A reconstruction-free anatomical imaging algorithm that localizes the Compton scattering of annihilation $\gamma$ rays within an object using ultrafast time-of-flight information and provides an anatomical cross-section image without tomographic reconstruction has been previously proposed. However, the spatial performance of generated images remains unclear. This study introduces a theoretical understanding and supporting simulation results of a Compton scattering-based reconstruction-free anatomical imaging method. First, we provide the exact algebraic solution, which was solved approximately in a previous study. Using the algebraic solution, the system performance can be quantitatively estimated via total differentiation. Total differentiation indicated that the difference in the path lengths of annihilation $\gamma$ rays, which can be theoretically interpreted as the Compton scattering angle in an object strongly impacting the spatial resolution of the anatomical image. Monte Carlo simulations demonstrated that the spatial resolution of the generated images is improved by selecting events with large Compton scattering angles, which is consistent with the theory derived based on the algebraic solution. Furthermore, the use of energy information helped enhance the image quality by eliminating multiple scattering events in the imaging object.

This paper proposes the concept and demonstrates the realization of acoustic logic networks (ALNs), a system that performs various logic operations using sound waves. Unlike traditional systems with separate components, ALNs integrate multiple logic functionalities with a single platform, performing operations like and, or, and xor gates, similar to those used in electronic computers. The design of ALNs leverages the non-Hermitian and nonlocal metagratings. By considering the intricate nonlocal interaction within the structure, the metagratings are designed to operate at exceptional points (EPs) to achieve the desired logic functions. Two metagratings are cascaded to construct the acoustic logic module (ALM), which acts as the basic building block of an ALN. The effectiveness of ALNs is confirmed through theoretical calculations, numerical simulations, and experiments. We demonstrate the ability to perform complex logic operations and realize various multichannel computing functions with the ALN of different configurations. These results showcase precise control over sound waves and significantly enhanced computing capabilities. The proposed ALNs represent a significant step forward in acoustic computing. This technology has the potential to revolutionize fields where traditional electronic computing might be impractical due to size, energy efficiency, or environmental limitations.

Cold-atom magnetometers exploit a dense ensemble of quanta with long coherence times to realize leading sensitivity on the micrometer scale. Configured as a Ramsey interferometer, a cold-atom sensor can approach atom shot-noise limited precision but suffers from fringe ambiguity, producing gross errors when the field falls outside a narrow predefined range. We describe how Hilbert-demodulated optical magnetometry can be realized on cold-atom sensors to provide field measurements both precise and unambiguous. Continuous reconstruction of the Larmor phase allows us to determine the dc magnetic field unambiguously in an unshielded environment, as well as measure ac variation of the field, in a single shot. The ac measurement allows us to characterize, and then neutralize, line-synchronous magnetic interference, extending reconstruction times. Using $1.6\times {10}^6{}^{87}\mathrm{Rb}$ atoms in a volume of $(68\text{μ}\mathrm{m}{)}^3$, we measure a test field to be $86.0121261(4)\text{μ}\mathrm{T}$ in a single shot, achieving dc sensitivity of $380\mathrm{fT}$ in a duration of $1000\mathrm{ms}$. Our results demonstrate that Hilbert-demodulated optical readout yields metrologically significant sensitivity without the fringe ambiguity inherent to Ramsey interferometry.

Increased demand for high-performance permanent magnets in the electric vehicle and wind-turbine industries has prompted the search for cost-effective alternatives. Discovering magnetic materials with the desired intrinsic and extrinsic permanent magnet properties presents a significant challenge to researchers because of issues with the global supply of rare-earth elements, material stability, and a low maximum magnetic energy product $B{H}_{\text{max}}$. While first-principles density functional theory (DFT) predicts materials’ magnetic moments, magnetocrystalline anisotropy constants, and exchange interactions, it cannot compute extrinsic properties such as coercivity ($H_c$). Although it is possible to calculate $H_c$ theoretically with micromagnetic simulations, the predicted value is larger than the experiment by almost an order of magnitude due to the Brown paradox. To circumvent these issues, we employ machine-learning (ML) methods on an extensive database obtained from experiments, DFT calculations, and micromagnetic modeling. The use of a large experimental dataset enables realistic $H_c$ predictions for materials such as $\text{Ce}$-doped ${\text{Nd}}_2{\text{Fe}}_{14}\text{B}$, comparing favorably against micromagnetically simulated coercivities. Remarkably, our ML model accurately identifies uniaxial magneto-crystalline anisotropy as the primary contributor to $H_c$. With DFT calculations, we predict the $\mathrm{Nd}$-site-dependent magnetic anisotropy behavior in ${\text{Nd}}_2{\text{Fe}}_{14}\text{B}$, confirming that $\text{Nd}4g$ sites mainly contribute to uniaxial magnetocrystalline anisotropy, and also calculate the Curie temperature ($T_{\mathrm{c}}$). Both calculated results are in good agreement with the experiments. The coupled experimental dataset and ML modeling with DFT input predict $H_c$ with far greater accuracy and speed than was previously possible using micromagnetic modeling. Further, we reverse engineer the grain-boundary and intergrain exchange coupling with micromagnetic simulations by employing the ML predictions.

Resonant states underpin a variety of metastructures that exhibit remarkable capabilities for the effective control of acoustic waves at subwavelength scales. The development of metamaterials relies on rigorous mode engineering, providing the implementation of the desired properties. At the same time, the application of metamaterials is still limited, as their building blocks are frequently characterized by complicated geometry and cannot be tuned easily. In this work, we consider a simple system of coupled Helmholtz resonators and study their properties associated with the tuning of coupling strength and symmetry breaking. We numerically and experimentally demonstrate the excitation of the quasibound state in the continuum in the resonators placed in free space and in a rectangular cavity. It is also shown that tuning the intrinsic losses via introducing porous inserts can lead to spectral splitting or merging of quasibound states in the continuum and the occurrence of exceptional points. The obtained results open opportunities for the development of simple and easily tunable metastructures based on Helmholtz resonances.

A crucial requirement for quantum computing—in particular, for scalable quantum computing and error correction—is fast and high-fidelity qubit readout. For semiconductor-based qubits, one limiting factor for local low-power signal amplification is the output swing of the charge sensor. We demonstrate $\mathrm{Ga}\mathrm{As}$ and Si asymmetric sensing dots (ASDs) specifically designed to provide a significantly larger response compared to conventional charge-sensing dots. Our ASD design features a drain reservoir strongly decoupled from the sensor dot, which mitigates negative feedback effects found in conventional sensors. This results in a boosted output swing of $3\mathrm{mV}$, which exceeds the response in the conventional regime of our device by more than a factor of 10. The enhanced output signal paves the way for employing very low-power readout amplifiers in close proximity to the qubit.

Nitrogen-vacancy ($\mathrm{N}$-$V$) centers in diamond enable nuclear magnetic resonance (NMR) spectroscopy of samples at the nano- and microscale. However, at typical tesla-scale NMR magnetic field strengths, $\mathrm{N}$-$V$–NMR protocols become difficult to implement due to the challenge of driving fast $\mathrm{N}$-$V$ pulse sequences sensitive to nuclear Larmor frequencies above a few megahertz. We perform simulations and theoretical analysis of the experimental viability of $\mathrm{N}$-$V$ NMR at tesla-scale magnetic fields using a measurement protocol called DRACAERIS (Double Rewound ACquisition Amplitude Encoded Radio Induced Signal). DRACAERIS detects the longitudinal magnetization of the NMR sample at a much lower driven Rabi frequency, more suitable technically for $\mathrm{N}$-$V$ detection. We discuss how pulse errors, finite pulse lengths, and nuclear spin-spin couplings affect the resulting NMR spectra. We find that DRACAERIS is less susceptible to pulse imperfections and off-resonance effects than previous protocols for longitudinal magnetization detection. We also identify reasonable parameters for experimental implementation.

We report a comprehensive first-principles study of the relative stability of the various possible crystal structures, and the electronic and optical properties of ternary alkali-metal copper chalcogenides $A\mathrm{Cu}X$ ($A=\mathrm{Na}/\mathrm{K}$ and $X=\mathrm{S}/\mathrm{Se}/\mathrm{Te}$) compounds through density functional theory (DFT) calculations. The energetics and phonon spectra of greater than 700 structures were compared, and eight possible stabilized structures of six $A\mathrm{Cu}X$ compounds were identified using the fixed composition evolutionary search method. Our electronic band structure simulation confirms that all the ternary $A\mathrm{Cu}X$ compounds are direct-band-gap semiconductors, with the band gap lying between 0.83 to 2.88 eV. These compounds exhibit directly allowed electronic transitions from the valence band to the conduction band, which leads to a significant strength of optical transition probability. This yields a sharp rise in the optical absorption spectra (ranging between ${10}^4$ to ${10}^5{\mathrm{cm}}^{-1}$) near the energy gap. The estimated spectroscopic limited maximum efficiency (SLME) is about 18% for an 8-$\text{μ}\text{m}$-thick $\mathrm{Na}\mathrm{Cu}\mathrm{Te}$ film. For other $A\mathrm{Cu}X$ compounds, the SLME ranges between 10% to 13%. In addition, we also explored the feasibility of these ternary $A\mathrm{Cu}X$ compounds for photocatalytic water-splitting applications and found that they can be promising candidates as photocathodes for hydrogen evolution reactions.

The negative capacitance (NC) effect observed in perovskite-based devices is a unique phenomenon that remains subject to debate regarding its origin. In this study, we present a unified model aimed at describing these NC features, providing valuable insights into the coupling of ionic and electronic properties that give rise to NC behavior. Through meticulous analysis of impedance spectra, capacitance-frequency relationships, and current-voltage profiles under varying conditions of light intensity and bias voltage, we aim to uncover the underlying sources of NC in perovskite solar cells (PSCs). Our findings suggest that the emergence of NC in PSCs directly correlates with the migration of both positive and negative ions toward the uppermost and lowermost surfaces of the perovskite layer. Moreover, the polarity of capacitance at low frequencies is determined by the predominant recombination pathway within the device. We present a unified model that comprehensively describes the evolution of NC in PSCs. This model not only elucidates experimental observations but also confirms the connection between NC and recombination mechanisms.

Quantum secure direct communications (QSDC) can directly transmit secret messages through a quantum channel without keys. The imperfect photon source is a major obstacle for QSDC’s practical implementation. The unwanted vacuum state and multiphoton components emitted from imperfect photon source largely reduce QSDC’s secrecy message capacity and even threaten its security. In the paper, we propose a high-efficient passive decoy-state QSDC protocol with the heralded single-photon source (HSPS). We adopt a spontaneous parametric down-conversion source to emit entangled photon pairs in two spatial modes. By detecting the photons in one of the two correlated spatial modes, we can infer the photon-number distribution of the other spatial mode. Meanwhile, our protocol allows a simple passive preparation of the signal states and decoy state. The HSPS can effectively reduce the probability of vacuum state and increase QSDC’s secrecy message capacity. Meanwhile, the passive decoy-state method can simplify the experimental operations and enhance QSDC’s robustness against the third-party side-channel attacks. Under the communication distance of 10 km, the secrecy message capacity of our QSDC protocol can achieve 81.85 times with average photon number of 0.1 and 12.79 times with average photon number of 0.01 of that in the original single-photon-based QSDC protocol without the HSPS. Our QSDC protocol has longer maximal communication distance about 17.975 km with average photon number of 0.01. Our work serves as a major step toward the further development of practical passive decoy-state QSDC systems.

Recent experimental progress on Bernal-stacked bilayer graphene (BLG) has shown its versatility not only in exotic physics but also in practical applications in electronic devices thanks to its gate-tunable band gap. Modeling and simulating quantum transport experiments on BLG samples, however, has so far remained rather limited. Here we present a four-band effective square-lattice model and demonstrate its power and reliability by revisiting BLG transport experiments, including the classic magnetotransport of quantum Hall effect and transverse magnetic focusing, and more recent studies of anti-Klein to Klein tunneling and Aharonov-Bohm effect. Our model originates from discretizing the continuum model but does not suffer from the fermion doubling due to accidental mapping of both $K$ and $K^{\prime }$ valleys from the original hexagonal Brillouin zone (BZ) to the square BZ of the effective model. Such a mapping is expected to exist in more two-dimensional materials of hexagonal lattices.

Covalently bonded carbon nanotube (CNT) networks offer promising potential for heat-dissipation applications due to their low interfacial thermal resistivity between connected CNTs. In this work, the thermal-transport properties of covalently bonded CNT networks were simulated by molecular dynamics. It was found that the thermal conductivity (TC) of the networked CNTs is periodic dependent. Although the TCs are reduced compared to those of pristine CNTs, they are considerably larger than the values in thermal interface materials. The TC reduction with respect to the pristine CNTs primarily stems from defect scattering at junctions and resonant scattering generated by CNTs oriented perpendicular to the transport direction. The two mechanisms operate over different frequency ranges and collectively contribute to a reduction in both phonon group velocity and relaxation time across the entire frequency range. Moreover, we demonstrate that increasing the network period in a specific direction increases the TC along this direction, while reduces TC in the perpendicular direction due to intensified resonant coupling. Such directional-dependent TC variations with period facilitate the regulation of thermal-transport anisotropy within CNT networks. Overall, our findings elucidate the underlying phonon transport mechanisms in CNT networks and offer valuable insights into the design of thermal interface materials, thermal insulation materials, and materials with tailored thermal anisotropy. By leveraging the identified mechanisms, it becomes possible to develop CNT-based materials with enhanced heat-dissipation capabilities and engineered TC profiles.

We demonstrate that it is possible to observe enhanced shot noise in field-effect transistors, i.e., a current noise spectral density $S>S_{\mathrm{Poisson}}$, where $S_{\mathrm{Poisson}}=2qI$ is the so-called “shot noise” spectral density associated to a Poissonian process of electrons traversing the channel. Whereas the effects responsible for shot-noise suppression have been broadly investigated, here we unveil the mechanism and the conditions leading to an enhancement of shot noise in field-effect transistors biased in the subthreshold or weak inversion regime, that have particular relevance in the case of short-channel metal-oxide-semiconductor field-effect transistors. The effect is due to the interplay between carrier backscattering in the channel and Coulomb repulsion among carriers. We evaluate quantitatively the effect with a semianalytical model for different types of transistors, and find a characteristic shape of the Fano factor $F=S/S_{\mathrm{Poisson}}$ as a function of gate bias, that enables us to look for the signature of this effect in experiments.

Ambient magnetic skyrmions stabilized in multilayer nanostructures are of immense interest due to their relevance to magnetic tunnel junction (MTJ) devices for memory and unconventional computing applications. However, existing skyrmionic nanostructures built using conventional metallic or oxide multilayer nanodots are unable to concurrently fulfill the requirements of nanoscale skyrmion stability and all-electrical readout and manipulation. Here, we develop a few-repeat hybrid multilayer platform consisting of metallic $[\mathrm{Pt}/\mathrm{Co}\mathrm{B}/\mathrm{Ir}{]}_3$ and oxide [$\mathrm{Pt}/\mathrm{Co}\mathrm{B}/\mathrm{Mg}\mathrm{O}$] components that are coupled to evolve together as a single, composite stack. Zero-field (ZF) skyrmions with sizes as small as 50 nm are stabilized in the hybrid multilayer nanodots, which are smoothly modulated by up to $2\times$ by varying $\mathrm{Co}\mathrm{B}$ thickness and dot sizes. Meanwhile, skyrmion multiplets are also stabilized by small bias fields. Crucially, we observe higher-order “target” skyrmions with varying magnetization rotations in moderately sized, low-anisotropy nanodots. These results provide a viable route to realize robust skyrmionic MTJs and alternative possibilities for multistate skyrmionic device concepts.

We identify a major source of quantum decoherence in three-dimensional superconducting radio-frequency (SRF) resonators and two-dimensional transmon qubits composed of oxidized niobium: oxygen vacancies in the niobium pentoxide, which drive two-level system (TLS) losses. By probing the effect of sequential in situ vacuum-baking treatments on the rf performance of bulk $\mathrm{Nb}$ SRF resonators and on the oxide structure of a representative $\mathrm{Nb}$ sample using TOF SIMS, we find a nonmonotonic evolution of cavity quality factor $Q_0$, which correlates with the interplay of ${\mathrm{Nb}}_2{\mathrm{O}}_5$ vacancy generation and oxide-thickness reduction. We localize this effect to the oxide itself and present the insignificant role of diffused interstitial oxygen in the underlying $\mathrm{Nb}$ by regrowing the oxide via wet oxidation, which reveals a mitigation of aggravated TLS losses. We hypothesize that such vacancies in the pentoxide serve as magnetic impurities and are a source of TLS-driven rf loss.

Large dipole moments between high Rydberg states have been utilized to measure microwave electric fields based on electromagnetically induced transparency, while nonlocal Rydberg interactions are not suitably addressed yet in most relevant works. Here we adopt a mean-field superatom model to investigate the main restrictions of Rydberg microwave electrometry due to nonlocal interactions in atomic samples of appropriate densities. It is found that accurate microwave measurements can be attained in the linear regime only for dilute enough atomic samples and not too strong probe fields, which jointly determine whether Rydberg excitations are insignificant. This then leads to the critical lines of atomic density and probe intensity, below which the discrepancy between measured and real values is negligible due to vanishing Rydberg interactions, for a fixed microwave or coupling field. Our findings are instructive to identify the optimal conditions for Rydberg microwave electrometry by reaching a compromise between high precisions and high accuracies, requiring, respectively, large and small numbers of Rydberg excitations, when measuring weaker microwave fields.

Continuous-variable quantum key distribution (CV-QKD) with plug-and-play design offers a promising route in simplifying the system implementation and shows intriguing prospects for quantum access network applications. However, such a scheme makes it possible for the eavesdropper (Eve) to completely control the source, helping her to gain more information since the laser travels through the unsecured channel before being modulated, which will severely compromise the performance of the system and limit its potential application. To fight against the security loophole, we propose a passive source monitoring scheme based on a combination of beam splitter and homodyne detector, as well as source noise suppression. The corresponding entanglement-based model is established to estimate the secret key rate for the proposed scheme. We show that the performance of the plug-and-play CV-QKD system can be significantly improved by using the source monitoring scheme compared with the untrusted source model. With typical parameters, the maximum transmission distance can be promoted by more than 50%, and the secret key rate can be increased by more than 25% when the transmission distance is longer than 50 km. This study provides a feasible approach for improving the security and performance of the plug-and-play CV-QKD and holds positive potential for practical applications.

Ion-ion coupling over long distances represents a highly useful resource for quantum technologies, for example, to sympathetically cool or interconnect qubits in ion-based quantum computing architectures. In this respect, the recently demonstrated wire-mediated ion-ion coupling stands due to the simplification of its trap layout and its prospects for deterministic entanglement. However, the strength of such coherent ion-wire-ion coupling is typically weak, hindering its practical utilization. Here, we propose a wire-mediated scheme for coherent ion-electron coupling. The scheme not only enables the sympathetic cooling of electrons via advanced ion-cooling techniques, but also allows promotion of the effective ion-ion coupling strength by orders of magnitudes via electron mediation. Our work thus paves a way toward quantum information processing in ion-electron hybrid quantum systems.

We present a versatile method for full-field x-ray scattering tensor tomography that is based on energy conservation and is applicable to data obtained using different wavefront modulators. Using this algorithm, we pave the way for speckle-based tensor tomography. The proposed model relies on a mathematical approach that allows tuning spatial resolution and signal sensitivity. We present the application of the algorithm to three different imaging modalities and demonstrate its potential for applications of x-ray directional dark-field imaging.