The output randomness from a random number generator can be certified by observing the violation of quantum contextuality inequalities based on the Kochen-Specker theorem. Contextuality can be tested in a single quantum system, which significantly simplifies the experimental requirements to observe the violation comparing to the ones based on nonlocality tests. However, it is not yet resolved as to how to ensure compatibilities for sequential measurements that is required in contextuality tests. Here, we employ a modified Klyachko-Can-Binicioğlu-Shumovsky contextuality inequality, which can ease the strict compatibility requirement on measurements. On a trapped single ${}^{138}{\mathrm{Ba}}^{+}$ ion system, we experimentally demonstrate violation of the contextuality inequality and realize quantum random number expansion by closing detection loopholes. We perform $1.29\times {10}^8$ trials of experiments and extract a randomness of $5.28\times {10}^5$ bits with a speed of $270\mathrm{bits}{\mathrm{s}}^{-1}$. Our demonstration paves the way for practical high-speed spot-checking quantum random number expansion and other secure information processing applications.

Acousto-optical devices, such as modulators, filters, or deflectors, implement a simple and effective way of light modulation and signal-processing techniques. However, their operation wavelengths are restricted to the visible and near-infrared frequency regions due to a quadratic decrease of the efficiency of acousto-optical interactions with increasing wavelength. At the same time, almost all materials with a high value of the acousto-optic figure of merit are nontransparent at wavelengths longer than 5 µm, while the transparent materials possess a significantly lower acousto-optic figure of merit. Here, we propose and demonstrate by calculations how these limitations can be overcome using specially designed planar semiconductor structures that support electromagnetic modes strongly coupled to the incident light in the Otto configuration. Such an approach can be used for an efficient acousto-optical device operating in the mid-infrared range of 8–14 µm. An acoustic wave excited by a piezoelectric transducer in a semiconductor prism is utilized to modulate the coupling coefficient of the incident light to the guided mode of the semiconductor structure, which results in up to 100% modulation of the transmitted light at a spatial scale less than the ultrasound wavelength. It allows the utilization of acoustic waves with a short decay distance, and therefore, it provides a unique possibility to achieve an efficient acousto-optical modulation at frequencies about 1 GHz, which are unreachable for traditional acousto-optics.

We demonstrate a largely enhanced nonreciprocal propagation of spin waves (SWs) magnetoelastically excited in $\mathrm{Ni}$/$\mathrm{Si}$ bilayer films. The nonreciprocity is attributed to the shear component of the strain tensor, which can be enlarged by increasing the thickness of the $\mathrm{Si}$ layer. By evaporating a 400-nm-thick $\mathrm{Si}$ layer on a $\mathrm{Ni}$ film, we enhanced the nonreciprocal ratio by up to 1200%. This finding paves the way for rectification of SWs in magnonic devices.

Electromagnetic wave interaction with confined metallic magnetic structures is complex due to the excitation of nonuniform electromagnetic fields and magnetic precession and spin-wave modes over length scales that are dependent on the geometric, electromagnetic, and micromagnetic properties of the magnetic structures. Here, we solve the coupled system of Maxwell’s equations and the Landau-Lifshitz-Gilbert equation using a stable algorithm based on the finite-difference time-domain method to study the transient wide-band electromagnetic propagation and resonance in infinitely long cobalt nanoprisms with a square cross section of side lengths 50–1000 nm. In particular, we identify the resonance mechanisms through studying the local transient and spectral distributions of magnetization in the prisms. The nanoprisms are excited by an axially polarized plane wave at normal incidence with a 70 GHz Gaussian pulse profile. For this incident wave condition, the simulations confirm that resonance in the cobalt prisms is excited mainly by the current-induced magnetic fields and indicate a magnetization curling resonance mode for prism side lengths less than 100 nm. The eigenfrequencies for the curling mode are confirmed theoretically using a model for an equivalent long circular cylinder with radial spin-wave modes. For prism side lengths longer than 100 nm (but less than the nonmagnetic skin depth), the magnetic response is confined to thin regions along the prism edges due to resonance-induced skin effects. The simulations indicate predominately uniform magnetization precession in the confined edge regions with increased pinning in the corners. The uniform resonance mode in the central part of the edge regions increases in intensity with prism size, with frequency described using Kittel’s ferromagnetic resonance frequency of a thin planar structure. A higher-frequency size-independent uniform resonance mode is observed in the corner regions with frequency determined by the local demagnetizing factors. Local and integrated power absorption spectra are calculated using the simulated transient magnetization and fields, and confirm the resonance modes and frequencies in the cobalt prisms and their size dependence. The profile of the local power absorption spectra is also used to estimate the thickness of the confined edge regions or magnetic skin depth in the cobalt prisms at the fundamental resonance mode and is found to be approximately 50 nm. The outcomes of this work would benefit the design and engineering of lightweight and compact materials and devices incorporating metallic magnetic nanostructures. This work also provides a foundation for further modeling and understanding of electromagnetic transmission and propagation in more complex metallic ferromagnetic structures and composites excited by different electromagnetic wave conditions.

Spin-charge interconversion in ferromagnets has recently drawn great attention due to its ability to generate spin current via the anomalous Hall, planar Hall, or intrinsic spin Hall effect. In this work, we demonstrate the magnetization switching of perpendicular anisotropy ferromagnets by the transversely polarized spin current generated from an easy-plane ferromagnet such as $(\mathrm{Fe},\mathrm{Mn})\mathrm{Pt}$ and $(\mathrm{Co},\mathrm{Fe})\mathrm{B}$. The spin-torque efficiency is characterized to be about 21% of that of the same structure with the easy-plane ferromagnet replaced by $\mathrm{Ta}$. In addition, we find that the magnetization switching consists of multiple intermediate states, which can be robustly set by the applied current pulse amplitude and repetition number. This behavior can be used to mimic synaptic devices that is potentially useful for neuromorphic computing.

Processes based on electrostatic projection are used extensively in industry, e.g., for mineral separations, electrophotography, or manufacturing of coated abrasives, such as sandpaper. Despite decades of engineering practice, there are still unanswered questions. In this paper, we present a comprehensive experimental study of the projection process of more than 1500 individual spherical alumina particles with a nominal size of $500\mu \mathrm{m}$, captured by high-speed video imaging and digital image analysis. Based on flight trajectories of approximately 1100 projected particles, we determine the acquired charge and dynamics as a function of the relative humidity (RH) and the electric field intensity and compare the results with classical theories. For RH levels of 50% and above, more than 85% of distributed particles are projected, even when the electric field intensity is at its minimum level. This suggests that, beyond a critical value of the electric field intensity, the RH plays a more critical role in the projection process. We also observe that the charging time is reduced dramatically for RH levels of 50% and above, possibly due to the buildup of thin water films around the distributed particles, which can facilitate charge transfer. In contrast, projected particles at the 30% RH level exhibit excessive amounts of electric charge, between 2 and 4 times than that of the saturation value, which might be attributed to triboelectric charging effects. Finally, the physics of electrostatic projection is compared and contrasted with those of induced-charge electrokinetic phenomena, which share similar field-square scaling, as the applied electric field acts on its own induced charge to cause particle motion.

Photonic resonators that allow an electromagnetic field to be confined in an ultrasmall volume with a long decay time are crucial to a number of applications requiring enhanced nonlinear effects. For applications to integrated photonic devices on chip, compactness and optimized in-plane transmission become relevant figures of merit as well. Here we optimize an encapsulated $\mathrm{Si}/{\mathrm{Si}\mathrm{O}}_2$ photonic-crystal nanobeam cavity at telecom wavelengths by means of a global optimization procedure, where only the first few holes surrounding the cavity are varied to decrease its radiative losses. This strategy allows us to theoretically achieve intrinsic quality factors close to 10 million, sub-diffraction-limited mode volumes, and in-plane transmission above 65%, in a structure with a very small footprint of about $8\mu {\mathrm{m}}^2$. We address and quantitatively assess the dependence of the main figures of merit on the nanobeam length and on fabrication disorder. Finally, we study a system of two optimized and laterally coupled nanobeam cavities with the goal of demonstrating an unconventional photon blockade at room temperature in a monolithic passive device. We estimate the single-photon nonlinearity of this device and discuss the relevant figures of merit, which lead to sub-Poissonian photon statistics of the transmitted signal. Our results hold promise for prospective experiments in low-power nonlinear and quantum photonics.

The steady-state charge-carrier mobility in semiconductors can be extracted from the space-charge-limited current in single-carrier devices. However, in many cases, a built-in voltage is present, which should be known accurately to obtain the correct mobility. Here, it is demonstrated that band bending at the injecting electrode has important consequences for the built-in voltage and the analytical description of the current-voltage characteristics. It is shown that the built-in voltage can be accurately determined from the slope of the current-voltage characteristics on a semilogarithmic scale. Knowing the effect of band bending on the injected current, a simple analytical equation for the drift-diffusion space-charge-limited current above the built-in voltage is derived, which allows for improved determination of the charge-carrier mobility from experimental data.

Valley splitting is a key feature of silicon-based spin qubits. Quantum dots in $\mathrm{Si}$/${\mathrm{Si}}_x{\mathrm{Ge}}_{1-x}$ heterostructures reportedly suffer from a relatively low valley splitting, limiting the operation temperature and the scalability of such qubit devices. Here, we demonstrate a robust and large valley splitting exceeding 200 $\mu \mathrm{eV}$ in a gate-defined single quantum dot, hosted in molecular-beam-epitaxy-grown ${}^{28}\mathrm{Si}/{\mathrm{Si}}_x{\mathrm{Ge}}_{1-x}$. The valley splitting is monotonically and reproducibly tunable up to 15% by gate voltages, originating from a 6-nm lateral displacement of the quantum dot. We observe static spin relaxation times $T_1>1$ s at low magnetic fields in our device containing an integrated nanomagnet. At higher magnetic fields, $T_1$ is limited by the valley hotspot and by phonon noise coupling to intrinsic and artificial spin-orbit coupling, including phonon bottlenecking.

Ferroelectric oxide perovskites are promising materials for use in photovoltaic devices, due to their ability to exploit the bulk photovoltaic effect to achieve high power-conversion efficiency. In this work, we use first-principles methods to investigate the ferroelectric perovskite $[{\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_3{]}_x\text{−}[{\mathrm{Na}\mathrm{Ti}}_{1/2}{\mathrm{Mo}}_{1/2}{\mathrm{O}}_3{]}_{1-x}$ and $[{\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_3{]}_x\text{−}[{\mathrm{Na}\mathrm{Ti}}_{1/2}{\mathrm{W}}_{1/2}{\mathrm{O}}_3{]}_{1-x}$ solid solutions for potential use in ferroelectric-based photovoltaics. We find that compositional variations change the band gap, shifting it to the edge of the visible range for the 25% ${\mathrm{Na}\mathrm{Ti}}_{1/2}{\mathrm{Mo}}_{1/2}{\mathrm{O}}_3$ composition and to the visible range for some Mo-cation and W-cation arrangements for the 50% ${\mathrm{Na}\mathrm{Ti}}_{1/2}{\mathrm{Mo}}_{1/2}{\mathrm{O}}_3$ and 50% ${\mathrm{Na}\mathrm{Ti}}_{1/2}{\mathrm{W}}_{1/2}{\mathrm{O}}_3$ compositions. Mo and W substitutions both maintain the ferroelectric properties of the parent ${\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_3$. While the A-site cation arrangement has a minor effect on the band gap, the variations in the B-site cation arrangement and the cation displacements affect the band gap by up to 0.8 eV. Analysis of the structures and the calculated band-gap values shows that the band gap is controlled by the identity of the substituent cation, the O-B-O angles, the relative orientations of the Mo and W substituent atoms, and the B-cation displacement. We demonstrate the thermodynamic feasibility of these solid solutions by formation energy analysis. The decrease of the band gap relative to the parent ${\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_3$ to the standard and transparent photovoltaic range combined with the ferroelectricity maintained make this earth-abundant-containing solid solution a promising candidate for use in high-performance ferroelectric-based photovoltaic devices.

Tetrahex carbon is a recently predicted two-dimensional (2D) carbon allotrope that is composed of tetragonal and hexagonal rings. Unlike flat graphene, this new 2D carbon structure is buckled and possesses a direct band gap of approximately 2.6 eV and a high carrier mobility of approximately ${10}^4\mathrm{c}{\mathrm{m}}^2/(\mathrm{V}\mathrm{s})$ with anisotropic features. In this work, we employ first-principles density-functional theory calculations to explore the mechanical properties of tetrahex C under uniaxial tensile strain. We find that tetrahex C demonstrates ultrahigh ideal strength, outperforming both graphene and pentagraphene. It shows superior ductility and sustains uniaxial tensile strain up to 20% (16%) until phonon instability occurs and the corresponding maximal strength is 38.3 N/m (37.8 N/m) in the zigzag (armchair) direction. It shows an intrinsically negative Poisson ratio. This exotic in-plane Poisson ratio takes place when the axial strain reaches a threshold value of 7% (5%) in the zigzag (armchair) direction. We also find that tetrahex C maintains a direct band gap of 2.64 eV at the center of the Brillouin zone. This direct-gap feature remains intact upon strain application, with no direct-indirect gap transition. The ultrahigh ideal strength, the negative Poisson ratio, and the integrity of the direct gap under strain in tetrahex C suggest that it may have potential applications in nanomechanics and nanoelectronics.

Single dopants in materials have strong influences on the fundamental properties in extensive fields. Annular dark-field scanning tranmission electron microscopy (ADF STEM) has been contributed to directly investigate the occupation site of single dopants and their spatial distributions at atomic scale. However, the resultant atomic resolution images are projected in two dimensions, lacking in depth information. Here, we demonstrate the direct determination of three-dimensional atomic locations of single $\mathrm{Ce}$ dopants embedded in cubic boron nitride by ADF STEM depth sectioning with a significantly large illumination angle of 63 mrad in semiangle. Furthermore, we directly evaluate the $\mathrm{Ce}$-$\mathrm{Ce}$ partial pair distribution function in the considerably long distance up to 8.5 nm, which opens the way to study a far-long-range interaction between impurities especially in a dilute doping system.

High accuracy recognition of the orbital angular momentum (OAM) of light based on petal interference patterns is demonstrated using a convolutional neural network (CNN) approach with an improved Alexnet structure. A type of hybrid beam carrying OAM is utilized to provide more controllable degrees of freedom to recognize the OAM of light. The relationship between the training sample resolution (or the number associated with the accuracy) and the training time of the model, is presented. The recognition accuracy is closely related with the quantum number l of OAM, the angular ratio n of the spire phase over the hybrid phase in one modulation period, and the propagation distance z. Our studies show that when l ranges from 1 to 10, and n varies from 0.02 to 0.99, the recognition accuracy rate of OAM is nearly 100%. The minimum interval of n recognized at the OAM modes decreases to 0.01, which shows the super-high bandwidth of the generation and detection of OAM modes. Such results suggest great potential for the next generation of CNN-based OAM optical communication applications.

There is an increasing and demanding interest in the realization of secure communication schemes. Here, we conceive and realize an approach for a message-encoding scheme between two parties, Alice and Bob, by exploiting the infinite number of polarization states of unpolarized thermal light on the Poincaré sphere together with their correlation properties to camouflage and recover a message, thus realizing ghost polarization communication. At first, we investigate the second-order correlation coefficient $g^{(2)}(\tau )$ of classical unpolarized broadband amplified-spontaneous-emission light emitted by an erbium-doped fiber amplifier at 1550 nm by manipulating its instantaneous polarization state utilizing various polarization optics in the beam paths of a Hanbury-Brown and Twiss like ghost polarimetry setup using ultra-fast two-photon absorption in a photomultiplier tube. The observed polarization state modifications are in excellent agreement with our developed model based on the Stokes vector dynamics and a Glauber protocol for $g^{(2)}(\tau )$. On the basis of these results we then proceed towards the realization of a message-encoding scheme. By changing the instantaneous polarization state using a half-wave plate, Alice encodes and subsequently transmits a message. The camouflaged message can be recovered uniquely by Bob measuring the second-order correlations of the modified instantaneous polarization state with his reference beam. By using the agreed keypad of the communication scheme, he is able to retrieve the bit values of the encoded message. Finally, we address real-world implementation of this first proof-of-principle demonstration and discuss its security issues. This scheme demonstrates a method of establishing a secure communication link between two parties directly on the physical layer based on polarization correlations of classical light. It is expected that by the realization of this correlated photon modality not only avenues for ghost modalities, in general, are opened but also insight into polarization, even more than 175 years after Stokes will be achieved.

In electronic transport through mesoscopic systems, the various resonances in quantities such as conductance and scattering cross sections are characterized by the universal Fano formula. Does a similar formula exist for spin transport? We provide an affirmative answer by deriving a Fano formula to characterize the resonances associated with two fundamental quantities underlying spin transport: spin-resolved transmission and the spin polarization vector. In particular, we generalize the conventional Green’s function formalism to spin transport and use the Fisher-Lee relation to obtain the spin-resolved transmission matrix, which enables the spin polarization vector to be calculated, leading to a universal Fano formula for spin resonances. Particularly, the theoretically obtained resonance width depends on the nature of the classical dynamics as determined by the geometric shape of the dot. We explicitly demonstrate this fact and argue that it can be used to smooth out or even eliminate Fano spin resonances by manipulating the classical dynamics, which can be realized by applying or withdrawing a properly designed local gate potential. Likewise, modulating the classical dynamics in a different way can enhance the resonance. This is of particular importance in the design of electronic switches that can control the spin orientation of the electrons associated with the output current through weakening or enhancement of a Fano resonance, which are a key component in spintronics.

A method of dynamic continuous-wave spectroscopy of coherent population trapping (CPT) resonances using phase modulation of the jump type is developed. The time evolution of the spectroscopic signal is investigated. A method for the formation of an error signal for frequency stabilization is proposed. We show that our approach has a reduced sensitivity to the line-shape asymmetry of the CPT resonance. The experimental results are in good qualitative agreement with theoretical predictions based on a mathematical model of a three-level $\mathrm{\Lambda }$ system in a bichromatic field. This method can be used in atomic frequency standards (including chip-scale atomic clocks).

We elucidate the dependence of the in-plane and interfacial thermal conduction of two-dimensional (2D) transition-metal dichalcogenide (TMDC) materials (including ${\mathrm{Mo}\mathrm{S}}_2$, ${\mathrm{WS}}_2$, and ${\mathrm{W}\mathrm{Se}}_2$) on the materials’ physical features, such as size, layer number, composition, and substrates. The in-plane thermal conductivity k is measured at suspended 2D TMDC materials and the interfacial thermal conductance g is measured at materials supported on substrates, both through Raman thermometry techniques. The thermal conductivity k increases with the radius R of the suspended area following a logarithmic scaling as k∼log(R). k also shows a substantial decrease from monolayer to bilayer, but only changes slightly with a further increase in the layer number. In contrast, the interfacial thermal conductance g has a negligible dependence on the layer number, but g increases with the strength of the interaction between 2D TMDC materials and the substrate, substantially varying among different substrates. The result is consistent with theoretical predictions and clarifies much inconsistence in the literature. This work provides useful guidance for thermal management in 2D TMDC materials and devices.