The zinc nitride ${\mathrm{Zn}}_3{\mathrm{N}}_2$ is composed of inexpensive and earth-abundant Zn and N elements and shows high electron mobility exceeding $100{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$. Although various technological applications of ${\mathrm{Zn}}_3{\mathrm{N}}_2$ have been suggested so far, the synthesis of high-quality ${\mathrm{Zn}}_3{\mathrm{N}}_2$ samples, especially single crystals, is still challenging, and therefore its basic properties are not yet well understood. Indeed, the reported band gaps of as-grown ${\mathrm{Zn}}_3{\mathrm{N}}_2$ widely scatter from 0.85 to 3.2 eV. In this study, we investigate the large gap variation of ${\mathrm{Zn}}_3{\mathrm{N}}_2$ in terms of the Burstein-Moss (BM) effect and point-defect energetics using first-principles calculations. First, we discuss the relation between electron carrier concentration and optical gaps based on the electronic structure obtained using the Heyd-Scuseria-Ernzerhof hybrid functional. The calculated fundamental band gap is 0.84 eV in a direct-type band structure. Second, thermodynamic stability of ${\mathrm{Zn}}_3{\mathrm{N}}_2$ is assessed using the ideal-gas model in conjunction with the rigid-rotor model for gas phases and first-principles phonon calculations for solid phases. Third, carrier generation and compensation by native point defects and unintentionally introduced oxygen and hydrogen impurities are discussed. The results suggest that a significant BM shift occurs mainly due to oxygen substitutions on nitrogen sites and hydrogen interstitials. However, gaps larger than 2.0 eV would not be due to the BM shift because of the Fermi-level pinning caused by acceptorlike zinc vacancies and hydrogen-on-zinc impurities. Furthermore, we discuss details of peculiar defects such as a nitrogen-on-zinc antisite with azidelike atomic and electronic structures.

Background forces are linear long-range interactions of the cantilever body with its surroundings that must be compensated for in order to reveal tip-surface force, the quantity of interest for determining material properties in atomic force microscopy. We provide a mathematical derivation of a method to compensate for background forces, apply it to experimental data, and discuss how to include background forces in simulation. Our method, based on linear-response theory in the frequency domain, provides a general way of measuring and compensating for any background force and it can be readily applied to different force reconstruction methods in dynamic AFM.

We measure the electroluminescence of light-emitting diodes (LEDs) on substrates with low dislocation densities (LDD) at ${10}^6{\mathrm{cm}}^{-2}$ and low ${10}^8{\mathrm{cm}}^{-2}$, and compare them to LEDs on substrates with high dislocation densities (HDD) closer to ${10}^{10}{\mathrm{cm}}^{-2}$. The external quantum efficiencies (EQEs) are fitted using the $ABC$ model with and without localization. The nonradiative-recombination (NR) coefficient $A$ is constant for HDD LEDs, indicating that the NR is dominated by dislocations at all wavelengths. However, $A$ strongly increases for LDD LEDs by a factor of 20 when increasing the emission wavelength from 440 to 540 nm. We attribute this to an increased density of point defects due to the lower growth temperatures used for longer wavelengths. The radiative recombination coefficient $B$ follows the squared wave-function overlap for all samples. Using the observed coefficients, we calculate the peak efficiency as a function of the wavelength. For HDD LEDs the change of wave-function overlap (i.e., $B$) is sufficient to reduce the EQE as observed, while for LDD LEDs also the NR coefficient $A$ must increase to explain the observed EQEs. Thus, reducing NR is important to improving the EQEs of green LEDs, but this cannot be achieved solely by reducing the dislocation density: point defects must also be addressed.

Entangling remote qubits is an essential technological element in the distributed quantum information processing. Here, we propose a deterministic scheme to generate maximal entanglement between remote superconducting atoms, using a propagating microwave photon as a flying qubit. The building block of this scheme is an atom-photon two-qubit gate, in which the photon qubit is encoded on its carrier frequencies. The gate operation completes deterministically upon reflection of a photon, and the gate type can be continuously varied (including swap, $\sqrt{\mathrm{SWAP}}$, and identity gates) through in situ control of the drive field. Applying such atom-photon gates sequentially, we can perform various gate operations between remote superconducting atoms.

We propose and theoretically study a parity-time ($\mathcal{P}\mathcal{T}$)-symmetric photonic-crystal coupled-resonator optical waveguide (CROW) based on buried heterostructure nanocavities which has potential scalability and controllability. We analytically reveal its spectral transport properties with a tight-binding model and show the possibility of the wide-range control of its group velocity using the $\mathcal{P}\mathcal{T}$ phase transition. While the group velocity at the $\mathcal{P}\mathcal{T}$ phase-transition point diverges, the group-velocity dispersion converges. A numerical estimation of the system response to temporal pulse inputs shows that the pulse broadening is not severe in a device of hundreds of micrometers in size. Furthermore, a longer pulse duration results in a higher upper limit of the pulse peak velocity, which can be, in principle, superluminal. We next perform numerical simulations on the considered photonic-crystal slab structures with the finite-element method, and we successfully observe $\mathcal{P}\mathcal{T}$ phase transitions. In the simulated parameter range, gain and loss coefficients of the order of $100{\mathrm{cm}}^{-1}$ meet the condition for the maximum group-velocity coefficient in the context of the tight-binding approach. A 9.3-fold increase in the group velocity at 1502 nm is obtained in a three-dimensional device by switching between the conventional and $\mathcal{P}\mathcal{T}$-symmetric CROWs. Meanwhile, we also encounter band smoothing around the phase transition, which hampers the group-velocity divergence. Our simulation result indicates that it arises from interfering evanescent waves decaying out of the device structure, and we discuss ways to suppress this effect.

The speed of quantum gates and measurements is a decisive factor for the overall fidelity of quantum protocols when performed on physical qubits with a finite coherence time. Reducing the time required to distinguish qubit states with high fidelity is, therefore, a critical goal in quantum-information science. The state-of-the-art readout of superconducting qubits is based on the dispersive interaction with a readout resonator. Here, we bring this technique to its current limit and demonstrate how the careful design of system parameters leads to fast and high-fidelity measurements without affecting qubit coherence. We achieve this result by increasing the dispersive-interaction strength, by choosing an optimal linewidth of the readout resonator, by employing a Purcell filter, and by utilizing phase-sensitive parametric amplification. In our experiment, we measure 98.25% readout fidelity in only 48 ns, when minimizing readout time, and 99.2% in 88 ns, when maximizing the fidelity, limited predominantly by the qubit lifetime of $7.6\mu \mathrm{s}$. The presented scheme is also expected to be suitable for integration into a multiplexed readout architecture.

Reservoir computing is a bioinspired computing paradigm for processing time-dependent signals. Its hardware implementations have received much attention because of their simplicity and remarkable performance on a series of benchmark tasks. In previous experiments, the output was uncoupled from the system and, in most cases, simply computed off-line on a postprocessing computer. However, numerical investigations have shown that feeding the output back into the reservoir opens the possibility of long-horizon time-series forecasting. Here, we present a photonic reservoir computer with output feedback, and we demonstrate its capacity to generate periodic time series and to emulate chaotic systems. We study in detail the effect of experimental noise on system performance. In the case of chaotic systems, we introduce several metrics, based on standard signal-processing techniques, to evaluate the quality of the emulation. Our work significantly enlarges the range of tasks that can be solved by hardware reservoir computers and, therefore, the range of applications they could potentially tackle. It also raises interesting questions in nonlinear dynamics and chaos theory.

In this paper, we investigate the lithiation of pristine amorphous silicon nanoclusters with diameters from 0.8 to 1.4 nm using first-principles molecular dynamics. It is found that this process occurs in two stages. In the first one, Li atoms accumulate at the surface of the nanocluster. A transfer of electrons from Li to the Si atoms takes place, which is accompanied by moderate structural changes. At a given Li coverage threshold that is related to the nanoparticle size, it becomes energetically favorable for any additional Li atoms to penetrate into the nanoparticle. Our results suggest that the driving force for this transition is the increasing electrostatic repulsion between positively charged surface Li ions. Beyond the threshold a second lithiation stage starts where the nanocluster is progressively filled with Li atoms. This leads to significant structural changes, but no sign of fracture is observed during the first lithiation cycle, suggesting that the amorphous silicon cluster remains intact at this size.

We present a method that enables performing x-ray phase-contrast imaging (XPCI) computed tomography with a laboratory setup using a single image per projection angle, eliminating the need to move optical elements during acquisition. Theoretical derivation of the method is presented, and its validity conditions are provided. The object is assumed to be quasihomogeneous, i.e., to feature a ratio between the refractive index and the linear attenuation coefficient that is approximately constant across the field of view. The method is experimentally demonstrated on a plastics phantom and on biological samples using a continuous rotation acquisition scheme achieving scan times of a few minutes. Moreover, we show that such acquisition times can be further reduced with the use of a high-efficiency photon-counting detector. Thanks to its ability to substantially simplify the image-acquisition procedure and greatly reduce collection times, we believe this method represents a very important step towards the application of XPCI to real-world problems.

We describe a macroscopic beam splitter for polar neutral molecules. A complex electrode structure is required for the beam splitter which would be very difficult to produce with traditional manufacturing methods. Instead, we make use of a nascent manufacturing technique: 3D printing of a plastic piece, followed by electroplating. This fabrication method opens a plethora of avenues for research, since 3D printing imposes practically no limitations on possible shapes, and the plating produces chemically robust, conductive construction elements with an almost free choice of surface material. It has the added advantage of dramatically reduced production cost and time. Our beam splitter is an electrostatic hexapole guide that smoothly transforms into two bent quadrupoles. We demonstrate the correct functioning of this device by separating a supersonic molecular beam of ${\mathrm{ND}}_3$ into two correlated fractions. It is shown that this device can be used to implement experiments with differential detection wherein one of the fractions serves as a probe and the other as a reference. Reverse operation would allow the merging of two beams of polar neutral molecules.

We propose a method to trap liquid (both oil and water) in a microchannel by sequentially injecting oil and water (or vice versa) over photopatterned obstacles with controlled wetting properties. We present a simple geometrical model to understand the liquid-entrapment process and predict the evolution of the water/oil interface over an obstacle. Our analysis provides an analytic solution that can successfully predict useful properties such as angular position of pinch-off and the amount of captured liquid. We show that we are able to obtain a liquid bridge over two circular obstacles, and we are also able to predict the condition for the formation of a bridge. We further demonstrate the effect of the obstacle shape and how a sudden change in gradient results in larger liquid entrapment. We also demonstrate the ability to entrap isolated liquid chambers for parallel experimentation.

Spintronic applications based on magnetic domain-wall (DW) motion, such as magnetic data storage, sensors, and logic devices, require approaches to reliably manipulate the magnetization in nanowires. In this paper, we report the direct dynamic experimental visualization of reliable switching from the onion to the vortex state by DW automotion at zero field in asymmetric ferromagnetic rings using a uniaxial field pulse. Employing time-resolved x-ray microscopy, we demonstrate that depending on the detailed spin structure of the DWs and the size and geometry of the rings, the automotive propagation can be tailored during the DW relaxation from the higher-energy onion state to the energetically favored vortex state, where both DWs annihilate. Our measurements show DW automotion with an average velocity of about $60\mathrm{m}/\mathrm{s}$, which is a significant speed for spintronic devices. Such motion is mostly governed by local forces resulting from the geometry variations in the device. A closer study of the annihilation process via micromagnetic simulations reveals that a new vortex is nucleated in between the two initial walls. We demonstrate that the annihilation of DWs through automotion in our scheme always occurs with the detailed topological nature of the walls influencing only the DW dynamics on a local scale. The simulations show good quantitative agreement with our experimental results. These findings shed light on a robust and reliable switching process of the onion state in ferromagnetic rings, which paves the way for further optimization of these devices.

The classical Hall effect in ordinary isotropic conducting materials describes the occurrence of a voltage perpendicular to the direction of the electric-current flow and perpendicular to the imposed magnetic-field vector. The Hall effect is routinely used in magnetic-field sensors. Here, we fabricate and characterize microstructured anisotropic metamaterials composed of a single semiconducting constituent ($n$-type ZnO) for which the direction and the sign of the Hall electric field can be tailored by microstructure. This class of metamaterials includes the possibility of a Hall voltage parallel—rather than perpendicular—to the magnetic-field vector. One possible future application arising from this far-reaching control of the effective electric-conductivity tensor is a sensor measuring the circulation of a magnetic field.

Microscopic imaging of local magnetic fields provides a window into the organizing principles of complex and technologically relevant condensed-matter materials. However, a wide variety of intriguing strongly correlated and topologically nontrivial materials exhibit poorly understood phenomena outside the detection capability of state-of-the-art high-sensitivity high-resolution scanning probe magnetometers. We introduce a quantum-noise-limited scanning probe magnetometer that can operate from room-to-cryogenic temperatures with unprecedented dc-field sensitivity and micron-scale resolution. The Scanning Quantum Cryogenic Atom Microscope (SQCRAMscope) employs a magnetically levitated atomic Bose-Einstein condensate (BEC), thereby providing immunity to conductive and blackbody radiative heating. The SQCRAMscope has a field sensitivity of 1.4 nT per resolution-limited point (approximately $2\mu \mathrm{m}$) or $6\mathrm{nT}/\sqrt{\mathrm{Hz}}$ per point at its duty cycle. Compared to point-by-point sensors, the long length of the BEC provides a naturally parallel measurement, allowing one to measure nearly 100 points with an effective field sensitivity of $600\mathrm{pT}/\sqrt{\mathrm{Hz}}$ for each point during the same time as a point-by-point scanner measures these points sequentially. Moreover, it has a noise floor of 300 pT and provides nearly 2 orders of magnitude improvement in magnetic flux sensitivity (down to ${10}^{-6}{\mathrm{\Phi }}_0/\sqrt{\mathrm{Hz}}$) over previous atomic probe magnetometers capable of scanning near samples. These capabilities are carefully benchmarked by imaging magnetic fields arising from microfabricated wire patterns in a system where samples may be scanned, cryogenically cooled, and easily exchanged. We anticipate the SQCRAMscope will provide charge-transport images at temperatures from room temperature to 4 K in unconventional superconductors and topologically nontrivial materials.

Neural networks have proven effective for solving many difficult computational problems, yet implementing complex neural networks in software is computationally expensive. To explore the limits of information processing, it is necessary to implement new hardware platforms with large numbers of neurons, each with a large number of connections to other neurons. Here we propose a hybrid semiconductor-superconductor hardware platform for the implementation of neural networks and large-scale neuromorphic computing. The platform combines semiconducting few-photon light-emitting diodes with superconducting-nanowire single-photon detectors to behave as spiking neurons. These processing units are connected via a network of optical waveguides, and variable weights of connection can be implemented using several approaches. The use of light as a signaling mechanism overcomes fanout and parasitic constraints on electrical signals while simultaneously introducing physical degrees of freedom which can be employed for computation. The use of supercurrents achieves the low power density ($1\mathrm{mW}/{\mathrm{cm}}^2$ at 20-MHz firing rate) necessary to scale to systems with enormous entropy. Estimates comparing the proposed hardware platform to a human brain show that with the same number of neurons ($10^{11}$) and 700 independent connections per neuron, the hardware presented here may achieve an order of magnitude improvement in synaptic events per second per watt.

The modes of silicon meta-atoms are investigated, motivated by their use as building blocks of Huygens metasurfaces. A model based on these modes is presented, giving a clear physical explanation of all features in the extinction spectrum. Counterintuitively, this model can show negative contributions to extinction, which are shown to arise from the interference between nonorthogonal modes. The direct and interference contributions to extinction are determined, showing that conservation of energy is preserved. The Huygens condition of matched electric- and magnetic-dipole moments leads to strong forward scattering and suppressed backscattering. It is shown that higher-order modes with appropriate symmetry generalize this condition, leading to multiple bands of directional scattering. The presented results are obtained using a robust approach to find the modes of nanophotonic scatterers, commonly referred to as quasinormal modes. By utilizing an integral formulation of Maxwell’s equations, this work avoids the problem of normalizing diverging far fields, which other approaches require. The model and presented results are implemented in open-source code.

We present a model study of an alternative implementation of a two-port Hall-effect microwave gyrator. Our setup involves three electrodes, one of which acts as a common ground for the others. Based on the capacitive-coupling model of Viola and DiVincenzo, we analyze the performance of the device and we predict that ideal gyration can be achieved at specific frequencies. Interestingly, the impedance of the three-terminal gyrator can be made arbitrarily small for certain coupling strengths, so that no auxiliary impedance matching is required. Although the bandwidth of the device shrinks as the impedance decreases, it can be improved by reducing the magnetic field; it can be realistically increased up to 150 MHz at $50\mathrm{\Omega }$ by working at the filling factor $\nu =10$. We also examine the effects of the parasitic capacitive coupling between electrodes and we find that, although, in general, they strongly influence the response of device, their effect is negligible at low impedance. Finally, we analyze an interferometric implementation of a circulator, which incorporates the gyrator in a Mach-Zender–like construction. Perfect circulation in both directions can be achieved, depending on frequency and on the details of the interferometer.

We report on the design and implementation of a field-programmable Josephson amplifier (FPJA)—a compact and lossless superconducting circuit that can be programmed in situ by a set of microwave drives to perform reciprocal and nonreciprocal frequency conversion and amplification. In this work, we demonstrate four modes of operation: frequency conversion (transmission of $-0.5$ dB, reflection of $-30$ dB), circulation (transmission of $-0.5$ dB, reflection of $-30$ dB, isolation of 30 dB), phase-preserving amplification (gain $>20\mathrm{dB}$, one photon of added noise) and directional phase-preserving amplification (reflection of $-10$ dB, forward gain of 18 dB, reverse isolation of 8 dB, one photon of added noise). The system exhibits quantitative agreement with the theoretical prediction. Based on a gradiometric superconducting quantum-interference device with $\mathrm{Nb}/\mathrm{Al}\text{−}\mathrm{Al}{\mathrm{O}}_x/\mathrm{Nb}$ Josephson junctions, the FPJA is first-order insensitive to flux noise and can be operated without magnetic shielding at low temperature. Owing to its flexible design and compatibility with existing superconducting fabrication techniques, the FPJA offers a straightforward route toward on-chip integration with superconducting quantum circuits such as qubits and microwave optomechanical systems.

Artificial molecular walkers beyond burnt-bridge designs are complex nanomachines that potentially replicate biological walkers in mechanisms and functionalities. Improving the man-made walkers up to performance for widespread applications remains difficult, largely because their biomimetic design principles involve entangled kinetic and mechanical effects to complicate the link between a walker’s construction and ultimate performance. Here, a synergic mechanical-kinetic model is developed for a recently reported DNA bipedal walker, which is based on a modular design principle, potentially enabling many directional walkers driven by a length-switching engine. The model reproduces the experimental data of the walker, and identifies its performance-limiting factors. The model also captures features common to the underlying design principle, including counterintuitive performance-construction relations that are explained by detailed balance, entropy production, and bias cancellation. While indicating a low directional fidelity for the present walker, the model suggests the possibility of improving the fidelity above 90% by a more powerful engine, which may be an improved version of the present engine or an entirely new engine motif, thanks to the flexible design principle. The model is readily adaptable to aid these experimental developments towards high-performance molecular walkers.

The near-infrared (NIR) photon pumped optical physics of nitrogen-vacancy (NV) centers in diamond is experimentally studied by considering both the charge-state conversion and stimulated emission. We find that the NIR laser can help to highly increase the charge-state conversion rate and, subsequently, can be applied to improve the performance of charge-state-depletion (CSD) nanoscopy. Using a doughnut-shaped visible laser beam and a Gaussian-shaped NIR laser beam for charge-state manipulation, we develop a low-power two-depletion-laser CSD nanoscopy for NV centers. Without dropping the imaging resolution, the visible depletion-laser intensity is approximately 10 times lower than that in the single-depletion-laser CSD. With high spatial resolution and low laser power, the CSD nanoscopy can be used for nanoscale quantum sensing with NV centers.