The miniaturization of optical systems is an ongoing challenge across the electromagnetic spectrum. While the thickness of optical elements themselves can be reduced using advances in metamaterials, it is the voids between these elements—which are necessary parts of an optical system—that occupy most of the volume. Recently, a novel optical element coined a “spaceplate” has been proposed, which replaces a region of free space with a thinner optical element that emulates the free-space optical response function—thus having the potential to substantially shrink the volume of optical systems. While there have been a few proof-of-principle demonstrations of spaceplates, they have not yet been deployed in a real-world optical system. In this work, we use a bespoke-designed spaceplate to reduce the length of a gradient-index- (GRIN) lens microwave antenna. Our antenna is designed to operate at 23.5 GHz and the incorporation of a nonlocal metamaterial spaceplate enables the distance between the antenna feed and the GRIN lens to be reduced by almost a factor of 2. We find that the radiation patterns from a conventional and space-squeezed antenna are very similar, with a very low cross-polarization, and only a minor increase in the side-lobe levels when introducing the spaceplate. Our work represents a demonstration of a spaceplate integrated into a real-world optical system operating in microwave spectral region, highlighting the potential for this concept to reduce the physical size of systems in applications including imaging, spectroscopy, radar, and communications.

We provide a systematic method for constructing effective dispersive Lindblad master equations to describe weakly anharmonic superconducting circuits coupled by a generic dissipationless nonreciprocal linear system, with effective coupling parameters and decay rates written in terms of the immittance parameters characterizing the coupler. This article extends the foundational work of Solgun et al. [IEEE Trans. Microw. Theory Techn. 67, 928 (2019)] for linear reciprocal couplers described by an impedance response. Notably, we expand the existing toolbox to incorporate nonreciprocal elements, account for direct stray coupling between immittance ports, circumvent potential singularities, and include collective dissipative effects that arise from interactions with external common environments. We illustrate the use of our results with a circuit of weakly anharmonic Josephson junctions coupled to a multiport nonreciprocal environment and a dissipative port. The results obtained here can be used for the design of complex superconducting quantum processors with nontrivial routing of quantum information, as well as analog quantum simulators of condensed matter systems.

This work presents a significant advancement in spin-wave computing devices utilizing magnetostatic surface spin waves. We demonstrate a micro-Y-shaped waveguide fabricated from yttrium iron garnet with a nanometer thickness. This intricately engineered design enables a novel logic device with two inputs and one output, enabling future cascading of such elements. Electrical measurements on the Y-shaped structure reveal that strategically introduced gaps effectively manipulate spin-wave propagation, as corroborated by detailed micromagnetic simulations. Notably, we achieve robust diagonal spin-wave transmission across 1.2 µm gaps, covering a distance of 120 µm. Furthermore, the gapped device exhibits clear phase-dependent spin-wave interference, surpassing the performance of a conventional Y-shaped design. This phenomenon, confirmed by mapping simulated magnetization components, signifies the potential of dipole-coupled devices for realizing efficient 2-input-1-output magnonic logic elements, laying the groundwork for future development in this field.

Hybrid quantum devices enable novel functionalities by combining the benefits of different subsystems. Particularly, point defects in nanomechanical resonators made of diamond or silicon carbide (SiC) have been proposed for precise magnetic field sensing and as versatile quantum transducers. However, the realization of a hybrid system may involve trade-offs in the performance of the constituent subsystems. In a spin-mechanical system, the mechanical properties of the resonator may suffer from the presence of engineered defects in the crystal lattice. This may severely restrict the performance of the resulting device and needs to be carefully explored. Here we focus on the impact of defects on high-$Q$ nanomechanical string resonators made of prestressed $3C$-SiC grown on Si(111). We use helium-ion implantation to create point defects and study their accumulated effect on the mechanical performance. Using Euler-Bernoulli beam theory, we present a method to determine Young’s modulus and the prestress of the strings. We find that Young’s modulus is not modified by implantation. Under implantation doses relevant for single-defect or defect-ensemble generation, both tensile stress and damping rate also remain unaltered. For a higher implantation dose, both exhibit a characteristic change.

Double half-Heusler (HH) alloys are a new class of compounds that can be seen as transmuted versions of two single half-Heusler alloys with higher flexibility of tunability of their properties. Here, we report a detailed study of the thermoelectric (TE) properties of two double HH alloys $X_2{\mathrm{Ni}}_2\mathrm{InSb}$ ($X$ = $\mathrm{Hf}$, $\mathrm{Zr}$), using first-principles calculation. These alloys exhibit a rich phase diagram with the possibility of tetragonal, cubic, and solid-solution phases. As such, a comparative study of the TE properties of all these phases is highly desired. The simulated band gap, obtained using a hybrid functional, of the ordered phase of ${\mathrm{Hf}}_2{\mathrm{Ni}}_2\mathrm{InSb}$ and ${\mathrm{Zr}}_2{\mathrm{Ni}}_2\mathrm{InSb}$ lies in the range 0.24–0.4 and 0.17–0.59 eV, respectively, while, for the disordered phase, it lies in between 0.05 and 0.06 eV. A simulated TE figure of merit ($ZT$) as high as 2.02 and 2.45 is obtained for ${\mathrm{Hf}}_2{\mathrm{Ni}}_2\mathrm{InSb}$ and ${\mathrm{Zr}}_2{\mathrm{Ni}}_2\mathrm{InSb}$, respectively. In both compounds, electronic transport plays the dominant role in achieving the promising $ZT$ values. We believe this study will attract the attention not only of experimentalists but also of theoreticians from the thermoelectric community to further investigate similar double HH alloys.

This paper presents a theoretical and experimental demonstration of a security analysis of a Trojan-horse attack (THA) on a real-world quantum key distribution (QKD) system. We show that the upper bound on the information leakage depends solely on the fidelity between the states of the adversary. We find the lower bound for fidelity between THA states in both the polarization- and phase-coding BB84 protocols, considering both pure and mixed states. Our bounds depend only on the mean photon number per pulse available to an adversary. We also present an experimental analysis of a QKD system, including optical time-domain reflectometry measurements with centimeter resolution and spectral transmittance measurements for optical defense elements ranging from 1100 to 1800 nm with a noise floor lower than $-100$ dB. Finally, by considering the optimal attack, we obtain the value of the mean photon number per pulse available to an adversary and calculate the key leakage that needs to be eliminated during the privacy amplification procedure.

Optimization problems are the core challenge in many fields of science and engineering, yet general and effective methods for finding optimal solutions remain scarce. Quantum computing has been envisioned to help solve such problems, with methods like quantum annealing (QA), grounded in adiabatic evolution, being extensively explored and successfully implemented on quantum simulators such as D-Wave’s annealers and some Rydberg arrays. In this work, we investigate the topological sector optimization (TSO) problem, which has attracted particular interest in the quantum simulation and many-body physics community. We reveal that the topology induced by frustration in the optimization model is an intrinsic obstruction for QA and other traditional methods to approach the ground state. We demonstrate that the difficulties of the TSO problem are not restricted to the gaplessness, but are also due to the topological nature, which was often ignored for the analysis of optimization problems before. To solve TSO problems, we utilize quantum imaginary-time evolution (QITE) with a possible realization on quantum computers, which leverages the property of quantum superposition to explore the full Hilbert space and can thus address optimization problems of topological nature. We report the performance of different quantum optimization algorithms on TSO problems and demonstrate that their capabilities to address optimization problems are distinct even when considering the quantum computational resources required for practical QITE implementations.

In quantum sensing of magnetic fields, ensembles of nitrogen-vacancy centers in diamond offer high sensitivity, high bandwidth and outstanding spatial resolution while operating in harsh environments. Moreover, the orientation of defect centers along four crystal axes forms an intrinsic coordinate system, enabling vector magnetometry within a single diamond crystal. While most vector magnetometers rely on a known bias magnetic field for full recovery of three-dimensional (3D) field information, employing external 3D Helmholtz coils or permanent magnets results in bulky, laboratory-bound setups, impeding miniaturization of the device. Here, a novel approach is presented that utilizes a fiber-integrated microscale coil at the fiber tip to generate a localized uniaxial magnetic field. The same fiber-tip coil is used in parallel for spin control by combining dc and microwave signals in a bias tee. To implement vector magnetometry using a uniaxial bias field, the orientation of the diamond crystal is preselected and then fully characterized by rotating a static magnetic field in three planes of rotation. The measurement of vector magnetic fields in the full solid angle is demonstrated with a shot-noise-limited sensitivity of $19.4{\text{nT/Hz}}^{1/2}$ and microscale spatial resolution while achieving a fiber sensor head cross section of less than $1{\text{mm}}^2$.

Interface-type, analog memristors have quite a reputation for real-time applications in edge sensorics, edge computing, and neuromorphic computing. The n-type conducting ${\mathrm{Bi}\mathrm{Fe}\mathrm{O}}_3$ (BFO) is such an interface-type, analog memristor, which is also nonlinear and can therefore not only store, but also process data in the same memristor cell without data transfer between the data-storage unit and the data-processing unit. Here we present a physical memristor model, which describes the hysteretic current-voltage curves of the BFO memristor in the small and large current-voltage range. Extracted internal state variables are reconfigured by the ion drift in the two write branches and are determining the electron transport in the two read branches. Simulation of electronic circuits with the BFO interface-type, analog memristors was not possible so far because previous physical memristor models have not captured the full range of internal state variables. We show quantitative agreement between modeled and experimental current-voltage curves exemplarily of three different BFO memristors in the small and large current-voltage ranges. Extracted dynamic and static internal state variables in the two full write branches and in the two full read branches, respectively, can be used for simulating electronic circuits with BFO memristors, e.g., in edge sensorics, edge computing, and neuromorphic computing.

Helical twists exist in liquid crystals as a result of the addition of chiral dopants or boundary conditions and play a role in several liquid-crystal phenomena and devices. To date, various methodologies have been explored, albeit with varying degrees of precision and complexity, often resulting in the measurement of just one parameter, such as helical sense. However, a consensus on the optimal technique remains elusive. We developed a technique to determine the total twist angle and handedness of the twist by using optical transmission spectroscopy. We present the theoretical background and experimental technique and test it against some other results and results from data published elsewhere. Furthermore, we investigate the use of three naturally occurring terpenoid materials as chiral dopants for liquid crystals and determine the helical handedness using the spectroscopy method.

We explore the exceptional points (EPs) in mass-spring oscillators with time-varying stiffness. The second-order EPs result from the coalescences of the fundamental and harmonic eigenmodes that can be achieved by a time modulation of the spring stiffness. The occurrence of EPs is first demonstrated in a theoretical model by solving the eigenvalue problem with the state-space method. The signature of the EP can be seen in the response spectrum when the system is subjected to an external excitation. The undamped and damped cases are both considered in this mass-spring oscillator, and the impact of the damping ratio and modulation amplitude on the EPs is systematically investigated. Based on an analogy of the equation of motion with the Mathieu differential equation, we discuss the stability of the undamped time-varying system under different modulation parameters and explore its connection to the EP phenomenon. As a typical property of a second-order EP, the square-root dependence of the frequency splitting response to a perturbation is studied by introducing an added mass. In a second part, the above theoretical concepts are established by simulation methods in an elastic solid mass-spring model and an equivalent time-varying stiffness is realized by shunting a piezoelectric patch with switch-controlled external negative capacitance circuits. This work should pave the way for applications of crack or perturbation detection in elastic media and inspire other elastic wave modulation functions in time-varying systems.

Growing interest in acoustic topological insulators arises from their robust edge states, which are resistant to defects and backscattering. Traditionally, the edge states of topological insulators are believed to be nearly immune to subwavelength-sized defects. As a result, there have been few studies focusing on utilizing defects to enhance wave control. Here, we investigate the impact of introducing periodic defects into acoustic pseudospin systems on the band structures and extend our analysis to nonperiodic structures. We discover that even a single subwavelength defect can significantly affect the topological boundary states, providing a platform for controlling and switching pseudospin edge states using defects. Expanding on this discovery, we develop a broadband topological sound switch (TSS) that achieves a high transmission ratio before and after switching by simply rotating a single scatterer. Additionally, we design three topological logic gates based on this TSS and experimentally verify their functionality. Our theory and experiments demonstrate that the spatial arrangement of defects can serve as a means of manipulating sound waves, potentially advancing acoustic computing and information processing.

A novel high-energy electron radiography system combined with an active plasma lens (APL) has been designed for the first time and validated by simulation studies. The system consists of a 50-MeV electron linear accelerator followed by a 3-cm-long capillary with a discharge current up to hundreds of amperes. With the APL, the distance from the object plane to the imaging plane can be reduced from 5 m to 45 cm with a magnification factor (MF) of 20, and pictures with 1.1-$\text{μ}\mathrm{m}$ spatial resolution can be obtained. The effects of lens chromatic aberrations, imaging blurring, and the uniformity of the plasma discharge current are shown to be significant in obtaining a high-spatial-resolution radiograph, which have been discussed in this work. Such a plasma-based imaging lens has a high tolerance for chromatic aberrations, is suitable for imaging thick target materials, and has radial symmetric focusing and adjustable focusing gradients. Furthermore, a cascaded high-MF radiography system based on APLs has been proposed to improve spatial resolution.