Multiparameter estimation is necessary for force sensing due to simultaneous and nontrivial small changes of position and momentum. The design of quantum probes that allow simultaneous estimation of all parameters is therefore an important task. The optimal methods for estimation of the conjugate changes of position and momentum of quantum harmonic oscillator employ probes in entangled or quantum non-Gaussian states. We show that the same results can be obtained in a significantly more feasible fashion by employing independent sets of differently squeezed Gaussian states classically correlated with position or momentum measurements. This result demonstrates an unexplored power of a classical correlation between the probe states and measurements directly applicable to force sensing.

Hot-carrier photodetectors have been extensively studied in recent years, but still exhibit particularly low efficiencies. The physics behind this is closely related to the intrinsic electronic structures of hot-carrier materials. Transition-metal carbides and nitrides (TMCNs) show intriguing potential for high-performance hot-carrier applications; however, a systematic study from the electronic structure perspective remains elusive. Herein, we study the underlying physics, including the electronic structures, hot-carrier energy distribution, electron-electron and electron-phonon scatterings, and hot-carrier injection for TMCNs by first-principles calculation and Monte Carlo simulations. We investigate the complete hot-carrier behavior from the microscopic dynamics (e.g., generation, transport, and injection) of hot carriers to the macroscopic responses (e.g., device responsivity) of the photodetectors. It is found that a well-manipulated electronic structure can greatly improve the performance of hot-carrier photodetectors. Our systematic study shows that hot-carrier systems based on $\mathrm{Hf}\mathrm{C}$, $\mathrm{Zr}\mathrm{N}$, or $\mathrm{Ta}\mathrm{C}$ display significantly higher efficiencies, benefitting from their better electronic structures; in particular, the unbiased responsivity of the $\mathrm{Zr}\mathrm{N}/{\mathrm{Ti}\mathrm{O}}_2$ system can be up to 20 mA/W at a wavelength of 1033 nm, which is about 70 times higher than that of the conventional $\mathrm{Au}$ system, enabling the realization of high-performance hot-carrier photodetectors by using non-noble metals.

Klein tunneling is a counterintuitive quantum-mechanical phenomenon, predicting perfect transmission of relativistic particles through higher-energy barriers. This phenomenon was shown to be supported at normal incidence in graphene due to pseudospin conservation. Here I show that Klein-tunneling analogy can occur in classical systems, and remarkably, does not rely on mimicking graphene’s spinor wave-function structure. Instead, the mechanism requires a particular form of constitutive parameters of the penetrated medium, yielding transmission properties identical to the quantum tunneling in graphene. I demonstrate this result by simulating tunneling of sound in a two-dimensional acoustic metamaterial. More strikingly, I show that by introducing a certain form of anisotropy, the tunneling can be made unimpeded for any incidence angle, while keeping most of its original Klein dispersion properties. This phenomenon may be denoted by the omnidirectional Klein-like tunneling. The suggested tunneling mechanism and its omnidirectional variant may be useful for applications requiring lossless and direction-independent transmission of classical waves.

The coupling between multiple modes in subwavelength scale has attracted a lot of attention due to the interesting physics favored by the enhancement of light-matter interaction. In this contribution, we propose a planar hybrid plasmonic-photonic crystal structure, which enables the tunable coupling between Tamm plasmon-polariton and Fabry-Perot cavity modes. Then, a strong coupling can be achieved through evoking the Tamm-mode dependence of the top-layer thickness of the hybrid structure, which is indicated by an anticrossing with a spectrum splitting of 9.298 meV. The effect enables the spatial confinement of polaritons and the formation of hybrid one-dimensional plasmon-polariton modes. In order to account for the splitting of the orthogonally polarized resonances, a detailed investigation on the TE-TM splitting is also performed through the angular-dependence analysis. We note that the value of the TE-TM splitting strongly depends on the thicknesses of dielectric layer and the distance between two distributed Bragg reflector (DBR) stacks. We show that the cavity modes are extremely sensitive to the distance between the DBR stacks and propose using this feature for strain sensing. Thus, our structure may not only offer a platform for study of interesting physics (e.g., spin optronics), but may also provide a route for high-accuracy strain sensors.

Typical acoustic metamaterial absorbers rely on relatively rigid materials for acoustic resonances, which, however, poses a challenge in reshaping and reconfiguring them. Here, we demonstrate soft reconfigurable acoustic absorbers, which are realized by the use of flexible tubular resonators. Owing to the structural compliance, the tube absorbers promote intriguing acoustic-structure coupling and exhibit hybrid resonances of acoustic and structural modes, which are characterized by Rabi splitting in the strong-coupling regime and Fano-like resonance in the weak-coupling regime. Also, the soft walls of the flexible resonators induce decreased resonance frequencies for low-frequency absorption by reducing the effective sound speed inside the tube. Moreover, we find that these hybrid resonances are critically influenced by the structural loss of the flexible resonators. While elucidating the role of the material rigidity and structural loss in acoustic resonances, this work allows us to design versatile broadband absorbers that can be reconfigured depending on the topology of a target substrate.

The properties of rare-earth elements diluted in nonmagnetic crystals make them promising candidates for a quantum memory due to their limited Hilbert space. The control and readout of the qubit states require a highly sensitive measurement and large coupling of the spin ensemble with the electromagnetic mode of a superconducting resonator. We report sensitive detection of forbidden transitions of electronuclear states from the minority species of ${}^{155}\mathrm{Gd}$ and ${}^{157}\mathrm{Gd}$ isotopes, which opens the possibility of connecting quantum states with very different spin projections. Cavity perturbation measurements seen in the reflected signal allow the detection of about $7.6\times {10}^7$ spins and the measurement of phase memory loss rate and spin-photon coupling strength.

Semiconductor BiOCl has a layered structure with ultralow lattice thermal conductivity [Q.D. Gibson et al., Science 373, 1017–1022 (2021)] and has potential applications in the field of thermoelectric materials. In the present study, the thermoelectric properties of BiOCl crystals are accurately predicted using the first-principles calculation combined with Boltzmann transport theory. The dimensionless figure of merit ($ZT$) of $p$-type BiOCl is found to be 0.2 at room temperature, reaching 1.1 at 800 K. In addition, applying in-plane biaxial tensile strain ${ϵ}_{xy}$ can lead to a further increase in the value of $ZT$ to 1.9 at 800 K. This means that $p$ BiOCl is an excellent high-temperature thermoelectric material. And this is due to the fact that biaxial strain drastically reduces the lattice thermal conductivity and the shift of the valence band toward the Fermi level can optimize the carrier concentration. Thus, the present work paves a way for the design of adjustable high-temperature thermoelectric materials.

The fluctuational electrodynamic investigation of thermal radiation from nonequilibrium or nonisothermal bodies remains largely unexplored because it necessarily requires volume integration over the fluctuating currents inside the emitter, which quickly becomes computationally intractable. Here, we put forth a formalism combining fast calculations based on modal expansion and fluctuational electrodynamics to accelerate research at this frontier. We employ our formalism on a sample problem: a long silica wire held under temperature gradient within its cross section. We discover that the far-field thermal emission carries a nonzero spin, which is constant in direction and sign, and interestingly, is transverse to the direction of the power flow. We clearly establish the origin of this transverse spin as arising from the nonequilibrium intermixing of the cylindrical modes of the wire, and not from any previously studied or intuitively expected origins such as chiral or nonisotropic materials and geometries, magnetic materials or fields, and mechanical rotations. This finding of nonequilibrium spin texture of emitted heat radiation can prove useful for advancing the noninvasive thermal metrology or infrared-imaging techniques.

Quantum entanglement is an important resource in quantum information processing tasks, such as quantum cryptography, quantum teleportation, and quantum computation. The main obstacle to faithful and reliable preparation of entangled states is environment-induced decoherence. Dissipative transition of a general quantum state to an entangled steady state provides an effective strategy to protect entanglement from decoherence, where the decoherence converts into an essential resource. In this paper, we put forward a scheme to prepare generalized Bell states based on trapped ions, in which unitary dynamics and spontaneous emission are combined to drive an arbitrary quantum state in the ground-state space to a unique steady state. The scheme is realized by using the renowned Sørensen-Mølmer setting and is therefore compatible with current experimental technology. Moreover, the vibrational degree of freedom is decoupled from the unitary dynamics so that the scheme does not need to cool the collective vibrational mode to its ground state and thus it works for thermally excited states. Besides, our scheme is demonstrated to be insensitive to variations of some parameters with the aid of numerical simulation.

Acoustic camouflage, which is realized by controlling wave propagation, has attracted great attention in recent years. Aided by a reflecting surface, the acoustic carpet cloak has become one of the most practically feasible invisibility devices and captured much experimental research enthusiasm. However, owing to the difficulty in the realization of ideal material parameters, the implemented underwater carpet cloaks could only work for a small incident angle. Here we design an underwater carpet cloak using a three-component metafluid composed of syntactic foam, steel, and water. The proposed syntactic foam, which is synthesized from epoxy resin and hollow glass microspheres, exhibits lower mass density and higher sound velocity relative to water. By periodically embedding the syntactic foam and steel rods in water, a carpet cloak is constructed and experimentally demonstrated in an anechoic water tank. Experimental results indicate that the designed carpet cloak works well under both normal and oblique incidences for broadband frequencies. The implementation of the designed underwater carpet cloak paves the way for future applications. Moreover, the introduction of syntactic foam in the design of the carpet cloak provides an extra degree of freedom for the acoustic parameter regulation of the metafluid in underwater acoustic devices.

Engineering the kinetic motion of magnetic skyrmions shows great potential in spintronics. Particularly, as a natural property, temperature plays a significant role in the dynamics of skyrmions. For instance, the nonlinear and the rectilinear motions of skyrmions driven by spin-transfer torque and local energy imbalance in temperature gradients, respectively, have been explored. Although existing studies have already implied the multiphysics field-controlled property of skyrmion thermal motion, the investigation of mechanically controlled skyrmion motions in temperature gradients is absent, and the kinematic equation is unclear. Here, we employ a phase-field model to simulate the effect of mechanical strain on skyrmion motion in a temperature gradient, and demonstrate that the strain-induced anisotropic deformation of skyrmions increases the driving force and accelerates their motion. To model the mechanical effect, we propose a kinematic equation for describing the relationship between skyrmion velocity and multiphysics field variables. Lastly, the potential application of the mechanically controlled skyrmion thermal motion is demonstrated via tip indentation simulations. This work provides the thermodynamic mechanism and kinematic equation for mechanically accelerated skyrmion thermal motions, which is expected to be a guidance for future research on skyrmion dynamics and application of functional devices.

Breaking the Abbe-Rayleigh diffraction limit and realizing acoustic superdiffraction focusing and imaging is of great significance in acoustic science and engineering. While a conventional focusing lens cannot even reach the Abbe-Rayleigh resolution limit due to the cutoff of spatial-frequency components of the regular lens. Superoscillation technology paves a way to generate superdiffraction for acoustic superresolution imaging. However, high-transmission superresolution is still a challenge issue in the imaging field of acoustics. Here, we propose a high-throughput phase-modulated acoustic-superoscillatory-lens (ASOL) focal-imaging technology. The ASOL is made up of many subwavelength spiral metamaterial elements for superimposing different spatial-frequency components to generate a superdiffraction focal spot. For proof of concept, the design and fabrication of the phase-modulated ASOL, a 0.326λ focal spot with a 0.9 transmission coefficient, is achieved through both simulation and experiment. Moreover, a superresolution focal-imaging experiment is successfully demonstrated by the Chinese traditional window-grille structure with a slit width of 0.33λ. It verifies that the proposed ASOL has the great technical potential to improve acoustic-focal-imaging resolution. The proposed method also opens up opportunities for developing high-throughput acoustic superresolution imaging, and it will reduce the reflection effect and improve the signal-to-noise ratio. The proposed ASOL has significant potential applications in underwater sonar scanning, acoustic diagnosis, and acoustic imaging technology.

We propose the implementation of the swap test using a charge qubit in a double quantum dot. The swap test is a fundamental quantum subroutine in quantum machine learning and other applications for estimating the fidelity of two unknown quantum states by measuring an auxiliary qubit. Our proposal uses a controlled three-qubit gate, which is natural to quantum dot charge qubits. It allows us to implement a swap test with a circuit depth of six layers, and an estimated gate time of less than 3 ns, that is below the coherence time of double quantum dots. This work paves the way for enhancing the toolbox of quantum machine learning developments in semiconductor qubits.

Acoustic imaging techniques suffer from the diffraction limit due to the loss of evanescent waves that carry subwavelength information of objects. To overcome the diffraction limit, the evanescent components have to be collected and measured. Most of the existing methods targeting this task rely on expensive detector arrays and inefficient near-field point-by-point scanning. Here, we propose and experimentally demonstrate the realization of a far-field acoustic subwavelength imaging method based on a single stationary detector. Specifically, we utilize a series of masks to structure the detected field, so that the evanescent wave information is encoded into the propagating waves due to spatial frequency convolution between the object and masks. Our study shows that, by combining the principles of computational imaging and metalens, high-quality images of a subwavelength object can be reconstructed in the far field, even in the presence of unwanted scatterers. Our work provides a robust method for far-field acoustic subwavelength imaging, which could bring possibilities for acoustic microscopy and could further be applied to medical ultrasonography, underwater sonar, and ultrasonic nondestructive evaluation.

Rydberg atoms have great potential in electric field measurement and have an advantage with a large frequency bandwidth from the kHz to the THz scale. However, the sensitivity for measuring a weak MHz electric field signal is limited by the spectroscopic resolution, because the weak electric field induces only a small perturbation of the population and energy-level shift of the Rydberg atoms. Here, we report highly sensitive measurement of a weak MHz electric field using electromagnetically induced transparency with Rydberg atoms in a thermal atomic system. Using the heterodyne method on a 30-MHz electric field, we successfully measure the minimum electric field strength to be $37.3\mu \mathrm{V}/\mathrm{cm}$ with a sensitivity up to $-65$ dBm/Hz and a linear dynamic range over 65 dB. Additionally, we measure an amplitude-modulated signal and demodulate the signal with a fidelity over 98%. This work extends the sensitivity of atomic sensors for measuring MHz electric fields, which advances atomic electric field-sensing technology.

The continuous discovery of ultrahigh-modulus materials has increased the record for the Young’s modulus and the gravimetric Young’s modulus. However, the theoretical bounds on these moduli are still unknown. The upper bounds depend on the limits of the stiffness, alignment, and density of chemical bonds. From these limits, we here develop theoretical expressions for predicting the Young’s modulus, $Y_{\max }$ = [${\hslash }^2$/($m_e^2{a}_B^2$)]${\rho }_e$, and the gravimetric Young’s modulus, $Y_{\rho ,\max }=[{\hslash }^2$/($m_e{m}_p{a}_B^2$)]($N_e/A$), for ideal extreme-modulus solids, where ħ, $m_e$, $m_p$, $a_B$, ${\rho }_e$, and $N_e/A$ are the reduced Planck constant, electron mass, proton mass, Bohr radius, mass density of valence electrons, and the ratio of valence-electron number to atomic mass. By substituting the values of the nonconstant parameters (${\rho }_e$ and $N_e/A$) for all elements into these expressions, the upper bounds on the Young’s modulus and gravimetric Young’s modulus are predicted to be 3074 GPa and 1036 GPa g${}^{-1}$ cm${}^3$. These predictions are supported by the fact that the Young’s modulus and gravimetric Young’s modulus from a large set of experiments and first-principles calculations fall within these bounds. Moreover, by applying lateral pressure to linear carbyne crystals, the first-principles-calculated maximum Young’s modulus and gravimetric Young’s modulus are 2973 GPa and 968 GPa g${}^{-1}$ cm${}^3$, respectively, which are near the predicted bounds. These carbyne crystals are predicted to have space group R-3m.

Small-angle x-ray scattering tensor tomography provides three-dimensional information on the unresolved material anisotropic microarchitecture, which can be hundreds of times smaller than an image pixel. We develop a direct filtered back-projection method based on algebraic filters that enables rapid tensor-tomographic reconstructions and is a few orders of magnitude faster compared to established techniques, given the same computational resources. We demonstrate the accuracy of the method on experimental data for a fiber-reinforced material sample. The achieved acceleration may pave the way toward the investigation of multiple large samples as well as rapid control and feedback during in situ tensor-tomographic experiments, opening perspectives for the understanding of the fundamental link between functional material properties and microarchitecture.