Silicon spin qubits are a promising quantum-computing platform offering long coherence times, small device sizes, and compatibility with industry-backed device-fabrication techniques. In recent years, high-fidelity single-qubit and two-qubit operations have been demonstrated in $\mathrm{Si}$. Here we demonstrate coherent spin control in a quadruple quantum dot fabricated from isotopically enriched ${}^{28}\mathrm{Si}$. We tune the ground-state charge configuration of the quadruple dot down to the single-electron regime and demonstrate tunable interdot tunnel couplings as large as 20 GHz, which enables exchange-based two-qubit gate operations. Site-selective single spin rotations are achieved with the use of electric dipole spin resonance in a magnetic field gradient. We execute a resonant controlled-not gate between two adjacent spins in 270 ns.

This paper reports the implementation of Doppler-broadening gas thermometry by use of line-shape analysis of a line-doublet. The two spectral components are vibration-rotation transitions belonging to a pair of acetylene combination bands at a wavelength of $1.39\mu \mathrm{m}$. Characterized by an extraordinary spectral fidelity in combination with high resolution, the spectrometer is based on two phase-locked extended-cavity diode lasers, one of them being referenced to an optical-frequency-comb synthesizer. The global analysis of 1180 spectra, which are recorded as a function of the ${\mathrm{C}}_2{\mathrm{H}}_2$ pressure at the constant temperature of the triple point of water, yields an optical determination of the thermodynamic temperature with a combined relative uncertainty (type A plus type B) of 23 parts per million. Similar results are obtained at the melting point of gallium (approximately $303\mathrm{K}$). Furthermore, we apply line-absorbance analysis to the acquired spectra, demonstrating a reduction by a factor of approximately $6$ of the statistical uncertainty for the retrieved gas temperature.

Nondiffracting Bessel, Mathieu, and Weber vector beams are generated by using ring-shaped plasmonic geometric metasurfaces. The nondiffracting vector beam is produced by the superposition of two off-axis right-handed and left-handed circularly polarized nondiffracting scalar beams described by the Whittaker integral, which are simultaneously generated by a single metasurface with the ring-shaped phase profile. The polarization states of the generated nondiffracting vector beams are analyzed by the Stokes parameters and the orbital angular momentum states are measured by the beam interference. In addition, the selfhealing properties of nondiffracting vector beams are further demonstrated, showing that not only the beam profiles but also the polarization states can be recovered behind the small opaque obstacle. The demonstrated ring-shaped plasmonic metasurfaces provide a compact and efficient platform to produce complex nondiffracting vector beams and pave the way to many promising applications related to spin and orbital angular momentum conversion, quantum information processing, optical manipulation, and optical communication.

Quantum networks are the ultimate target in quantum communication, where many connected users can share information carried by quantum systems. The keystones of such structures are the reliable generation, transmission, and manipulation of quantum states. Two-dimensional quantum states, qubits, are steadily adopted as information units. However, high-dimensional quantum states, qudits, constitute a richer resource for future quantum networks, exceeding the limitations imposed by the ubiquitous qubits. The generation and manipulation of such $D$-level systems have been improved over the last ten years, but their reliable transmission between remote locations remains the main challenge. Here, we show how a recent air-core fiber supporting orbital angular momentum (OAM) modes can be exploited to faithfully transmit $D$-dimensional states. Four OAM quantum states and their superpositions are created, propagated in a 1.2-km-long fiber and detected with high fidelities. In addition, three quantum-key-distribution protocols are implemented as concrete applications to assert the practicality of our results. This experiment enhances the distribution of high-dimensional quantum states, attesting the orbital angular momentum as a vessel for the future quantum network.

A method to generate spatiotemporally controllable plasma lattice structures via dielectric barrier discharge is proposed by utilizing a latticed water electrode. A variety of plasma lattice structures including triangle, hexagon, honeycomb, and complex superlattices are obtained by changing three parameters including the applied voltage, the gas pressure, and the gas compositions. Moreover, these plasma lattices can be quickly reconstructed, allowing for active control both in space and time. Two-dimensional particle-in-cell simulations are carried out to demonstrate the formation of plasma structures under different voltages, which are in good agreement with experimental observations. Our method provides a unique way for the fabrication of controllable plasma lattice structures, which may enable tunable control of microwave radiation for wide applications.

Form birefringence, where the orientation of periodic structures yields effective refractive-index differences between orthogonal polarizations, is integral to multidimensional optical data storage, is apparent for certain metamaterials, yet is also critical to the inspection of periodic devices in nanoelectronics. A traditional assumption behind form birefringence—that the wavelength is larger than the periodicity—challenges several applications at shorter wavelengths. In this paper, the form dependence of high spatial-frequency scattering off structures with two-dimensional periodicity is numerically assessed among deep-UV (DUV), vacuum-UV (VUV), and EUV illumination wavelengths. At each wavelength, the available optical hardware is incorporated as a constraint and the impact of known laboratory source intensities is addressed. Evaluations are presented for both ideal periodic arrays and arrays with an added bridging defect that breaks the local periodic symmetry. Form-dependent-scattering (FDS) intensity ratios in the ideal case decrease with wavelength in the DUV and VUV, but dramatically increase for the 47-nm EUV wavelength, due to the wavelength-dependent optical properties of the conformal layers modeled. This increase at the 47-nm wavelength shifts the ideal polarization for FDS off the bridging defect and enhances sensitivity to these patterning faults. The results of these realistic simulations in this challenging wavelength range should be extensible, not only to structures in nanoelectronics, but also to emerging engineered optical materials for DUV and shorter wavelengths.

We report a hybrid multichannel metal-cladded slab-capillary microcavity, which supports simultaneous red, green, and blue lasing from an individual microcavity at room temperature. White and tunable lasing over the full range of visible colors is achieved through controlled injection of different dye solutions into the metal-cladded capillaries of the microcavity. The results show that RGB lasing beams with a line width of approximately 1 nm are observed from the metal-cladded slab-capillary microcavity. The experimental observations show the realization of white light and lasing simultaneously in a single hybrid microcavity. Moreover, the improvement in the laser structure enables the elimination of background noise. Our work describes a simplification in fabricating microlasers with dynamically color-controllable emissions and provides an important route toward the potential of realizing a microlaser device generating both white and full color light directly from a single monolithic structure.

Laser-induced keyholing occurs in additive manufacturing and welding processes, but the keyhole dynamics have not been well understood. A multiphase and multiphysics numerical model is used to predict the keyhole shapes recorded in the experimental observations and to predict transient and nonuniform distributions of laser absorption, temperature, and flow velocity in the process. When compared against data from a state-of-the-art dynamic x-ray radiography technique, good agreement is found for the keyhole shapes and fluctuation of the gas-liquid interface, thereby validating the simulation method. A detailed discussion is then given to elucidate the effects of laser absorption on the dynamic behavior of the front and rear keyhole walls. A quantitative comparison of different driving forces on the keyhole is also given to evaluate their significance to the keyhole dynamics.

Superconducting coplanar-waveguide resonators that can operate in strong magnetic fields are important tools for a variety of high-frequency superconducting devices. Magnetic fields degrade resonator performance by creating Abrikosov vortices that cause resistive losses and frequency fluctuations or suppress the superconductivity entirely. To mitigate these effects, we investigate lithographically defined artificial defects in resonators fabricated from $\mathrm{Nb}$-$\mathrm{Ti}\text{−}\mathrm{N}$ superconducting films. We show that by controlling the vortex dynamics, the quality factor of resonators in perpendicular magnetic fields can be greatly enhanced. Coupled with the restriction of the device geometry to enhance the superconductors critical field, we demonstrate stable resonances that retain quality factors $\simeq {10}^5$ at the single-photon power level in perpendicular magnetic fields up to $B_{\perp }\simeq 20\mathrm{mT}$ and parallel magnetic fields up to $B_{\parallel }\simeq 6\mathrm{T}$. We demonstrate the effectiveness of this technique for hybrid systems by integrating an $\mathrm{In}$-$\mathrm{Sb}$ nanowire into a field-resilient superconducting resonator and use it to perform fast charge readout of a gate-defined double quantum dot at $B_{\parallel }=1\mathrm{T}$.

Breaking reciprocity with spatiotemporal modulation provides an opportunity to design unprecedented optical, acoustic, and mechanical waveguides. A main challenge is to physically realize continuum-based metamaterials whose properties can be rapidly tuned in both space and time at the length and time scales of the propagated waves. We design a tunable elastic metamaterial by embedding in a beam a set of permanent magnets, and placing oscillating electrical coils coaxially adjacent to each magnet. By programming in space and time the ac input of the coils, the magnet-coil effective coupling stiffness is modulated along with the resonance frequency. Distinctly nonreciprocal flexural wave propagation is then experimentally observed. In addition, robust tunability of unidirectional band gaps and wave energy bias are quantitatively analyzed by applying different modulation current amplitudes, material damping coefficients, and modulation frequencies. Both simplified analytical and finite-element-method-based numerical models of the modulated metamaterial are suggested and analyzed in support of the experimental work. Specifically, unidirectional frequency conversions and band gaps due to the second-order mode interactions are discussed for the first time when the large modulation amplitude is implemented. The suggested prototype sheds light on nonreciprocal waveguiding, which could be applied in advanced wave diodes, phononic logic, energy localization, trapping, and harvesting.

The dynamics of a ${90}^{\circ }$ domain wall in an antiferromagnetic nanostrip driven by the current-induced spin-orbit torque are theoretically examined in the presence of thermal fluctuations. A soliton-type equation of motion is developed on the basis of energy balance between the driving forces and dissipative processes in terms of the domain-wall velocity. Comparison with micromagnetic simulations in the deterministic conditions shows good agreement in both the transient and steady-state transport. When the effects of thermal fluctuations are included via a stochastic treatment, the results clearly indicate that the dispersion in the domain-wall position can be controlled electrically by tailoring the strength and duration of the driving current mediating the spin orbital torque in the antiferromagnet. More specifically, the standard deviation of the probability distribution function for the domain-wall movement can be tuned widely while maintaining the average position unaffected. Potential applications of this unusual functionality include probabilistic computing such as Bayesian learning.

The antiferroelectric-ferroelectric phase transition in ${\mathrm{Pb}\mathrm{Zr}\mathrm{O}}_3$-based oxides is of both fundamental and practical importance. In ceramics in which such a transition readily occurs, the antiferroelectric and the ferroelectric phases often coexist in individual grains with a coherent interphase interface. In this work, the electric biasing in situ transmission electron microscopy technique is employed to directly observe a unique microstructural dynamic when ferroelectric and antiferroelectric domains are driven by a moderate electric field to interact. It is found that, under monotonic loading, the ferroelectric domain grows until it is blocked by the ferroelectric-antiferroelectric interface. At the same time, a kink is formed on the interface at the contact point. The interaction of the growing domain with the interface is interpreted in terms of depolarization field-assisted phase transition, which is supported by our phase-field simulation. Upon further bipolar cycling, the ferroelectric domain becomes less mobile and no longer reaches the ferroelectric-antiferroelectric interface, indicative of electric fatigue of the ferroelectric phase.

The major challenge to improve deterministic single-ion sources is to control the position and momentum of each ion. On the basis of the extra information given by the electron created in a photoionization process, the trajectory of the correlated ion can be corrected with use of a fast real-time feedback system. In this paper, we report on a proof-of-principle experiment that demonstrates the performance of this feedback control with individual cesium ions. The electron produced is detected with a time- and position-sensitive detector, the information from which is used to quickly infer the position of the corresponding ion. Then the feedback system drives the ion trajectory through steering plates, and the individual ion can thus be sent to any dedicated location. This enables us to perform deterministic patterning and achieve a factor-of-1000 reduction in spot area. The single-ion feedback control is versatile and can be applied to different kinds of ion sources. It provides a powerful tool to optimize the ion beam and offers a complete portfolio for quantum systems and applications in materials science.

We present an ultrahigh gain, narrow bandwidth, and low noise optical amplifier via the radiation pressure induced by the four-wave mixing (FWM) effect based on a cavity optomechanical system in the unresolved sideband regime. It offers a favorable platform to amplify a weak signal field and simultaneously generate a conjugate field with tunable gain and bandwidth. We achieve an ultranarrow bandwidth (9 Hz) and ultrahigh gain (40 dB) optical field amplification at a weak pump power (10 µW). Due to the unresolved sideband regime, the system supports simultaneous multimode amplification. The presented optomechanical FWM mechanism-based amplifier can provide standard quantum-noise-limit amplification, in principle, and is applicable to a broad range of the electromagnetic spectrum ranging from the microwave band to the ultraviolet band.

Spin and valley are two coupled degrees of freedom relevant for low-energy electrons in heavy group-IV monolayers with strong spin-orbit couplings, such as silicene, germanene, and stanene. Using these materials, we propose that a spin-diode effect driven by longitudinal bias can be realized in a ferromagnetic/antiferromagnetic junction, where an interlayer electric field with strength $E_z$ is applied in the antiferromagnetic region. This effect shows a unique bipolar-unipolar transition strongly depending on $E_z$. When $E_z=0$, only electrons from one valley contribute to electric current, with one spin only moving forward and the opposite spin only moving backward, thus corresponding to a bipolar spin diode. When $E_z$ is increased, the active valley can be switched to the other valley, and only electrons of one spin orientation are involved in transport and only move forward, corresponding to a unipolar spin diode. This transition is attributed to the characteristic band-to-band tunneling mechanism of spin-valley matching. We also find that, by reversing the direction of the interlayer electric field, the bipolar or unipolar regime of the spin diode does not change as a result of the symmetry in transport. By electrically adjusting the longitudinal bias and $E_z$, we further prove that the signals produced by the spin diode are strong enough to be observed even when the magnetic exchange fields are on the order of 1 meV, which is fully available in experimentally reported samples with $\mathrm{Eu}$ ($\mathrm{Gd}$)-intercalated materials or those in proximity to ferromagnetic insulators. Our findings on the valley-mediated controllability and switching between the bipolar and unipolar regimes of the spin-diode operation suggest promising applications of heavy group-IV monolayers in improvement of reprogrammable spin logic and nonvolatile memory.

Current-induced spin-orbit torque (SOT) driven magnetization switching has relied mostly on 4d and 5d heavy metals with strong spin-orbit coupling to generate large spin currents to deliver SOT and a $\mathrm{Mg}\mathrm{O}$ layer to acquire perpendicular magnetic anisotropy (PMA). We demonstrate 3d $\mathrm{Cr}$ metal can generate PMA and deliver SOT switching with more prowess than $\mathrm{Ta}$. In certain $\mathrm{Cr}$-based heterostructures, even field-free SOT switching has been achieved due to the subtle microstructure in the layered structure. The field-free SOT switching device based on 3d $\mathrm{Cr}$ provides significant advantages for next generation high-efficiency and low-cost nonvolatile spintronic devices.

The nondestructive detection and evaluation of crystallographic properties of nanocrystals is of great significance for both fundamental physics research and further development of high-performance functional devices employing nanostructured materials. Synchrotron radiation-based CXD using a nanoscale x-ray beam is shown to be a powerful tool to explore the crystallographic properties of $\mathrm{In}\mathrm{P}$ nanocrystals (NCs) selectively grown on $\mathrm{Si}$ nanotip wafers. CXD characterization clearly clarifies, with atomic sensitivity and without complex sample preparation, the crystallographic properties of the selected $\mathrm{In}\mathrm{P}$ NC such as the structure of the facets, the strain, the existence of defects (stacking faults and microtwins), and the size of the defected crystallites. Several selected $\mathrm{In}\mathrm{P}$ NCs explored by CXD reveal homogeneous structures. The CXD results are in good agreement with electron microscopy. These results not only confirm that nanoheteroepitaxy is a promising approach to monolithically integrate high-quality III-V compounds on silicon wafers, but also opens a pathway to nondestructively explore the crystallinity of materials on the nanometer scale, particularly in nano-electronic and nano-optoelectronic devices.

Two-dimensional (2D) semiconductors, graphene, ${\mathrm{Mo}\mathrm{S}}_2$, and black phosphorus, have attracted tremendous interest for their unique mechanical, electronic, optical, and magnetic properties. However, few of the previously studied systems are perfect for practical electronic and optoelectronic applications due to limitations such as zero band gap, low carrier mobility, severe instability, or low absorption coefficient. Here, we predict an alternative ultrathick 2D semiconducting ${\mathrm{Sn}\mathrm{Sb}}_2{\mathrm{Te}}_4$ with high mobility ($2.782\times {10}^4{\mathrm{cm}}^2\cdot {\mathrm{V}}^{-1}\cdot {\mathrm{s}}^{-1}$) and large absorption coefficient ($1.29\times {10}^6$). Intriguingly, the electronic band gap of 2D ${\mathrm{Sn}\mathrm{Sb}}_2{\mathrm{Te}}_4$ exhibits an indirect-direct transition with increasing thickness, which is opposite to that observed in many 2D transition metal disulfides. Importantly, compared with the monolayer, bilayer ${\mathrm{Sn}\mathrm{Sb}}_2{\mathrm{Te}}_4$ possesses stronger light harvesting, higher mobility, and larger on-state current by about one order of magnitude due to electronic states’ rehybridization with charge accumulation in the interlayer region. All these features suggest 2D ${\mathrm{Sn}\mathrm{Sb}}_2{\mathrm{Te}}_4$ as a promising competitor for future high-performance infrared electronic and optoelectronic applications.

Universal unitary photonic devices can apply arbitrary unitary transformations to a vector of input modes and provide a promising hardware platform for fast and energy-efficient machine learning using light. We simulate the gradient-based optimization of random unitary matrices on universal photonic devices composed of imperfect tunable interferometers. If device components are initialized uniform randomly, the locally interacting nature of the mesh components biases the optimization search space toward banded unitary matrices, limiting convergence to random unitary matrices. We detail a procedure for initializing the device by sampling from the distribution of random unitary matrices and show that this greatly improves convergence speed. We also explore mesh architecture improvements such as adding extra tunable beam splitters or permuting waveguide layers to further improve the training speed and scalability of these devices.

There has been recent interest in neuromorphic photonics, a field with the promise to access pivotal and unexplored regimes of machine intelligence. Progress has been made on isolated neurons and analog interconnects; nevertheless, this renewal of interest has yet to produce a demonstration of a silicon photonic neuron capable of interacting with other like neurons. We report a modulator-class photonic neuron fabricated in a conventional silicon photonic process line. We demonstrate the behaviors of transfer-function configurability, fan-in, inhibition, time-resolved pulse processing, and, crucially, autaptic cascadability—a sufficient set of behaviors for a device to act as a neuron participating in a network of like neurons. The silicon photonic modulator neuron constitutes the final piece needed to make photonic neural networks fully integrated on currently available silicon photonic platforms.