Computational reversibility is necessary for quantum computation and inspires the development of computing systems, in which information carriers are conserved as they flow through a circuit. While conservative logic provides an exciting vision for reversible computing with no energy dissipation, the large dimensions of information carriers in previous realizations detract from the system efficiency, and nanoscale conservative logic remains elusive. We therefore propose a nonvolatile reversible computing system in which the information carriers are magnetic skyrmions, topologically-stable magnetic whirls. These nanoscale quasiparticles interact with one another via the spin Hall and skyrmion Hall effects as they propagate through ferromagnetic nanowires structured to form cascaded conservative logic gates. These logic gates can be directly cascaded in large-scale systems that perform complex logic functions, with signal integrity provided by clocked synchronization structures. The feasibility of the proposed system is demonstrated through micromagnetic simulations of Boolean logic gates, a Fredkin gate, and a cascaded full adder. As skyrmions can be transported in a pipelined and nonvolatile manner at room temperature without the motion of any physical particles, this skyrmion logic system has the potential to deliver scalable high-speed low-power reversible Boolean and quantum computing.

We present measurements of the capacitive coupling energy and the interdot capacitances in a linear quadruple-quantum-dot array in undoped $\mathrm{Si}$/$\mathrm{Si}\mathrm{Ge}$. With the device tuned to a regime of strong ($>1\mathrm{GHz}$) intra-double-dot tunnel coupling, as is typical for double-dot qubits, we measure a capacitive coupling energy of $20.9\pm 0.3\mathrm{GHz}$. In this regime, we demonstrate a fitting procedure to extract all the parameters in the four-dimensional Hamiltonian for two capacitively coupled charge qubits from a two-dimensional slice through the quadruple-dot charge-stability diagram. We also investigate the tunability of the capacitive coupling energy, using interdot barrier gate voltages to tune the inter- and intra-double-dot capacitances, and change the capacitive coupling energy of the double dots over a range of 15–32 GHz. We provide a model for the capacitive coupling energy based on the electrostatics of a network of charge nodes joined by capacitors, which shows how the coupling energy should depend on inter-double-dot and intra-double-dot capacitances in the network, and find that the expected trends agree well with the measurements of coupling energy.

The optimal segmentation of three-dimensional permanent-magnet systems into uniformly magnetized blocks to generate a desired field is a question that has been studied extensively in recent years. We present a procedure that generates the theoretically optimal shape and magnetization direction of all the magnet segments in the system, given only the total number of blocks as a constraint. Materials of arbitrary magnetic permeability can be included in this optimization framework. As an optimization objective we consider any functional that is linear with respect to the magnetic field distribution. We furthermore assume that all magnetic materials obey a linear constitutive relation. We show that with these assumptions calculating an optimal three-dimensional segmentation is equivalent to determining a centroidal Voronoi tessellation. We use the most-well-known and most-straightforward algorithm for the generation of centroidal Voronoi tessellations, known as “Lloyd’s method,” which can also be thought of as a fixed-point iteration. We show that the procedure is guaranteed to lead to a configuration that is at least locally optimal with respect to the assumed linear objective functional. However, we present results providing a strong indication that for most design problems applying our technique a few times starting from different random configurations is very likely to lead to the globally optimal solution.

Piezoacoustic drop-on-demand (DOD) inkjet printing is widely applied in high-end digital printing due to its unprecedented precision and reproducibility. Micron-sized droplets of a wide range of chemical compositions can be deposited; however, the stability of piezoacoustic DOD inkjet printing can sometimes be compromised through the stochastic entrainment of bubbles within the ink channel. Here, bubble nucleation, translation, and growth are studied in an experimental silicon-based printhead with a glass nozzle plate using high-speed imaging that is triggered by changes in the ink-channel acoustics. It is found that impurities in the ink can trigger bubble nucleation upon their interaction with the oscillating meniscus. Cavitation inception on a dirt particle during the rarefaction pressure wave is identified as a second mechanism for bubble formation. The acoustic driving pressure within the ink channel, and its change upon bubble nucleation, are obtained from a fit of a Rayleigh-Plesset-type bubble-dynamics equation to the measured time-resolved radial dynamics of the bubble. The measured decrease in channel resonance frequency after bubble entrainment results in a 24% increased ink-jet length. The nucleated bubbles translate toward the ink-channel walls due to acoustic radiation forces and ink streaming. The convective ink flow is characterized using high-speed particle-tracking velocimetry. The vortical flow near the oscillating meniscus is shown to trap the impurities, thereby increasing the particle-to-meniscus interaction probability and, correspondingly, the bubble-entrainment probability.

The orbital angular momentum (OAM) of light has attracted a great amount of interest in recent times, due to the wide variety of applications it has made possible. Generally, the OAM spectrum is produced by coaxial superposition of several vortex beams, which might increase the complexity of the system. Here, we propose and experimentally demonstrate a simple way to produce a state of light with a controllable OAM spectrum using a binary array of pinholes. More specifically, we show that a spiral structure can convert a plane wave into a beam with a wide OAM spectrum, which can be easily tuned to pure or multiple OAM modes by adapting the structure of the pinhole plate. Furthermore, we show that a simple pinhole plate can produce structured beams with particular OAM states, such as photonic gears (superposition of OAM modes with opposite topological charges ±ℓ) and OAM combs (an optical mode formed by a series of discrete and equally spaced OAM modes, akin to an optical frequency comb). It is worth noting that we demonstrate the OAM comb experimentally. This study provides an avenue for the flexible generation of OAM spectra and the simplicity of the setup could make this approach convenient for many applications, such as optical communications and quantum information.

One long-term goal of phononic communications is to achieve the controlled transport of elastic wave signals with enhanced information capacity and improved robustness, which is still challenging given the complexity of elastic waves in terms of vectorial movement and multiple polarizations. Here, we present our theoretical and experimental study of the robust transport of elastic wave signals along interfaces between distinct topological phases. These topological edge states are robust against defects and disorders because they are jointly protected by both pseudospin and valley degrees of freedom (DOFs); thus naturally providing doubled information carriers within a single channel. A topology-based beam splitter is experimentally demonstrated, where the signal's propagation path is uniquely determined and topologically protected. Edge states between distinct topological phases not only offer a route towards new topological phenomena, but also open up an avenue for the design of high-capacity and robust phononic communication devices through discrete DOF multiplexing.

While wired-power-transfer devices ensure robust power delivery even if the receiver position or load impedance changes, achieving the robustness of wireless power transfer (WPT) is challenging. Conventional solutions are based on additional control circuits for dynamic tuning. Here we propose a robust WPT system in which no additional tuning circuitry is required for robust operation. This is achieved by our systematically designing the load and the coupling link to be parts of the feedback circuit. Therefore, the WPT operation is automatically adjusted to the optimal working condition under a wide range of load and receiver positions. In addition, pulsed oscillations instead of single-harmonic oscillation are adopted to increase the overall efficiency. An example system is designed with use of a capacitive coupling link. It realizes a virtual, nearly-ideal oscillating voltage source at the load site, giving efficient power transfer comparable to that of the ideal wired-connection scenario. We numerically and experimentally verify the robustness of the WPT system under the variations of load and coupling, where coupling is changing by our varying the alignment of aluminum plates. The working frequency and the transferred power agree well with analytical models. The proposed paradigm can have a significant impact on future high-performance WPT devices. The designed system can also work as a smart table supporting multiple receivers with robust and efficient operation.

We investigate the spin-orbit torque in a $\mathrm{Ta}/\mathrm{Ir}\text{−}\mathrm{Mn}/\mathrm{Cu}/\mathrm{Ni}\text{−}\mathrm{Fe}$ multilayer heterostructure and relate it to spin current transmission through the $\mathrm{Ir}\text{−}\mathrm{Mn}$ layer. We identify several spin current transport regimes as a function of the temperature and the thickness of the $\mathrm{Ir}\text{−}\mathrm{Mn}$ layer. To interpret this experiment, we develope a drift-diffusion model accounting for both electron and magnon transport in the heterostructures. This model allows us to discriminate between the contributions of electrons and magnons to the total spin current in $\mathrm{Ir}\text{−}\mathrm{Mn}$. We find that the electron-magnon spin convertance is one order of magnitude larger than the interfacial electronic spin conductance, while the magnon diffusion length is about ten times longer than the electronic spin relaxation length. This study demonstrates that magnonic spin transport dominates over electronic spin transport even in disorder metallic antiferromagnets.

We present a protocol for directly detecting time-dependent magnetic field waveforms with a quantum two-level system. Our method is based on a differential refocusing of segments of the waveform using spin echoes. The sequence can be repeated to increase the sensitivity to small signals. The frequency bandwidth is intrinsically limited by the duration of the refocusing pulses. We demonstrate detection of arbitrary waveforms with approximately $20\mathrm{ns}$ time resolution and approximately $4\mu \mathrm{T}/\sqrt{\mathrm{Hz}}$ field sensitivity using the electronic spin of a single nitrogen-vacancy center in diamond.

Parasitic crosstalk in superconducting quantum devices is a leading limitation for quantum gates. We demonstrate the suppression of static ZZ crosstalk in a two-qubit, two-coupler superconducting circuit, where the frequency of a tunable coupler can be adjusted such that the ZZ interactions from each coupler destructively interfere. We verify the crosstalk elimination with simultaneous randomized benchmarking, and use a parametrically activated iswap interaction to achieve a Bell-state preparation fidelity of 98.5% and a $\sqrt{\mathrm{i}\mathrm{swap}}$-gate fidelity of 94.8% obtained via quantum-process tomography.

A time scale is a procedure for accurately and continuously marking the passage of time. It is exemplified by Coordinated Universal Time (UTC) and provides the backbone for critical navigation tools such as the Global Positioning System. Present time scales employ microwave atomic clocks, whose attributes can be combined and averaged in a manner such that the composite is more stable, accurate, and reliable than the output of any individual clock. Over the past decade, clocks operating at optical frequencies have been introduced that are orders of magnitude more stable than any microwave clock. However, in spite of their great potential, these optical clocks cannot be operated continuously, which makes their use in a time scale problematic. We report the development of a hybrid microwave-optical time scale, which only requires the optical clock to run intermittently while relying upon the ensemble of microwave clocks to serve as the flywheel oscillator. The benefit of using a clock ensemble as the flywheel oscillator instead of a single clock can be understood by the Dick-effect limit. This time scale demonstrates for the first time subnanosecond accuracy over a few months, attaining a fractional frequency stability of 1.45 × ${10}^{-16}$ at 30 days and reaching the ${10}^{-17}$ decade at 50 days, with respect to UTC. This time scale significantly improves the accuracy in timekeeping and could change the existing time-scale architectures.

The recently discovered phenomena of photomagnetic recording using femtosecond laser pulses in magnetic dielectrics opens up intriguing opportunities for ultrafast least-dissipative magnetic writing. Fundamental questions remain, however, about the processes, which limit the repetition rate of such writing. We report the magnetization dynamics and switching caused by a double-pulse excitation with various orientations of pump polarization. In particular, we study write and erase sequences, showing that mutual interference of the effects caused by two laser pulses can occur if the pulses are temporally separated by less than 60 ps. This imposes the fundamental limit for the repetition rate of writing (20 GHz), but this could be increased even further by strengthening the photoinduced magnetic anisotropy.

High-doping-induced Urbach tails and band-gap narrowing play a significant role in determining the performance of tunneling devices and optoelectronic devices such as tunnel field-effect transistors, Esaki diodes, and light-emitting diodes. In this work, Urbach tails and band-gap-narrowing values are calculated explicitly for $\mathrm{Ga}\mathrm{As}$, $\mathrm{In}\mathrm{As}$, $\mathrm{Ga}\mathrm{Sb}$, and $\mathrm{Ga}\mathrm{N}$ as well as for ultrathin bodies and nanowires of the same materials. Electrons are solved for by the nonequilibrium Green’s function method in multiband atomistic tight binding. Scattering on polar optical phonons and charged impurities is solved for in the self-consistent Born approximation. The corresponding nonlocal scattering self-energies and numerically efficient formulations for them are introduced for ultrathin bodies and nanowires. The predicted Urbach band tails and conduction-band-gap narrowing agree well with the experimental literature for a range of temperatures and doping concentrations. Polynomial fits of the Urbach tail and band-gap narrowing as a function of doping are tabulated for quick reference.

Transparent conducting oxides (TCOs) with the functionality of transparency plus conductivity lie at the center of many technological applications. Commonly used TCOs are $n$-type. However, the performances of $p$-type TCOs lag far behind their $n$-type counterparts and the development of high-performance $p$-type TCOs remains an arduous challenge. The electron correlation due to on-site Coulomb interaction in the correlated metal oxides can not only promote modification of the valence band, which forms the basis for $p$-type conduction, but also shift the screened plasma energy to below the visible region for enhancing optical transparency. In this paper, the electron correlation is utilized as a key material design parameter to improve the performance of $p$-type TCOs. This approach is experimentally verified by decreasing electron correlation from ${\mathrm{Cr}}_2{\mathrm{O}}_3$ to ${\mathrm{V}}_2{\mathrm{O}}_3$, which, despite its high hole concentration ($>{10}^{22}{\mathrm{cm}}^{-3}$), has low screened plasma energy (approximately 0.97 eV) and significantly boosts the performance through good balance between the electrical conductivity and optical transparency. These results indicate that the strategy of electron correlation engineering paves an effective way for exploring high-performance $p$-type TCOs in strongly correlated oxides.

Current-induced dynamics of twisted domain walls and skyrmions in ferromagnetic perpendicularly magnetized multilayers is studied through three-dimensional micromagnetic simulations and analytical modeling. It is shown that such systems generally exhibit a Walker-breakdown-like phenomenon in the presence of a current-induced dampinglike spin-orbit torque. Above a critical current threshold, corresponding to typical velocities of the order of tens of m/s, domain walls in some layers start to precess with frequencies in the gigahertz regime, which leads to oscillatory motion and a significant drop in mobility. This phenomenon originates from complex stray field interactions and occurs for a wide range of multilayer materials and structures that include at least three ferromagnetic layers and finite Dzyaloshinskii-Moriya interaction. An analytical model is developed to describe the precessional dynamics in multilayers with surface-volume stray field interactions, yielding qualitative agreement with micromagnetic simulations.

We present techniques to model and design membrane phononic crystals with low-mass defects, optimized for force sensing. Further, we identify the importance of the phononic crystal mass contrast as it pertains to the size of acoustic band gaps and to the dissipation properties of defect modes. In particular, we quantify the tradeoff between high-mass-contrast phononic crystals, with their associated robust acoustic isolation, and a reduction of soft clamping of the defect mode. We fabricate a set of phononic crystals with a variety of defect geometries out of high-stress stoichimetric silicon-nitride membranes and measure at both room temperature and 4 K in order to characterize the dissipative pathways across a variety of geometries. Analysis of these devices highlights a number of design principles integral to the implementation of low-mass low-dissipation mechanical modes into optomechanical systems.

Understanding and controlling chirality are important because of its ubiquity in nature. In recent years, we have witnessed the rapid development of chiral metamaterials with exceptional light-manipulating capabilities. Here, we present a design for photonic heterostructures in chiral photonics, demonstrating that photonic heterostructures stacked from achiral metasurfaces can create distinctive chiroptical functionalities. Analyses based on the symmetry group and microscopic dipolar interactions at two-dimensional boundaries reveal the implicit mechanism of the interlayer coupling in photonic heterostructures. By flip stacking, the photonic heterostructure gives rise to an asymmetric chiral phase from achiral composites, and hence, provides the full spin-flipped scattering of incident photons. Incident electromagnetic waves with arbitrary polarization orientations can be efficiently separated into two orthogonal circularly polarized ones with orthogonal trajectories, leading to an ideal circular beam splitter. This effect is experimentally demonstrated at microwave frequencies via the measurement of a scattering matrix. Due to its generality and scalability, we expect the methodology of photonic heterostructure might open up an alternative pathway for exploring intriguing and advanced metamaterials over a broad wavelength range.

A transducer capable of converting quantum information stored as microwaves into telecom-wavelength signals is a critical piece of future quantum technology as it promises to enable the networking of quantum processors. Cavity optomechanical devices that are simultaneously coupled to microwave fields and optical resonances are being pursued in this regard. Yet even in the classical regime, developing optical modulators based on cavity optomechanics could provide alternatives to current technology. Here we demonstrate a magnetically mediated wavelength-conversion technique based on mixing high-frequency tones with an optomechanical torsional resonator. This process can act either as an optical phase modulator or an amplitude modulator depending on the experimental configuration, and the carrier modulation is always coherent with the input tone. Such coherence allows classical information transduction and transmission via the technique of phase-shift keying. We demonstrate that we can encode up to eight bins of information, corresponding to three bits, simultaneously and demonstrate the transmission of a 52 500-pixel image over 6 km of optical fiber with just 0.67% error. Furthermore, we show that magneto-optomechanical transduction can be described in a fully quantum manner, implying that this is a possible approach to signal transduction in the quantum regime.

Applying nonharmonic driving-field waveforms to nanoparticles for magnetic hyperthermia has beneficial effects on their ability to generate heating power. The response of superparamagnetic nanoparticles described as double-well systems with randomly distributed easy axes is studied by means of a rate-equation approach in the typical range of frequencies adopted in therapeutic applications. Magnetic hysteresis loops obtained at various frequencies and vertex fields show that sawtooth and square field waveforms substantially enhance the specific loss power (SLP) with respect to a harmonic excitation of the same frequency and amplitude. This improvement is related to the presence of fast, quasiadiabatic transformations where the occupancy numbers in the two potential wells are virtually frozen, resulting in largely off-equilibrium magnetization states and in hysteresis loops with a greater area. The square driving-field waveform is particularly promising for magnetic hyperthermia, the SLP being not only most effectively enhanced but also independent of particle size over a large interval of diameters. The special features of the hysteresis loops generated by the square driving-field waveform are studied in detail; a peculiar effect of loop instability is described, and a condition for having stable hysteresis loops is defined. The considered driving-field waveforms can be made to comply with the physiological restrictions imposed by magnetic hypertermia treatments in vivo.

A micropump is the heart of any microfluidic device that finds applications in several lab-on-chip devices. Passive micropumps are highly desirable for this purpose due to their ease of integration, low energy requirements, and simplistic design and operation. The design of a plant leaf serves as natural inspiration for the development of an evaporation-assisted passive micropump. The presence of a branching-channel-like venation pattern ensures water distribution to the spongy mesophyll cells, increasing the surface area for evaporation. However, because of its multiscale design and the complexity of the venation pattern, emulating a leaf’s design is challenging. Apart from the lack of understanding of design parameters that affect fluid flow, manufacturing limitations impede the development of such bioinspired micropumps. Inspired by the multiscale design of the leaf, in this work we propose a passive micropump mimicking the structure of a leaf. Using evaporation and capillary pressure as the pumping mechanism, our leaf-mimicking micropump consists of a microporous membrane integrated with a branched, fractal channel network resembling a leaf’s venation pattern. Our proposed fabrication method is simple, scalable, and inexpensive and uses readily available materials. We demonstrate a significant increase in the fluid flow rate due to the addition of this branched-channel network. We support our experimental observations using an analytical model, wherein we discuss the design parameters that affect the pumping rate. Correspondingly, the performance of these micropumps can be optimized on the basis of intrinsic and extrinsic factors as per the desired applications.