As the predominant information carrier, acoustic waves play an essential role in underwater communications, with significantly less scattering and attenuation than that of electromagnetic waves. Integrating the gradient-index metamaterials (GIMs) with circular waveguide, we propose a design to achieve a multimodal acoustic converter with multifrequency and great efficiency in a circular waveguide. Numerical simulations and analytical results demonstrate that the zeroth order of acoustic mode can be converted to the first-order mode or even higher-order modes. Moreover, optimizing the parameter of GIMs, it is further demonstrated that GIMs, attached to the rigid wall of waveguide, can be utilized as a passive orbital-angular-momentum (OAM) generator. In the future, this mode-conversion scheme with suppressed backscattering and high mode-conversion efficiency may find many applications in integrated acoustics and underwater communications.

Engineering light absorption in the extended short-wave infrared (ESWIR) range using scalable materials is a long-sought-after capability that is crucial to implement cost-effective and high-performance sensing and imaging technologies. Herein, we demonstrate enhanced, tunable ESWIR absorption using silicon-integrated platforms consisting of ordered arrays of metastable ${\mathrm{Ge}}_{1-x}{\mathrm{Sn}}_x$ nanowires with Sn content reaching 9 at.% and variable diameters. Detailed simulations are combined with experimental analyses to systematically investigate light−${\mathrm{Ge}}_{1-x}{\mathrm{Sn}}_x$ nanowire interactions to tailor and optimize the nanowire-array geometrical parameters and the corresponding optical response. The diameter-dependent leaky-mode resonance peaks are theoretically predicted and experimentally confirmed with a tunable wavelength from 1.5 to 2.2 µm. A threefold enhancement in the absorption with respect to ${\mathrm{Ge}}_{1-x}{\mathrm{Sn}}_x$ layers at 2.1 µm is achieved using nanowires with a diameter of 325 nm. Finite-difference time-domain simulations unravel the underlying mechanisms of the ESWIR-enhanced absorption. The coupling of the ${\mathrm{HE}}_{11}$ and ${\mathrm{HE}}_{12}$ resonant modes to nanowires is observed at diameters above 325 nm, while at smaller diameters and longer wavelengths the ${\mathrm{HE}}_{11}$ mode is guided into the underlying Ge layer. The presence of tapering in nanowires further extends the absorption range while minimizing reflection. This ability to engineer and enhance ESWIR absorption lays the groundwork to implement alternative photonic devices exploiting all-group-IV platforms.

Decoherence induced by laser frequency noise is one of the most important obstacles in quantum information processing. In order to suppress this decoherence, the noise power spectral density needs to be accurately characterized. In particular, the noise spectrum measurement based on the coherence characteristics of qubits would be a meaningful and still challenging method. Here, we theoretically analyze and experimentally obtain the spectrum of laser frequency noise based on the continuous dynamical decoupling technique. We first estimate the mixture-noise (including laser and magnetic noises) spectrum up to $(2\pi )530$ kHz by monitoring the transverse relaxation from an initial state $+X$, followed by a gradient descent data process protocol. Then the contribution from the laser noise is extracted by encoding the qubits on different Zeeman sublevels. We also investigate two sufficiently strong noise components by making an analogy between these noises and driving lasers whose linewidth is assumed to be negligible. This method is verified experimentally and finally helps to characterize the noise.

Precise spectroscopy of oscillating fields plays a significant role in many fields. Here, we propose an experimentally feasible scheme to measure the frequency of a fast-oscillating field using a single-qubit sensor. By invoking a stable classical clock, the signal phase correlations between successive measurements enable us to extract the target frequency with extremely high precision. In addition, we integrate dynamical decoupling technique into the framework to suppress the influence of slow environmental noise. Our framework is feasible with a variety of atomic and single solid-state spin systems within the state-of-the-art experimental capabilities as a versatile tool for quantum spectroscopy.

Manipulation of magnetic domain walls (DWs) in an antiferromagnet plays a fundamental role in developing antiferromagnetic (AFM) devices with small size and high velocity for information processing. On the other hand, in addition to the ultrahigh-frequency (THz) AFM magnetization precession, recent investigation shows that an antiferromagnet may also exhibit unique dynamical behaviors at a lower frequency (GHz) by an optical method. In this work, we predict the characteristic frequency of AFM magnetization dynamics can be further widened to MHz by triggering the oscillation of an AFM DW under a stress that is a sinusoidal function in a spatial coordinate. This low frequency is due to the mismatch of size between the spatial period of stress and DW width. Based on this low-frequency oscillation, DW can move at an almost constant velocity by shifting the phase of stress. The proposition in this work paves the way to develop AFM devices with small size and ultralow dissipation.

Annealing schedule control provides opportunities to better understand the manner and mechanisms by which putative quantum annealers operate. By appropriately modifying the annealing schedule to include a pause (keeping the Hamiltonian fixed) for a period of time, we show that it is possible to more directly probe the dissipative dynamics of the system at intermediate points along the anneal and examine thermal relaxation rates, for example, by observing the repopulation of the ground state after the minimum spectral gap. We provide a detailed comparison of experiments from a D-Wave device, simulations of the quantum adiabatic master equation, and a classical analogue of quantum annealing, spin-vector Monte Carlo, and we observe qualitative agreement, showing that the characteristic increase in success probability when pausing is not a uniquely quantum phenomena. We find that the relaxation in our system is dominated by a single timescale, which allows us to give a simple condition for when we can expect pausing to improve the time to solution, the relevant metric for classical optimization. Finally, we also explore in simulation the role of temperature whilst pausing as a means to better distinguish quantum and classical models of quantum annealers.

In the device-independent quantum-information approach, the implementation of a given task can be self-tested solely from the recorded statistics and without detailed models for the employed devices. Even though experimentally demanding, it provides appealing verification schemes for advanced quantum technologies that naturally fulfil the associated requirements. In this work, we experimentally study whether self-testing protocols can be adopted to certify the proper functioning of quantum devices built with modern space-division multiplexing optical fiber technology. Specifically, we consider the prepare-and-measure protocol of Farkas and Kaniewski [Phys. Rev. A 99, 032316 (2019)] for self-testing measurements corresponding to mutually unbiased bases (MUBs) in a dimension $d>2$. In our scheme, the state preparation and measurement stages are implemented using a multiarm interferometer built with multicore optical fibers and related components. Due to the high overlap of the interferometer’s optical modes achieved with this technology, we are able to reach the required visibilities for self-testing the implementation of two four-dimensional MUBs. We also quantify two operational quantities of the measurements: (i) the incompatibility robustness, connected to Bell violations, and (ii) the randomness extractable from the outcomes. Since MUBs lie at the core of several quantum-information protocols, our results are of practical interest for future quantum works relying on space-division multiplexing optical fibers.

Developing two-dimensional multifunctional materials is highly desirable for nanoscale device applications. Herein, by means of first-principles calculations, we demonstrate a promising two-dimensional multifunctional semiconductor with ferroelectricity, ultrahigh carrier mobility, and negative Poisson’s ratio in single-layer $\mathrm{BI}$. We show that single-layer $\mathrm{BI}$ exhibits intrinsic ferroelectricity, derived from its asymmetric structure, and the ferroelectricity features an in-plane electric polarization as large as $3.3\times {10}^{-10}{\mathrm{C}}_{{\mathrm{m}}^{-1}}$, which is beneficial for nonvolatile memories. Owing to its large band dispersion, single-layer $\mathrm{BI}$ is also found to harbor an extremely high carrier mobility ($1.17\times {10}^5\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$), even comparable to that of graphene, suggesting its potential for high-performance electronics and bulk photovoltaic effect. This, combined with the moderate band gap, renders single-layer $\mathrm{BI}$ with a high absorption coefficient (${10}^5\mathrm{cm}{\mathrm{m}}_{-1}$) from near-infrared to ultraviolet light. In addition, we unveil that single-layer $\mathrm{BI}$ is a long-sought-after auxetic material with a negative Poisson’s ratio of −0.31. All of these findings make single-layer $\mathrm{BI}$ a compelling multifunctional material, offering a versatile platform for diverse nanoscale devices applications.

We propose and experimentally demonstrate a parity-time-symmetric electronic system exhibiting the self-dual emitter-absorber property with a remarkable modulation depth in the radio-frequency (rf) region. The dramatically different rf responses between the emitter and absorber modes may allow detection of ultrasmall conductive or reactive perturbations. Our measurement results show that even a perturbation on the order of ${10}^{-2}$ can greatly change the system’s output intensity by more than 30 dB, consistent with the theoretical prediction. The measured sensitivity is far beyond the sensitivity of traditional sensors based on a Fabry-Perot resonator, and may lead to monotonic rf sensors with high sensitivity and resolvability.

The topological edge state (TES) may emerge at the interface between acoustic crystals with distinct topological properties. It supports robust wave phenomena against geometry imperfections. By imposing holistic onsite gains and losses in PT symmetric Su-Schrieffer-Heeger lattices, zero mode can be obtained from the convergence of two splitting TES modes induced by adjacent coupling. However, the nonlocal non-Hermiticity requires exorbitant configuration on each atom, resulting in complex systems. Here, we demonstrate the effect of local non-Hermitian modulation to the coupled TESs, utilizing passive acoustic crystals with sandwiched arrangements. Local non-Hermiticity is introduced at the position of one of the TESs to manipulate the splitting TESs. By suitably adjusting the non-Hermitian strength, the splitting TESs experience the coalescence process along with the asymmetric reflection and absorption near and beyond the exceptional point. Due to the topological essentials, the emergence of the exceptional point also can be proved to be robust to the geometrical disorders. Our results reveal that coupled TESs can be modulated by local non-Hermiticity to realize the extraordinary scattering phenomena, which may inspire more acoustic functional devices based on the topological or non-Hermitian characteristics.

We study frequency fluctuations in self-sustained oscillators based on nonlinear underdamped resonators. Important types of such resonators are nano- and microelectromechanical systems. Various noise sources are considered, with the emphasis on the fundamentally unavoidable noise that comes along with dissipation from the coupling to a thermal reservoir. The formulation in terms of the action-angle variables of the resonator allows us to analyze a deeply nonlinear regime. In this regime the vibration frequency as a function of the action can have an extremum. We show that frequency fluctuations can be strongly reduced by choosing the operation point at this extremum. We suggest a practical implementation of a nanoresonator that has the appropriate property and show explicit results for the corresponding model.

Quantum-system characterization techniques represent the front line in the identification and mitigation of noise in quantum computing but can be expensive in terms of quantum resources and time to repeatedly employ. Another challenging aspect is that the parameters governing the performance of various operations tend to drift over time and monitoring these is hence a difficult task. One of the most promising characterization techniques, gate-set tomography (GST), provides a self-consistent estimate of the completely positive trace-preserving (CPTP) maps for a complete set of gates, as well as preparation and measurement operators. We develop a method for performance optimization seeded by tomography (POST), which couples the power of GST with a classical optimization routine to achieve a consistent gate improvement in just a short number of steps within a given calibration cycle. By construction, the POST procedure finds the best available gate operation given the hardware and is therefore robust to the effects of drift. Further, in comparison to other quantum error-mitigation techniques, it builds upon a one-time application of GST. To demonstrate the performance of this method on a real quantum computer, we map out the operations of six qubit pairs on the superconducting ibmq_poughkeepsie quantum device. Under the restriction of logical-only control, we monitor the performance of the POST approach on a chosen controlled-not (cnot) gate over a period of six weeks. In this time, we achieve a consistent improvement in gate fidelity, averaging a fidelity increase of 21.1% as measured by randomized benchmarking. The POST approach should find wide applicability as it is hardware agnostic and can be applied at the upper logical level or at a deeper pulse-control level.

We investigate the effect of small spatiotemporal modulations in subwavelength-dimensioned phononic crystals with large band gaps, on the frequency spectrum for elastic waves polarized in the plane of periodicity. When the radius of cylinders periodically placed inside a matrix of highly contrasting elastic properties is time varying, we find that due to the appearance of frequency harmonics throughout the spectrum, the notion of a band gap is destroyed in general, although with the appropriate tuning of parameters, in particular the modulation frequency, it is possible that some band-gap region is retained, making such systems possible candidates for tunable bandpass filters or phononic isolators, accordingly, and for sensor applications.

Injection from metallic electrodes serves as a main channel of charge generation in organic semiconducting devices and the quantum effect is normally regarded as essential. We develop a dynamic approach based upon the surface-hopping (SH) algorithm and classical device modeling, by which both quantum tunneling and thermionic emission of charge-carrier injection at metal-organic interfaces are concurrently investigated. The injected charges from a metallic electrode are observed to spread quickly onto the organic molecules, followed by an accumulation close to the interface, induced by the built-in electric field. We compare the Ehrenfest dynamics on the mean-field level and the SH algorithm by simulating the temperature dependence of the charge-injection dynamics and it is found that the former leads to an improper result, that the injection efficiency decreases with increasing temperature in the room-temperature regime while the SH results are credible. The relationship between the injected charges and the applied bias voltage suggests that it is the quantum tunneling that dominates the low-threshold injection characteristics in molecular crystals, which is further supported by the calculation results of a small entropy change during the injection process. An optimum interfacial width for the charge-injection efficiency at the interface is also quantified and can be utilized to understand the role of the interfacial buffer layer in practical devices.

Magnetic skyrmions are topologically protected whirling spin textures nucleated from other ferromagnetic states that have great potential applications for nanoscale devices. To realize this potential, it is important to find useful methods to generate skyrmions in a controlled fashion and to identify tools to quantitatively describe their formation. Here we generate skyrmions in perpendicularly magnetized $\mathrm{Mg}\mathrm{O}/{\mathrm{Mn}}_2\mathrm{Co}\mathrm{Al}/\mathrm{Pd}$ ultrathin films and investigate their formation by fractal analysis. Using an in-plane field, we modify the effective magnetic anisotropy of the films to control the nucleation of skyrmions from two distinct domain states in the films: labyrinth domains typically found in perpendicular materials and a featureless monodomain ferromagnetic phase. By using fractal analysis, we obtain a magnetic phase diagram related to the fractal dimension of the domains that can describe the evolution of the magnetic states and can be used to categorize the formation of skyrmions. Moreover, we find that the skyrmion density is determined not only by the critical material parameter $\kappa$ but also by a repulsive skyrmion-skyrmion interaction at high in-plane magnetic fields. We thus establish a reliable and powerful method to generate skyrmions and a way to analyze the evolution of skyrmions.

High-density low-crosstalk waveguide superlattices can potentially help shrink the pitch of an optical phased array to half-wavelength, which may lead to superior beam characteristics. While a superlattice’s longitudinal coherence is exploited to create phase mismatch and reduce crosstalk, its transverse coherence is also modified and will inevitably disturb the beam forming of the optical phased array. Our theory shows that the transverse and longitudinal coherence can be balanced in a nontrivial superlattice such that superlattice-induced disturbance of the beam can be suppressed without compromising crosstalk. Large supercells less restrained by symmetry offer more degrees of freedom for finding a desirable superlattice structure. The supercell configuration order and structural modulation strength are crucial to balancing coherence and approaching the ideal characteristics of a half-wavelength pitch optical phased array. Experimental results show a high main-beam energy ratio, low sidelobe levels, and a wide angular scanning range. Such optical phased arrays may potentially open up further opportunities for light detection and ranging (LIDAR), wireless optical communications, and biomedical scanning imaging.

We propose lateral optical trapping of Rayleigh particles using tailored anisotropic and hyperbolic metasurfaces illuminated with a linearly polarized Gaussian beam. This platform permits optical traps to be engineered at the beam axis with a response governed by nonconservative and giant lateral recoil force coming from the directional excitation of confined surface plasmons during the light-scattering process. Compared to optical traps set over uniform metals, either in bulk or thin-layer configurations, the proposed traps are broadband in the sense that they can be set with beams oscillating at any frequency within a wide range in which the metasurface supports surface plasmons. Over that range, the metasurface dispersion evolves from an anisotropic elliptic to a hyperbolic regime going through a topological transition and enables optical traps with distinctive spatially asymmetric potential distribution, local potential barriers arising from the momentum imbalance of the excited plasmons, and an enhanced potential depth that permits stable trapping of nanoparticles using low-intensity laser beams. To investigate the performance of this platform, we develop a rigorous formalism based on Lorentz force within the Rayleigh approximation combined with anisotropic Green’s functions and calculate the trapping potential of nonconservative lateral forces using the Helmholtz decomposition method. Tailored anisotropic and hyperbolic metasurfaces, commonly implemented by nanostructuring thin metallic layers, permit to use low-intensity laser sources operating in the visible or infrared frequencies to trap and manipulate particles at the nanoscale, and may enable a wide range of applications in bioengineering, physics, and chemistry.

In this work, we study magnetization switching induced by spin-orbit torque in $\mathrm{W}$ ($\mathrm{Pt}$)/$\mathrm{Co}$/$\mathrm{Ni}\mathrm{O}$ heterostructures with variable thickness of $\mathrm{W}$ and $\mathrm{Pt}$ heavy-metal layers, a perpendicularly magnetized $\mathrm{Co}$ layer, and an antiferromagnetic $\mathrm{Ni}\mathrm{O}$ layer. Using current-driven switching and magnetoresistance and anomalous-Hall-effect measurements, we determine the perpendicular and in-plane exchange-bias field. Several Hall-bar devices possessing in-plane exchange bias from both systems are selected and analyzed in relation to our analytical switching model of the critical current density as a function of $\mathrm{Pt}$ and $\mathrm{W}$ thickness, resulting in estimation of the effective spin Hall angle and perpendicular effective magnetic anisotropy. We demonstrate in both the $\mathrm{Pt}$/$\mathrm{Co}$/$\mathrm{Ni}\mathrm{O}$ system and the $\mathrm{W}$/$\mathrm{Co}$/$\mathrm{Ni}\mathrm{O}$ system deterministic $\mathrm{Co}$ magnetization switching without an external magnetic field, which is replaced by an in-plane exchange-bias field. Moreover, we show that due to a higher effective spin Hall angle in the $\mathrm{W}$-based system than in the $\mathrm{Pt}$-based system, the relative difference between the resistance states in the magnetization current switching to the difference between the resistance states in magnetic field switching determined by the anomalous Hall effect ($\mathrm{\Delta }R/\mathrm{\Delta }{R}_{\mathrm{AHE}}$) is about twice as high in $\mathrm{W}$-based devices than in $\mathrm{Pt}$-based devices, while the critical switching-current density in $\mathrm{W}$-based devices is 1 order lower than in $\mathrm{Pt}$-based devices. The current-switching stability and the training process are discussed in detail.