Considerable work has been undertaken for the improvement of wave-energy converters and array design. It has recently been suggested that by extracting wave energy, these farms could also serve to protect shorelines from wave damage. The present work focuses on the local effects of wave-structure interactions within an array of oscillating absorbers to optimize global effects, such as reflection, damping, and energy absorption. We use a model system of flexible blades, subjected to monochromatic waves, and develop a simplified one-dimensional model to predict optimal configurations, depending on various parameters, which include the number of blades, their spacing, and their flexibility. Optimal configurations are found to be close to regular patterns, and the impact of array configurations is shown to be limited regarding wave dissipation, mainly due to a competition between reflection and absorption.

The twin-field quantum key distribution (TFQKD) protocol and its variants, e.g., phase-matching (PM) QKD and TFQKD based on sending or not sending, are highly attractive since they are able to overcome the well-known rate-loss limit for QKD protocols without a repeater: $R=O(\eta )$, with $\eta$ standing for the channel transmittance. However, all these protocols require active phase randomization and postselection that play an essential role together in their security proof. Counterintuitively, we find that in TFQKD, beating the rate-loss limit is still possible even if phase randomization and postselection in the coding mode are both removed, which means our final secure key rate $R=O(\sqrt{\eta })$. Furthermore, our protocol is more feasible in practice and more promising according to its higher final key rate in the valid distance. Our security proof counters a collective attack and can also counter a coherent attack in the asymptotical case.

The development of alternative mm-wave high-gradient, >200 MV/m, accelerating structures offers a promising path to reduce the cost and footprint of future TeV-scale linear colliders, as well as linacs for industrial, medical, and security applications. The major factor limiting accelerating gradient is vacuum rf breakdown. The probability of such breakdowns increases with pulse length. For reliable operation, millimeter-wave structures require nanoseconds-long pulses at the megawatt level. This power is available from gyrotrons, which have a minimum pulse length on the order of microseconds. To create shorter pulses and to reliably detect rf breakdowns, we developed the following devices: a laser-based rf switch capable of selecting 10 ns long pulses out of the microseconds long gyrotron pulses, thus enabling the use of the gyrotrons as power sources for mm-wave high-gradient linacs, and a shot-to-shot sub-THz spectrometer with high-frequency resolution, capable of detecting pulse shortening due to rf breakdowns. We will describe the principle of operation of these devices and their achieved parameters. We also report on the experimental demonstration of these devices with the high-power gyrotron at the Massachusetts Institute of Technology. In the experiments, we demonstrate nanosecond rf power modulation, shot-to-shot measurements of the pulse spectra, and detection of rf breakdowns.

When electric and magnetic fields are applied together on a magnetoelectric antiferromagnet, the domain state is subject to reversal. Although the initial and final conditions are saturated single-domain states, the process of reversal may decompose into local multidomain switching events. In thin films of ${\mathrm{Cr}}_2{\mathrm{O}}_3$, the magnetoelectric coercivity and the switching speed found from experiments are considerably lower than expected from magnetic anisotropy, similar to Brown’s paradox in ferromagnetic materials. Multidomain effects originate because antiferromagnetic domain walls are metastably pinned by lattice defects, not due to reduction of magnetostatic energy, which is negligible. This paper theoretically analyzes domain reversal in thin-film magnetoelectric antiferromagnets in the form of nucleation, domain-wall propagation, and coherent rotation. The timescales of reversal mechanisms are modeled as a function of applied magnetoelectric pressure. The theory is assessed with reference to the latest experimental works on magnetoelectric switching of thin-film ${\mathrm{Cr}}_2{\mathrm{O}}_3$: domain-wall propagation is found to be dominant and responsible for switching in the experiments. The results bear implications for the energy-delay performance of magnetoelectric memory devices utilizing antiferromagnetic insulators, which are prospective for nonvolatile technology.

We fabricate and investigate a stacked two-chip device, consisting of a lumped-element resonator on one chip, which is side coupled to a coplanar waveguide transmission line on a second chip. We present a full model to predict the behavior of the device dependent on the position of the lumped-element resonator with respect to the transmission line. We identify different regimes in which the device can be operated. One of them can be used to tune the coupling between the two subsystems. Another regime enables frequency tunability of the device, without leaving the overcoupled limit for internal quality factors of about ${10}^4$, while in the last regime the resonator properties are insensitive against small variations of the position. Finally, we measure the transmission characteristics of the resonator for different positions, demonstrating a good agreement with the model.

Use of the spatial degree of freedom vastly enhances the informational capacity of light at the cost of stringent requirements on the processing devices. Multimode quantum memories are a viable candidate for quantum and classical information processing; however, full use of the assets of high-dimensionality requires a flexible processing technique. We use a spatially varying ac Stark effect to perform arbitrary one-dimensional phase modulation of a coherent spin-wave state stored in a wavevector-multiplexed quantum memory. A far-field characterization and an interferometric near-field characterization of the introduced phase profiles are presented. Additionally, coherence between temporally separated partial readouts of a single coherent spin-wave state is demonstrated, offering possible applications in adaptive measurements via conditional spin-wave modulation.

Suggesting the AB-stacked bilayer ${\mathrm{WS}}_2/{\mathrm{Mo}\mathrm{S}}_2$ heterostructure as an ideal architecture for Berry curvature engineering, we theoretically demonstrate that it is possible to manipulate valley polarizations at K and −K valleys on each layer in a distinguishable fashion and then attempt an operation using four channels of valley manipulations, that is, four pairwise incorporations of ${\mathbf{K}}_{\mathrm{W}{\mathrm{S}}_2}$ and $(-\mathbf{K}{)}_{\mathrm{W}{\mathrm{S}}_2}$, ${\mathbf{K}}_{\mathrm{Mo}{\mathrm{S}}_2}$ and $(-\mathbf{K}{)}_{\mathrm{Mo}{\mathrm{S}}_2}$,${\mathbf{K}}_{\mathrm{W}{\mathrm{S}}_2}$ and ${\mathbf{K}}_{\mathrm{Mo}{\mathrm{S}}_2}$, and $(-\mathbf{K}{)}_{\mathrm{W}{\mathrm{S}}_2}$ and $(-\mathbf{K}{)}_{\mathrm{Mo}{\mathrm{S}}_2}$, within a frame of the electro-optic method. Four-channel valley manipulation, which provokes varied anomalous Lorentz effects, conveys a development of multimode inverse valley Hall currents comprising two intralayer and one interlayer modes, each of which has a different nonlocal resistance. This finding proposes an alternative discipline of valley valve of valleytronics to manage a variable (multilevel) nonlocal resistance to the valley-mediated nonlocal charge current, in analogy with the spin valve of spintronics.

Spin-based coherent information processing and encoding utilize the precession phase of spins in magnetic materials. However, the detection and manipulation of spin precession phases remain a major challenge for advanced spintronic functionalities. By using simultaneous electrical and optical detection, we demonstrate the direct measurement of the spin precession phase of a permalloy device driven by the spin-orbit torques from adjacent heavy metals. The spin Hall angle of the heavy metals can be independently determined from concurrent electrical and optical signals. The phase-sensitive optical detection also allows spatially-resolved measurements of local spin-torque parameters and ferromagnetic resonance with comprehensive amplitude and phase information. Our study offers a route toward future advanced characterizations of spin-torque oscillators, magnonic circuits, and tunneling junctions, where measuring the current-induced spin dynamics of individual nanomagnets is required.

Spin waves in insulating materials such as Yttrium Iron Garnet (YIG) can be used for signal propagation and processing using the spin of the electrons rather than transport of their charge. Planar YIG films can be integrated with silicon technology to realize devices such as tunable filters, frequency selective limiters, and signal-to-noise enhancers. However, such films suffer from spin-wave damping, which limits their use in such applications. Here, we show that spin currents in topological insulators (TI) can be used to reduce spin-wave damping. TI supports surface spin currents, potentially making it an efficient source of antidamping torque. We show that in a YIG/${\mathrm{Bi}}_2{\mathrm{Se}}_3$ bilayer, the spin-wave damping rate can be reduced by 60% at a current density of $8\times {10}^5\mathrm{A/c}{\mathrm{m}}^2$. Furthermore, we show that the damping reduction has a strong dependence on spin-wave frequency and we demonstrate that this dependence arises from nonlinear magnon scattering.

We report on the experimental implementation of topological edge-state arrays with power confinement in the edge state as high as 29 dB relative to its farthermost channel. Average insertion losses of <2 dB in the edge channel vs total output power in the whole array are measured. The device follows a Su-Shrieffer-Heeger construction, consisting of seven waveguides fabricated on (100) liquid-phase-epitaxy-grown bismuth-substituted lutetium iron garnet films on a gadolinium gallium garnet substrate. The delocalization of the field can be realized by magnetizing only the single edge-state channel, thereby diverting the backscattered signal to serve as a high-performing optical isolator with a predicted isolation ratio as high as −50 dB. We discuss methods and challenges for full experimental implementation of a SSH topology-based optical isolator.

Wavelengths in the telecommunication window (approximately 1.25–1.65 $\mu \mathrm{m}$) are ideal for quantum communication due to low transmission loss in fiber networks. To realize quantum networks operating at these wavelengths, long-lived quantum memories that couple to telecom-band photons with high efficiency need to be developed. We propose coupling neutral ytterbium atoms, which have a strong telecom-wavelength transition, to a silicon photonic crystal cavity. Specifically, we consider the ${}^3{P}_0{\leftrightarrow }^3{D}_1$ transition in neutral ${}^{171}\mathrm{Yb}$ to interface its long-lived nuclear spin in the metastable ${}^3{P}_0$ “clock” state with a telecom-band photon at $1.4\mu \mathrm{m}$. We show that $\mathrm{Yb}$ atoms can be trapped using a short-wavelength (approximately $470$ nm) tweezer at a distance of 350 nm from the silicon photonic crystal cavity. At this distance, due to the slowly decaying evanescent cavity field at a longer wavelength, we obtain a single-photon Rabi frequency of $g/2\pi \approx 100$ MHz and a cooperativity of $C\approx 47$ while maintaining a high photon collection efficiency into a single mode fiber. The combination of high system efficiency, telecom-band operation, and long coherence times makes this platform well suited for quantum optics on a silicon chip and long-distance quantum communication.

Optics naturally provides us with some powerful mathematical operations. Here, we experimentally demonstrate that during reflection or refraction at a single optical planar interface, the optical computing of spatial differentiation can be realized by analyzing specific orthogonal polarization states of light. We show that the spatial differentiation is intrinsically due to the spin Hall effect of light and generally accompanies light reflection and refraction at any planar interface, regardless of material composition or incident angles. The proposed spin-optical method takes advantages of a simple and common structure to enable vectorial-field computation and perform edge detection for ultrafast image processing.

Since their first synthesis in 2015, the all-inorganic lead halide perovskite nanocrystals ${\mathrm{Cs}\mathrm{Pb}\mathrm{X}}_3\left(\mathrm{X}=\mathrm{Cl},\mathrm{Br},\mathrm{I}\right)$ have attracted a great deal of attention due to their outstanding electronic and optical properties as well as their performances, which outclass the ones of their II–VI conterparts in many application fields. In addition to these properties, the understanding of the emission features in these systems at the single object scale is crucial, e.g., for nanophotonics and quantum optics devices. Here, the details of the band-edge excitonic emission are theoretically explored. The contribution of the long-range exchange interaction to the bright-exciton splittings is computed in strong and weak confinement regimes using group theory arguments and k$\cdot$p approximation. We show that the shape anisotropy with the real crystalline (cubic, tetragonal, or orthorhombic) structures of nanocrystals explain well their emission properties. In the weak confinement regime, splittings are inversely proportional to the cube of the exciton Bohr radius and we observe an increase of the splittings from iodide, to bromide, and then to chloride perovskite compounds. However, in the strong confinement regime, splittings increase inversely proportional to the nanocrystal volume and, for a given nanocrystal size, the splitting values are comparable for the three-halide-perovskite materials. The present theoretical developments lead to quantitative contributions in good agreement with available experimental data mainly in the weak confinement regime.

Stimulated Raman-scattering (SRS) microscopy has emerged as a powerful tool for quantitative vibrational imaging of live tissues and cells with biochemical specificity, but the spatial resolution of SRS microscopy is still diffraction limited. We present work on saturated stimulated-Raman-scattering (SSRS) microscopy to break the diffraction limit for far-field superresolution imaging in biological systems. A virtual sinusoidal modulation (VSM) method is also proposed for retrieval of superresolution SSRS images. We show an experimental implementation of SSRS microscopy for enhancing the spatial resolution of SRS imaging in samples (e.g., plant cells, live HeLa cells) with a spectral focusing hyperspectral SRS system. With saturation occurring in coherent Raman processes under a certain level of excitation powers, it is anticipated that the SSRS technique has the ability to become an appealing tool for label-free far-field superresolution SRS imaging in biological and biomedical systems.

We present a spin-exchange relaxation-free vector magnetometer with suppressed $1/f$ probe noise, achieved by applying a small dc bias field and a comb of magnetic dc $\pi$ pulses along the pump direction. This results in a synchronous orthogonal ac response for each of its two sensitive axes. The magnetometer is particularly well suited to applications such as biomagnetism in which the signal to be measured carries a dominant component of its power at low frequencies. The magnetometer reaches a technical noise floor of 8.4 fT ${\mathrm{Hz}}^{-1/2}$ ($\hat{\mathbf{x}}$) and 11 fT ${\mathrm{Hz}}^{-1/2}$ ($\hat{\mathbf{y}}$) at 0.01 Hz. A single-axis dc spin-exchange relaxation-free (SERF) magnetometer sharing the same experimental apparatus attains 61 fT ${\mathrm{Hz}}^{-1/2}$ at the same frequency. A noise minimum of 1.1 fT ${\mathrm{Hz}}^{-1/2}$ ($\hat{\mathbf{x}}$) and 2.0 fT ${\mathrm{Hz}}^{-1/2}$ ($\hat{\mathbf{y}}$) is reached by the magnetometer at 10 Hz, compared to 0.7 fT ${\mathrm{Hz}}^{-1/2}$ at 25 Hz for a dc SERF magnetometer.

Synthetic microswimmers mimicking biological movements at the microscale have been developed in recent years. Actuating helical magnetic materials with a homogeneous rotating magnetic field is one of the most widespread techniques for propulsion at the microscale, partly because the actuation strategy revolves around a simple linear relationship between the actuating field frequency and the propeller velocity. However, full control of the swimmers' motion has remained a challenge. Increasing the controllability of micropropellers is crucial to achieve complex actuation schemes that, in turn, are directly relevant for numerous applications. However, the simplicity of the linear relationship limits the possibilities and flexibilities of swarm control. Using a pool of randomly shaped magnetic microswimmers, we show that the complexity of shape can advantageously be translated into enhanced control. In particular, directional reversal of sorted micropropellers is controlled by the frequency of the actuating field. This directionality change is linked to the balance between magnetic and hydrodynamic forces. We further show an example of how this behavior can experimentally lead to simple and effective sorting of individual swimmers from a group. The ability of these propellers to reverse swimming direction solely by frequency increases the control possibilities and is an example for propeller designs, where the complexity needed for many applications is embedded directly in the propeller geometry rather than external factors such as actuation sequences.

Reaction-driven interfacial growth causes significant strain in layered architectures accompanied by mass transfer and moving boundaries. We present an analytical construct of the stress generated in a multilayer film, which incorporates the elastic-plastic strain of the growth layer, which suggests its strong dependence on the mechanical properties and thickness. This analytical formalism is further applied to a layered all-solid-state lithium battery architecture. This study demonstrates that mechanical stability can be enhanced by using a positive electrode material with high stiffness, porous hosts for lithium plating, and small external elastic constraints to buffer the volumetric changes in the electrode material. Our results also reveal that small surface flaws in the solid electrolyte and high internal hydrostatic pressure can alleviate lithium dendrite growth through surface cracks.

Using perovskite-type charge-transfer oxide thin films of ${\mathrm{Nd}\mathrm{Ni}\mathrm{O}}_3$ (NNO) as a model system, we demonstrate that the effects of lattice strain on the electronic transport properties can be more comprehensively understood by growing NNO films on a number of (001)-, (011)-, and (111)-cut single-crystal substrates with different lattice mismatches including the relaxor-based $0.31\mathrm{Pb}({\mathrm{In}}_{1/2}{\mathrm{Nb}}_{1/2}){\mathrm{O}}_3\text{−}0.35\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_3\text{−}0.34{\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_3$ (PIN-PMN-PT) and 0.71$\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_3\text{−}0.29{\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_3$ (PMN-PT) ferroelectric (FE) single crystals. In addition to the static lattice strains from conventional substrates (e.g., ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$, ${\mathrm{La}\mathrm{Al}\mathrm{O}}_3$), we in situ impose in-plane compressive or tensile strains to NNO films using FE/ferroelastic domain switching of FE substrates. An unprecedented electric-field-induced large out-of-plane compressive strain (−0.53%) and in-plane tensile strain (+0.81%) are achieved in the 25-nm NNO film by switching the polarization direction of the PIN-PMN-PT substrate at T = 200 K. This value is approximately 7.4 to 45 times larger than those previously reported in FE substrate-based heterostructures. As a result of the induced large lattice strain, the resistivity of the NNO film is modulated up to 125%. Further, taking advantage of the linear piezoelectric strain, a quantitative relationship between the resistivity and the in-plane strain of the NNO film is established, with a gauge fact of $(\mathrm{\Delta }\rho /\rho )/\delta {\epsilon }_{xx}\sim 40.8$. Moreover, using the domain-engineered FE/ferroelastic switching of PMN-PT substrates, multiple stable resistance states with good retention and endurance properties can be obtained at room temperature and the metal-to-insulator transition temperature (T) of NNO films can be modified by precisely controlling the electric-field-pulse sequence as a result of the nonvolatile remnant strain transferring from the PMN-PT to the NNO film. Our results demonstrate that the electric-field-tunable ferroelastic/piezoelectric strain approach can be utilized to gain deeper insight into the intrinsic strain-property relationship of perovskite nickelate films and provide a simple and energy efficient way to construct multistate resistive memories.

Compositionally graded interfaces in power electronic devices eliminate dislocations but they can also decrease thermal conduction, leading to overheating. We quantify the thermal resistances of $\mathrm{Ga}\mathrm{N}$/$\mathrm{Al}\mathrm{N}$ graded interfaces of varying thickness using ab initio Green’s functions and compare them with the abrupt interface case. A non-trivial power dependence of the thermal resistance versus the interface thickness emerges from the interplay of alloy and mismatch scattering mechanisms. We show that the overall behavior of such graded interfaces is very similar to that of a thin film of an effective alloy on the length scales relevant to real interfaces.

We theoretically investigate a near-quantum-limited parametric amplifier based on the nonlinear dynamics of quasiparticles flowing through a superconductor-insulator-superconductor junction. Photon-assisted tunneling, resulting from the combination of dc- and ac-voltage bias, gives rise to a strong parametric interaction for the electromagnetic modes reflected by the junction coupled to a transmission line. We show phase-sensitive and phase-preserving amplification, together with single- and two-mode squeezing. For an aluminum junction pumped at twice the center frequency, ${\omega }_0/2\pi =6\mathrm{GHz}$, we predict narrow-band phase-sensitive amplification of microwaves signals to more than 20 dB, and broadband phase-preserving amplification of 20 dB over a 3-dB bandwidth of 1.2 GHz. We also predict single- and two-mode squeezing reaching more than $-12\mathrm{dB}$ over a 3-dB bandwidth of 5.3-GHz. Moreover, with a simple impedance-matching circuit, we demonstrate a 3-dB bandwidth reaching 4.3 GHz for 20 dB of gain. A key feature of the device is that its performance can be controlled in situ with the applied dc- and ac-voltage biases.