We experimentally investigate the properties of hyperentangled states displaying simultaneous entanglement in multiple degrees of freedom and find that Bell tests beyond the standard Clauser-Horne-Shimony-Holt inequality can reveal a higher-dimensional nature in a device-independent way. Specifically, we show that hyperentangled states possess more than just simultaneous entanglement in separate degrees of freedom but also entanglement in a higher-dimensional Hilbert space. We also verify the steerability of hyperentangled quantum states by steering different photonic degrees of freedom.

We devise nanoscale-designed ferromagnetic thin films with graded exchange-coupling (J) profiles that allow us to obtain constant coercivities over a very extended temperature range. The magnetic structure designed here is composed of two ferromagnetic layers, which are separated by a spacer layer with a linear exchange-graded profile. Our detailed magnetometry analysis on single-crystal $\mathrm{Co}\text{−}\mathrm{Ru}$ alloy thin films reveals a coercivity plateau with a relative change of less than 2% over a temperature range of 75 K, whereas conventional films exhibit changes larger than 20%. This remarkable behavior is achieved by means of a strongly temperature-dependent exchange bias between adjacent ferromagnetic portions of our film structures, which are designed to fully compensate for the conventional temperature dependence of the coercive field. This strongly temperature-dependent bias field is mediated by a paramagnetic interlayer in our film design and can be adjusted by a suitable choice of both the spacer-layer composition profile and thickness. Our particular design can be applied to a large variety of magnetic materials and adapted to relevant temperature ranges that are needed to maintain stable coercive field values over a wide operation range in real applications.

Ionic gating is a powerful technique for tuning the physical properties of a material via electric-field-induced charge doping, but is prone to introduce extrinsic disorder and undesired electrochemical modifications in the gated material beyond pure electrostatics. Conversely, reversible, volatile, and electrostatic modulation is pivotal in the reliable design and operation of novel device concepts enabled by the ultrahigh induced charge densities attainable via ionic gating. Here we demonstrate a simple and effective method to achieve reversible and volatile gating of surface-sensitive ultrathin niobium nitride films via controlled oxidation of their surface. The resulting niobium oxide encapsulation layer exhibits a capacitance comparable to that of nonencapsulated ionic transistors, withstands gate voltages beyond the electrochemical stability window of the gate electrolyte, and enables a fully reversible tunability of both the normal-state resistivity and the superconducting transition temperature of the encapsulated films. Our approach should be transferable to other materials and device geometries where more standard encapsulation techniques are not readily applicable.

${\mathrm{Bi}}_2{\mathrm{Te}}_3$ is a widely used thermoelectric material with strong anharmonicity. Determination of its thermal conductivity requires consideration of the high-order phonon scattering process, which makes it extremely time consuming and challenging to accurately calculate its thermal conductivity by obtaining high-order force constants based on density-functional theory. In this work, a deep-neural-network potential is developed to reproduce phonon dispersion and predict the lattice thermal conductivity of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. The equilibrium molecular dynamics simulations combined with this potential are performed to calculate the lattice thermal conductivity and the results nicely match the experimental values. Meanwhile, we find the generalized gradient approximation with the DFT-D3 functional can accurately reproduce the experimental lattice constants of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ and provide a description of the phonon dispersion in ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ as well as the local density approximation. Furthermore, we explore the influence of the native point defects on thermal conductivity, and find that $\mathrm{Te}$ vacancies have the most significant effect on the reduction of thermal conductivity, owing to the appreciable inhibition of phonon propagation speed by $\mathrm{Te}$ vacancies, and the additional scattering among original low-frequency optical phonons and the fresh low-frequency optical phonons moving downward from high frequency region, which provides some theoretical guidance for reducing thermal conductivity in experimental research.

Bound states in the continuum (BICs) provide a viable way of achieving high-Q resonances in both photonics and acoustics. In this work, we propose a general method of constructing Friedrich-Wintgen (FW) BICs and accidental BICs in a coupled acoustic waveguide-resonator system. We demonstrate that FW BICs can be achieved with arbitrary two degenerate resonances in a closed resonator, regardless of whether they have the same or opposite parity. Moreover, their eigenmode profiles can be arbitrarily engineered by adjusting the position of the attached waveguide. This suggests an effective way of continuously switching the nature of the BICs from FW BICs to symmetry-protected BICs or accidental BICs. Also, such BICs are sustained in the coupled waveguide-resonator system with shapes such as rectangles, ellipses, and rhomboids. These interesting phenomena are well explained by the two-level effective non-Hermitian Hamiltonian, where two strongly coupled degenerate modes play a major role in forming such FW BICs. Additionally, we find that such an open system also supports accidental BICs in geometry space instead of momentum space via tuning the position of the attached waveguide, which is attributed to the quenched coupling between the waveguide and eigenmodes of the closed cavity. Finally, we fabricate a series of three-dimensional coupled resonator waveguides and experimentally verify the existence of FW BICs and accidental BICs by measuring the transmission spectra. Our results complement the current BIC library in acoustics and provide nice routes for designing acoustic devices, such as acoustic absorbers, filters, and sensors.

The lack of certain crystalline symmetries in strong spin-orbit-coupled nonmagnetic materials allows for the existence of unconventional spin Hall responses, with electrically generated transverse spin currents possessing collinear flow and spin directions. The injection of such spin currents into an adjacent ferromagnetic layer can excite magnetization dynamics via unconventional spin-orbit torques, leading to deterministic switching in ferromagnets with perpendicular magnetic anisotropy. We study the interplay between conventional and unconventional spin-orbit torques on the magnetization dynamics of a perpendicular ferromagnet in the small intrinsic damping limit, and identify a rich set of dynamical regimes that includes deterministic and probabilistic switching, precessional and pinning states. Contrary to common belief, we find that there exists a critical conventional spin Hall angle, beyond which deterministic magnetization switching transitions to a precessional or pinned state. Conversely, we show that larger unconventional spin Hall angle is generally beneficial for deterministic switching. We derive an approximate expression that qualitatively describes the state-diagram boundary between the full deterministic switching and precessional states and discuss a criterion for searching symmetry-broken spin Hall materials in order to maximize switching efficiency. Our work offers a roadmap towards energy-efficient spintronic devices, which might open the door for applications in advanced in-memory computing technologies.

We demonstrate quantum sensing of dc magnetic fields that exceeds the sensitivity of conventional $T_2^{\ast }$-limited dc magnetometry by more than an order of magnitude. We use nitrogen-vacancy centers in a diamond rotating at periods comparable to the spin coherence time, and characterize the dependence of magnetic sensitivity on measurement time and rotation speed. Our method up-converts only the dc field of interest and preserves the quantum coherence of the sensor. These results definitively improve the sensitivity of a quantum magnetometer to dc fields, demonstrating that sensitivity below the $T_2^{\ast }$ limit is possible and can be applied to any diamond magnetometer where $T_2\gg T_2^{\ast }$ to yield an order-of-magnitude sensitivity improvement.

Plasmon antennas allow subwavelength confinement and enhancement of electromagnetic fields at the “hotspot” where the radiation efficiency of emitters can be substantially enhanced. Such enhancement, however, is often polarization dependent. Consequently, the radiation behaviors (e.g., radiation pattern and polarization states) of the emitter placed at the hotspot are also modified significantly. Enhancing the radiation efficiency without altering the original radiation pattern and polarization state of the emitter is highly desired for many sought-after applications involving chiral emitters but remains a challenging task, especially at low frequencies. To this end, spoof-plasmon antennas with fourfold and sixfold rotational symmetries are designed and realized experimentally. These plasmon antennas support polarization-independent localized plasmon resonances, which can significantly enhance the local density of photonic states at the structural center without changing the polarization state of the emitter. As a typical example, the structure with sixfold rotational symmetry is coupled with a half-wave dipole antenna. The measurement results show that the far-field radiation pattern of the dipole antenna is maintained with the radiation efficiency enhanced by more than 2 orders of magnitude, irrespective of the dipole orientation.

Charge detection is a crucial component used in semiconductor quantum circuits. It allows the noninvasive readout of charge and spin qubits in quantum information protocols and can also be leveraged for quantum sensing applications. In this work, we demonstrate that van der Waals heterostructure devices with gated nanoconstrictions in monolayer ${\mathrm{WSe}}_2$ can be used as charge detectors for nearby quantum dots. This is a demonstration of charge detection using transition-metal dichalcogenides, and we find that the detection sensitivity is comparable to that of other two-dimensional (2D) material charge detectors based on graphene. These results open the possibility of implementing charge detection schemes based on 2D materials in complex quantum circuits.

Mapping the positions of single electron spins is a highly desired capability for applications such as nanoscale magnetic resonance imaging and quantum network characterization. Here, we demonstrate a method based on rotating an external magnetic field to identify the precise location of single electron spins in the vicinity of a quantum spin sensor. We use a nitrogen-vacancy center in diamond as a quantum sensor and modulate the dipolar coupling to a proximate electron spin in the crystal by varying the magnetic field vector. The modulation of the dipolar coupling contains information on the coordinates of the spin, from which we extract its position with an uncertainty of 0.9 Å. We show that the method can be used to locate electron spins with nanometer precision up to 10 nm away from the sensor. We discuss the applicability of the method to mapping hyperfine coupled electron spins and show that it may be applied to locating nitroxide radicals. The magnetic tomography method can be utilized for distance measurements for studying the structure of individual molecules.

Fully coupled photon-electron-positron computational schemes are needed in a broad range of domains, from radiotherapy and radiosurgery treatment planning to nuclear-reactor safety analysis. Industrial deployment of such schemes remains problematic because of the prohibitively time-consuming charged-particle-transport aspects of both Monte Carlo algorithms and Boltzmann-Fokker-Planck solvers. In this paper, a partially coupled scheme based on reactor-physics models for secondary photons (RPSPs) is presented. RPSPs include thick-target-bremsstrahlung, annihilation, and full atomic cascade models, recovering long-range effects of both electrons and positrons. The RPSP-based scheme is compared with state-of-the-art fully coupled and uncoupled schemes in medical, nuclear, and multidisciplinary contexts. Within tissues, the average discrepancies from the fully coupled scheme are about $0.5\mathrm{\%}$, with a maximum error below $1.3\mathrm{\%}$ at high energy. The CPU time of the coupled schemes is reduced by 58 and 78 times for 3- and 6-MeV beams, respectively. The discrepancies are also within the accuracy of the cross sections for ${}_4\mathrm{Be}$, ${}_6\mathrm{C}$, ${}_{13}\mathrm{Al}$, ${}_{26}\mathrm{Fe}$, ${}_{29}\mathrm{Cu}$, ${}_{32}\mathrm{Ge}$, ${}_{47}\mathrm{Ag}$, ${}_{79}\mathrm{Au}$, ${}_{82}\mathrm{Pb}$, and ${}_{92}\mathrm{U}$ slabs. The same discrepancies are about $0.1\mathrm{\%}$, $0.4\mathrm{\%}$ and $0.8\mathrm{\%}$, respectively, in the reactor's shielding, vessel and fuel slabs. A power peaking factor reduction of 4.35 times from the state-of-the-art uncoupled scheme is demonstrated for uranium oxide and mixed plutonium-uranium oxide slabs.

We report $\mathrm{Ga}\mathrm{As}$/$(\mathrm{Al},\mathrm{Ga})\mathrm{As}$ nanowires in the one-dimensional (1D) quantum limit. The ultrathin wurtzite $\mathrm{Ga}\mathrm{As}$ cores between 20 and 40 nm induce large confinement energies of several tens of millielectronvolts, allowing us to experimentally resolve up to four well-separated subband excitations in microphotoluminescence spectroscopy. Our detailed experimental and theoretical polarization-resolved study reveals a strong diameter-dependent anisotropy of these transitions: We demonstrate that the polarization of the detected photoluminescence is governed, on the one hand, by the symmetry of the wurtzite 1D quantum wire subbands but also, on the other, by the dielectric mismatch of the wires with the surrounding material. The latter effect leads to a strong attenuation of perpendicularly polarized light in thin-dielectric wires, making the thickness of the $(\mathrm{Al},\mathrm{Ga})\mathrm{As}$ shell an important factor in the observed polarization behavior. Including the dielectric mismatch to our $\mathbf{k}\cdot \mathbf{p}$-based simulated polarization-resolved spectra of purely wurtzite $\mathrm{Ga}\mathrm{As}$ quantum wires, we find excellent agreement between experiment and theory. These findings reveal insights relevant to spectroscopic studies of the 1D quantum regime and the design of photonic quantum wire applications such as lasers and hopefully open up paths for selective subband excitations.

We report on a wave-driven microfluidic ultrasonic shaker that can stably trap and oscillate microparticles in water in a microchannel. The microfluidic ultrasonic shaking system mainly consists of a microfluidic chip and two orthogonally excited piezoelectric transducers: one for creating radiation force potential wells for three-dimensional trapping of microparticles and the other for generating sound destabilization to shake the trapped microparticles. It is shown that two standing waves of different but close frequencies provide a shaking force [approximately $F_0$cos(2πΔft), with $F_0$ and Δf being the force amplitude and the frequency difference, respectively] that can repeatedly perturb a trapped microparticle and cause it to execute spontaneous oscillations. The dynamic behavior of microparticle oscillation driven by two sound waves is found to be equivalent to a forced harmonic oscillator (i.e., a mass on a spring) that has been extensively described in the literature. Specifically, experiments at various ultrasonic excitations show that the oscillating frequency of a stably trapped microparticle is determined by the frequency difference between the two sound waves, and the shaking amplitude scales inversely to the frequency difference and is affected by pressure amplitudes of both sound waves. We foresee that the illustrative microfluidic ultrasonic shaker presented here opens perspectives for dynamic manipulation of single microparticles or clusters in microscale acoustofluidic systems.

When two identical periodic structures are stacked at a series of discrete rotation angles, the resulting moiré pattern brings an extreme flat band with transport enhancement. It provides insights into the fascinating physics of insulating states and unconventional superconductivity with delocalization-localization transitions and commensurable-incommensurable phases. However, the exploration of symmetry- and geometry-independent flat-band physics with moiré patterns is still rare, limited by the stringent requirement in high dimension. Here, we experimentally observe the localized flat-band state by mapping a moiré model into a one-dimensional photonic lattice using a femtosecond laser direct writing technique. By accurately controlling the external periodic field, we construct moiré photonic lattices with different moiré band structures. We successfully observe the photon-walker evolution from splitting with dispersion to nondispersive propagation with larger localization in the input ports as the result of a flat band induced by moiré patterns with larger period. Our approach to engineering moiré model, together with the integrated photonic implementation, establishes a powerful tool for exploring the effects accompanying the transition from commensurate to incommensurate phases.

The emergence of ratchet effects in two-dimensional materials is strongly correlated with the introduction of asymmetry into the system. In general, dual-grating-gate structures forming lateral asymmetric superlattices provide a suitable platform for studying this phenomenon. Here, we report on the observation of ratchet effects in $\mathrm{Hg}\mathrm{Te}$-based dual-grating-gate structures hosting different band-structure properties. Applying polarized terahertz laser radiation we detect linear and polarization-independent ratchets, as well as a radiation-helicity-driven circular ratchet effect. Studying the ratchet effect in devices made of quantum wells (QWs) of different thickness, we observe that the magnitude of the signal substantially increases with decreasing QW width with a maximum value for devices made of QWs of critical thickness hosting Dirac fermions. Furthermore, sweeping the gate voltage amplitude we observe sign-alternating oscillations for gate voltages corresponding to $p$-type conductivity. The amplitude of the oscillations is more than two orders of magnitude larger than the signal for $n$-type conducting QWs. The oscillations and the signal enhancement is shown to be caused by the complex valence band structure of $\mathrm{Hg}\mathrm{Te}$-based QWs. These peculiar features of the ratchet currents make these materials an ideal platform for the development of terahertz applications.

A physics-based model that predicts the emitted current from thermionic cathodes is developed, which accurately spans from the temperature-limited (TL) to full-space-charge-limited (FSCL) regions. Experimental observations of thermionic electron emission demonstrate a smooth transition between TL and FSCL regions of the emitted-current-density-versus-temperature (J-T) (Miram) curve and the emitted-current-density-versus-voltage (J-V) curve. Knowledge of the temperature and shape of the TL-FSCL transition is important in evaluating the thermionic electron-emission performance of cathodes, including predicting the lifetime. However, there are no first-principles physics-based models that predict the smooth TL-FSCL transition region for real thermionic cathodes without applying a priori assumptions or empirical phenomenological equations that are physically difficult to justify. Previous work detailing the nonuniform thermionic emission found that the effects of three-dimensional space charge, patch fields (electrostatic potential nonuniformity on the cathode surface based on local work-function values), and Schottky barrier lowering can lead to a smooth TL-FSCL transition region from a model thermionic cathode surface with a checkerboard spatial distribution of work-function values. In this work, we construct a physics-based nonuniform emission model for commercial dispenser cathodes. This emission model is obtained by incorporating the cathode surface grain orientation via electron-backscatter diffraction and the facet-orientation-specific work-function values from density-functional-theory calculations. The model enables the construction of two-dimensional emitted-current-density maps of the cathode surface and corresponding J-T and J-V curves. The predicted emission curves show excellent agreement with experiment, not only in the TL and FSCL regions but, crucially, also in the TL-FSCL transition region. This model provides a method to predict the thermionic emission from the microstructure of a commercial cathode and improves the understanding of the relationship between thermionic emission and cathode microstructure, which is beneficial for the design of vacuum electronic devices.

The dissemination of atomic clocks with fiber-based techniques finds application in the fields of metrology, fundamental physics, navigation, and spectroscopy but is a challenge in terms of reliability, maintenance, and performance. Here, we describe the realization of a 1023-km-long fiber link between the metrological institutes of Italy and France that shares the infrastructure with the Internet traffic and exploits segmentation in shorter, cascaded spans to fight optical losses exceeding 280 dB. With four months of quasicontinuous operation of this link, we compared the $\mathrm{Cs}$, $\mathrm{Rb}$, and $\mathrm{Yb}$ atomic clocks at our laboratories, highlighting the potential of this tool to assess the clock uncertainty budgets, characterize advanced satellite techniques, and develop optical timescales. The integration of the metrological, fiber-based infrastructures in the two countries, connecting photonics and spectroscopy laboratories as well as telescope facilities, provides the research community with a physical layer over which applications can be built on.

Here we theoretically and experimentally demonstrate a way to control the intensity of optical vortices by a magnetic field directed along the beam axis. We describe the principle of an optical vortex modulator and switch based on the Faraday effect in an iron garnet film. Importantly, the magnetic field influences only the polarization plane of the vortex and keeps its topological charge.