We analyze the cross-resonance effect for fluxonium circuits and investigate a two-qubit gate scheme based on selective darkening of a transition. In this approach, two microwave pulses at the frequency of the target qubit are applied simultaneously with a proper ratio between their amplitudes to achieve a controlled-not operation. We study in detail coherent gate dynamics and calculate gate error. With nonunitary effects accounted for, we demonstrate that gate error below ${10}^{-4}$ is possible for realistic hardware parameters. This number is facilitated by long coherence times of computational transitions and strong anharmonicity of fluxoniums, which easily prevents excitation to higher excited states during the gate microwave drive.

We perform comprehensive experimental tests of various composite pulse sequences using one of the open-access IBM quantum processors, based on superconducting transmon qubits. We implement explicit pulse control of the qubit by making use of the opportunity of low-level access to the backend, provided by IBM Quantum. We obtain the excitation profiles for a huge variety of broadband, narrowband, and passband composite pulses, producing any prechosen target probabilities, ranging from zero to one. We also test universal composite pulses that compensate errors in any experimental parameter. In all experiments, we find excellent agreement between theoretical and experimental excitation profiles. This proves both the composite pulses as a very efficient and flexible quantum control tool and the high quality of the IBM quantum processor. As an extreme example, we test and observe a pronounced narrowband excitation profile for a composite sequence of as many as 1001 pulses.

Due to the similarities in electrochemical and physicochemical properties of sodium and lithium, sodium-ion batteries are an attractive substitute for lithium-ion batteries. The selection of appropriate electrode materials for $\mathrm{Na}$-ion batteries is an important step to achieve high stability and theoretical capacity. Here, the effects of heteroatom doping and charge variation on the ability of the ${\mathrm{Mo}\mathrm{S}}_2$ substrate to capture $\mathrm{Na}$ ions are studied using first-principles calculations. The results show that the interaction between $\mathrm{Na}$ ions and the ${\mathrm{Mo}\mathrm{S}}_2$ substrate is enhanced by electron deficiency or substituting a $\mathrm{S}$ atom with a dopant atom (X) with fewer valence electrons (X = $\mathrm{B}$, $\mathrm{Al}$, $\mathrm{Ga}$, $\mathrm{In}$, $\mathrm{C}$, $\mathrm{Si}$, $\mathrm{Ge}$, $\mathrm{Sn}$, $\mathrm{N}$, $\mathrm{P}$, $\mathrm{As}$, $\mathrm{Sb}$). In contrast, adding electrons to the ${\mathrm{Mo}\mathrm{S}}_2$ substrate weakens its interaction with $\mathrm{Na}$. We further analyze the diffusion barrier, open-circuit voltage, and theoretical capacity of hypothetical $\mathrm{Na}$-ion batteries using doped $X\text{−}{\mathrm{Mo}\mathrm{S}}_2$ (X = heteroatom) as electrode materials. We find that $\mathrm{Al}\text{−}$ or $\mathrm{Si}$-doped ${\mathrm{Mo}\mathrm{S}}_2$ can serve as good electrode materials for $\mathrm{Na}$-ion batteries, with a high theoretical capacity that exceeds 700 mAh/g. This work provides practical guidance for the development of electrode materials of $\mathrm{Na}$-ion batteries using two-dimensional materials.

Quantum-optical coherence tomography (QOCT) is an optical sectioning modality based on the quantum interference of entangled photon pairs in the well-known Hong-Ou-Mandel interferometer. Despite its promise, the technique is far from being competitive with current classical OCT devices due to the long required acquisition times, derived from the low photon-pair emission rates. In this work, we, on the one hand, demonstrate a quantum optical coherence microscopy technique, based on full-field QOCT, which employs entangled collinear photon pairs in a Linnik interferometer designed to overcome some of the limitations of previous QOCT implementations, and, on the other hand, test it on representative samples, including glass layers with manufactured transverse patterns and metal-coated biological specimens. In our setup, while the idler photon is collected with a multimode fiber, the signal photon is detected by an intensified charge-coupled device camera, leading to full-field transverse reconstruction through a single-axial acquisition sequence. Interestingly, our setup permits concurrent OCT and QOCT trace acquisition, the former with greater counts and the latter with the benefit of quantum-conferred advantages. We hope that our current results will represent a significant step forward towards the actual applicability of QOCT, e.g., in clinical settings.

Antineutrinos are an inextricable element of the fission process. The kiloton-scale KamLAND experiment has demonstrated a capability to detect reactor antineutrinos at a range of a few hundred kilometers. But to detect or rule out the existence of a single small reactor over many kilometers requires a large detector, so large in fact that the optical opacity of the detection medium itself becomes an important factor. If the detector is so large that photons cannot traverse the detector medium to an optical detector, then it becomes impractical. For this reason, gadolinium-doped-water Cherenkov detectors have been proposed for large volumes, due to their appealing light-attenuation properties. Even though Cherenkov emission does not produce many photons and the energy resolution is poor, there may be a place for Gd-doped-water detectors in far-field nuclear-reactor monitoring. In this paper, we focus on the reactor-discovery potential of large-volume Gd-doped-water Cherenkov detectors for nuclear-nonproliferation applications. Realistic background models for the worldwide reactor flux, geoneutrinos, cosmogenic fast neutrons, and detector-associated backgrounds are included. We calculate the detector run time required to detect a small 50-MWt reactor at a variety of stand-off distances as a function of detector size. We highlight that, at present, the photomultiplier-tube dark rate and event reconstruction algorithms are the limiting factors to extending such detectors beyond a fiducial mass of approximately 50 kt.

Integration of active elements into silicon wafers is the first step towards their usage in modern electronic devices based on nanometric structures. Spintronic terahertz emitters, typically composed of nanometer-thin magnetic multilayer, have the outstanding capability of producing high-quality, broadband terahertz pulses using extremely simple heterostructures. A question remains on whether an efficient and cheap integration with other silicon-based technologies can be achieved. We show here that simply having a ferromagnetic layer on silicon produces remarkably efficient spintronic terahertz emission despite the low spin-orbit coupling of the individual components. We achieve this by leveraging on the natural formation of silicides at the interface of a transition metal and silicon. The cobalt silicide layer has good spin-to-charge conversion efficiency that reaches around 1/6 as that of the prototypical spintronics THz-emitter heterostructure cobalt/platinum.

Hyperspectral imaging (HSI) records a series of two-dimensional (2D) images for different wavelengths to provide the chemical fingerprint at each pixel. Combining HSI with a tomographic data-acquisition method, we can obtain the chemical fingerprint of a sample at each point in three-dimensional (3D) space. The so-called 3D HSI typically suffers from low imaging throughput due to the requirement of scanning the wavelength and rotating the beam or sample. In this paper, we present an optical system that captures the entire four-dimensional (4D; i.e., 3D structure and one-dimensional spectrum) data set of a sample with the same throughput of conventional HSI systems. Our system works by combining snapshot projection optical tomography (SPOT), which collects multiple projection images with a single snapshot, and Fourier-transform spectroscopy (FTS), which results in superior spectral resolution by collecting and processing a series of interferogram images. Using this hyperspectral SPOT system, we image the volumetric absorbance of dyed polystyrene microbeads, oxygenated red blood cells (RBCs), and deoxygenated RBCs. The 4D optical system demonstrated in this paper provides a tool for high-throughput chemical imaging of complex microscopic specimens.

An ultrathin underwater metasurface with low frequency, broadband, high-efficiency absorption, and high-hydrostatic-pressure-resistance performance is proposed based on the integrated principle of local resonance and cavity resonance. The design-parameter complexity is overcome by the global-optimization capability of the genetic algorithm. As a demonstration, an average sound-absorption coefficient of 92.3% at 500–10 000 Hz with an ultrathin thickness of 32 mm is achieved, including more than 99% sound-absorption coefficient at 4670–8630 Hz. In the proposed metasurface, the complex surface impedances of the three subsurfaces provide unique views for the high-efficiency sound-absorption coupling mechanism by the equivalent-circuit model. Further investigation demonstrates that the excellent property of high-efficiency sound absorption is maintained under 4.5-MPa hydrostatic pressure. The proposed metasurface provides more possibilities for underwater noise suppression.

We assess the viability of topological semimetals for application in advanced interconnect technology, where conductor size is on the order of a few nanometers and grain boundaries are expected to be prevalent. We investigate the electron transport properties and grain-boundary scattering in thin films of the topological semimetals $\mathrm{Co}\mathrm{Si}$ and $\mathrm{Co}\mathrm{Ge}$ using first-principles calculations combined with the nonequilibrium Green’s function (NEGF) technique. Unlike conventional interconnect metals such as $\mathrm{Cu}$ and $\mathrm{Al}$, we find that $\mathrm{Co}\mathrm{Si}$ and $\mathrm{Co}\mathrm{Ge}$ conduct primarily through topologically protected surface states in thin-film structures even in the presence of grain boundaries. The area-normalized resistance decreases with decreasing film thickness for $\mathrm{Co}\mathrm{Si}$ and $\mathrm{Co}\mathrm{Ge}$ thin films both with and without grain boundaries; a trend opposite to that of the conventional metals $\mathrm{Cu}$ and $\mathrm{Al}$. The surface-dominated transport mechanisms in thin films of topological semimetals with grain boundaries does not follow the classic resistivity size effect, and suggests that these materials may be promising candidates for applications as nanointerconnects where high electrical resistivity acts as a major bottleneck limiting semiconductor device performance.

Ultrafast acoustic imaging experiments are a powerful tool to investigate, at the nanometer scale, cell mechanical properties such as stiffness, viscosity, and adhesion, properties that play some roles in the life and death of cells. However, due to cell complex structures, the ultrafast acoustic signal analysis is quite intricate and depends on multiple parameters. Complex data analysis with poorly known parameters can be handled by a data clustering method as already shown in particle physics and biology. In this work, ultrafast acoustic data analysis is tackled by a spectral clustering method followed by a hierarchical agglomerating method. Coupled to conventional microscopy performed on the very same cell, the clustered data can be assigned to inner-cell features such as the nucleus, the cytoplasm, and the cytoskeleton. The signal dependency on the cell thickness and stiffness is highlighted. Moreover, thanks to the improvement of the signal-to-noise ratio, the nature of the adhesion is also assessed through observation and characterization of a polymerlike layer as thin as a few nanometers.

Recent studies have revealed that atomically thin two-dimensional (2D) materials exhibit outstanding nonlinear optical (NLO) properties compared with traditional NLO crystals, which provides great potential in numerous photonic devices such as integrated nonlinear photonic chips and modulators. However, the evolution of NLO response with layer number and pump intensity of various 2D materials remains unclear, but offers a basis for hunting powerful NLO materials. Herein, controllable synthesis of a series of $\mathrm{Re}$$X_2$ (X = $\mathrm{Se}$, $\mathrm{S}$) films with different numbers of layers is achieved by chemical vapor deposition. Z-scan techniques are used to investigate the nonlinear absorption coefficient (β) and nonlinear refractive index ($n_2$). Empirically, the absolute values of both β and $n_2$ show a downtrend as the power exponential function with the layer and pump intensity. The NLO parameters of ${\mathrm{Re}\mathrm{Se}}_2$ film are β ∼ 4156 cm/GW and $n_2$ ∼ 1.819 × ${10}^{-10}{\mathrm{cm}}^2/\mathrm{W}$, which are more significant than those of ${\mathrm{Re}\mathrm{S}}_2$ film (β ∼ 806 cm/GW and $n_2$ ∼ 2.22 × ${10}^{-11}{\mathrm{cm}}^2/\mathrm{W}$). This result is ascribed to the smaller band gap, higher carrier density, and larger ground-state absorption of ${\mathrm{Re}\mathrm{Se}}_2$ than that of ${\mathrm{Re}\mathrm{S}}_2$, which is confirmed by the theoretical analysis of the band structure and a three-energy-level system. It is worth pointing out that the β values for $\mathrm{Re}$$X_2$ films are 10–100 times larger than those of ${\mathrm{WS}}_2$, ${\mathrm{Mo}\mathrm{Se}}_2$, and ${\mathrm{Mo}\mathrm{Te}}_2$. The $n_2$ values for $\mathrm{Re}$$X_2$ films are 3–4 orders of magnitude larger than for traditional semiconductors such as $\mathrm{Ga}\mathrm{As}$ and $\mathrm{Si}$. Our results suggest that $\mathrm{Re}$$X_2$ semiconductors are greatly anticipated in designing high-performance on-chip photonic devices.

Phase-change materials with temperature-dependent thermal conductivity have drawn lots of attention recently. They have exhibited promising potentials in devices such as the thermal diode, thermal memristor, etc. Here we investigate the heat exchange between two reservoirs through a bare phase-change material. In the presence of temperature variation in one of the reservoirs, we find a unidirectional thermal flux between those two reservoirs though they share the same average temperature, i.e., no temperature bias. By mapping such a system to the heat exchange between two reservoirs through a regular material with a constant thermal conductivity, we obtain an effective temperature difference $T_{\mathrm{eff}}$, which should present to maintain the unidirectional thermal flux. We then provide a semianalytical formula for $T_{\mathrm{eff}}$, which works well over an extensive parameter range. The parameter dependence of $T_{\mathrm{eff}}$ and the temperature dependence of the thermal capacity are then studied in detail. Our work thus provides a useful tool for calibration of heat conduction within phase-change materials and leads to potential applications related to energy extraction from the thermal variation of the environment.

Isolated attosecond pulses (IAPs) in the soft-x-ray (SXR) region have been successfully produced via high-order harmonic generation in a gas medium at several laboratories, but pulse energies are much lower compared to the IAPs in the extreme ultraviolet. Here, we show that in a gas-filled hollow waveguide, an efficient gating method using a few-cycle three-color synthesizer is able to generate stronger SXR IAPs. We show that an 104-as IAP in the SXR region (central energy of 210 eV) with a higher pulse energy (0.9 nJ) and a lower divergence (within 1.5 mrad) can be generated with a three-color laser beam for input pulse energy at 0.62 mJ, under the condition of optimal gas pressure and waveguide length. The enhancement of the IAP is attributed to favorable phase matching of SXR high harmonics caused by the evolution of the three-color waveform during its propagation in the waveguide, which can be understood by analyzing the time shift of the waveform due to the interplay of the waveguide mode, atomic dispersion, and plasma dispersion. We check that the gating method is quite robust with respect to the phase jitter in the waveform. The advantage of this method is further demonstrated by comparing the generation of SXR high harmonics and attosecond pulses with two other schemes: the three-color waveform in a gas cell and a previously optimized two-color waveform in a gas-filled waveguide.

Along with the scaling of dimensions in quantum systems, transitions between the system’s energy levels would become close in frequency, which are conventionally resolved by weak and lengthy pulses. Here, we extend and experimentally demonstrate analytically based swift quantum control techniques on a four-level trapped ion system, where we perform individual or simultaneous control over two pairs of spectrally nearby transitions with tailored time-varied drive, achieving operational fidelities ranging from 99.2(3)% to 99.6(3)%. We achieve approximately an order of magnitude speed up compared with the case of a weak square pulse for a general control. Therefore, our demonstration may be beneficial to a broad range of quantum systems with crowded spectrum, for spectroscopy, quantum information processing, and quantum simulation.

The interfacial Dzyaloshinskii-Moriya interaction (DMI) has long been observed in normal metal/ferromagnetic multilayers, enabling the formation of chiral domain walls, skyrmions, and other two-dimensional antisymmetric spin textures confined within a single ferromagnetic layer, while more recent works on the interlayer DMI reveal alternative pathways for realizing chiral three-dimensional spin textures by extending the DMI to two spatially distinct magnetic layers. Here, we report on the interlayer DMI between two orthogonally magnetized ferromagnetic layers ($\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}/\mathrm{Co}$) mediated by a $\mathrm{Pt}$ layer and confirm the chiral nature of the observed effective field of up to 37 Oe through asymmetric hysteresis loops under an in-plane field. We highlight the importance of growth-induced in-plane symmetry breaking, resulting in a sizable interlayer DMI and a universal characteristic vector through wedge deposition of the samples. We further perform deterministic current-driven magnetization switching in the perpendicularly magnetized $\mathrm{Co}$ layer utilizing solely the effective field from the interlayer DMI. These results demonstrate the interlayer DMI’s potential to facilitate deterministic field-free switching in spin-memory applications.

Several electronic and optoelectronic devices have been proposed in recent years based on vertical heterostructures of two-dimensional (2D) materials. The large number of combinations of available 2D materials and the even larger number of possible heterostructures require effective and predictive device-simulation methods, to inform and accelerate experimental research and to support the interpretation of experiments. Here, we propose a computationally effective and physically sound method to model electron transport in 2D van der Waals heterostructures, based on a multiscale approach and quasiatomistic Hamiltonians. The method uses ab initio simulations to extract the parameters of a simplified tight-binding Hamiltonian based on a uniform three-dimensional lattice geometry that enables device simulations using the nonequilibrium Green’s function approach in a computationally effective way. We describe the application and limitations of the method and discuss the examples of two use cases of practical electronic devices based on 2D materials, such as a field-effect transistor and a floating-gate memory, composed of molybdenum disulphide, hexagonal boron nitride and graphene.

We demonstrate a nonlinear photomodulation spectroscopy method to image the mode profile of a high-$Q$ photonic crystal resonator (PhCR). This far-field imaging method is suitable for ultrahigh-$Q$ cavities which we demonstrate on a $Q=619000$ PhCR. We scan the PhCR surface with a 405-nm pump beam that modulates the refractive index by local thermal tuning, while probing the response of the resonance. We enhance resolution by probing at high power, using the thermo-optical nonlinearity of the PhCR. Spatial resolution of the thermo-optical effect is typically constrained by the broad thermal profile of the optical pump. Here we go beyond the thermal limit and show that we can approach the diffraction limit of the pump light. This is due to free carrier absorption that heats up the PhCR only when there is overlap between the optical pump spot and the optical mode profile. This is supported with a thermo-optical model that reproduces the high-resolution mode mapping. Results reveal that the observed enhanced resolution is reached for surprisingly low carrier density.