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.

The scalability of spin-qubit devices is conditioned by qubit-to-qubit variability. Disorder in the host materials indeed affects the wave functions of the confined carriers, which leads to variations in their charge and spin properties. Charge disorder in the amorphous oxides is particularly detrimental owing to its long-range influence. Here we analyze the effects of charge traps at the semiconductor-oxide interface, which are generally believed to play a dominant role in variability. We consider multiple random distributions of these interface traps and numerically calculate their impact on the chemical potentials, detuning, and tunnel coupling of two adjacent quantum dots in $\mathrm{Si}\mathrm{Ge}$ heterostructure. Our results highlight the beneficial screening effect of the metal gates. The surface of the heterostructure shall, therefore, be covered as much as possible by the gates in order to limit variability. We propose an alternative layout with tip-shaped gates that maximizes the coverage of the semiconductor-oxide interface and outperforms the usual planar layout in some regimes. This highlights the importance of design in the management of device-to-device variability.

Quantum circuit mapping is a crucial process in the quantum circuit compilation pipeline, facilitating the transformation of a logical quantum circuit into a list of instructions directly executable on a target quantum system. Recent research has introduced a postcompilation step known as remapping, which seeks to reconfigure the initial circuit mapping to mitigate quantum circuit errors arising from system variability. As quantum processors continue to scale in size, the efficiency of quantum circuit mapping and the overall compilation process has become of paramount importance. In this work, we introduce a quantum circuit remapping algorithm that leverages the intrinsic symmetries in quantum processors, making it well suited for large-scale quantum systems. This algorithm identifies all topologically equivalent circuit mappings by constraining the search space using symmetries and accelerates the scoring of each mapping using vector computation. Notably, this symmetry-based-circuit-remapping algorithm exhibits linear scaling with the number of qubits in the target quantum hardware and is proven to be optimal in terms of its time complexity. Moreover, we conduct a comparative analysis against existing methods in the literature, demonstrating the superior performance of our symmetry-based method on state-of-the-art quantum hardware architectures and highlighting the practical utility of our algorithm, particularly for large-scale quantum computing.

${\mathrm{Hf}\mathrm{O}}_2$-based ferroelectrics ($\mathrm{FE}\text{−}{\mathrm{Hf}\mathrm{O}}_2$) have triggered extensive research interest because they are promising materials for beyond-Moore electronic device applications. However, the origin of ferroelectricity in ${\mathrm{Hf}\mathrm{O}}_2$-based ferroelectrics remains elusive. Here, we demonstrate that the ordered oxygen vacancies (${\mathrm{V}}_{\mathrm{O}}$) in $\mathrm{FE}$-${\mathrm{Hf}\mathrm{O}}_2$ thin films contribute to ferroelectricity through the formation of a stable-polar phase. In $\mathrm{FE}$-${\mathrm{Hf}\mathrm{O}}_2$ thin films, oxygen vacancy (${\mathrm{V}}_{\mathrm{O}}$) defects induce local electric dipoles and the ordered ${\mathrm{V}}_{\mathrm{O}}$ distribution results in the stable-polar phases. Using first-principles calculations, a thermodynamically stable-polar phase $M_1$ with the spontaneous polarization of 27 µC/${\mathrm{cm}}^2$ is predicted in monocliniclike ${\mathrm{Hf}}_4{\mathrm{Zr}}_4{\mathrm{O}}_{15}$ structures. Moreover, the predicted $M_1$ structures are directly visualized in ${\mathrm{Hf}}_{0.5}{\mathrm{Zr}}_{0.5}{\mathrm{O}}_{2-\delta }$ (HZO) ferroelectric thin films using spherical aberration (Cs)-corrected scanning transmission electron microscopy. The proposed mechanism for ${\mathrm{V}}_{\mathrm{O}}$-induced order-disorder ferroelectricity in $\mathrm{FE}$-${\mathrm{Hf}\mathrm{O}}_2$ opens a new pathway to explore the underlying physics of the ferroelectric characteristics in $\mathrm{FE}$-${\mathrm{Hf}\mathrm{O}}_2$.

Efficient optical quantum memories are a milestone required for several quantum technologies, including repeater-based quantum key distribution and on-demand multiphoton generation. We present an efficiency optimization of an optical electromagnetically induced transparency (EIT) memory experiment in a warm cesium vapor using a genetic algorithm and analyze the resulting wave forms. The control pulse is represented either as a Gaussian or free-form pulse and the results from the optimization are compared. We see an improvement factor of 3(7)% when using optimized free-form pulses. By limiting the allowed pulse energy in a solution, we show an energy-based optimization giving a 30% reduction in energy, with minimal efficiency loss.

Parametric converters are parametric amplifiers that mix two spatially separate nondegenerate modes and are commonly used for amplifying and squeezing microwave signals in quantum computing and sensing. In Josephson parametric converters, the strong localized nonlinearity of the Josephson junction limits the amplification and squeezing, as well as the dynamic range, in current devices. In contrast, a weak distributed nonlinearity can provide higher gain and dynamic range, when implemented as a kinetic inductance (KI) nanowire of a dirty superconductor, and has additional benefits such as resilience to magnetic field, higher-temperature operation, and simplified fabrication. Here, we propose, demonstrate, and analyze the performance of a KI parametric converter that relies on the weak distributed nonlinearity of a Nb-Ti-N KI nanowire. The device utilizes three-wave mixing induced by a dc current bias. We demonstrate its operation as a nondegenerate parametric amplifier with high phase-sensitive gain, reaching two-mode amplification and deamplification of approximately 30 dB for two resonances separated by 0.8 GHz, in excellent agreement with our theory of the device. We observe a dynamic range of $-108$ dBm at 30 dB gain. Our device can significantly broaden applications of quantum-limited signal processing devices including phase-preserving amplification and two-mode squeezing.

Pancreatic ductal adenocarcinoma is a devastating disease with very low survival rates 5 years after diagnosis. The main reason for this dismal prognosis is the thick stroma which both protects tumor cells from drug penetration and supports tumor development. Ultrasound inertial cavitation is a promising treatment with potential for stromal disruption, enhancing tumor cells’ sensitivity to chemical agents and biomodulators. Our goal was to develop a dedicated microelastography setup allowing us to measure the elasticity of in vitro tumor models called spheroids. In a second step, the impact of cavitation treatment on their mechanical properties was assessed. A transcranial magnetic stimulation clinical device was used to induce shear waves in the spheroids containing magnetic nanoparticles. Using an inverted optical microscope, particle imaging velocimetry, and noise correlation algorithms, the shear wave velocity, indicative of the medium’s elasticity, could be measured. Shear waves generated by the magnetic pulse inside the spheroids were detected and their velocity was measured using noise correlation elastography. This allowed the estimation of the spheroids’ elasticity. Cavitation treatment softened them significantly, and the impact of the exposure conditions and the spheroids’ composition have been studied. In the future, such a method could be used to monitor cavitation treatments. In addition, since it is now well established that mechanical constraints and elasticity play an important role in tumor growth, it is of great interest to measure the elasticity of tumor models to better understand the mechanisms of tumor growth.