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.

Shortcut to adiabatic passage (STAP), initially proposed to accelerate adiabatic quantum-state transfer, has been widely explored and applied in quantum optics and integrated optics. Here we bring STAP into the field of acoustics to design compact couplers and functional metamaterial. The space-varying coupling strengths of acoustic waveguides (WGs) are tailored by means of dressed states in a three-level system, accounting for the desirable acoustic energy transfer among three WGs with short length. We show that the acoustic coupler has a one-way feature when loss is introduced into the intermediate WG. More uniquely, when the propagation of acoustic wave is designed to mimic a Hadamard transformation, an acoustic metamaterial can be constructed by arraying several couplers, possessing the beam-splitting function and unidirectional, broadband performances. Our work bridges STAP and the acoustic coupler as well as metamaterial, which may have profound impacts on exploring quantum technologies for promoting advanced acoustic devices.

We theoretically study the behavior of the critical current of a thermally biased tunnel Josephson junction with a particular design, in which the electrodes of the junction are enclosed in two different superconducting loops pierced by independent magnetic fluxes. In this setup, the superconducting gaps can be modified independently through the magnetic fluxes threading the loops. We investigate the response of the device as a function of the magnetic fluxes, by changing the asymmetry parameter, i.e., the ratio between the zero-temperature superconducting gaps $\delta ={\mathrm{\Delta }}_{10}/{\mathrm{\Delta }}_{20}$ and the temperatures of the two rings. We show a magnetically controllable steplike response of the critical current, which emerges even in a symmetric junction, $\delta =1$. Finally, we discuss the optimal working conditions and the high response of the critical current to small changes in the magnetic flux, reporting good performance of a magnetic flux-to-critical current transducer, with a high transfer function that depends on the operating point, the temperature gradient, and the quality of the junction.

Atomic spin sensors offer precision measurements using compact microfabricated packages, placing them in a competitive position for both market and research applications. The performance of these sensors, such as the dynamic range, may be enhanced through magnetic field control. In this work, we discuss the design of miniature coils for three-dimensional localized field control by direct placement around the sensor, as a flexible and compact alternative to global approaches used previously. Coils are designed on biplanar surfaces using a stream-function approach and then fabricated using standard printed-circuit techniques. Application to a laboratory-scale optically pumped magnetometer of sensitivity approximately $20\mathrm{fT}/\sqrt{\mathrm{Hz}}$ is shown. We also demonstrate the performance of a coil set measuring $7\times 17\times 17{\mathrm{mm}}^3$ that is optimized specifically for magnetoencephalography, where multiple sensors are operated in close proximity to one another. Characterization of the field profile using ${}^{87}$$\mathrm{Rb}$ free-induction spectroscopy and other techniques show $>96\mathrm{\%}$ field homogeneity over the target volume of a MEMS vapor cell and a compact stray-field contour of approximately $1\mathrm{\%}$ at 20 mm from the center of the cell.

Phononic crystals (PnC) are spatially periodic materials with band gaps that form by Bragg scattering of elastic waves. Within the frequency range of a band gap, wave propagation is not admitted. A long-standing limitation of this class of materials is that the wavelength for band-gap formation must be on the order of the unit-cell size. This restricts the presence of band gaps to relatively high frequencies for a given lattice spacing. Locally resonant metamaterials, on the other hand, enable the opening of low-frequency, subwavelength band gaps through resonance hybridization. However, their band gaps are characteristically narrow and require large or massive local resonators to form. Here, we break both limitations using beam-based PnCs by (1) locking the rotation degree of freedom at the edges of the primitive unit cell, and (2) coiling the PnC by applying full beam-axis rotations at the locked locations. These respective kinematic and geometric transformations convert a conventional beam PnC from its extended form with a nominal lattice constant to an extremely compact coiled configuration with a greatly reduced lattice constant. With the periodic rotational locking, the band gaps remain intact and are still large, and in fact increase noticeably in size. With the subsequent coiling, the band gaps remain based on Bragg scattering and are quantitatively conserved except now appearing at lower frequencies as dictated by the ratio of the extended-to-coiled lattice constants. This ratio defines a coiling factor, which is a measure of the reduction in the PnC unit-cell length in the direction of wave transmission while maintaining the band structure of its original extended form except for the favorable changes induced by the periodic rotational locking. A coiling factor of $\beta$ lowers, by construction, the location of the normalized central frequency of any given band gap by a factor of $\beta$. The only limitation is the need for lateral space to accommodate the coiling of the beam segments. The vibration behavior of a finite version of the coiled structure is experimentally tested demonstrating a matching band-gap response, despite the reduction in length, to that obtained by finite-element analysis of the extended rotationally locked version. This concept creates effectively subwavelength Bragg band gaps. It clears the path for PnCs to serve in applications that are orders-of-magnitude smaller in scale than are currently possible, while featuring band gaps that are significantly larger than those generated by locally resonant metamaterials.

Elastic waves traveling through polycrystalline ice Ih can experience distortion due to the transversely isotropic structure of its crystals. This can pose a challenge to nondestructive ultrasonic testing when ice is used as a coupling medium to transfer energy between an ultrasonic source and the solid to be inspected for the presence of damage. Here, it is shown that when ice is grown on a metallic solid using directional freezing techniques, it develops a coarse columnar grain structure, which causes the ultrasonic signals to vary greatly depending on which region of the ice volume is interrogated. On the other hand, if small ice particles are compacted around the solid and saturated with degassed liquid water before directional freezing, an equiaxed grain structure is obtained, which behaves as a homogeneous and isotropic medium. Furthermore, it is demonstrated that ultrasonic transmission through an ice-metal interface can be achieved regardless of the angle formed between the ultrasonic beam and the interface. Therefore, it is possible to overcome one of the fundamental limitations of conventional water coupling, which prevents transmission when the beam is not close to orthogonal to the interface due to the total reflection phenomenon. This property is useful to expand the scope of the application of ultrasonic testing to solids with complex geometries such as those obtained with additive manufacturing methods.

The developing frequency-comb spectral technologies have potential applications and prospects in the wide fields of cosmology, meteorology, and microwave measurement. Here, we demonstrate a Rydberg microwave-frequency-comb spectrometer via multiple-microwave field dressing, providing precise microwave measurement. The Rydberg microwave-frequency-comb spectrum provides a real-time and absolute-frequency measurement with a range of 125 MHz and gives the relative phase of a single-frequency microwave signal. The reported experiment helps real-time sensing of an unknown microwave signal over a wide range using Rydberg atoms, which are vital for Rydberg-atom-based microwave metrology.