Two-level system (TLS) defects constitute a major decoherence source of quantum information science, but they are generally less understood at material interfaces than in deposited films. Here we study surface TLSs at the metal-air interface, by probing them using a quasiuniform field within vacuum-gap (VG) capacitors of resonators. The VG capacitor has a nanogap that creates an order-of-magnitude larger contribution from the metal-air interface than typical resonators used in circuit QED. We measure three phenomena and find qualitative agreement with an interacting TLS model, where near-resonant TLSs experience substantial frequency jitter from the state switching of far-detuned low-frequency TLSs. First, we find that the loss in all of our VG resonators is weakly or logarithmically power dependent, in contrast to data from deposited dielectric films. Second, we add a saturation tone with power $P_{\mathrm{in}}$ to a transmission measurement and obtain the TLS Rabi frequency ${\mathrm{\Omega }}_0$. These data show a substantially weaker $P_{\mathrm{in}}$ dependence of ${\mathrm{\Omega }}_0$ than the prediction from the standard noninteracting TLS model. Lastly, we increase the temperature and find an increased TLS jitter rate and dephasing rate from power-dependent loss and phase noise measurements, respectively. We also anneal samples, which lowers the low-frequency TLS density and jitter rate, but the single-photon loss is found to be unchanged. The results are qualitatively consistent with a fast-switching interacting TLS model and they contrast the standard model of TLSs that describes TLSs independently.

Unidirectional magnetoresistance (UMR) has garnered extensive attention for its rich physics and potential applications. The prevailing belief is that indispensable ferromagnetic films serve as scattering sources of polarized electrons or that noncentrosymmetric systems cause spin band splitting, driving most research toward ferromagnet/normal-metal bilayer films or nonmagnetic Rashba systems. However, our observations reveal a significant UMR in bilayer films consisting solely of the oxidized light metal $\mathrm{Al}$/$\mathrm{Cu}$. Remarkably, the UMR signal of 0.073% is approximately one order of magnitude larger than that of most structures. Such a UMR is attributed to the dual functionality of copper oxide, which not only generates polarized electrons—a recognized function—but also scatters these electrons in a weak magnetization manner. Our findings provide a fresh strategy to generate the UMR, facilitating its application through the use of more readily available materials.

The goal of ultrathin lightweight photophoretic flyers, or light flyers for short, is to levitate continuously in Earth’s upper atmosphere using only sunlight for propulsive power. We previously reported light flyers that levitated by utilizing differences in thermal accommodation coefficient between the top and bottom of a thin film, made possible by coating their lower surfaces with carbon nanotubes (CNTs). Such designs, though successful, had relatively high thermal emissivity (>0.5), which prevented them from achieving high temperatures and resulted in their transferring relatively low amounts of momentum to the surrounding gas. To address this issue, we have developed light flyers with ultrathin undoped germanium layers that selectively absorb nearly 80% of visible light but are mostly transparent in the thermal infrared, with an average thermal emissivity of <0.1. Our experiments show that germanium-coated light flyers could levitate at up to 43% lower light irradiances than mylar-CNT disks with identical sizes. In addition, we simulated our experiments using a semiempirical model, which allowed us to predict that our 2-cm-diameter disk-shaped germanium-coated light flyers can levitate in the mesosphere (altitudes 67–75 km) under the natural sunlight (1.36 kW/${\mathrm{m}}^2$). Similar ultrathin selective-absorber coatings can also be applied to three-dimensional light flyers shaped like solar balloons, allowing them to carry significant payloads and thereby revolutionize long-term atmospheric exploration of Earth or Mars.

Cat state, is not only interesting for testing the fundamentals of quantum mechanics, but also for wide applications ranging from fault-tolerant quantum computation to quantum metrology. Here, we propose a scheme to generate the magnon cat state by the indirect magnon nonlinearity. We show that the effective two-magnon loss process can be induced by the photon parametric coupling, steering the magnon into the cat-state manifold. In our work, the size of the magnon cat state can be easily adjusted, and the large-size cat state can be generated with high fidelity. At the same time, the speed of cat-state generation can be greatly improved by enhancing the photon-magnon interaction with maintaining a larger size of cat state. Further, we find that the effective two-magnon loss process can strongly suppress the decoherence of the environment under appropriate parameter conditions. The lifetime of the magnon cat state can be expected to be $t\sim 3\text{μ}\mathrm{s}$ under the current experimental techniques. This work provides a proposal for the preparation of nonclassical states of magnons without the direct magnon nonlinearity, which is meaningful for the development of quantum technology in the future.

Optical clocks have been demonstrated to be good choices to redefine the SI “second.” With this goal, we have constructed a quantum-logic-based ${}^{27}$$\mathrm{Al}$${}^{+}$ ion optical clock with a fractional frequency uncertainty of $1.6\times {10}^{-18}$. A ${}^{25}$$\mathrm{Mg}$${}^{+}$ ion is used as the logic ion for sympathetic cooling, state readout, and frequency-shift measurement of the ${}^{27}$$\mathrm{Al}$${}^{+}$ ion. The stability of the optical clock is $2.6\times {10}^{-15}/\sqrt{\tau }$, measured with a self-comparison method.

Fast measurements of quantum devices is useful in areas such as quantum sensing, quantum computing, and nanodevice quality analysis. Here, we develop a superconductor-semiconductor multimodule microwave assembly to demonstrate charge-state readout at the state of the art. The assembly consist of a superconducting readout resonator interfaced to a silicon-on-insulator (SOI) chiplet containing quantum dots (QDs) in a high-$\kappa$ nanowire transistor. The superconducting chiplet contains resonant and coupling elements as well as $LC$ filters that, when interfaced with the silicon chip, result in a resonant frequency $f=2.12$ GHz, a loaded quality factor $Q=850$, and a resonator impedance $Z=470\mathrm{\Omega }$. Combined with the large gate lever arms of SOI technology, we achieve a minimum integration time for single and double QD transitions of 2.77 and 13.5 ns, respectively. We utilize the assembly to measure charge noise over 9 decades of frequency up to 500 kHz and find a $1/f$ dependence across the whole frequency spectrum as well as a charge-noise level of $4\text{μ}\mathrm{eV}/\sqrt{\text{Hz}}$ at 1 Hz. The modular microwave circuitry presented here can be directly utilized in conjunction with other quantum devices to improve the readout performance as well as enable large bandwidth noise spectroscopy, all without the complexity of superconductor-semiconductor monolithic fabrication.

High-performance self-powered photodetectors are in high demand for many applications in wireless sensor networks, Internet-of-things devices, wearable electronics, optical communication, and biomedical sensing. The self-powered photodetector design requires a type-II staggered heterojunction between two semiconductors. Hence, conventional heterojunction-free self-powered photodetector designs remain a challenging task. Here, we report a self-powered photodetector of significantly high performance using an all-inorganic ${\mathrm{Cs}\mathrm{Pb}\mathrm{Br}}_3$ monocrystal grown in situ directly over the substrate with prepatterned interdigitated gold ($\mathrm{Au}$) electrodes. Interestingly, we achieved an extremely low dark current of about 1 fA at room temperature, a dark-to-light current ratio of ${10}^7$, a responsivity of 2 A/W, and a specific detectivity of ${10}^{14}$ Jones in self-powered mode. The self-powered nature of the $\mathrm{Au}/{\mathrm{Cs}\mathrm{Pb}\mathrm{Br}}_3/\mathrm{Au}$ photodetector with asymmetric contact geometry is investigated theoretically using a back-to-back Schottky diode model and experimentally using the Kelvin-probe force microscopy technique. Both approaches converged to predict the existence of a sufficient contact potential difference between the metal contacts due to having an interfacial area difference, which remains the source of built-in potential in the device to dissociate photogenerated excitons effectively for self-powered operation. Our innovative approach in this study opens up an inventive direction for developing a cost-effective heterojunction-free self-powered photodetector.

The creation of fluorescent defects in silicon is a key stepping stone toward assuring the integration perspectives of quantum photonic devices into existing technologies. Here, we demonstrate the creation, by femtosecond laser annealing, of W and G centers in commercial silicon on insulator (SOI) previously implanted with ${}^{12}$$\mathrm{C}$${}^{+}$ ions. Their quality is comparable to that found for the same emitters obtained with conventional implantation processes; as quantified by the photoluminescence radiative lifetime, the broadening of their zero-phonon line (ZPL), and the evolution of these quantities with temperature. In addition to this, we show that both defects can be created without carbon implantation and that we can erase the G centers by annealing while enhancing the W-center emission. These demonstrations are relevant to the deterministic and operando generation of quantum emitters in silicon.

The search for material combinations that exhibit highly efficient current-induced spin-orbit torque (SOT) is crucial for the development of next-generation nonvolatile spintronics devices. Existing research results suggest that the spin Hall effect (SHE) in heavy metals with strong spin-orbit coupling or the interfacial Rashba effect at the heavy-metal–ferromagnet interface are the primary sources of spin currents. Here, we present experimental evidence that ferrimagnetic $\mathrm{Tb}$-$\mathrm{Co}$ multilayers, engineered with an artificially designed composition gradient, exhibit a bulk Rashba effect originating from gradient-related spatial inversion symmetry breaking, which provides sizable spin currents for magnetization switching. The two types of spin currents, originating separately from the SHE and the bulk Rashba effect, would cooperate or compete with each other, depending highly on the gradient direction, ultimately resulting in a weakening or enhancement of SOT efficiency. Our findings regarding the gradient-related manipulation of current-induced SOT provide valuable insights for the development of highly efficient spintronics devices based on ferrimagnetic materials.

In this work, we develop a supervised learning model for implementing robust quantum control in composite-pulse systems, where the training parameters can be either phases, detunings, or Rabi frequencies. This model exhibits great resistance to all kinds of systematic errors, including single, multiple, and time-varying errors. We propose a modified gradient descent algorithm for adapting the training of phase parameters, and show that different sampling methods result in different robust performances. In particular, there is a trade-off between high fidelity and robustness for a given number of training parameters, and both can be simultaneously enhanced by increasing the number of training parameters (pulses). For its applications, we demonstrate that the current model can be used for achieving high-fidelity arbitrary superposition states and universal quantum gates in a robust manner. This work provides a highly efficient learning model for fault-tolerant quantum computation by training various physical parameters.

Recent studies have unveiled that the electronically insulating ${\mathrm{Cd}\mathrm{PS}}_3$ can serve as a versatile solid ion conductor, exhibiting exceptionally high conductivity at room temperature towards various cations. $\mathrm{Cd}$ vacancies are believed to play a crucial role in achieving high ionic conductivity, but their specific assisting mechanisms remain unknown. Here, we investigated the synergistic mechanism of $\mathrm{Cd}$-vacancy and $\mathrm{Li}$-ion intercalation using first-principles calculations based on density-functional theory. We found that under a $\mathrm{Cd}\text{−}$ or $\mathrm{P}$-poor chemical potential, as the Fermi energy approaches the conduction band of ${\mathrm{Cd}\mathrm{PS}}_3$, the formation of $\mathrm{Cd}$ vacancies with the −1 charge state $V_{\text{Cd}}^{-}$ is energetically favored. The reason for achieving high electrical conductivity is the introduction of a high density of $\mathrm{Li}$ ions into $\mathrm{Cd}$ vacancies. Keeping the concentration (x) of $\mathrm{Cd}$ vacancies below 0.5 in ${\mathrm{Cd}}_{1-x}{\mathrm{PS}}_3{\mathrm{Li}}_{2x}$ is crucial to ensure high $\mathrm{Li}$-ion conductivity, as concentrations above this threshold lead to a loss of long-range thermal stability and increased energy barriers that impede the diffusion of $\mathrm{Li}$ ions. In particular, the $\mathrm{Li}$-ion diffusion barrier in ${\mathrm{Cd}}_{0.833}{\mathrm{PS}}_3{\mathrm{Li}}_{0.333}$ is determined to be 0.2 eV, which is highly consistent with the experimental value of 0.21 ± 0.022 eV. Further analysis revealed that $\mathrm{Li}$ ions on the surface show a preference for moving along the $\mathrm{Cd}$ or $\mathrm{S}$ atoms rather than the $\mathrm{P}$ atoms. Our research provides insights into the role of vacancies in facilitating cation diffusion, establishing a theoretical framework for the design of high-ionic-conductivity materials.

Quantum entanglement is a fundamental physical resource that enables a range of information-processing tasks with no efficient solution on classical computers. In this paper, we construct a class of general maximum entangled states by parameterizing the coefficients of Bell states. By calculating, we obtain the local unitary operations required to realize the mutual transformation of the constructed states, analogous to Pauli operators acting on one particle of a Bell state. Interestingly, the local unitary operations on the two particles of Bell states are identical, whereas for the constructed states, they must differ in certain cases. To explore the potential applications of these maximum entangled states and the obtained local unitary operations, we design a class of measurement-device-independent quantum secure direct communication protocols based on entanglement swapping. In the class of designed protocols, entanglement swapping is realized between a constructed maximum entangled state and a Bell state. We demonstrate that the class of designed protocols have no information leakage and can resist intercept-and-resend attacks, measure-resend attacks, and collective attacks. In addition, the unconditional security of the class of designed protocols is proved. We believe that the constructed maximum entangled states and the obtained local unitary operations may find applications in other quantum information-processing fields.

Terahertz (THz) spin waves or their quanta, magnons, can be efficiently excited by acoustic phonons because these excitations have similar wave vectors in the THz regime. THz acoustic phonons can be produced using photoacoustic phenomena but typically have a low population, and thus, a relatively low displacement amplitude. The magnetization amplitude and population of the acoustically excited THz magnons are thus usually small. Using analytical calculations and dynamical phase-field simulations, we show that a freestanding metal/magnetic insulator (MI)/dielectric multilayer can be designed to produce large-amplitude THz spin waves via cavity-enhanced magnon-phonon interactions. The amplitude of the acoustically excited THz spin wave in the freestanding multilayer is predicted to be more than 10 times larger than in a substrate-supported multilayer. Acoustically excited nonlinear magnon-magnon interactions are demonstrated in the freestanding multilayer. The simulations also indicate that the magnon modes can be detected by probing the charge current in the metal layer generated via spin-charge conversion across the MI/metal interface and the resulting THz radiation. Applications of the freestanding multilayer in THz optoelectronic transduction are computationally demonstrated.

The research experimentally demonstrates the direct electric current control of hyperchaotic packets of dissipative dark envelope solitons that are self-generated in a microwave active ring resonator containing two nonlinear elements: a one-dimensional magnonic crystal (MC) with dynamic line defect and a transistor amplifier. The hyperchaotic packets of dissipative dark envelope solitons are formed at a magnetostatic surface spin wave (MSSW) propagating in the MC and taking part both in three-wave nonlinear spin-wave processes of decay and confluence. The transistor amplifier operates in output-power saturation mode. The direct electric current flowing through a copper wire placed along the longitudinal axis of the MC creates the dynamic line defect that effectively controls a duty factor of dissipative dark envelope soliton packets and their hyperchaotic properties at the MC band gap only. The hyperchaotic nature of dissipative soliton packets is confirmed by an estimation of the two highest Lyapunov exponents from the experimental time series. The modified Vyshkind-Rabinovich model demonstrates the self-generation of a dark pulse train at a decaying wave (MSSW) and a bright pulse train at the parametrically excited waves (the exchange spin waves) due to two saturation mechanisms. One of them is connected with the parametric instability, whereas the other one is caused by the increment nonlinearity. The developed self-generator may be of great interest to reservoir-computing that is based on the concept of “computation at the edge of chaos.”

Incorporating time-varying elements into electromagnetic systems has been shown to be a powerful approach to challenge well-established performance limits, for example, bounds on absorption and impedance matching. So far, most of these studies have concentrated on time-switched systems, where the material undergoes instantaneous modulation in time while the input field is entirely contained within it. This approach, however, necessitates accurate timing of the switching event and limits how thin the system can ultimately be because of the spatial width of the impinging pulse. To address these challenges, here we investigate the periodic temporal modulation of highly lossy materials, focusing on their relatively unexplored parametric absorption aspects. Our results reveal that, by appropriate selection of the modulation parameters, the absorption performance of a periodically modulated absorber can be greatly improved compared with that of its time-invariant counterpart, and can even exceed the theoretical bound for conventional electromagnetic absorbers, namely, the “Rozanov bound.” Our findings thus demonstrate the potential of periodic temporal modulations to enable significant improvements in absorber performance while circumventing the limitations imposed by precise timing and material thickness in time-switched schemes, opening up new opportunities for the design and optimization of advanced electromagnetic absorber systems for various applications.

Metasurfaces provide unique capability in guiding surface waves and controlling their polarization and dispersion properties. One way to study their guiding properties is by analyzing their equifrequency contours. Equifrequency contours are the 2D projections of the 3D dispersion diagram. Since they are $\mathbf{k}$-space-map representations of the surface, many of the wave properties can be understood from the equifrequency contours. In this paper, we investigate numerically and experimentally the engineering of equifrequency contours using a C-shaped metasurface design. We show the ability to provide high self-collimation as well as spin-dependent wave splitting for the same metasurface by tuning the frequency of operation. We also show the ability to steer the wave along a defined curved path by rotating the C-shaped unit cells, which results in rotation of its equifrequency contours. This work demonstrates how engineering equifrequency contours can be used as a powerful tool for controlling the surface wave propagation properties.

Edge-dependent states in electronic and synthetic graphenes have stimulated extensive research interests in recent years. The orbital degree of freedom (ODOF), facilitating the understanding of exotic phenomena in condensed matter, supplies a powerful base to explore boundary topological properties, which has not yet been investigated extensively in classical-wave realms such as acoustic systems. Here, we experimentally demonstrate orbital-dependent acoustic twig edge states in multi-ODOF systems as degenerate-orthogonal p orbitals are introduced. We analyze and discuss the band structures of orbital strip supercell lattices with the twig and zigzag edges by using the tight-binding method (TBM) and full-wave simulation (FWS). We visualize fourfold zero-energy orbital twig edge states that form the complete flat band across the Brillouin zone, characterized by nontrivial topological windings. Moreover, we unravel the selective excitation of the orbital twig edge states in experiments. Our findings advance the field for understanding the interplay between graphene edge states and acoustic orbitals as well as broaden the potential application opportunities of Dirac materials such as trapping and sensing.

Recent advances in nonlocal metasurfaces have enabled unprecedented success in shaping the wave front of light with spectral selectivity, offering alternative solutions for many emerging nanophotonics applications. The ability to tune both the spectral and spatial properties of such a class of metasurfaces is highly desirable, but the dynamic nonvolatile control remains elusive. Here, we demonstrate active narrowband wave-front manipulation by harnessing quasi-bound states in the continuum (quasi-BICs) in phase-change nonlocal metasurfaces. The proof-of-principle metasurfaces made of ${\mathrm{Sb}}_2{\mathrm{S}}_3$ allow for nonvolatile, reversible, and tunable spectral control over wave front and switchable spatial response at a given wavelength in the near-infrared regime. The design principle mainly builds upon the combination of the geometry phase of quasi-BICs and the dynamic tunability of phase-change meta-atoms to tailor the spatial response of light at tunable resonant wavelengths. By tuning the crystallization level of ${\mathrm{Sb}}_2{\mathrm{S}}_3$ meta-atoms through controlling the external stimuli, the dynamic nonlocal wave-front-shaping functionalities of beam steering, one-dimensional, and two-dimensional focusing, and holographic imaging are achieved exclusively at resonant wavelengths, with functionally transparent off resonance. This work represents a critical advance towards developing an integrated dynamic nonlocal metasurface for future augmented and virtual reality wearables.