Ultrathin films of the topological Dirac semimetal ${\mathrm{Na}}_3\mathrm{Bi}$ have recently been revealed as unusual electronic materials with field-tunable topological phases. Here we investigate the electronic and transport properties of ultrathin ${\mathrm{Na}}_3\mathrm{Bi}$ as an electrical contact to a two-dimensional (2D) metal (i.e., graphene) and 2D semiconductors (i.e., ${\mathrm{Mo}\mathrm{S}}_2$ and ${\mathrm{WS}}_2$ monolayers). Using combined first-principles density-functional theory and nonequilibrium Green’s function simulation, we show that the electrical coupling between bilayer-${\mathrm{Na}}_3\mathrm{Bi}$ thin film and graphene results in a notable interlayer charge transfer, thus inducing sizable $n$-type doping in the ${\mathrm{Na}}_3\mathrm{Bi}/$graphene heterostructures. In the case of ${\mathrm{Mo}\mathrm{S}}_2$ and ${\mathrm{WS}}_2$ monolayers, the lateral Schottky transport barrier is significantly lower than for many commonly studied bulk metals, thus revealing bilayer ${\mathrm{Na}}_3\mathrm{Bi}$ to be a high-efficiency electrical contact material for 2D semiconductors. These findings open up an avenue for utilizing topological semimetal thin film as an electrical contact to 2D materials, and further expand the family of 2D heterostructure devices into the realm of topological materials.

We show that the divergence of extreme ultraviolet pulses from high-harmonic generation, which is directly linked to the shape and size of the refocused beam, can be controlled by the relative delay between the fundamental and its intense orthogonally polarized second harmonic in two-color high-harmonic generation. We find that the divergence is minimized close to the delays where the number of emitted photons is maximized. These findings are rationalized as the suppression and enhancement of long and short electron trajectories as a function of the phase of the two-color laser field, respectively. The orthogonally polarized second harmonic introduces a lateral-momentum component that can select one trajectory, whereas it deflects the other. At the same time, the second harmonic profoundly modifies the tunnel-ionization process that initiates high-harmonic generation, which provides another trajectory gate. Our scheme for controlling the divergence facilitates imaging and metrology of embedded structures through the associated reduction of the focus size. In addition, the reduction of the focus size is beneficial for extreme ultraviolet nonlinear optics and spectroscopy.

State-of-the-art optical microscopy fails to image quantitatively complex anisotropic specimens, which are essential in the life sciences, materials science, and optical engineering. In this context, the recent development of Jones-imaging techniques, such as vectorial ptychography, could open up exciting perspectives provided that the measured Jones maps are fully exploited. Here we present a generalized definition of the isotropic phase in the context of arbitrary Jones matrices. We show how to deduce, from combined knowledge of isotropic and anisotropic properties, the underlying structure of the specimen. The power of the method is confirmed by investigations on a biomineral polycrystalline oyster-shell specimen, where we are able to map variations of the three-dimensional orientation of the crystal axis and to highlight structural defects in depth.

Quantum key distribution (QKD) networks can provide unconditional secure communications among many remote users. In real QKD networks, with the dynamic access and quit of nodes, qualities of various user links and the link lengths among different users, such as transmission loss, are different and changing. Customized optimization that aims at various channel environments offers a network performance benefit. In this paper, we propose including the detection efficiency, dark-count probability, and afterpulse rate of the single-photon avalanche photodiode (SPAD) into the parameter optimization by controlling its voltage and temperature to optimize the secure key rates in dynamic QKD networks. The result shows that our method improves the network adaptability to the variation and asymmetry of channel and profits the establishment of QKD networks in a complex channel environment. This method is suitable for arbitrary SPAD-based QKD system and network architecture.

The investigation of the switching-current probability distribution of a Josephson junction is a conventional tool to gain information on the dynamics of the phase slips as a function of the temperature. Here we adopt this well-established technique to probe the impact of an external static electric field on the occurrence of phase slips in gated all-metallic titanium (Ti) Josephson weak links. We show, in a temperature range between 20 and 420 mK, that the evolution of the dynamics of the phase slips as a function of the electrostatic field starkly differs from that observed as a function of the temperature. This fact demonstrates, on the one hand, that the electric field suppression of the critical current is not simply related to a conventional thermal-like quasiparticle overheating in the weak-link region. On the other hand, our results may open the way to operate an electrostatic-driven manipulation of phase slips in metallic Josephson nanojunctions, which can be pivotal for the control of decoherence in superconducting nanostructures.

We experimentally study the strain effect on electron spin relaxation in a semiconductor using nonlocal spin-transport measurements in lateral spin-valve devices. Application of in-plane and biaxial tensile strain to a (111)-oriented and heavily doped $n\text{−}\mathrm{type}{\mathrm{Si}}_{0.1}{\mathrm{Ge}}_{0.9}$ layer leads to lifting of the valley degeneracy in the conduction band and to reduction of the electron’s effective mass, resulting in increased electron mobility. Nonlocal four-terminal spin signals in the strained-${\mathrm{Si}}_{0.1}{\mathrm{Ge}}_{0.9}$ lateral spin-valve devices are markedly enhanced and the estimated spin lifetime becomes 3 times longer than that in strain-free devices at low temperatures. On the basis of a comparison of the experimental data and recent theories, we propose that only the donor-driven intervalley spin-flip scattering of electrons at low temperatures is partly suppressed for the strained and heavily doped ${\mathrm{Si}}_{0.1}{\mathrm{Ge}}_{0.9}$.

Using three-dimensional kinetic simulations, we examine the emission of collimated $\gamma$-ray beams from structured laser-irradiated targets with a prefilled cylindrical channel and its scaling with laser power (in the multi-PW range). The laser power is increased by increasing the laser energy and the size of the focal spot while keeping the peak intensity fixed at $5\times {10}^{22}\mathrm{W}/{\mathrm{cm}}^2$. The channel radius is increased proportionally to accommodate the change in laser spot size. The efficiency of conversion of the laser energy into a beam of MeV-level $\gamma$ rays (with a ${10}^{\circ }$ opening angle) increases rapidly with the incident laser power $P$ before it roughly saturates above $P\approx 4\mathrm{PW}$. Detailed particle tracking reveals that the power scaling is a result of enhanced electron acceleration at higher laser powers. One application that directly benefits from such a strong scaling is pair production via two-photon collisions. We investigate two schemes for generating pairs through the linear Breit-Wheeler process: colliding two $\gamma$-ray beams and colliding one $\gamma$-ray beam with black-body radiation. The two scenarios project up to ${10}^4$ and ${10}^5$ pairs, respectively, for the $\gamma$-ray beams generated at $P=4\mathrm{PW}$. A comparison with a regime of laser-irradiated hollow channels corroborates the robustness of the setup with prefilled channels.

We develop a means for speckle-based phase imaging of the projected thickness of a single-material object, under the assumption of illumination by spatially random time-independent x-ray speckles. These speckles are generated by passing x rays through a suitable spatially random mask. The method makes use of a single image obtained in the presence of the object, which serves to deform the illuminating speckle field relative to a reference speckle field (which only needs to be measured once) obtained in the presence of the mask and the absence of the object. The method implicitly rather than explicitly tracks speckles and utilizes the transport-of-intensity equation to give a closed-form solution to the inverse problem of determining the complex transmission function of the object. Implementation using x-ray synchrotron data shows the method to be robust and efficient with respect to noise. Applications include x-ray phase-amplitude radiography and tomography, as well as time-dependent imaging of dynamic and radiation-sensitive samples using low-flux sources.

We address the properties of nonlinear-Fourier-transform (NFT)-based fiber-optic communications systems and, particularly, study how the presence of noise deteriorates the performance of these systems. Similarly to the case of linear Fourier modes evolving independently under the action of chromatic dispersion alone, NFT-based systems employ so-called “nonlinear modes,” forming a nonlinear spectrum, as data carriers, and these nonlinear modes evolve independently in an uncoupled manner under the joint action of nonlinearity and dispersion. However, the influence of amplified-spontaneous-emission (ASE) noise on these nonlinear modes is still relatively poorly studied. In this paper, dealing with a continuous nonlinear spectrum, we scrutinize the properties of the effective noise emerging in the nonlinear Fourier domain. We also show that in the transmission stages, where the signal peak power is relatively high (e.g., at the receiver, in back-to-back transmission, or at short distances), the performance of an NFT system is mostly degraded not by the inline ASE noise, but by the imperfections of the digital sampling and the forward and backward NFT algorithms, i.e., by the NFT processing noise.

We analytically model the magnetization switching time of a biaxial ferromagnet driven by an antidampinglike spin torque. The macrospin magnetization dynamics is mapped to an energy-flow equation, wherein a rational-function approximation for the elliptic integrals for moderate spin current and small damping results in a closed-form expression for the switching time. The randomness in the initial angle of magnetization gives the distribution function of the switching time. The analytical model conforms to results obtained from Monte Carlo simulation for a broad range of material parameters. Our results can improve the design and benchmarking of in-plane spin-torque magnetic memory by obviating the need for expensive numerical computation.

Electrostatically-defined semiconductor quantum dot arrays offer a promising platform for quantum computation and quantum simulation. However, crosstalk of gate voltages to dot potentials and interdot tunnel couplings complicates the tuning of the device parameters. To date, crosstalk to the dot potentials is routinely and efficiently compensated using so-called virtual gates, which are specific linear combinations of physical gate voltages. However, due to exponential dependence of tunnel couplings on gate voltages, crosstalk to the tunnel barriers is currently compensated through a slow iterative process. In this work, we show that the crosstalk on tunnel barriers can be efficiently characterized and compensated for, using the fact that the same exponential dependence applies to all gates. We demonstrate efficient calibration of crosstalk in a quadruple quantum dot array and define a set of virtual barrier gates, with which we show orthogonal control of all interdot tunnel couplings. Our method marks a key step forward in the scalability of the tuning process of large-scale quantum dot arrays.

Silicon-vacancy qubits in silicon carbide ($\mathrm{Si}\mathrm{C}$) are emerging tools in quantum-technology applications due to their excellent optical and spin properties. In this paper, we explore the effect of temperature and strain on these properties by focusing on the two silicon-vacancy qubits, V1 and V2, in $4H$-$\mathrm{Si}\mathrm{C}$. We apply density-functional theory beyond the Born-Oppenheimer approximation to describe the temperature-dependent mixing of electronic excited states assisted by phonons. We obtain a polaronic gap of around 5 and 22 meV for the V1 and V2 centers, respectively, which results in a significant difference in the temperature-dependent dephasing and zero-field splitting of the excited states, which explains recent experimental findings. We also compute how crystal deformations affect the zero-phonon line of these emitters. Our predictions are important ingredients in any quantum applications of these qubits sensitive to these effects.

We report on terahertz emission from $\mathrm{Fe}\text{−}\mathrm{Mn}\text{−}\mathrm{Pt}/\mathrm{Ru}/\mathrm{Fe}\text{−}\mathrm{Mn}\text{−}\mathrm{Pt}$ and $\mathrm{Pt}/\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}/\mathrm{Ru}/\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}/\mathrm{Pt}$ synthetic antiferromagnet (SAF) structures upon irradiation by a femtosecond laser; the former is via the anomalous Hall effect, whereas the latter is through the inverse spin Hall effect. The antiparallel alignment of the two ferromagnetic layers leads to a terahertz emission peak amplitude that is almost double that of a corresponding single-layer or bilayer emitter with the same equivalent thickness. In addition, we demonstrate by both simulation and experiment that terahertz emission provides a powerful tool to probe the magnetization reversal processes of individual ferromagnetic layers in a SAF structure, as the terahertz signal is proportional to the vector difference $({\mathbf{M}}_1-{\mathbf{M}}_2)$ of the magnetizations of the two ferromagnetic layers.

Discrimination of coherent states beyond the standard quantum limit (SQL) is an important task not only for quantum information processing but also for optical coherent communication. To optimize long-distance optical fiber networks, it is of practical importance to develop a quantum receiver beating the SQL and approaching the quantum bound at telecom wavelength. In this paper, we experimentally demonstrate a receiver beating the conventional SQL at telecom wavelength. Our receiver is composed of a displacement operation, a single-photon counter, and a real-time adaptive-feedback operation. By using a high-performance single-photon detector operating at the telecom wavelength, we achieve a discrimination error beyond the SQL. The demonstration in the telecom band is an enabler for quantum and classical communication beyond the SQL using a coherent state alphabet, and we envision that the technology can be used for long-distance quantum-key distribution, effective quantum-state preparation, and quantum estimation.

We demonstrate a quantitative disentanglement of current-induced spin-orbit torques (SOTs) in $\mathrm{Pt}$/$\mathrm{Co}$ bilayers, where both the spin Hall effect (SHE) and the Rashba-Edelstein effect (REE) are present. The SOTs originating from the SHE in bulk $\mathrm{Pt}$ and from the REE at substrate (sub.)/$\mathrm{Pt}$ and $\mathrm{Pt}$/$\mathrm{Co}$ interfaces are successfully disentangled and quantified utilizing harmonic Hall measurements, by taking advantage of different characteristic lengths between the two effects during spin-current transport. The fieldlike (FL) torque in our samples originates from the REE, while both SHE and REE contribute to the dampinglike (DL) torque. The experimentally extracted transport characteristic lengths in bulk $\mathrm{Pt}$ (an effective spin-diffusion length) are 1.8–3.0 nm, while those at $\mathrm{Pt}$ interfaces (an effective REE thickness) are 0.2–0.6 nm. The extracted REE-induced FL torque (and DL torque) efficiencies at sub./$\mathrm{Pt}$ and $\mathrm{Pt}$/$\mathrm{Co}$ interfaces are of the same order of magnitude, but of opposite signs, which is consistent with the REE scenario. The origin of the observed temperature-dependent SOT sign reversal is clarified with our disentanglement analysis, demonstrating the significant role of the REE at both sub./$\mathrm{Pt}$ and $\mathrm{Pt}$/$\mathrm{Co}$ interfaces, in contributing to the overall SOTs, as the thickness of the heavy-metal layer is thinner than its spin-diffusion length or comparable to a REE thickness.

Vortex beams carrying orbital angular momentum (OAM) have recently attracted great interest due to their capability for providing a new method to increase the communication channels in wireless communications. It is expected that the unbounded eigenstates of OAM can bring a large number of channels to enhance the transmission capacity. Here, we propose an effective strategy to generate vortex beams by using spoof surface plasmon polaritons (SSPPs) in the microwave region. A periodically modulated SSPP ring resonator is applied to convert near-field SSPP waves into radiating a vector vortex beam. The emitted beam can possess different OAM modes at different frequencies through rational design. In addition, circularly polarized vortex beams are demonstrated. Both numerical simulations and experimental characterizations confirm the theoretical predictions. Owing to the deep subwavelength characteristic of SSPP, the proposed device can be readily integrated into miniaturized circuits and systems. We anticipate that our design will enable a wide range of applications for future wireless communication technologies.

In this work, we theoretically propose and experimentally demonstrate a kind of nonreflecting acoustic metamaterial fence that is perfectly impedance matched to air over a wide range of incident angles and a broad frequency band. The metamaterial fence is composed of an array of rigid cylinders with alternating layers of different cylinder diameters. It is designed by a spatially dispersive effective medium theory and experimentally demonstrated by airborne sound reflectance measurements. Our work provides a unique approach to design and realize rigid acoustic structures that can robustly prohibit reflection and reverberation. Such structures may have important applications in general acoustics.

In this paper we present experimental verification of the magnitude of the imaginary part of the spin-mixing conductance in bilayers comprising heavy metals. We present results of broadband ferromagnetic resonance studies on heterostructures consisting of Finemet (${\mathrm{Fe}}_{66.5}{\mathrm{Cu}}_1{\mathrm{Nb}}_3{\mathrm{Si}}_{13.5}{\mathrm{B}}_6{\mathrm{Al}}_7$) thin films covered by $\mathrm{Pt}$ and $\mathrm{Ta}$ wedge layers with the aim of observing spin-pumping effects and evaluating both the real ($\mathrm{Re}[g_{\mathrm{eff}}^{\uparrow \downarrow }]$) and imaginary ($\mathrm{Im}[g_{\mathrm{eff}}^{\uparrow \downarrow }]$) parts of the spin-mixing conductance. In particular, we show that the imaginary part of the spin-mixing conductance cannot be regarded as negligible, and we discuss its influence on magnetization dynamics. For Finemet/$\mathrm{Ta}$ bilayers, the ratio $\mathrm{Re}[g_{\mathrm{eff}}^{\uparrow \downarrow }]/\mathrm{Im}[g_{\mathrm{eff}}^{\uparrow \downarrow }]=0.38$; that is, the fieldlike torque dominates over the dampinglike torque in such a system. The experimental results are analyzed in the framework of a recent microscopic theory that allows us to estimate the value of the interfacial spin-orbit interaction and confirm its important role. The role of material and interface parameters is also discussed.