A flux qubit can interact strongly when it is capacitively coupled to other circuit elements. This interaction can be separated into two parts, one acting on the qubit subspaces and one in which excited states mediate the interaction. The first term dominates the interaction between the flux qubit and an $LC$ resonator, leading to ultrastrong couplings of the form ${\sigma }^y(a+a^{†})$, which complement the inductive ${\sigma }^{x}i(a^{†}-a)$ coupling. However, when coupling two flux qubits capacitively, all terms need to be taken into account, leading to complex nonstoquastic ultrastrong interaction of types ${\sigma }_1^y{\sigma }_2^y$, ${\sigma }_1^z{\sigma }_2^z$, and ${\sigma }_1^x{\sigma }_2^x$. Our theory explains all these interactions, describing them in terms of general circuit properties—coupling capacitances, qubit gaps, inductive, Josephson and capacitive energies—that apply to a wide variety of circuits and flux qubit designs.

Diverse external stimuli, such as light irradiation, an electric field, and a stress field, are employed to trigger the large modulation of electronic properties in Mott insulators. Here, we demonstrate that multiple nonvolatile and reversible resistivity evolution can be realized by adjusting the magnitude of ferroelastic strain in voltage-actuated ${\mathrm{LaVO}}_3/0.7\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_3\text{−}0.3{\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_3$ heterostructures. The electrically driven ferroelastic strain tunability of resistivity, with a giant gauge factor of 494, can be effectively modified by light stimulus. Moreover, the visible-light-excited photoresistivity response can be ferroelastically enhanced by up to 65%. This discovery illustrates the strong interplay between the ferroelastic-strain-induced and light-induced effects, mediated by lattice-charge-orbital coupling. Our work implies the potential application of correlated oxide-ferroelectric systems for future low-power high-density versatile electronic storage devices with light-sensing capability.

Acoustic manipulation has been a topic of significant research in recent years. However, the trapping scale in the radiative acoustic field domain is severely limited by the diffraction limit of the classic wave. Here, we report on a simple strategy to create multiple acoustic diffraction-limit-broken spots at different spatial positions in the far-field region for a wide frequency range. The subwavelength acoustic field comes from the interference of the plentiful diffractive beams specific to a quasiperiodic structure plate. We analyze the acoustic radiation force of the gradient field at multiheight plane above the plate for different frequencies and further demonstrate acoustic pushing and ring-shaped trapping of polystyrene particles within this subwavelength region. This work could be envisaged to pave a way for high-resolution manipulation of tiny particles in biomedical and cell research fields.

Demonstrating a quantum computational advantage will require high-fidelity control and readout of multiqubit systems. As system size increases, multiplexed qubit readout becomes a practical necessity to limit the growth of resource overhead. Many contemporary qubit-state discriminators presume single-qubit operating conditions or require considerable computational effort, limiting their potential extensibility. Here, we present multiqubit readout using neural networks as state discriminators. We compare our approach to contemporary methods employed on a quantum device with five superconducting qubits and frequency-multiplexed readout. We find that fully connected feedforward neural networks increase the qubit-state-assignment fidelity for our system. Relative to contemporary discriminators, the assignment error rate is reduced by up to 25% due to the compensation of system-dependent nonidealities such as readout crosstalk, which is reduced by up to one order of magnitude. Our work demonstrates a potentially extensible building block for high-fidelity readout relevant to both near-term devices and future fault-tolerant systems.

An optical dyadic Green’s function framework to describe the transverse electromagnetic fields in a planar perovskite solar-cell stack is coupled to an electronic drift-diffusion model for rigorous treatment of photon recycling in the wave-optics regime for a realistic photovoltaic device. The optical model provides the local reabsorption rate as well as a detailed-balance compatible radiative prefactor, which are used in the electronic model to achieve a self-consistent solution that yields the full optoelectronic device characteristics. The presented approach provides detailed insights into the impact of photon recycling on device performance under different regimes of charge transport and recombination and can help identify the various electronic and optical losses for nonideal, realistic devices. The global efficiency of photon recycling is quantified by defining quantum efficiencies of reabsorbed radiation, while the local efficiency can furthermore be quantified by defining an effective local radiative prefactor. The model introduced here can be used to guide the design of future devices that exploit the full potential of photon recycling.

We use a science-grade skipper charge-coupled device (skipper CCD) operating in a low-radiation background environment to develop a semiempirical model that characterizes the origin of single-electron events in CCDs. We identify, separate, and quantify three independent contributions to the single-electron events, which were previously bundled together and classified as “dark counts”: dark current, amplifier light, and spurious charge. We measure a dark current, which depends on exposure, of $(5.89\pm 0.77)\times {10}^{-4}{e}^{-}/\mathrm{pix}/\mathrm{day}$, and an unprecedentedly low spurious charge contribution of $(1.52\pm 0.07)\times {10}^{-4}{e}^{-}/\mathrm{pix}$, which is exposure independent. In addition, we provide a technique to study events produced by light emitted from the amplifier, which allows the detector’s operation to be optimized to minimize this effect to a level below the dark-current contribution. Our accurate characterization of the single-electron events allows one to greatly extend the sensitivity of experiments searching for dark matter or coherent neutrino scattering. Moreover, an accurate understanding of the origin of single-electron events is critical to further progress in ongoing research and development efforts of skipper and conventional CCDs.

Quantum diamond microscope (QDM) magnetic field imaging is an emerging interrogation and diagnostic technique for integrated circuits (ICs). To date, the ICs measured with a QDM have been either too complex for us to predict the expected magnetic fields and benchmark the QDM performance or too simple to be relevant to the IC community. In this paper, we establish a 555 timer IC as a “model system” to optimize QDM measurement implementation, benchmark performance, and assess IC device functionality. To validate the magnetic field images taken with a QDM, we use a spice electronic circuit simulator and finite-element analysis (FEA) to model the magnetic fields from the 555 die for two functional states. We compare the advantages and the results of three IC-diamond measurement methods, confirm that the measured and simulated magnetic images are consistent, identify the magnetic signatures of current paths within the device, and discuss using this model system to advance QDM magnetic imaging as an IC diagnostic tool.

The transport of tiny particles is of great interest in the field of lab-on-a-chip. Here, by drawing inspiration from electric materials, we introduce magnetometamaterials, which, as opposed to electrons, transport magnetic particles (i.e., matter). The proposed metamaterial is composed of lithographically patterned disk-shaped micromagnets arranged in rows forming linear magnetic tracks on a silicon substrate. It is shown that by applying an external rotating magnetic field with the right frequency, the particles move along the magnetic tracks. The magnetic matter transport rate (i.e., magnetic matter conductivity) is tuned by adjusting the external magnetic field. We show that the particle transport is similar to the classical electron transport through a periodic lattice. At low frequencies, similar to the electrons at low temperatures, the particles move in closed loops around single magnets, resulting in an insulating regime. At higher frequencies, similar to the electrons at higher temperatures, some particles show the same behavior (i.e., move in closed loops); however, some others move along the magnetic tracks (i.e., demonstrate open trajectories). This behavior resembles a magnetic matter semiconductor. At even higher frequencies, all the particles show open trajectories along the magnetic tracks, and the device resembles a magnetic matter conductor. We define the operational ranges, both theoretically and experimentally, for both one- and two-dimensional magnetometamaterials. We show that, in an appropriate vertical bias field, the two-dimensional magnetometamaterials can transport the particles in arbitrary paths. The proposed metamaterial in this work can be used in designing circuits for transporting particles with crucial applications in biomedical engineering.

Nonequilibrium Green’s function modeling of the structure used for III-V quantum cascade lasers emitting at approximately 5-$\mu \mathrm{m}$ wavelength is performed to get insight into and analyze the processes that determine the linewidth of the optical (inter-sub-band) transition. Theoretical results are compared with electroluminescence data collected for real structures. Excellent agreement is found for both the transition energy of approximately $234\mathrm{meV}$ and the linewidth of approximately $32\mathrm{meV}$, typical for this sort of device. Careful inspection of simulation data shows that, for these structures, the interface-roughness (IR) scattering contributes to the optical linewidth $\cong 50\mathrm{\%}$. The decreased broadening due to IR scattering is not due to very smooth interfaces, but rather due to the finite carrier concentration and the decreased population of final states ${\mathbf{k}}^{{}^{\prime }}$ available for the initial state $\mathbf{k}$ in the limit of vanishing in-plane momentum $\mathbf{k}\to 0$. Further line narrowing is caused by nonparabolicity coupled with the highly nonthermal occupation of the lower laser sub-band, which destroys the population inversion for the hot carriers.

We investigate how a combination of a nonmagnetic impurity scattering rate $\gamma$ and finite subgap states parametrized by Dynes $\mathrm{\Gamma }$ affects various physical quantities relevant to superconducting devices made from extreme type-II $s$-wave superconductors. All the calculations are based on the Eilenberger formalism of the BCS theory. It is well known that the optimum impurity concentration minimizes the surface resistance $R_s$. We find the optimum $\mathrm{\Gamma }$ can also reduce $R_s$ by one order of magnitude for a clean superconductor ($\gamma /{\mathrm{\Delta }}_0<1$) and a few tens of % for a dirty superconductor ($\gamma /{\mathrm{\Delta }}_0>1$). Here, ${\mathrm{\Delta }}_0$ is the pair potential for the idealized ($\mathrm{\Gamma }\to 0$) BCS superconductor for $T\to 0$. Also, we find a nearly ideal ($\mathrm{\Gamma }/{\mathrm{\Delta }}_0\ll 1$) clean-limit superconductor exhibits a frequency-independent $R_s$ for a broad range of frequency $\omega$, which can significantly improve $Q$ of a compact cavity with a few tens of GHz frequency. As $\mathrm{\Gamma }$ or $\gamma$ increases, $R_s$ obeys the ${\omega }^2$ dependence. The subgap-state-induced residual surface resistance $R_{\mathrm{res}}$ is also studied, which can be detected by a high-$Q$ three-dimensional resonator. We calculate the kinetic inductance $L_k(\gamma ,\mathrm{\Gamma },T)$ and the depairing current density $J_d(\gamma ,\mathrm{\Gamma },T)$, which are monotonic increasing and decreasing functions of $(\gamma ,\mathrm{\Gamma },T)$, respectively. Measurements of $(\gamma ,\mathrm{\Gamma })$ of device materials can give helpful information on improving $Q$, engineering $L_k$, and ameliorating $J_d$ via materials processing.

In this paper we study the electromagnetic scattering, absorption, and performance bounds for short time modulated pulses that impinge on a time-varying lossy layer that is sandwiched between vacuum and a perfect electric conductor. The electric characteristics of the layer, namely, the conductivity, permittivity, and permeability, are assumed to change abruptly or gradually in time. We demonstrate numerically that a time-varying absorbing layer that undergoes temporal switching of its permittivity and conductance can absorb the power of a modulated, ultrawideband, as well as a quasimonochromatic, pulsed wave beyond what is dictated by the time-invariant Rozanov bound when integrating over the whole frequency spectrum. We suggest and simulate a practical metamaterial realization that is constructed as a three-dimensional array of resistor loaded dipoles. By switching only the dipole’s load resistance, desired effective media properties are obtained. Furthermore, we show that Rozanov’s bound can be bypassed with abrupt and a more practical gradual, soft, switching, thus overcoming some possible causality issues in abrupt switching.

Single-crystalline α-$\mathrm{Sn}$ films on the wafer scale are epitaxially grown on $\mathrm{In}\mathrm{Sb}(001)$ substrates by molecular-beam epitaxy. The Berry phase for α-$\mathrm{Sn}$ is extracted to be −0.64π from the quantum oscillations of the magnetoconductivity. Angle- and temperature-dependent Shubnikov–de Haas oscillations substantiate that the α-$\mathrm{Sn}$ film has a spherical Fermi surface and small effective mass (0.039$m_0$; $m_0$ is the resting mass of an electron), which are in agreement with the features of three-dimensional Dirac semimetals. In addition, an extremely large magnetoresistance of over $4.5\times {10}^5\mathrm{\%}$ at 1.5 K is observed. These α-$\mathrm{Sn}$ films, with wafer-scale dimensions and extremely large magnetoresistance, may pave the way for device applications of topological materials.

Electrical currents flowing through magnetic domain walls tend to encounter additional extraordinary resistance. At the thin-wall limit, the occurrence of anomalous-field antisymmetric magnetoresistance breaks Onsager symmetry, which seems to be determined by the combination of single-wall and perpendicular-current-injection geometry. However, the delicate magnetic structure and measurement configuration are hard to implement, and the abnormal transport properties observed in multidomain structures are not fully understood. Here, we report the observation of antisymmetric magnetoresistance due to domain-wall tilting in a multidomain structure controlled by a magnetic field gradient. We demonstrate, both experimentally and theoretically, that the anomalous magnetoresistance arises from the nonequilibrium current in the vicinity of tilting domain walls, which are basically governed by the geometry factor of the tilting walls.

Selective shielding of surfaces from flow and diffusion is essential in many domains of physics and engineering. We propose and characterize experimentally a framework to shield both flow and diffusion simultaneously within a reshapable fluid volume. To do so, we directly use flow singularities to generate conformal transforms of the diffusion profile, taking inspiration from classical cloaking devices. The properties of microfluidic surface shields as open-space diffusion filters (O-filters) are further investigated. An analytical model is provided, which yields an outstanding match with experimental measurements of both flow and tracer diffusion. Via conformal mapping, these measurements reveal a strong topological link with classical H-filters in microfluidics.

Perovskite oxides are extremely interesting for their possible use in high-mobility thin-film transistors (TFTs) suitable for high-performance large-area circuits. Here we present a semianalytical model of a recently fabricated TFT based on ${\mathrm{La}\text{−}\mathrm{Ba}\mathrm{Sn}\mathrm{O}}_3$, and explore the possibilities for technology optimization and the intrinsic potential of the device concept for applications in transparent and flexible electronics. We show that ${\mathrm{La}\text{−}\mathrm{Ba}\mathrm{Sn}\mathrm{O}}_3$ TFTs can outperform existing TFT device technologies through a detailed benchmarking exercise, and that these devices can be promising for display electronics and for a broader range of applications.

Absorption spectroscopy has long been hailed as an absolute necessity for detecting the properties of complex chemical and biological samples. While conventional spectroscopy using classical light is severely limited by shot noise and lacks robustness against experimental imperfections, quantum light offers an avenue to perform absorption spectroscopy with provable advantages in the technical operations and the measurement precision. Here, we present an experimental approach to implement entanglement-based absorption spectroscopy with the assistance of Hong-Ou-Mandel interference. Since the temporal interference fringe is determined by the spectrum pattern, a maximum-likelihood decision is sufficient for revealing the relevant quantum information about the absorptive properties of the test samples. These experimental results may significantly facilitate the use of quantum interferometric spectroscopy to practical applications, which may be particularly relevant for photon-sensitive biological and chemical samples.