Obtaining the density matrix that fully describes a quantum system is central to quantum science. Recently, the technique of “direct measurement” has been proposed to characterize a quantum state in situ; however, all the existing schemes require sequential weak and strong measurements, which strongly limits their practical application. Here, we present an efficient scheme for directly measuring arbitrary quantum matrix elements with only one strong measurement or weak measurement on each qudit. Our scheme is experimentally feasible for the direct measurement of arbitrary quantum states; in particular, multiparticle quantum entangled states. This scheme has applications in the characterization of large-scale quantum systems.

The hardware overhead associated with microwave control is a major obstacle to the scale-up of superconducting quantum computing. An alternative approach involves irradiation of the qubits with trains of single-flux-quantum (SFQ) pulses, pulses of voltage whose time integral is precisely equal to the superconducting flux quantum. Here, we describe the derivation and numerical validation of compact SFQ pulse sequences in which classical bits are clocked to the qubit at a frequency that is roughly a factor 5 higher than the qubit oscillation frequency, allowing for variable pulse-to-pulse timing. The control sequences are constructed by repeated streaming of short subsequence registers that are designed to suppress leakage out of the computational manifold. With a single global clock, high-fidelity ($>99.99$%) control of qubits resonating at over 20 distinct frequencies is possible. SFQ pulses can be stored locally and delivered to the qubits via a proximal classical Josephson digital circuit, offering the possibility of a streamlined, low-footprint classical coprocessor for monitoring errors and feeding back to the qubit array.

Materials suitable for magnonic crystals demand low magnetic damping and long spin-wave propagation distance. In this context $\mathrm{Co}$-based Heusler compounds are ideal candidates for magnonic based applications. In this work, antidot arrays (with different shapes) of epitaxial ${\mathrm{Co}}_2{\mathrm{Fe}}_{0.4}{\mathrm{Mn}}_{0.6}\mathrm{Si}$ Heusler-alloy thin films are prepared using e-beam lithography and sputtering technique. Magneto-optic Kerr effect (MOKE) and ferromagnetic resonance analysis confirm the presence of dominant cubic and moderate uniaxial magnetic anisotropies in the thin film. Domain imaging via x-ray photoemission electron microscopy on the antidot arrays reveals chainlike switching or correlated bigger domains for different antidot shapes. Time-resolved MOKE microscopy is performed to study the precessional dynamics and magnonic modes of the antidots with different shapes. We show that the optically induced spin-wave spectra in such antidot arrays can be tuned by changing the shape of the holes. The variation in internal-field profiles, pinning energy barrier, and anisotropy modifies the spin-wave spectra dramatically within the antidot arrays with different shapes. We further show that by combining the magnetocrystalline anisotropy with the shape anisotropy, an extra degree of freedom can be achieved to control the magnonic modes in such antidot lattices.

In this work, a single-laser-beam-based atomic Faraday-rotation magnetometer (FRM) scheme is studied in which atoms interact with a triple-chromatic laser beam. The circularly polarized carrier component of the beam prepares the atoms while its two orthogonal linearly polarized $\pm 1\text{st}$ sidebands are rotated oppositely and then measured. The study result reveals that the FRM scheme is suitable for implementation of a compact sensor supported by a fiber-coupled laser and is also suitable for implementation of a chip-scale sensor with a self-contained light source when using microelectromechanical systems (MEMS) technology.

The nitrogen-vacancy (N-V) center in diamond has been recognized as a high-sensitivity nanometer-scale metrology platform. Thermometry has been a recent focus, with attention largely confined to room-temperature applications. Temperature sensing at low temperatures, however, remains challenging as the sensitivity decreases for many commonly used techniques, which rely on a temperature-dependent frequency shift of the N-V center's spin resonance and its control with microwaves. Here we use an alternative approach that does not require microwaves, ratiometric all-optical thermometry, and demonstrate that it may be utilized to liquid-nitrogen temperatures without deterioration of the sensitivity. The use of an array of nanodiamonds embedded within a portable polydimethylsiloxane sheet provides a versatile temperature-sensing platform that can probe a wide variety of systems without the configurational restrictions needed for applying microwaves. With this device, we observe a temperature gradient over tens of microns in a ferromagnetic-insulator substrate (yttrium iron garnet) under local heating by a resistive heater. This thermometry technique provides a cryogenically compatible, microwave-free, minimally invasive approach capable of probing local temperatures with few restrictions on the substrate materials.

One of the main strategies to control magnetism by voltage is the use of magnetostrictive-piezoelectric hybrid materials, such as ferromagnetic-ferroelectric heterostructures. When such heterostructures are subjected to an electric field, piezostrain-mediated effects, electronic charging, and voltage-driven oxygen migration (magnetoionics) may simultaneously occur, making the interpretation of the magnetoelectric effects not straightforward and often leading to misconceptions. Typically, the strain-mediated magnetoelectric response is symmetric with respect to the sign of the applied voltage because the induced strain (and variations in the magnetization) depends on the square of the ferroelectric polarization. Conversely, asymmetric responses can be obtained from electronic charging and voltage-driven oxygen migration. By engineering a ferromagnetic-ferroelectric hybrid consisting of a magnetically soft 50-nm thick ${\mathrm{Fe}}_{75}{\mathrm{Al}}_{25}$ (at. %) thin film on top of a 〈110〉-oriented $\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_3$-$32{\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_3$ ferroelectric crystal, a highly asymmetric magnetoelectric response is obtained and the aforementioned magnetoelectric effects can be disentangled. Specifically, the large thickness of the ${\mathrm{Fe}}_{75}{\mathrm{Al}}_{25}$ layer allows dismissing any possible charge accumulation effect, whereas no evidence of magnetoionics is observed experimentally, as expected from the high resistance to oxidation of ${\mathrm{Fe}}_{75}{\mathrm{Al}}_{25}$, leaving strain as the only mechanism to modulate the asymmetric magnetoelectric response. The origin of this asymmetric strain-induced magnetoelectric effect arises from the asymmetry of the polarization reversal in the particular crystallographic orientation of the ferroelectric substrate. These results are important to optimize the performance of artificial multiferroic heterostructures.

Spin-torque ferromagnetic resonance (ST-FMR) provides a useful tool to investigate various magnetic properties in spintronic systems, where one excites FMR by applying a rf current and detects the dc voltage generated through rectification effects. While this scheme has been used to characterize spin-orbit torques (SOTs) that have attracted much attention recently, it is known that dc voltages can also be generated by spin pumping that overlaps with the signal from the rectification effects. Here, we show a method to determine the SOT generation efficiency by ST-FMR free from spin pumping using two representative material systems, $\mathrm{W}/\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}/\mathrm{Mg}\mathrm{O}$ and $\mathrm{Pt}/\mathrm{Co}/\mathrm{Mg}\mathrm{O}$. In addition, using the values determined by the method, which are confirmed to agree well with the results of a separately performed extended harmonic Hall measurement, we also quantify the amount of overestimation if the results obtained by a conventional ST-FMR setup are analyzed without considering the spin pumping. The present finding offers a useful insight to obtain a reliable value of SOT efficiency using ST-FMR, in particular for systems that exhibit large SOT, which accompanies large spin pumping.

In this work, we develop an extended uniform potential (UP) model for a membrane nanopore by including two different charging mechanisms of the pore walls, namely by electronic charge and by chemical charge. These two charging mechanisms generally occur in polymeric membranes with conducting agents, or membranes made of conducting materials like carbon nanotubes with surface ionizable groups. The electronic charge redistributes along the pore in response to the gradient of electric potential in the pore, while the chemical charge depends on the local pH via a Langmuir-type isotherm. The extended UP model shows good agreement with experimental data for membrane potential measured at the zero-current condition. When both types of charge are present, the ratio of the electronic charge to the chemical charge can be characterized by the dimensionless number of surface groups and the dimensionless capacitance of the dielectric Stern layer. The performance of the membrane pore in converting osmotic energy from a salt concentration difference into electrical power can be improved by tuning the electronic charge.

In this work, we introduce machine-learning (ML) methods to implement fast and high-fidelity readout of a trapped-ion qubit on a hardware module, which is based on field-programmable gate arrays (FPGAs) and an ARM (Advanced RISC Machines) processor. An average readout fidelity of 99.5% (with ${10}^5$ magnitude trials) within $171\mu \mathrm{s}$ is achieved in experiments. Different ML architectures including convolutional neural networks and fully connected neural networks are implemented to compare with traditional methods, demonstrating higher fidelity and more robust readout results in a relatively short time with the proposed methods. Our hardware implementation of the proposed ML methods with FPGAs improves the readout efficiency to a higher level, through reducing the communication time between the trapped-ion system and central processing units (CPUs) in general personal computer (PCs) and the hardware can be devolved to a functional module, which is compatible with the real-time readout and feedback control of qubit states.

The Hodgkin-Huxley model describes the behavior of the cell membrane in neurons, treating each part of it as an electric circuit element; namely, capacitors, memristors, and voltage sources. We focus on the activation channel of potassium ions, due to its simplicity, while keeping most of the features displayed by the original model. This reduced version is essentially a classical memristor, a resistor whose resistance depends on the history of electric signals that have crossed it, coupled to a voltage source and a capacitor. We consider a quantized Hodgkin-Huxley model based on a quantum-memristor formalism. We compare the behavior of the membrane voltage and the potassium-channel conductance when the circuit is subjected to ac sources, in both the classical realm and the quantum realm. Numerical simulations show an expected adaptation of the considered channel conductance depending on the signal history in all regimes. Remarkably, the computation of higher moments of the voltage shows purely quantum features related to the circuit zero-point energy. Finally, we study the implementation of the Hodgkin-Huxley quantum memristor as an asymmetric rf superconducting quantum-interference device in superconducting circuits. This study may allow the construction of quantum neuron networks inspired by the brain function, as well as the design of neuromorphic quantum architectures for quantum machine learning.

Spoof surface plasmon polaritons (SSPPs) can be converted to leaky-wave radiations by a spoof surface plasmon (SSP) waveguide with periodically modulated surface impedance, which is also known as a SSPP leaky-wave antenna (LWA). However, most of the reported SSPP LWAs can only radiate electromagnetic (em) waves with linear polarizations parallel to the SSP waveguide due to the limitation of perpendicular grooving structures, and the polarizations of LWA radiations are difficult to arbitrarily design. We propose an efficient method to achieve the leaky-wave radiations with arbitrarily customizable polarizations based on an alternative SSPP LWA, which is made of a periodically modulated SSP waveguide with bilateral 45°-tilted grooves to generate two orthogonally polarized em waves. The grooves on both sides of the SSPP LWA can be designed with proper displacement to provide the required phase difference between two orthogonally polarized radiation waves in order to customize the polarization of the leaky-wave radiations. Both simulation and experimental results are provided to validate the feasibility of tailoring arbitrary polarizations for the radiation waves based on the proposed SSPP LWA.

We perform picosecond absorption spectroscopy on $\mathrm{Ba}\mathrm{Br}\mathrm{Cl}$ samples that are undoped, doped with europium to make activated scintillators, and codoped with gold (as ${\mathrm{Au}\mathrm{Br}}_3$ in the melt) to increase light yield. The time-resolved measurements provide insights into the effects of interband excitation in $\mathrm{Ba}\mathrm{Br}\mathrm{Cl}$, including identification of self-trapped-exciton absorption bands, the mechanism of energy transport from the host to the $\mathrm{Eu}$ activator, and the effects of $\mathrm{Au}$ codoping. First-principles electronic structure calculations of self-trapped-exciton relaxations in $\mathrm{Ba}\mathrm{Br}\mathrm{Cl}$ are presented. $\mathrm{Ba}\mathrm{Br}\mathrm{Cl}$:$\mathrm{Eu}$,$\mathrm{Au}$ is a beneficiary of two material modifications for improvement of performance that have been intensely investigated recently: a mixed-halide solid solution that is further modified by aliovalent codoping.

We introduce a method for generating benchmark problem sets for Ising machines—devices designed to solve discrete-optimization problems cast as Ising models. In our approach, linear systems of equations are cast as Ising cost functions. While linear systems are easily solvable, the corresponding optimization problems are known to exhibit some of the salient features of nondeterministic polynomial-time hardness, such as strong exponential scaling of heuristic solvers’ runtimes and extensive distances between ground and low-lying excited states. We show how the proposed technique, which we refer to as “equation planting,” can serve as a useful tool for evaluating the utility of Ising solvers functioning either as optimizers or as ground-state samplers. We further argue that equation-planted problems can be used to probe the mechanisms underlying the operation of Ising machines.

Liquid-rope coiling occurs when a thin jet of viscous fluid falls from a sufficient height onto a rigid substrate. Coiling can also be induced by an electric field, and this precise control over coiling has inspired alternative applications in direct writing, such as the printing of nano-objects and the mixing of viscous fluids. However, the physical mechanism of electric field-assisted coiling remains inadequately understood, challenging the optimization of the design of printing setups. We identify the subtle role of the electric field profile on the jet morphology and the onset of coiling, especially the height, by modifying the electrode configuration for controlling an electrified jet. Based on our finding, we demonstrate that the viscous coiling effect can be either induced or suppressed by changing the configuration of the electrodes, which tunes the electric field profile. The exquisite control of the coiling of viscous fluids can enhance applications that require dispensing of viscous liquids and on-demand direct writing for a wide range of working conditions and various printed patterns.

A nonlinear analytical model for the pressure dynamics in a vacuum chamber, pumped with a sputter ion pump (SIP), is proposed, discussed and experimentally evaluated. The model describes the physics of the pumping mechanism of SIPs in the context of a cold-atom experiment. By using this model, we fit pump-down curves of our vacuum system to extract the relevant physical parameters characterizing its pressure dynamics. The aim of this investigation is the optimization of cold-atom experiments in terms of reducing the dead time for quantum sensing using atom interferometry. We develop a calibration method to improve the precision in pressure measurements via the ion current in SIPs. Our method is based on a careful analysis of the gas conductance and pumping in order to reliably link the pressure readings at the SIP with the actual pressure in the vacuum (science) chamber. Our results are in agreement with the existence of essentially two pumping regimes determined by the pressure level in the system. In particular, we find our results in agreement with the well-known fact that for a given applied voltage, at low pressures, the discharge current efficiently sputters pumping material from the pump’s electrodes. This process sets the leading pumping mechanism in this limit. At high pressures, the discharge current drops and the pumping is mainly performed by the already sputtered material.

Real-time high-resolution three-dimensional (3D) reconstruction of scenes hidden from the direct field of view is a challenging field of research, with applications in real-life situations related, e.g., to surveillance, self-driving cars, and rescue missions. Most current techniques recover the 3D structure of a non-line-of-sight (NLOS) static scene by detecting the return signal from the hidden object on a scattering observation area. Here, we demonstrate the full color retrieval of the 3D shape of a hidden scene by coupling back-projection imaging algorithms with the high-resolution time-of-flight information provided by a single-pixel camera. By using a high-efficiency single-photon avalanche-diode (SPAD) detector, this technique provides the advantage of imaging with no mechanical scanning parts, with acquisition times down to the subsecond range.

We report experimental studies on acoustic scatterers consisting of a pair of coupled decorated membrane resonators that exhibit near extreme contrast in reflection asymmetry and enhanced Willis coupling coefficient with an amplitude of 2.5 and a polarizability cross term of $1.1\times {10}^{-8}{\mathrm{ms}}^2$. The design is based on the theoretical analysis using the Green’s function formulism we developed earlier. The line shapes of the real and the imaginary parts of the Willis coefficient clearly show that the Kramers-Kronig relations are satisfied.

Whether an electrified jet breaks up while the jet maintains a straight line (varicose), or alternatively, the jet trajectory becomes chaotic (whipping), depends on the competition of the interfacial forces. Starting from the competition of normal stresses on the jet surface, we derive scaling laws in different electro-hydrodynamic operating regimes as a function of fluid properties and the flow rate. The onset of whipping occurs when this scaling function reaches a threshold, which is independent of the electric field strength. However, experimental evidence indicates that this onset condition applies only when the viscosity and electrical conductivity of a liquid are small enough. As a result, we further introduce a general parameter to incorporate viscous effects into the scaling law. A unified threshold value for this parameter is found through a substantial number of experiments for liquids having a wide range of properties and under a wide variety of operational conditions of flow rate.

The characteristic impedance of a microwave transmission line is typically constrained to a value $Z_0=50\mathrm{\Omega }$, in part because of the low impedance of free space and the limited range of permittivity and permeability realizable with conventional materials. Here we suggest the possibility of constructing high-impedance transmission lines by exploiting the plasmonic response of edge states associated with the quantum Hall effect in gated devices. We analyze various implementations of quantum Hall transmission lines based on distributed networks and lumped-element circuits, including a detailed account of parasitic capacitance and Coulomb drag effects, which can modify device performance. We additionally conceive of a metamaterial structure comprising arrays of quantum Hall droplets and analyze its unusual properties. The realization of such structures holds promise for efficiently wiring-up quantum circuits on chip, as well as engineering strong coupling between semiconductor qubits and microwave photons.

We report for the first time on the implications of the dielectric nature of transparent electrodes, not only their low conductivity, but also on charge injection and collection. Transparent electrodes are common in organic-based optoelectronic devices because of the small diffusion length of charged carriers that requires complete coverage by the electrode. The commonly reported drawback of transparent electrodes is their low conductivity, which leads to unwanted power dissipation. However, there is no reference to the influence of the dielectric nature on the contact properties. The effect of the dielectric nature is introduced by modifying the expression for the image potential and the resulting barrier lowering at the transparent contact. The model is verified by fabricating and measuring simple unipolar devices, which show excellent agreement with the proposed modification. Without a modification, in order to have a good fitting, there should have been unrealistic values for some parameters, for example, a dielectric constant of 23 for the organic layer. The modified expression is then inspected in numerical simulations to study its nonlinear influence on the device. We find that using our model with typical device parameters, the predicted current density of the device is around half of the value calculated for a metallic electrode. Similar results are obtained by reducing the value of the mobility while keeping the injection condition for a metallic electrode. This explains why this property has been overlooked and is an indication of the extreme care needed when fitting experimental results.