ intermediate layer also leads to a transition between abrupt- and gradual-switching modes. The $\mathrm{Ta}/{\mathrm{Mo}\mathrm{S}}_2/{\mathrm{Ta}}_2{\mathrm{O}}_5/\mathrm{ITO}$ device exhibits digital switching behavior with ultralow power consumption. Moreover, the $\mathrm{Ta}/{\mathrm{Ta}}_2{\mathrm{O}}_5/{\mathrm{Mo}\mathrm{S}}_2/\mathrm{ITO}$ device with an analog reset process can be applied to realize synaptic functions.

High-performance artificial synaptic devices are key building blocks for developing efficient neuromorphic computing systems. However, the nonlinear and asymmetric weight update of existing devices has restricted their practical applications. Herein, floating gate nonvolatile memory (FG NVM) devices based on two-dimensional (2D) ${\mathrm{Hf}\mathrm{S}}_2$/$h$-BN/FG-graphene heterostructures have been designed and investigated as multifunctional NVM and artificial optoelectronic synapses. Benefiting from the FG architecture, the ${\mathrm{Hf}\mathrm{S}}_2$-based NVM device exhibits competitive performances, such as a high on:off ratio ($>{10}^5$), large memory window (approximately 100 V), excellent charge retention ability ($>{10}^4\mathrm{s}$), and robust durability ($>{10}^3$ cycles). Notably, the artificial optoelectronic synapses based on ${\mathrm{Hf}\mathrm{S}}_2$ FG NVM show an impressive large conductance margin and good linearity, owing to the ultrahigh photoresponsivity and photogain of ${\mathrm{Hf}\mathrm{S}}_2$. The energy consumption of per spike in our artificial synapse is as low as 0.2 pJ. Therefore, a high recognition accuracy up to 91.5% of the artificial neural network on the basis of our ${\mathrm{Hf}\mathrm{S}}_2$-based optoelectronic synapse at the system level has been achieved, which is superior to other reported 2D artificial optoelectronic synapses. This work paves the way forward for all 2D material-based memory for developing efficient optogenetics-inspired neuromorphic computing in brain-inspired intelligent systems.

${\mathrm{Hf}\mathrm{O}}_2$-based films have attracted much attention due to their robust ferroelectric properties at the nanometer scale and their great potential for data storage. The interface between the substrate and the ${\mathrm{Hf}\mathrm{O}}_2$-based film has a significant effect on its ferroelectric behavior. A seed layer between the films and substrates, such as a ${\mathrm{Zr}\mathrm{O}}_2$ seed layer, is proved to be an effective way to realize the beneficial interface effect. However, the interfacial tuning mechanism for the ferroelectric properties of ${\mathrm{Hf}\mathrm{O}}_2$-based thin films brought by the substrate or seed layer remains unclear so far. By performing first-principles calculations, the phase stability and crystalline orientation of ${\mathrm{Hf}\mathrm{O}}_2$ thin films on ${\mathrm{Zr}\mathrm{O}}_2$ seed layers are systematically studied. Results indicate that the ${\mathrm{Zr}\mathrm{O}}_2$ seed layer, especially the [111]-oriented one, substantially stabilizes the orthorhombic ferroelectric phase and the high-symmetry tetragonal phase of the ${\mathrm{Hf}\mathrm{O}}_2$ thin film compared with the most stable monoclinic phase. Furthermore, the polarization magnitude of the ${\mathrm{Hf}\mathrm{O}}_2$ film on the ${\mathrm{Zr}\mathrm{O}}_2$ seed layer is in good agreement with experiment. The [100]-oriented ${\mathrm{Zr}\mathrm{O}}_2$ seed layer also has a great effect on the stability of the orthorhombic ferroelectric phase of the ${\mathrm{Hf}\mathrm{O}}_2$ thin film. These results are meaningful for the understanding of the interfacial effects of the ferroelectricity of ${\mathrm{Hf}\mathrm{O}}_2$-based thin films and the development of ferroelectric devices.

Achieving strong repulsive Casimir forces through engineered coatings can pave the way for micro- and nanoelectromechanical applications where adhesive forces currently cause reliability issues. Here, we exploit Lifshitz theory to identify the requirements for repulsive Casimir forces in gyrotropic media for two limiting cases (ultrastrong gyroelectric and nongyroelectric). We show that the origin of repulsive force in media with strong gyrotropy such as Weyl semimetals arises from the giant interconversion of polarization of vacuum fluctuations.

We show that a self-organized quantum dot (QD) embedded in a quantum well (QW) can provide better carrier localization compared to the bare QD due to a complex action of three processes, namely, an increase in the QD volume, a change of the QD aspect ratio, and strain redistribution provided by the QW. For an $\mathrm{In}\mathrm{As}$ QD and an ($\mathrm{In}$,$\mathrm{Ga}$)As QW, we calculate the possible energy benefits of these three contributions and examine the optimal dot-in-well configuration sustaining the interface coherency.

We report on a robust method to achieve strong coupling between a superconducting flux qubit and a high-quality quarter-wavelength coplanar waveguide resonator. We demonstrate the progression from the strong to ultrastrong coupling regime by varying the length of a shared inductive coupling element, ultimately achieving a qubit-resonator coupling strength of 655 MHz, $10\mathrm{\%}$ of the resonator frequency. We derive an analytical expression for the coupling strength in terms of circuit parameters and also discuss the maximum achievable coupling within this framework. We experimentally characterize flux qubits coupled to superconducting resonators using one- and two-tone spectroscopy methods, demonstrating excellent agreement with the proposed theoretical model.

Recent years have seen a dramatic rise in global cooling demand, driven by economic growth and climate change, and resulting in an increasing share of the total electric power consumption. Meanwhile, the ubiquitous vapor-compression air conditioners use refrigerants, which contribute greatly to global emission of greenhouse gases. In order to reduce the strain on electric grids, heat-driven technologies must be developed. Here, an acoustic driven sorption cooling device is examined experimentally and theoretically. The device can potentially utilize heat or electricity as a power input, uses environmentally benign working fluids, and offers simple, reliable construction with little to no moving parts. The results demonstrate the sorption-mediated, time-averaged heat-transfer mechanism, driven by the acoustic field. An unoptimized, proof-of-concept device is operated using a mixture of atmospheric air and either water or methanol, with cordierite and zeolite sorbents. The device is able to achieve temperature differentials $>30{}^{\circ }\mathrm{C}$. Moreover, a coefficient of performance of approximately 3 (based on the acoustic power input) is achieved at a temperature difference of $10{}^{\circ }\mathrm{C}$. Theoretical calculations provide an outlook on the operation of such technology, compared with existing cooling technologies, demonstrating its potential for achieving high efficiencies. Finally, prospects for further development are discussed.

Organic field-effect transistors (FETs) exhibit excellent switching characteristics when they involve trap-eliminated semiconductor interfaces with highly hydrophobic gate dielectric layers. In this study, we investigate the excitonic electro-optic response by delocalized carrier accumulation at the trap-eliminated interfaces of pentacene single-crystal FETs. We find that gate-modulation (GM) imaging, which sensitively visualizes the variation in optical microscope images between the gate-on and gate-off states, exclusively reveals the unique enhancement of electro-optic response under the application of drain voltages ($V_d$). The $V_d$-unbiased GM image exhibits a uniform spatial distribution, which is consistent with the accumulated carrier density in the channel. In contrast, the $V_d$-biased GM image presents a peculiar spatial distribution with fairly sharp increases around the edges of the source and drain electrodes. Furthermore, the following intriguing features are observed: (1) The sharp increase in the GM signal distribution around the electrode edge is similar to the lateral electric field distribution as measured by Kelvin probe force microscopy, and (2) the GM spectra, extracted from the respective GM images measured at different wavelengths, present a second-derivative-like shape that implies the broadening of exciton absorption. Based on these observations, we investigate the origin of this unique effect in terms of the enhanced violation of exciton coherence by delocalized carrier accumulations under drain bias. The gate-induced holes that are weakly bound to shallow traps should be detrapped by lateral electric fields, which eventually generate valence band holes and thus enhance the electro-optic response. These findings should elucidate the spatial coherence of the molecular excitons that are responsible for the various unique photoelectric characteristics of organic electronic devices.

With the increasing size of quantum processors, submodules that constitute the processor hardware will become too large to accurately simulate on a classical computer. Therefore, one would soon have to fabricate and test each new design primitive and parameter choice in time-consuming coordination between design, fabrication, and experimental validation. Here we show how one can design and test the performance of next-generation quantum hardware—by using existing quantum computers. Focusing on superconducting transmon processors as a prominent hardware platform, we compute the static and dynamic properties of individual and coupled transmons. We show how the energy spectra of transmons can be obtained by variational hybrid quantum-classical algorithms that are well suited for near-term noisy quantum computers. In addition, single- and two-qubit gate simulations are demonstrated via Suzuki-Trotter decomposition. Our methods pave a promising way towards designing candidate quantum processors when the demands of calculating submodule properties exceed the capabilities of classical computing resources.

We propose a fiber-based environmental sensor that exploits the reflection-phase-shift tunability provided by the use of layered coatings composed of dielectric slabs spaced by conducting membranes. A transfer-matrix study is done in a simplified theoretical model, for which an enhanced sensitivity of the reflection interference pattern to the output medium is demonstrated, in the typical refractive index range of liquid media. An experimental configuration using a cascaded Fabry-Perot microcavity coated by a graphene oxide/polyethylenimine (GO/PEI) multilayered structure is demonstrated. Its cost-effective chemical production method makes graphene oxide–based hybrid coatings excellent candidates for future real-life sensing devices.

A variational theory is presented for beam loading in microwave cavities. The beam-field interaction is formulated as a dynamical interaction whose stationarity according to Hamilton’s principle will naturally lead to steady-state solutions that indicate how a cavity’s resonant frequency, $Q$, and optimal coupling coefficient will detune as a result of the beam loading. A driven cavity Lagrangian is derived from first principles, including the effects of cavity-wall losses, input power, and beam interaction. The general formulation is applied to a typical klystron input cavity to predict the appropriate detuning parameters required to maximize the gain (or modulation depth) in the average Lorentz factor boost, $⟨\mathrm{\Delta }\gamma ⟩$. Numerical examples are presented, showing agreement with the general detuning trends previously observed in the literature. The developed formulation carries several advantages for the analysis and design of beam-loaded cavity structures. It provides a self-consistent model for the dynamical (nonlinear) beam-field interaction, a procedure for maximizing gain under beam-loading conditions, and a useful set of parameters to guide cavity-shape optimization during the design of beam-loaded systems. Enhanced clarity of the physical picture underlying the problem seems to be gained using this approach, allowing straightforward inclusion or exclusion of different field configurations in the calculation and expressing the final results in terms of measurable quantities. Two field configurations are discussed for the klystron input cavity, using finite magnetic confinement or no confinement at all. Formulating the problem in a language that is directly accessible to the powerful techniques found in Hamiltonian dynamics and canonical transformations may potentially carry an additional advantage in terms of analytical computational gains, under suitable conditions.

Neuroevolution, a field that draws inspiration from the evolution of brains in nature, harnesses evolutionary algorithms to construct artificial neural networks. It bears a number of intriguing capabilities that are typically inaccessible to gradient-based approaches, including optimizing neural-network architectures, hyperparameters, and even learning the training rules. In this paper, we introduce a quantum neuroevolution algorithm that autonomously finds near-optimal quantum neural networks for different machine-learning tasks. In particular, we establish a one-to-one mapping between quantum circuits and directed graphs, and reduce the problem of finding the appropriate gate sequences to a task of searching suitable paths in the corresponding graph as a Markovian process. We benchmark the effectiveness of the introduced algorithm through concrete examples including classifications of real-life images and symmetry-protected topological states. Our results showcase the vast potential of neuroevolution algorithms in quantum architecture search, which would boost the exploration towards quantum-learning advantage with noisy intermediate-scale quantum devices.

Quantum cascade lasers (QCLs) constitute a leading source of coherent radiation in the mid-IR region. However, their performance outside this region remains unsatisfactory. Indeed, there are currently no QCLs in the near-IR region. Here, we propose that a superlattice of atomically thin layers held together by van der Waals forces may operate near room temperature as a compact and powerful near-IR QCL emitting at a wavelength of $1.66\mu \mathrm{m}$. It can compress over 100 stages within $0.5\mu \mathrm{m}$. The electric field required for operation is about $3\times {10}^6{\mathrm{V~cm}}^{-1}$, while the lasing threshold current density is about $22.4{\mathrm{kA~cm}}^{-2}$ depending on parameters. Rate-equation analysis reveals that the peak power per unit volume can reach over $0.1\mathrm{mW}\mu {\mathrm{m}}^{-3}$ in cw operation. Unlike existing QCLs, our device is $p$-type working with holes.