In this work we present a simulation of the semiconductor electrodes of photoelectrochemical (PEC) water-splitting cells based on the nonequilibrium Green’s function (NEGF) formalism. While the performance of simple PEC cells can be adequately explained with semiclassical drift-diffusion theory, the increasing interest towards thin-film cells and nanostructures, in general, requires theoretical treatment that can capture the quantum phenomena influencing the charge carrier dynamics in these devices. Specifically, we study a $p$-type ${\mathrm{Cu}}_2\mathrm{O}$ electrode and examine the influence of the bias voltage, reaction kinetics, and the thickness of the ${\mathrm{Cu}}_2\mathrm{O}$ layer on the generated photocurrent. The NEGF equations are solved in a self-consistent manner with the electrostatic potential from Poisson’s equation, sunlight-induced photon scattering and the chemical overpotential required to drive the water-splitting reaction. We show that the NEGF simulation accurately reproduces experimental results from both voltammetry and impedance spectroscopy measurements, while providing an energy-resolved solution of the charge carrier densities and corresponding currents inside the semiconductor electrode at nanoscale.

Twisted kagome lattices have unusual physical effects upon regulating the twist angle, an additional degree of freedom, which has drawn attention to these lattices. Here, we extend twisted kagome lattices to continuous elastomers and study the dynamics of flexural waves. We confirm that the twist angle can introduce boundary-obstructed topological insulators into topological trivial elastic kagome metamaterials by topological quantum chemistry theory. The kagome metamaterials have degenerate boundary states and twist-angle-dependent corner states. According to corner states corresponding to the inner angles of the modes, the corner states can be divided into two categories, obtuse-corner states and acute-corner states. The operating frequencies of the corner state corresponding to different inner angles are nondegenerate. The twist angle leads to rich and diverse regulations of the corner state. These results reveal hidden topological properties in trivial lattices and add an additional degree of freedom, the twisted angle, to tune higher-order topological insulators.

Frequency-mixing processes in spin systems have a variety of applications in meteorology and in quantum data processing. Spin spectroscopy based on frequency mixing offers some advantages, including the ability to eliminate crosstalk between driving and detection. We experimentally explore nonlinear frequency-mixing processes with negatively charged nitrogen-vacancy defects in diamond at low temperatures, and near level anticrossing. The experimental setup allows simultaneously applying magnetic driving in the longitudinal and transverse directions. Magnetic resonance detection is demonstrated using both Landau-Zener-Stückelberg interferometry and two-tone driving spectroscopy. The experimental results are compared with predictions of a theoretical analysis based on the rotating wave approximation.

Magnetic field sensors are used intensively in many areas of our lives and the demands on their properties are constantly growing. One of the biggest challenges is the development of an offset-free sensor that provides a zero signal at zero field, regardless of the manufacturing accuracy. Here, we propose the concept of an offset-free sensor, based on the use of standing spin waves. The dispersion of the spin waves strongly depends on the applied magnetic field, so that the excitation maximum of the standing wave shifts with the change of the field. Spin pumping in combination with the inverse spin Hall effect is used to detect the sensor signal directly, in the form of a dc voltage. The offset-free functionality is achieved by reversing the magnetization direction of the spin-wave waveguide. The functionality of the sensors is proven numerically and concepts for practical implementation are presented.

We explore the scattering of waves in designed asymmetric one-dimensional waveguide networks. We show that the reflection between two ports of an asymmetric network can be identical over a broad frequency range, as if the network was mirror-symmetric, under the condition of so-called latent symmetry between the ports. This broadband equireflectionality is validated numerically for acoustic waveguides and experimentally through measurements on microwave transmission-line networks. In addition, introducing a generalization of latent symmetry, we study the properties of an $N$-port scattering matrix $S$. When the powers of $S$ fulfill certain relations, which we coin scaled cospectrality, the setup is guaranteed to possess at least one zero eigenvalue of $S$, so that the setup features coherent perfect absorption. More importantly, scaled cospectrality introduces a scaling factor which controls the asymmetry of the incoming wave to be absorbed. Our findings introduce a novel approach for designing tunable wave manipulation devices in asymmetric setups. As evidenced by our acoustic simulations and microwave experiments, the generality of our approach extends its potential applications to a wide range of physical systems.

Determining Hamiltonian parameters from noisy experimental measurements is a key task for the control of experimental quantum systems. An interesting experimental platform where precise knowledge of device parameters is useful is the quantum-dot-based Kitaev chain. In these systems, the fine tuning of Hamiltonian parameters is crucial in order to reach the desired regime with stable midgap modes. In this work, we demonstrate an adversarial machine-learning algorithm to determine the parameters of a quantum-dot-based Kitaev chain. We train a convolutional conditional generative adversarial neural network (CCGAN) with simulated differential conductance data and use the model to predict the parameters at which Majorana bound states are predicted to appear. In particular, the CCGAN model facilitates a rapid, numerically efficient exploration of the phase diagram describing the transition between elastic co-tunneling and crossed Andreev reflection regimes. We verify the theoretical predictions of the model by applying it to experimentally measured conductance obtained from a minimal Kitaev chain consisting of two spin-polarized quantum dots coupled by a superconductor-semiconductor hybrid. Our model accurately predicts, with an average success probability of 97%, whether the measurement was taken in the elastic co-tunneling or crossed Andreev reflection-dominated regime. Our work constitutes a stepping stone towards fast, reliable parameter prediction for tuning quantum dot systems into distinct Hamiltonian regimes. Ultimately, our results yield a strategy to support Kitaev-chain tuning that is scalable to longer chains.

Temporal filtering is widely recognized as an effective strategy for improving the visibility of the Hong-Ou-Mandel interferometer, an important tool for observing the phenomenon of two-photon quantum interference. However, such manipulation may lead to additional effects on the measurement. In addition to demonstrating the degradation of photon-detection efficiency, this paper demonstrates that temporal filtering can significantly broaden the interference width, especially in scenarios where photons experience substantial dispersion. This broadening effect is also presented in the nonlocal Franson interferometer. The good agreement between experimental results and theoretical simulations not only reveals the mechanism behind dispersion-induced broadening in two-photon quantum interference but also provides an alternative and convenient method for measuring dispersion without the need for extra equipment.

We propose a novel device concept that uses spin-orbit torques to realize a magnetic field sensor, where we eliminate the sensor offset using a differential measurement concept. We derive a simple analytical formulation for the sensor signal and demonstrate its validity with numerical investigations using macrospin simulations. The sensitivity and the measurable linear sensing range in the proposed concept can be tuned by varying the effective magnetic anisotropy or the magnitude of the injected currents. We show that undesired perturbation fields normal to the sensitive direction preserve the zero-offset property and only slightly modulate the sensitivity of the proposed sensor. Higher harmonic voltage analysis on a Hall cross experimentally confirms the linearity and tunability via current strength. Additionally, the sensor exhibits a nonvanishing offset in the experiment, which we mostly attribute to the anomalous Nernst effect.

Broadband voltage rectification is an interesting effect that was recently discovered in magnetic tunnel junction (MTJ) structures. Usually, this effect occurs for certain types of MTJ structures and for special configurations of the external magnetic field (namely, perpendicular to the plane component of magnetization). We report on an alternative type of broadband rectification effect with a frequency range up to 6 GHz that is observed in MTJ samples with in-plane magnetization and without an external out-of-plane magnetic field. We compare the experimental study with numerical calculations and theoretical analysis to explain the results. Furthermore, we demonstrated that for the appearance of broadband rectification, it is sufficient to create a nonzero equilibrium angle between the polarizer and the free layer. Our work paves the way for improved energy efficiency in wireless microwave energy-harvesting applications.

The initialization of superconducting qubits is one of the essential techniques for the realization of quantum computation. In previous research, initialization above 99% fidelity has been achieved at 280 ns. Here, we demonstrate the rapid initialization of a superconducting qubit with a quantum circuit refrigerator (QCR). Photon-assisted tunneling of quasiparticles in the QCR can temporally increase the relaxation time of photons inside the resonator and helps release energy from the qubit to the environment. Experiments using this protocol have shown that 99% of initialization time is reduced to 200 ns. This initialization time depends strongly on the relaxation rate of the resonator, and faster initialization is possible by reducing the resistance of the QCR, which limits the on:off ratio, and by strengthening the coupling between the QCR and the resonator.

The modest spin-orbit coupling arising from the n-π* transitions of phenazine complexes offers the unique means to activate the dual emission of singlets and triplets, i.e., fluorescence and phosphorescence, in organic light-emitting diodes (OLEDs) at room temperature. By spectrally resolving OLED electroluminescence, we investigate the optically detected magnetic resonance (ODMR) of such a metal-free dual-emitting host-guest OLED in separate fluorescence and phosphorescence channels. The phosphorescence component of ODMR exhibits two distinctive resonance lineshapes, which differ between the in-phase and quadrature channels. To rationalize these features, we devise a comprehensive quantum stochastic model relating the phase-sensitive components of the lock-in-detected ODMR signal to the forward and reverse transfer of triplets between the host molecules of the OLED, where they are dark, and the guest molecules, from which phosphorescence occurs. Within the model proposed, the observed resonance lines result from the interplay of spin-dependent electron-hole polaron-pair (PP) recombination and triplet-exciton–polaron (TEP) reaction processes. The analytical description of the model is based on the stochastic Liouville equation treatment of the PP and TEP spin dynamics. Quantitative analysis is performed by means of numerical simulations exploiting solutions of the corresponding Liouville equations based on Floquet theory. The resulting accurate match to experiments allows us to draw plausible conclusions with regards to the numerical values of parameters that characterize the system, including the phosphorescence lifetime and the polaron hyperfine interaction strengths. Based on the theoretical framework used to describe the phosphorescence ODMR lineshape, we compute the resonance line recorded in the conjugated detection channel, i.e., in the fluorescence. Surprisingly, this analysis reveals a substantial contribution of the TEP mechanism to the singlet fluorescence channel, implying that fluorescent species are generated from triplets in the course of the TEP reaction. We argue that the TEP process occurring in our device is a spin-allowed polaron-assisted upconversion of triplets to singlets, in which free polarons scatter off trapped triplets, losing their energy to transfer triplets to singlets of higher energy.

Bound states in the continuum (BICs) have attracted increasing interest in recent years owing to their intriguing physical characteristics such as the infinitely high quality factor ($Q$ factor) and the enhanced wave-matter interaction. In this study, we report the theoretical and experimental observation of a Friedrich-Wintgen BIC with bridging near-field coupling in an asymmetric two-state acoustic system. Through the temporal coupled-mode theory, we comprehensively elucidate the role of bridging near-field coupling in constructing Friedrich-Wintgen BICs. Then, we present a two-state system consisting of two asymmetric cavities with a bridging tube. By tuning the diameter and position of the bridging tube, we can effectively modulate the near-field coupling effect of the presented system and achieve a Friedrich-Wintgen BIC. Furthermore, the presented system is modulated to deviate from the BIC, which provides a quasi-BIC and allows for a high-$Q$ perfect absorption when the radiation and dissipation of the quasi-BIC-supporting system achieve the critical coupling condition. The experimental results validate the theoretical and simulation results, demonstrating both the existence of a BIC and a quasi-BIC-based perfect absorption. Our work opens up an avenue to investigate acoustic Friedrich-Wintgen BICs with bridging near-field coupling in asymmetric systems, which enriches the field of acoustic BICs and offers opportunities for the development of acoustic devices with high $Q$ factor and asymmetric wave control.

Noisy mean-field annealing (NMFA) is an algorithm that mimics a coherent Ising machine (CIM), which is an optical system for solving Ising problems. The NMFA has reproduced the solver performance of the CIM for systems of limited size even though it simplifies the interaction between spins with a mean-field approximation. However, recent experiments observing various thermodynamic quantities have revealed that the CIM can capture the phase transitions of the two-dimensional Ising models that the mean field cannot capture. This situation leads to a fundamental question as to how well the NMFA can capture the features of the thermodynamic quantities around the phase transition. This paper answers that the NMFA reproduces the thermodynamic features of the mean field, but cannot reproduce the CIM results. This suggests that, in terms of sampling, the level of performance of the CIM is beyond that of the NMFA.

The amount of work that can be extracted from a quantum system can be increased by exploiting the information obtained from a measurement performed on a correlated ancillary system. The concept of daemonic ergotropy has been introduced to properly describe and quantify this work extraction enhancement in the quantum regime. We here explore the application of this idea in the context of continuously monitored open quantum systems, where information is gained by measuring the environment interacting with the energy-storing quantum device. We first show that the corresponding daemonic ergotropy takes values between the ergotropy and the energy of the corresponding unconditional state. The upper bound is achieved by assuming an initial pure state and a perfectly efficient projective measurement on the environment, independently of the kind of measurement performed. On the other hand, if the measurement is inefficient or the initial state is mixed, the daemonic ergotropy is generally dependent on the measurement strategy. This scenario is investigated via a paradigmatic example of an open quantum battery: a two-level atom driven by a classical field and whose spontaneously emitted photons are continuously monitored via either homodyne, heterodyne, or photodetection.

The accuracy of microwave measurements is not only critical for applications in telecommunication and radar but also for future quantum computers. Qubit technologies such as superconducting qubits or spin qubits require the detection of minuscule signals, typically achieved by reflecting a microwave tone off a resonator that is coupled to the qubit. Noise from cabling and amplification, e.g., from temperature variations, can be detrimental to readout fidelity. We present an approach to detect phase and amplitude changes of a device under test based on the differential measurement of microwave tones generated by two first-order sidebands of a carrier signal. The two microwave tones are sent through the same cable to the measured device that exhibits a narrow-band response for one sideband and leaves the other unaffected. The reflected sidebands are interfered by down-conversion with the carrier. By choosing the amplitude and phases of the sidebands, suppression of either common-amplitude or common-phase noise can be achieved, allowing for fast, stable measurements of frequency shifts and quality factors of resonators. Test measurements were performed on $\mathrm{NbN}$ superconducting resonators at 25 mK to calibrate and characterize the experimental setup and to study time-dependent fluctuations of their resonance frequency.

Ultrasonic three-dimensional printed holograms are getting increasing interest for transcranial therapies since they can correct skull aberrations and, simultaneously, adapt the acoustic field to particular brain targets. However, evaluating the targeting performance of these systems requires the measurement of complex volumetric acoustic fields, which in many practical situations cannot be estimated by direct hydrophone measurements. In this work, we apply single-plane holographic measurement techniques to experimentally calibrate and measure the full volumetric field produced by holographic lenses. Two ex vivo test cases are presented, a four-foci lens and a preclinical case, both targeting through a macaque skull for potential applications in blood–brain barrier opening (BBBO) studies. Time-reversal and angular spectrum projection methods are compared to direct experimental measurements. Results show that holographic projection methods can reconstruct the complex acoustic images produced by holographic lenses, matching direct measurements in all test cases. However, while direct measurements are restricted to transverse-field cross sections, holographic projection allows estimating the field on the whole targeting volume. In this way, the location and the full three-dimensional shape of all acoustic foci can be obtained. Furthermore, these techniques can provide the field at the surface of the lens to compare it to the design phase distribution. Using this procedure, complex volumetric acoustic fields can be reconstructed, saving significant measurement time and computational resources, and enabling an accurate characterization of phase plates and other holographic lens topologies.

A central goal in quantum technologies is to maximize $G{T}_2$, where $G$ stands for the coupling of a qubit to control and readout signals and $T_2$ is the qubit’s coherence time. This is challenging, as increasing $G$ (e.g., by coupling the qubit more strongly to external stimuli) often leads to deleterious effects on $T_2$. Here, we study the coupling of pure and magnetically diluted crystals of ${\mathrm{Ho}\mathrm{W}}_{10}$ magnetic clusters to microwave superconducting coplanar waveguides. Absorption lines give a broadband picture of the magnetic energy level scheme and, in particular, confirm the existence of level anticrossings at equidistant magnetic fields determined by the combination of crystal field and hyperfine interactions. Such “spin clock transitions” are known to shield the electronic spins against magnetic field fluctuations. The analysis of the microwave transmission shows that the spin-photon coupling also becomes maximum at these transitions. The results show that engineering spin-clock states of molecular systems offers a promising strategy to combine sizable spin-photon interactions with a sufficient isolation from unwanted magnetic noise sources.

Semiconductor devices often require band-gap engineering, which in turn requires the use of alloys and the tuning of their composition to achieve the required electrical or optical behavior. While their compositional fluctuations are generally treated as a perturbation in most conventional III-V semiconductors, the effects of alloy disorder are much more significant in the III-nitrides. Here, the effects of alloy disorder on carrier localization are compared for different III-V semiconductors, particularly for holes for which localization effects are more significant due to their heavier effective mass. This study is conducted using three-dimensional computation for III-V alloys with natural (random) compositional fluctuations. Given the complexity of the problem, we carry out the computations relying on a simplified Hamiltonian in the envelope wave-function approximation with a single heavy-hole valence band. We investigate the effects of compositional fluctuations on carrier localization using two methods. The first method, based on the localization landscape theory, is used to solve the localization landscape equations and obtain the effective potentials acting on carriers. This potential acts as a confining potential that predicts the regions of spatial localization of carriers, and thus allows a comparison of the effects of alloy fluctuations between the conventional III-V and the III-nitride semiconductors. We find that the effective potential of the III-nitride semiconductors exhibits much larger fluctuations compared to the other III-V semiconductors. This might point to a higher degree of carrier localization in the nitrides, particularly for holes. This is verified through the second method, solving Schrödinger’s equation and obtaining the electron and hole wave functions. We find that for ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ the electron wave functions are delocalized even for the ground state, whereas the low-energy hole states are localized. This is in contrast with the behavior of holes in the common alloy ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$, which are found to be always delocalized. Thus, our study shows the importance of accounting for alloy disorder in the nitrides.

We present results of direct maskless magnetic patterning of ferromagnetic nanostructures using a cobalt focused-ion-beam (FIB) system. The liquid-metal ion source of the FIB was made of a ${\text{Co}}_{36}{\text{Nd}}_{64}$ alloy. A Wien mass filter allows the ion species to be selected. Using the FIB, we implanted narrow tracks of $\mathrm{Co}$ ions into a nominal $5000\times 1100\times 53{\mathrm{nm}}^3$ Permalloy strip. We observed the $\mathrm{Co}$-induced changes of the magnetic properties by measuring the sample with microresonator ferromagnetic resonance before and after the implantation. Regions as small as 40 nm can be implanted up to concentrations of 10 at % near the surface. This allows for easy magnetic modification of edge-localized spin waves with a lateral resolution otherwise hard to reach. The direct-write maskless FIB process is quick and convenient for optical measurement techniques, as it does not involve the virtually impossible removal of the ion-hardened resist masks one would face when using lithography with broad-beam ion implantation.