Inverse design (InvDes) combines simulation and optimization to algorithmically discover physical device layouts beyond conventional intuition. However, typical InvDes approaches require hundreds or thousands of simulations and numerous initializations to adequately explore complex design spaces. This computational burden is especially pronounced in metallic microwave-frequency devices, which involve highly nonconvex optimization landscapes and demand computationally expensive, high-resolution full-wave electromagnetic simulations. Here, we introduce a significantly accelerated inverse design method tailored for microwave-frequency devices. Our approach leverages rigorous full-wave simulations to precompute scattering parameters of small, simple elements, which are subsequently combined to form larger devices during optimization. This scattering-matrix-based approach enables simulations that are approximately $10000$ times faster and use approximately $100$–$10000$ times less memory than conventional full-wave methods, while maintaining high accuracy. Coupling this accelerated simulator with an adaptive random search optimization algorithm allows rapid, thorough exploration of highly nonconvex design spaces. We demonstrate the effectiveness and versatility of our method by efficiently designing a range of commonly used microwave devices, including couplers, power splitters, phase shifters, impedance-matching circuits, and demultiplexers.

The development of quantum circuits based on hybrid superconductor-semiconductor Josephson junctions holds promise for exploring their mesoscopic physics and for building innovative superconducting devices. The gate-tunable superconducting transmon qubit (gatemon) is the paradigmatic example of such a superconducting circuit; however, gatemons typically suffer from unstable and hysteretic qubit frequencies with respect to the applied gate voltage and reduced coherence times compared to conventional transmons fabricated from aluminum-oxide-based Josephson junctions. Here, we develop methods for characterizing these challenges in gatemons and deploy these methods to compare the impact of shunt-capacitor designs on gatemon performance. Our results indicate a strong frequency- and design-dependent behavior of the qubit stability, hysteresis, and dephasing times. Moreover, we achieve highly reliable tuning of the qubit frequency with 1-MHz precision over a range of several gigahertz, along with improved stability in grounded gatemons compared to gatemons with a floating capacitor design.

Entangled photon pairs are of crucial importance in quantum networks. For the future demands of large-scale and secure quantum communication, integrated photon sources are highly effective solutions. Here, we report entanglement distribution over 30 km in a metropolitan area using an on-chip broadband silicon nanowire biphoton polarization-entangled source based on a silicon-on-insulator platform. This source generates a continuous spectrum spanning the entire C band (4.5 THz), achieving locally detected coincidence counts of about 154 kHz within a 100-GHz bandwidth, making it suitable for long-distance entanglement distribution among multiple users. By combining this source with quantum entanglement, enhanced by high-precision clock synchronization that achieves a standard deviation of 56.8 ps over 600 s, we observe a violation of the Clauser-Horne-Shimony-Holt inequality by 27.8 standard deviations. Our results showcase the potential of silicon photonic technology as a scalable and practical platform for quantum technologies.

Angle-selective devices that filter electromagnetic (EM) waves from free space are highly desirable for microwave and optical applications. However, most existing designs face challenges in simultaneously achieving a low profile, broad bandwidth, and polarization insensitivity. In this paper, we propose a uniaxial zero-refractive-index metamaterial (UZIM) that offers ultrabroadband transmission under normal incidence while exhibiting narrowband strong reflection for oblique incidence across all polarizations. This unique angular response enables ultrabroadband angular selectivity by cascading UZIM layers with different operating frequencies. By cascading six UZIMs that share the same transmission band under normal incidence but have different reflection bands at oblique incidence, we achieve an angularly selective composite with a 15.56% relative bandwidth. The composite, with a total thickness of $1{\lambda }_0$, maintains high transmission within 5° of incidence and strong suppression beyond 30° in the whole operating frequency band. The measured results of the fabricated sample show good agreement with full-wave simulations. Our approach addresses the limitations of Brewster-effect-based designs, which suffer from bulky profiles, and overcomes the bandwidth limitations of traditional zero-refractive-index-material-based (ZIM) and frequency-selective-surface-based (FSS) designs. This makes the proposed structure highly suitable for applications such as spatial filtering and antenna sidelobe suppression.

Quantum technologies provide many applications for information processing tasks that are impossible to realize within classical physics. These capabilities include such fundamental resources as generating secure, i.e., private and unpredictable random values. Yet, the problem of quantifying the amount of generated randomness is still not fully solved. This work presents a comprehensive analysis of the design and performance optimization of a quantum random number generator (QRNG) based on Bell inequality violations. We investigate key protocol parameters, including the smoothing parameter (${ϵ}_{\mathrm{s}}$), test round probability ($\gamma$), and switching delays, and their effects on the generation rate and quality of randomness. We identify optimal ranges for $\gamma$ and $p_{\mathrm{\Omega }}$ (the protocol’s nonaborting probability) to balance the trade-off between randomness consumption and net randomness generation. We investigate the impact of the laser power on the overall generation rate. We explore the impact of switching delays on the system’s performance, providing strategies to mitigate these effects. The work provides practical guidelines for the efficient and secure design of QRNG systems and other cryptographic protocols.

In fluid-filled microchannels embedded in solid devices and driven by megahertz ultrasound transducers, the thickness of the viscous boundary layer in the fluid near the confining walls is typically three to four orders of magnitude smaller than the acoustic wavelength and five orders of magnitude smaller than the longest dimension of the device. This large span in length scale renders direct numerical simulations of such devices prohibitively expensive in terms of computer memory requirements, and consequently the so-called boundary-layer models are introduced. In such models, approximate analytical expressions of the boundary-layer fields are found and inserted in the governing equations and boundary conditions for the remaining bulk fields. Since the bulk fields do not vary across the boundary layers, they can be computed numerically using the resulting boundary-layer model without resolving the boundary layers. However, current boundary-layer models are only accurate for hard solids (e.g., glass and silicon) with relatively small oscillation amplitudes of the confining wall, and they fail for soft solids (e.g., polymers) with larger wall oscillations. In this work, we extend the boundary-layer model of Bach and Bruus, J. Acoust. Soc. Am. 144, 766 (2018) to enable accurate simulation of soft-walled devices. The extended model is validated by comparison (1) with direct numerical simulations in three and two dimensions of tiny submillimeter- and larger millimeter-sized polymer devices, respectively, and (2) with previously published experimental data.

In nondestructive and biomedical imaging, spatial patterns inside a sample are imaged without destroying it. Therefore, propagating waves, including electromagnetic or ultrasonic signals or even diffuse heat, are generated or modified by these internal patterns and transmit this structural information to the sample surface. There, the signals can be detected, and an image of the internal structure can be reconstructed from the measured signals. The amount of information about the interior of the sample that can be obtained from the detected signals at the sample surface is significantly influenced by the propagation from the internal structure to the surface. In the real world, all signal propagation is more or less irreversible. The entropy generated during propagation corresponds to the loss of information. In an idealized model, such propagating waves, called virtual waves, are described by the wave equation. They remain valid solutions of this equation when the direction of time is reversed, thus exhibiting reversibility and showing no entropy production and therefore no loss of information during propagation. A Fredholm integral equation of the first kind relates the diffusion fields locally for each point in space to the virtual waves as their propagating duals. These virtual waves can be calculated from the measured diffusion field at each detection point by inverting this integral equation. In the past, this Fredholm integral was derived using so-called “thermal waves” for thermography. This was often confusing because thermal diffusion cannot be described by the wave equation. Here we derive this relationship for the first time in real space without having to use Fourier frequency space. We have used the locally computed virtual waves from the measured diffusive surface signals for image reconstruction using established time-of-flight methods from ultrasound or radar imaging. This improves the spatial resolution in thermography and compensates for the dispersion of quantum wave packets in atom probe tomography.

We study, both experimentally and theoretically, the impact of various parameters on the characteristics of signals from a two-axis Hanle magnetometer based on the $\mathrm{Cs}$ ${\mathrm{D}}_1$ transition illuminated by linearly polarized pump and probe beams. The probe propagates through the cell such that its polarization vector can be switched between the $zy$ and $xz$ planes to measure the magnetic field along the $x$ and $y$ axes, respectively. When the probe polarization is additionally reflected about the pump polarization vector and the two resulting absorption signals are subtracted, dispersion signals centered around zero are obtained. These dispersion signals can be accurately reproduced by the theoretical model, and the information from the experiments and modeling can be used to optimize the magnetometer characteristics.

Centrosymmetric ${\mathrm{La}\mathrm{Fe}\mathrm{O}}_3$ (LFO) shows a photovoltaic effect when sandwiched between asymmetric electrodes, which stems from the built-in potential induced by the different work functions of the top and bottom electrodes, interfacial polar discontinuity, or even flexoelectric fields. However, the complex microstructure of epitaxial LFO film contributes to its photoresponse differently depending on the crystallographic domain landscape and the nature of the grain boundaries originating between them. In this work, to minimize the presence of strain gradients and isolate the substrate-induced lattice mismatch effects on microstructure and photoresponse, ultrathin (10 nm) LFO films are grown on substrates with different compressive and tensile in-plane strain. Structural characterization allows us to disentangle the contributions of the domain distribution and strain state to the measured photocurrent of the LFO films. Four-dimensional scanning transmission electron microscopy orientation mapping proves a strong correlation between the strain imposed by the substrate, the LFO domain distribution, and the photocurrent performance. Films grown on substrates imposing a compressive strain [${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$ (STO) (001), $({\mathrm{La}\mathrm{Al}\mathrm{O}}_3{)}_{0.3}({\mathrm{Sr}}_2{\mathrm{Ta}\mathrm{Al}\mathrm{O}}_6{)}_{0.7}$(001), and ${\mathrm{La}\mathrm{Al}\mathrm{O}}_3$(001)] exhibit predominantly twin boundaries with low disorder, whereas films grown on substrates imposing tensile strain (${\mathrm{Dy}\mathrm{Sc}\mathrm{O}}_3$ (DSO) (110)) display grain boundaries formed by crystallites with orthogonal c-axis orientations, which we hypothesize are associated with the observed enhanced photocurrent. Further validation is provided by LFO films grown on STO(110), where controlled grain boundary formation replicates the crystallite texture observed in films grown on DSO(110) and concomitantly show a large short-circuit photocurrent ($J_{SC}$ ≈ 50 mA/${\mathrm{cm}}^2$). This work highlights grain boundary engineering as a strategy for optimizing the photoelectronic properties of oxide thin films, as well as an experimental approach applicable to other orthorhombic Pbnm systems to investigate the performance of perovskite-based optoelectronic devices through crystallographic domain configurations.

Multilayer ferromagnetic nanostructures comprising nonmagnetic conducting and insulating materials, which provide the generation of a strong spin-orbit torque (SOT) by a charge flow and the emergence of voltage-controlled magnetic anisotropy (VCMA) of interfacial origin, are promising for the electrical excitation and control of spin dynamics at the nanoscale. Here we present a theoretical study of the SOT-induced generation of spin waves and their propagation in the ${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$ ($\mathrm{Co}\mathrm{Fe}\mathrm{B}$) nanolayer sandwiched between the topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ ($\mathrm{Bi}\mathrm{Se}$) and the dielectric $\mathrm{Mg}\mathrm{O}$ creating VCMA. Using micromagnetic simulations, we first describe the spin dynamics excited in the narrow $\mathrm{Co}\mathrm{Fe}\mathrm{B}$ waveguide by a radio-frequency electric current locally created in the underlying $\mathrm{Bi}\mathrm{Se}$ strip. Owing to giant efficiency of charge-to-spin density conversion in the $\mathrm{Bi}\mathrm{Se}$/$\mathrm{Co}\mathrm{Fe}\mathrm{B}$ bilayer, the current-induced SOT generates a large-angle magnetization precession even at low current densities in the excitation region. Moreover, high-amplitude traveling spin waves emerge in the $\mathrm{Co}\mathrm{Fe}\mathrm{B}$ waveguide when the excitation frequency exceeds the frequency of uniform ferromagnetic resonance in the $\mathrm{Co}\mathrm{Fe}\mathrm{B}$ strip. Next, we model the propagation of a monochromatic spin wave through the long waveguide in the presence of a direct electric current flowing along the $\mathrm{Bi}\mathrm{Se}$ strip. Remarkably, the simulations demonstrate that the current-induced SOT can provide temporal amplification of spin waves in the $\mathrm{Co}\mathrm{Fe}\mathrm{B}$ waveguide even at room temperature. Finally, we study the spin-wave routing through the joint of one and three $\mathrm{Co}\mathrm{Fe}\mathrm{B}$ waveguides covered by an $\mathrm{Mg}\mathrm{O}$ nanolayer with two gate electrodes. It is shown that the regulation of interfacial VCMA by appropriate dc voltages applied to these electrodes makes it possible to direct the incoming spin wave into any of the three outgoing waveguides. Our theoretical predictions offer guidelines for the development of electrically controlled magnonic devices providing information transmission and processing with a low power consumption.

The quasiparticle density observed in low-temperature superconducting circuits is several orders of magnitude larger than the value expected at thermal equilibrium. The tunneling of this excess of quasiparticles across Josephson junctions is recognized as one of the main loss and decoherence mechanisms in superconducting qubits. Here we present a unified impedance theory that accounts for quasiparticle energy loss in circuit regions both far from and near to (across) junctions. Our theory leverages the recent experimental demonstration that the excess quasiparticles are in quasiequilibrium [Connolly et al., Phys. Rev. Lett. 132, 217001 (2024)] and uses a generalized fluctuation-dissipation theorem to predict the amount of charge and flux noise generated by them. We compute the resulting energy relaxation time $T_1$ in transmon qubits with and without junction asymmetric gap engineering, and show that quasiparticles residing away from junctions can play a dominant role in the former case. They also may provide an upper limit for resonator quality factors if the density of amorphous two-level systems is reduced. In addition, we show that charge noise from quasiparticles leads to flux noise that is logarithmic in frequency, giving rise to a “nearly white” contribution that is comparable to the flux noise observed in flux qubits. This contrasts with amorphous two-level systems, whose associated flux noise is shown to be super-Ohmic. We discuss how this quasiparticle flux noise can limit $T_2^{\ast }$ coherence times in flux-tunable qubits. The final conclusion is that asymmetric gap engineering can greatly reduce noise and increase coherence times in superconducting qubits.

The single parabolic band (SPB) model is a popular, easy-to-use, and somewhat powerful tool to describe thermoelectric transport and is often employed for material optimization and the extraction of band structure parameters. While the fundamental limitations of the model are well-known and usually recognized, a discussion on the uncertainty in the derived parameters when the model is valid is missing, but indispensable for a qualified comparison and analysis of reported values. In this work, we derive analytical expressions for the uncertainty in the modeling parameters—the reduced Fermi level, density of states (DOS) mass, carrier concentration (n), mobility parameter (${\mu }_0$), Lorenz number (L), and electronic thermal conductivity ($K_{\mathrm{E}}$) based on experimental uncertainty in the Seebeck coefficient (α), electrical conductivity (σ), and Hall coefficient ($R_{\mathrm{H}}$). As the uncertainty in the SPB-derived quantities resulting from the Seebeck coefficient depends not only on the uncertainty of the Seebeck coefficient but also on the Seebeck coefficient value itself, it is discussed in detail. The uncertainty in the most cited parameter derived from the SPB model, i.e., the DOS mass, is dominated by the uncertainty arising due to α. For a relative uncertainty of 5% in α alone, the uncertainty in the DOS mass ranges from 5% to 15% for 50–400 µV/K, whether scattering of electrons by acoustic phonons, polar optical phonons, ionized impurities, or neutral impurities is assumed. Associated challenges to accurate and insightful modeling of the thermoelectric (TE) materials are discussed and exemplified by an analysis of DOS masses for ${\mathrm{Mg}}_2\mathrm{Si}$ from various literature sources, where the observed large scatter can be rationalized by large but realistic uncertainties in α and $R_{\mathrm{H}}$, thereby highlighting the need for better measurement protocols.

Aluminum gallium nitride [($\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{N}$] presents an ideal platform for designing ultraviolet (UV) light emitters across the entire UV spectral range. However, in the deep-UV spectral range ($<280$ nm) these emitters exhibit very low quantum efficiencies, which in part is linked to the light-polarization characteristics of ($\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{N}$ quantum wells (QWs). In this study, we provide insight into the degree of optical polarization of ($\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{N}$ QW systems operating across the UV-C spectral range by means of an atomistic multiband electronic structure model. Our model not only captures the difference in valence-band ordering in $\mathrm{Al}\mathrm{N}$ and $\mathrm{Ga}\mathrm{N}$ but it also accounts for alloy-disorder-induced band-mixing effects originating from random alloy fluctuations in ($\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{N}$ QWs. The latter aspect is often not captured in widely employed continuum-based models. The impact of alloy disorder on the electronic structure is studied in terms of Urbach-tail energies, which reflect the broadening of the valence-band density of states due to carrier-localization effects. We find that especially in wider wells, Urbach-tail energies are reduced with increasing carrier densities in the well, highlighting that alloy-disorder-induced carrier-localization effects in ($\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{N}$ QWs are also tightly linked to electrostatic built-in fields. Our calculations show that for QWs designed to emit at the longer-wavelength end of the UV-C spectrum, carrier density and well width are of secondary importance for their light-emission properties, meaning that one observes mainly transverse electrical polarization. However, for ($\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{N}$ QWs with high $\mathrm{Al}$ contents, we find that both well width and carrier density will impact the degree of optical polarization. Our calculations suggest that wider wells will increase the degree of optical polarization and may therefore be an option to improve the light-extraction efficiency in deep-UV light emitters.

Interferometric particle imaging (IPI) enables the sizing of small transparent particles, from a few microns to several millimeters, across a large field of view, which is not feasible with typical imaging-based methods. However, current IPI setups typically require two separate optical accesses—one for coupling the illuminating laser beam into the measurement volume, and another to allow the scattered light to exit toward the camera—thereby limiting their applicability in constrained environments. This work presents a generalized IPI approach suitable across all scattering regimes—including backscatter for single-optical-access configurations—and is applicable to both droplets and bubbles. This study demonstrates how the scattering angle influences signal-to-noise ratio (SNR), measurable size range, and uncertainty. Polarization significantly affects signal quality, particularly in backscatter. Variations in light intensity with angular variations can introduce position-dependent uncertainties, and the sensitivity of the size measurement (transfer function) varies with scattering angle. Experimental validation with bubbles confirms the feasibility of backscatter IPI despite lower SNR. For droplets, the reduction in SNR is less pronounced; however, the transfer function behaves differently compared to bubbles, making both evaluation and uncertainty estimation more complex. Additional effects like glare-point splitting can add redundancy but increase complexity. Overall, this study demonstrates that IPI in backscatter is not only feasible but also a key step toward single-sided optical access, expanding the method’s practical applicability of the IPI method.