Quantum key distribution (QKD) based on the laws of quantum physics allows the secure distribution of secret keys over an insecure channel. Unfortunately, imperfect implementations of QKD compromise its information-theoretical security. Measurement-device-independent quantum key distribution (MDI QKD) is a promising approach to remove all side channels from the measurement unit, which is regarded as the “Achilles’ heel” of QKD. An essential assumption in MDI QKD is, however, that the sources are trusted. Here we experimentally demonstrate that a practical source based on a semiconductor laser diode is vulnerable to a laser-seeding attack, in which light injected from the communication line into the laser results in an increase of the intensities of the prepared states. The unnoticed increase of intensity may compromise the security of QKD, as we show theoretically for the prepare-and-measure decoy-state BB84 and MDI QKD protocols. Our theoretical security analysis is general and can be applied to any vulnerability that increases the intensity of the emitted pulses. Moreover, a laser-seeding attack might be launched as well against decoy-state-based quantum cryptographic protocols beyond QKD.

Tunable electronic properties upon doping make laboratory-grown nanodiamond films a promising and intriguing platform for not only refining conventional electronics, but also developing superconducting quantum devices. In a variety of superconducting systems, mostly in layered materials and heterointerfaces, superconductivity exhibits a two-dimensional (2D) character as evidenced by the anisotropy factor $\epsilon ={\mu }_0{H}_{\mathrm{c}2}^{\parallel }/{\mu }_0{H}_{\mathrm{c}2}^{\mathrm{\perp }}\gg 1$, where ${\mu }_0{H}_{\mathrm{c}2}^{\parallel }$ and ${\mu }_0{H}_{\mathrm{c}2}^{\mathrm{\perp }}$ are the in-plane and out-of-plane upper critical fields, respectively. Here, we report on the observation of an anomalous anisotropy of superconductivity in heavily boron-doped nanodiamond films, which had been considered a purely three-dimensional (3D) material owing to the minute value of its coherence length in contrast to the film thickness. We investigate the resistive superconducting transition as a function of the angle of the applied magnetic field. The angular dependence of the resistive superconducting transition reveals ɛ < 1 in the nanodiamond films, indicating an anomalous anisotropy of the superconductivity. Our structural analysis shows that the grain boundaries, particularly twin boundaries, divide the nanodiamond film into nanoscale fragments standing out of the plane. The counterintuitive anisotropic superconductivity is interpreted as a result of the quantum confinement of the superconducting order parameter in the presence of the columnar grain boundaries and intragrain twin boundaries. This research provides physical insight for developing nanodiamond-based miniaturized superconducting quantum devices by making use of the as-grown grain boundaries and/or twin boundaries.

Fermion-to-qubit mappings that preserve geometric locality are especially useful for simulating lattice fermion models (e.g., the Hubbard model) on a quantum computer. They avoid the overhead associated with geometric nonlocal parity terms in mappings such as the Jordan-Wigner transformation and the Bravyi-Kitaev transformation. As a result, they often provide quantum circuits with lower depth and gate complexity. In such encodings, fermionic states are encoded in the common $+1$ eigenspace of a set of stabilizers, akin to stabilizer quantum error-correcting codes. Here, we discuss several known geometric locality-preserving mappings and their abilities to correct and detect single-qubit errors. We introduce a geometric locality-preserving map, whose stabilizers correspond to products of Majorana operators on closed paths of the fermionic hopping graph. We show that our code, which we refer to as the Majorana loop stabilizer code (MLSC) can correct all single-qubit errors on a two-dimensional square lattice, while previous geometric locality-preserving codes can only detect single-qubit errors on the same lattice. Compared to existing codes, the MLSC maps the relevant fermionic operators to lower-weight qubit operators despite having higher code distance. Going beyond lattice models, we demonstrate that the MLSC is compatible with state-of-the-art algorithms for simulating quantum chemistry, and can offer those simulations the same error-correction properties without additional asymptotic overhead. These properties make the MLSC a promising candidate for error-mitigated quantum simulations of fermions on near-term devices.

Some spin filters and switchers are designed with the topological-insulator (TI) heterojunctions made from the zigzag silicenelike nanoribbons (zSiNR). Different external fields applied on the left, central, or right region results in the expected TI edge state. The spin filtering property depends on the directions of the topological edge states across these junctions. We also find that the group velocity mismatch of the edge states, as well as the device length, plays a very important role in designing these types of spin filter and switcher. The realistic effects, such as dephasing and disorder, are also considered to ensure the robust and validity of our systems. The local current distributions are utilized to analyze the transport mechanism. Some interesting transport paths such as the spatial-separation Fano and Fabry-Perot resonances are also discovered, which cause a unit transmission in these topological junctions.

Charge transport in disordered organic semiconductors occurs by hopping of charge carriers between localized sites that are randomly distributed in a strongly energy-dependent density of states. Extracting disorder and hopping parameters from experimental data, such as temperature-dependent current-voltage characteristics, typically relies on parametrized mobility functionals that are integrated in a drift-diffusion solver. Surprisingly, the functional based on the extended Gaussian disorder model (eGDM) is extremely successful at this, despite it being based on the assumption of nearest neighbor hopping (nnH) on a regular lattice. We here propose a variable-range hopping (VRH) model that is integrated in a freeware drift-diffusion solver. The mobility model is calibrated using kinetic Monte Carlo calculations and shows good agreement with the Monte Carlo calculations over the experimentally relevant part of the parameter space. The model is applied to temperature-dependent space-charge-limited current (SCLC) measurements of different systems. In contrast to the eGDM, the VRH model provides a consistent description of both p- and n-type devices. We find a critical ratio of $a_{\mathrm{NN}}/\alpha$ (mean intersite distance:localization radius) of about three, below which hopping to non-nearest neighbors becomes important around room temperature and the eGDM cannot be used for parameter extraction. Typical (Gaussian) disorder values in the range 45–120 meV are found, without any clear correlation with photovoltaic performance, when the same active layer is used in an organic solar cell.

Thermodynamic properties, in particular, the temperature dependencies of magnetization of asymmetrically sandwiched ultrathin cobalt films with perpendicular anisotropy are investigated. The experimental results are described theoretically in the frame of magnon thermodynamics consistently accounting for the finite thickness of the films. The analysis includes both three-dimensional (Bloch’s $T^{3/2}$ law) and two-dimensional (the 2D Bloch law) theories as limiting cases. By fitting the experimental temperature dependencies of magnetization to the theoretical model, the exchange stiffness parameter is extracted. This approach provides access to the exchange stiffness without the need to know the strength of the Dzyaloshinskii-Moriya interaction in the stack. The exchange stiffness of sub-nm-thick $\mathrm{Co}$ films is found to be about three times smaller compared to the case of bulk cobalt. In the temperature range $T<170\pm 30$ K the temperature dependencies of magnetization follow the 2D Bloch law. The applicability of Bloch’s $T^{3/2}$ law and analysis of the Curie temperature (two-dimensional and three-dimensional pictures) to extract the exchange stiffness for sub-nm-thick $\mathrm{Co}$ films are tested as well. The closest value of the exchange stiffness to the magnon thermodynamics turned out to be within the analysis of the Curie temperature in the three-dimensional picture.

Dispersive coupling based on the Rabi model with large detuning is widely used for qubit quantum nondemolition (QND) readout in quantum computation. However, the measurement speed and fidelity are usually significantly limited by the Purcell effects (i.e., Purcell decay, critical photon numbers) and qubit-dependent Kerr nonlinearity. To avoid these effects, we propose how to realize an ideal QND readout of a gradiometric flux qubit with a tunable gap via its nonperturbative dispersive coupling (NPDC) to a frequency-tunable measurement resonator. We show that this NPDC-based readout mechanism is free of dipole-field interactions and that the qubit QND measurement is not deteriorated by intracavity photons. Both qubit-readout speed and qubit-readout fidelity can avoid the Purcell limitations. Moreover, NPDC can be conveniently turned on and off via an external control flux. We show how to extend this proposal to a multiqubit architecture for a joint qubit readout.

We report the extraction, capture, and manipulation of a droplet from a liquid-liquid surface using the acoustic near field from a fraxicon lens transducer. The lens is submerged in water, which shares a planar boundary with a reservoir of carbon tetrachloride (${\mathrm{C}\mathrm{Cl}}_4$). The ultrasonic beam exerts a negative radiation pressure, drawing the ${\mathrm{C}\mathrm{Cl}}_4$ up into the water, and a droplet of wavelength order is extracted out of the bulk liquid and sonically trapped. Our work demonstrates the means of producing and manipulating droplets from a fluid interface without mechanical contact using a near-field acoustic tweezer.

In recent years, magnetoelectric Delta-E-effect sensors have proven the potential to detect low-frequency and low-amplitude magnetic fields, while being fully integrable. The magnetic field sensors use the magnetization-induced change of the effective Young's modulus, referred to as the Delta-E effect. Although several sensor types have been demonstrated, little is reported about the dependence of the design parameters and the resulting sensor characteristics, such as sensitivity and noise. In this Paper, we comprehensively analyze and discuss theoretically and experimentally the magnetic sensitivity and the Delta-E effect of bending-mode Delta-E-effect sensors. Analytical expressions are derived for the magnetic sensitivity and compared with the results of a more elaborate numerical mean-field model. The mean-field model additionally considers spatial variations of the magnetic properties, including the internal stray field of the real shape. Measurements are performed on state-of-the-art bending-mode Delta-E-effect sensors. We find that the Delta-E effect significantly depends on the resonance mode and, moreover, second-order bending modes increase the magnetic sensitivity compared to first-order modes. This can be explained well by the spatial variation of the internal stray field in agreement with the numerical simulations and magneto-optical measurements. Overall, we provide a comprehensive description of the magnetic sensitivity for the improvement of bending-mode Delta-E-effect sensors. We show that the weighting of local properties by the mode shape is not only key for modeling and understanding the Delta-E effect of different bending modes, but also an important factor for improving magnetic sensitivities.

In ferromagnetic/heavy-metal bilayer systems, spin-current transmission with high efficiency between the heavy-metal layer and the ferromagnetic layer across the interface is crucial for power-efficient spin-orbit torque magnetic random access memory (SOT MRAM). The spin-mixing conductance is one of the critical parameters that determines such efficiency at the interface. For the $\mathrm{Co}$-$\mathrm{Fe}$-$\mathrm{B}/\mathrm{W}$ bilayer system, which is a promising candidate for SOT MRAM, improving the spin-current transmission efficiency at the interface by interface engineering, however, is not readily well studied. Here, we report that, by inserting an atomically thin α-W layer at the interface of $\mathrm{Co}$-$\mathrm{Fe}$-$\mathrm{B}/\mathrm{W}$ to construct the $\mathrm{Co}$-$\mathrm{Fe}$-$\mathrm{B}$/α-W/β-W trilayer system, the spin-mixing conductance can be enhanced by about 45%. A modified theoretical model is proposed that, in accordance with the experimental results, by considering the spin-backflow current as an equivalent spin resistance, the enhancement of spin-mixing conductance is attributed to suppression of the spin-backflow current by introducing α-W. This study demonstrates that interfacial phase engineering between multiphases of the same heavy metal is effective at enhancing the spin-mixing conductance in ferromagnetic/heavy metal bilayer systems, which may further improve spin-current transmission efficiency and help realize power-efficient SOT MRAM based on $\mathrm{Co}$-$\mathrm{Fe}$-$\mathrm{B}/\mathrm{W}$ systems.

The optimal segmentation of three-dimensional permanent-magnet systems into uniformly magnetized blocks to generate a desired field is a question that has been studied extensively in recent years. We present a procedure that generates the theoretically optimal shape and magnetization direction of all the magnet segments in the system, given only the total number of blocks as a constraint. Materials of arbitrary magnetic permeability can be included in this optimization framework. As an optimization objective we consider any functional that is linear with respect to the magnetic field distribution. We furthermore assume that all magnetic materials obey a linear constitutive relation. We show that with these assumptions calculating an optimal three-dimensional segmentation is equivalent to determining a centroidal Voronoi tessellation. We use the most-well-known and most-straightforward algorithm for the generation of centroidal Voronoi tessellations, known as “Lloyd’s method,” which can also be thought of as a fixed-point iteration. We show that the procedure is guaranteed to lead to a configuration that is at least locally optimal with respect to the assumed linear objective functional. However, we present results providing a strong indication that for most design problems applying our technique a few times starting from different random configurations is very likely to lead to the globally optimal solution.

Antiferromagnetic (AFM) skyrmionium composed of two topological AFM skyrmions shares the merits of an AFM skyrmion, for example, high mobility and no skyrmion Hall effect. Here, we analytically and numerically study the dynamics of the AFM skyrmion and skyrmionium induced by spin currents. Our calculations demonstrate that the current-induced spin-transfer torques can drive AFM skyrmion and skyrmionium with the same speed, while their steady motion speeds induced by spin-orbit torques are different. Furthermore, it is found that due to the existence of the effective AFM texture mass, the AFM skyrmion and skyrmionium obey the momentum theorem, and the time evolution of the position induced by alternating currents presents a phase. Besides, a spin torque nano-oscillator based on the AFM skyrmionium can produce high frequencies, similar to that based on the AFM skyrmion. Numerical simulations are in good agreement with the analytical solutions. Our results demonstrate the inertial dynamics of the AFM skyrmion and skyrmionium and may provide guidelines for building skyrmion-based spintronic devices.

Van der Waals heterostructures formed by stacking different atomically thin layered materials have emerged as the sought-after device platform for electronic and optoelectronic applications. Determination of the spatial extent of all the encapsulated components in such vertical stacks is key to optimal fabrication methods and improved device performance. Here, we employ electrostatic force microscopy as a fast and noninvasive microscopic probe that provides compelling images of two-dimensional layers buried over 30 nm below the sample surface. We demonstrate the versatility of the technique by studying heterojunctions comprising graphene, hexagonal boron nitride, and transition-metal dichalcogenides. The work function of each constituent layer acts as a unique fingerprint during imaging, thereby providing important insights into the charge environment, disorder, structural imperfections, and doping profile. The technique holds great potential for gaining a comprehensive understanding of the quality and flatness as well as local electrical properties of buried layers in a large class of nanoscale materials and vertical heterostructures.

We demonstrate various approaches to manipulate interfacial Dzyaloshinskii-Moriya interaction (IDMI) in magnetic films based on magnetic dynamic measurements, including Brillouin-light-scattering and broadband-ferromagnetic-resonance experiments. It is found that IDMI can be significantly enhanced with the ferromagnet (FM) layer sandwiched between heavy-metal (HM) layers. The induced IDMI clearly exhibits different chiralities for ${\mathrm{HM}}_1/\mathrm{FM}$ and ${\mathrm{HM}}_2/\mathrm{FM}$, respectively, resulting in enhancement of IDMI after stacking in a sandwich structure. We show that IDMI can be greatly increased by capping the HM/FM system with an oxide layer. The strong Rashba effect due to an interfacial electric effect at the FM/oxide interface is responsible for the increased IDMI. Furthermore, such an IDMI induced by the Rashba effect can be efficiently tuned by an electric field. Finally, we find that IDMI can be effectively tailored with a strain, and the strength of IDMI increases linearly with strain in the sample. We further speculate that IDMI can be enhanced by more than 100% by combining several approaches demonstrated here. We also find that the extracted IDMI strength is more accurate by introducing asymptotic values of the gyromagnetic ratio (γ) with a frequency approaching infinity from ferromagnetic resonance data. Our findings will benefit the spintronic community in the exploration of novel devices with chirality dependence.

In this article, we propose transformation caustics by applying coordinate transformation to the caustic effect. The limit boundaries of caustics can be used to design devices with asymmetric light propagation or light confinement, thus providing an alternative degree of freedom for metamaterials.

Recently, using good photoconductive, as-deposited $\mathrm{Zn}\mathrm{Se}$ thin-film-coated ITO glass plates, excellent vertical-alignment (VA) liquid-crystal (LC) cells are prepared and excitation of visible surface plasmons is also evidenced. The interface microcosmic conditions of LCs and the alignment layer dominates macroscopical optical properties. Here, we take the mechanism as unilateral anchoring of LC molecules to the $\mathrm{Zn}\mathrm{Se}$ interface mainly of (111) and using density-functional theory, the charge contributions at the interface are calculated. A strong adhesion of cyan and $\mathrm{Zn}$ atoms supported surface anchoring for molecules. LC cells are thus fabricated and exhibited fine original alignment with a leaked transmission as low as 0.0095% (40.2 dB) under crossed polarization. Meanwhile, the cell’s bright-dark contrast ratio can reach over 1150 under a transverse electric field, along with high transparency, providing a bright prospect in readily integrating the LC layer into photonic systems, which are compatible to currently commercially successful semiconductor-based photonic and sensing applications. The cells are quite easy to fabricate and the use of a semiconductor layer makes this promising for future development.

Recently, there have been several proposals on exciton-polariton topological insulators, most of which have required strong external magnetic fields induced by bulky superconducting coils. We propose an alternative design for a polariton topological insulator, where excitons are in the proximity of an additional layer of a ferromagnetic material, with a predefined magnetic moment located between the pillars of the cavity. Our design supports a variety of topological phases and transitions between Chern numbers $\pm 2$ and $\pm 1$ by varying either the magnetic moment of the ferromagnetic material or the spin-orbit coupling between different spin projections, thus enabling compact polariton devices harnessing switchable topological edge modes.

Electromagnetic (EM) cloaks have promising potentials in many real-world applications such as noninvasive detection, EM invisibility, etc. Available methods for concealing large (wavelength scale) objects have still been rarely demonstrated by experiments, especially with sufficient bandwidth and ultrathin cloak thickness. Here, we report a composite strategy for realizing backward-scattering reduction of a wavelength-scale cylindrical target by an ultrathin metasurface. This composite strategy integrates the theories of Mie scattering and microwave network to achieve improved operating bandwidth. As a proof of concept, we experimentally present an ultrathin (0.0156λ, with λ as the working wavelength) conformal metasurface to reduce the backward scattering of a metallic cylindrical object (diameter of 1.217λ) within a frequency band of 7.67%. Further bandwidth improvement is also realized by using two kinds of meta-atoms, working respectively in two neighboring frequency bands and placed in an interleaved form to cover over the cylindrical object. Both simulated and measured results show that more than 10-dB backward-scattering reduction can be obtained within an improved bandwidth of approximately 11.10%. The proposed metasurface encompasses advantages of ultrathin thickness and good performances in backward-scattering reduction, which may open promising perspectives for practical applications such as stealth technique, camouflaging, etc.

Various protocols implementing a quantum computer are being pursued, one of which is the adiabatic quantum computer. Natural interactions in electromagnetic environments are only two-body local interactions, but the construction or simulation of higher-order couplers is indispensable for a universal adiabatic quantum computer using conventional flux qubits (no nonstoquastic interactions). Here we show that in a specific flux-qubit coupler design without ancilla qubits, four-body stoquastic interactions are induced by virtual coupler excitations. For specific parameter regimes they are the leading effect and can be tuned up to the gigahertz range.

The central question in the field of two-dimensional (2D) materials is how a material behaves when it is patterned at nanometer scale with different edge geometries. Due to the anisotropy inherent in the puckered structure, black phosphorene nanostructures may have more varieties of edge geometries. Here, we present a comprehensive 2D planar crystallographic characterization of phosphorene uniformly by a chiral vector (angle), from which we report a type of edge atomic configuration, the slope-edge geometry. The phosphorene nanoribbons (PNRs) with slope edges, like previously noticed zigzag and skewed-armchair PNRs, also possess interesting twofold-degenerate edge states. These three marginal directions, together with the skewed-zigzag and armchair directions without edge states, divide a phosphorene into four regions, among which the electronic properties are different from each other. Further, for a PNR cutting along any possible direction, by taking into account the $z$ direction in the structure, whether or not it possesses edge states depends on the existence of periodic zigzaglike morphology along the edges. For application, moreover, we propose a PNR-based Z-shaped homogenous junction with scale approximately 100 nm, where the central scattering region between two PNR electrodes is a phosphorene quantum dot (PQD) of a PNR segment with various edge morphologies. Interestingly, the calculated conductance by the kwant code, based on tight-binding combined with a scattering matrix for the junction, relies sensitively on the central PQD edge states. In specification, for the junctions of PQDs with edge states, the conductance exhibits a terrace with resonant oscillations near the Fermi energy; otherwise, the electron transport is blocked due to the absence of edge states. Remarkably, the number of oscillating peaks exactly matches the number of sawtooth along an edge of the PQD, since the discrete energy levels of the zigzaglike edge provide the transporting channels for electrons. The results here may be extended to the group-VA 2D materials in order to observe the electronic properties of nanostructures in experiments and provide a reference for the preparation of better nanodevices.