Optomechanical crystal cavities (OMCCs) are fundamental nanostructures for a wide range of phenomena and applications. Usually, optomechanical interaction in such OMCCs is limited to a single optical mode and a unique mechanical mode. In this sense, eliminating the single-mode constraint—for instance, by adding more mechanical modes—should enable more complex physical phenomena, giving rise to a context of multimode optomechanical interaction. However, a general method to produce in a controlled way multiple mechanical modes with large coupling rates in OMCCs is still missing. In this work, we present a route to confine multiple GHz mechanical modes coupled to the same optical field with similar optomechanical coupling rates—up to 400 kHz—by OMCC engineering. In essence, we increase the number of unit cells (consisting of a silicon nanobrick perforated by circular holes with corrugations at both its sides) in the adiabatic transition between the cavity center and the mirror region. Remarkably, the mechanical modes in our cavities are located within a full phononic band gap, which is a key requirement to achieve ultrahigh mechanical $Q$ factors at cryogenic temperatures. The multimode behavior in a full phononic band gap and the easiness of realization using standard silicon nanotechnology make our OMCCs highly appealing for applications in the classical and quantum realms.

The evaluation of the specific power absorption of magnetic nanoparticles (MNPs) for magnetic hyperthermia (MH) applications has been performed through either local temperature probing or magnetic measurements so far. Each of these methods has advantages and drawbacks, and the concurrent use of both techniques offers the most reliable results. In this work, we propose an alternative strategy based on thermographic images to obtain two-dimensional temperature maps that allow the determination of the power absorption and other relevant thermodynamic parameters in MH experiments in a noninvasive way. This procedure and analysis are convenient to determine the heating performance of MNPs under the viscous conditions of in vitro and in vivo assays and to follow the time evolution of the temperature spatial distribution in the sample simultaneously. For this purpose, iron-oxide MNPs with 25-nm average diameter are coated with glucose and dispersed into different 8% polyacrylamide gels, which serve as phantoms that emulate intracellular viscosity. Power absorption experiments are performed under ac magnetic fields ($H=$ 32 kA/m; $f=$ 350 kHz) and the temperature evolution of the sample is monitored through a commercial thermographic camera (resolution, $240\times 180$ pixels; temperature accuracy, 2 K). To complement this simple setup, we design a program consisting of a detailed procedure for extracting graphical information from the video frames and obtaining spatiotemporal temperature profiles. The analysis of these profiles allows us to gather information on temperature, energy, power, and heat flux during the MH experiments. This method and analysis allows us to identify spatial inhomogeneities in samples, such as different local MNP density, which is extremely useful for the development of the therapy in vitro and the application in vivo where MNP aggregation is often present.

Nondiffracting light beams receive substantial attention in various areas due to their properties of diffractionless propagation and achievable subwavelength beam size. Extending the propagation distance and compressing the beam size of nondiffracting beams are crucial to their practical applications. However, it remains challenging to achieve nondiffracting light beams with a subwavelength subdiffracting transverse size and a long propagation distance with controllable intensity profiles in both transverse and longitudinal directions. Here, we propose an optimization method to generate an ultralong-superoscillation nondiffracting light beam with full control of the beam intensity profile. A 45λ-long-superoscillation nondiffracting light beam, the transverse sizes of which are within the range of 0.37λ–0.4λ along the propagation axis, is experimentally demonstrated by a metalens based on continuous phase modulation using geometrical phase metasurfaces. Our method provides an attractive way to controllably generate ultralong-superoscillation nondiffracting beams on a subwavelength scale and might find fascinating applications in optical manipulation, materials processing, data storage, microscopy, and spectroscopy.

To design fast memory devices, we need material combinations that can facilitate fast read and write operations. We present a heterostructure comprising a two-dimensional (2D) magnet and a 2D topological insulator (TI) as a viable option for designing fast memory devices. We theoretically model the spin-charge dynamics between 2D magnets and 2D TIs. Using the adiabatic approximation, we combine the nonequilibrium Green’s function method for spin-dependent electron transport and a time-quantified Monte Carlo method for simulating magnetization dynamics. We show that it is possible to switch a magnetic domain of a ferromagnet using the spin torque from spin-polarized edge states of a 2D TI. We show further that the switching of 2D magnets by TIs is strongly dependent on the interface exchange ($J_{\mathrm{int}}$), and an optimal interface exchange, is required for efficient switching. Finally, we compare experimentally grown Cr compounds and show that Cr compounds with higher anisotropy (such as $\mathrm{Cr}{\mathrm{I}}_3$) result in a lower switching speed but a more stable magnetic order.

The two-dimensional homojunction is promising in electronic and optoelectronic applications. However, its performance is restricted by lattice mismatch and the depletion region. Here, we construct a monolayer $\mathrm{In}\mathrm{Se}$-based p-i-n homojunction photodetector. It presents high photoelectric conversion with a wide light response in the photon-energy range from 1.4 to 5 eV, which induces a photocurrent density of 13.13 nA ${\mathrm{m}}^{-1}$ and a photoresponsivity of 0.022 A ${\mathrm{W}}^{-1}$. Moreover, the gate voltage improves the photocurrent and photoresponsivity almost fourfold. Additionally, the increasing dielectric constant of the substrate decreases the photocurrent spectral width, and a low doping concentration is suitable for photodetectors due to high mobility. These results indicate that the two-dimensional p-i-n homojunction is promising for future photoelectric devices.

We report the characteristic magnetic properties of several members of the family of rare-earth garnets, ${\mathrm{Gd}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ (GGG), ${\mathrm{Dy}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$, ${\mathrm{Tb}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$, and ${\mathrm{Nd}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$, and compare their relative potential utility for magnetocaloric cooling, including their minimal adiabatic demagnetization refrigeration (ADR) temperatures and relative cooling parameters. A main objective of this work concerns the identification of potential improvements over the magnetocaloric properties of GGG for use in low-temperature ADR cryostats. Using ${\mathrm{Tb}}^{+3}$ and ${\mathrm{Dy}}^{+3}$ at the rare-earth site offers, in principle, a higher saturation magnetization and ${\mathrm{Nd}}^{+3}$ gives a lower de Gennes factor and therefore potentially reduced magnetic transition temperatures, limiting the useful temperature range. Our results show that ${\mathrm{Dy}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ yields an optimal relative cooling parameter at low applied fields and low limiting temperatures, which would allow for the design of more efficient ADR cryostats.

We present a flexible method to study the optical properties of an ${\mathrm{Er}}^{3+}$ ensemble in $\mathrm{Si}$ accessed via resonant excitation and in situ single-photon detection. The technique allows an efficient resonant photoluminescence detection of optically active centers with weak oscillator strength in transparent crystals without the need for nanofabrication on the sample. We observe 70 ${\mathrm{Er}}^{3+}$ resonances in $\mathrm{Si}$, of which 62 resonances have not been observed in the literature, with optical lifetimes ranging from 0.5 to 1.5 ms. We observe inhomogeneous broadening of less than 400 MHz and an upper bound on the homogeneous linewidth of 0.75 and 1.4 MHz for two separate resonances. These narrow, stable resonances confirm ${\mathrm{Er}}^{3+}$ in $\mathrm{Si}$ as a promising quantum information candidate. We discuss the use of this technique for rapid characterization of future samples, with the aim of enhancing the prevalence of sites which are favorable for quantum information applications.

Quantum secure direct communication (QSDC) can directly transmit secrete messages through a quantum channel. Device-independent (DI) QSDC can guarantee the communication security relying only on the observation of the Bell-inequality violation, but not on any detailed description or trust of the inner workings of users’ devices. In the paper, we propose a DI-QSDC protocol with practical highly efficient single-photon sources. The communication parties construct the entanglement channel from single photons by adopting the heralded architecture, which makes the message-leakage rate independent of the photon-transmission loss. The secure communication distance and the practical communication efficiency of the current DI-QSDC protocol are about 6 times and 600 times of those in the original DI-QSDC protocol. Combining with the entanglement purification, the parties can construct the nearly perfect entanglement channel and completely eliminate the message leakage. This DI-QSDC protocol may have useful applications in the future quantum communication field.

Nanosecond electron pulses are appealing for ultrafast imaging and electron-gating applications, where tunable currents and narrow energy spreads are desirable. Here, we demonstrate photoemission from a Schottky emitter triggered by nanosecond laser pulses. Using photon energies optimally tuned to the emission potential barrier, we generate pulses containing over ${10}^5$ electrons with energy spreads below 1 eV by means of a prompt single-photon photoemission process. These results are consistent with a theoretical model of laser-triggered electron emission and energetic broadening during propagation and can be widely implemented.

The thermoelectric energy-conversion efficiency is significantly enhanced by introducing magnetism. Doping $\mathrm{Cr}$ atoms in a ${\mathrm{Cu}}_2{\mathrm{Sn}\mathrm{S}}_3(\mathrm{CTS})$ matrix modifies the electron density of state (e-DOS) and causes a special electron-transport mechanism by localized impurity spin moments. The localized spin moments cause a spin-spin exchange interaction with the spin of itinerant electrons of the CTS matrix. This is clearly verified in M-T measurement, which deviated from Curie’s law. The figure of merit (zT) of the magnetic nanocomposite was 8 times higher than that of pristine CTS by the synergetic effect of modifying e-DOS near the Fermi level and electron transport by localized spin moments.

In the design optimization of ion-cavity interfaces for quantum networking applications, difficulties occur due to the many competing figures of merit and highly interdependent design constraints, many of which present “soft limits,” which are amenable to improvement at the cost of engineering time. In this work, we present a systematic approach to this problem that offers a means to identify efficient and robust operating regimes and to elucidate the trade-offs involved in the design process, allowing engineering efforts to be focused on the most sensitive and critical parameters. We show that in many relevant cases it is possible to approximately separate the geometric aspects of the cooperativity from those associated with the atomic system and the mirror surfaces themselves, greatly simplifying the optimization procedure. Although our approach to optimization can be applied to most operating regimes, here we consider cavities suitable for typical ion-trapping experiments and with substantial transverse misalignment of the mirrors. We find that cavities with mirror misalignments of many micrometers can still offer very high photon extraction efficiencies, offering an appealing route to the scalable production of ion-cavity interfaces for large-scale quantum networks.

Several current searches for physics beyond the standard model are based on measuring the electron-capture (EC) decay of radionuclides implanted into cryogenic high-resolution sensors. The sensitivity of these experiments has already reached the level where systematic effects related to atomic state energy changes from the host material are a limiting factor. One example is a neutrino mass study based on the nuclear EC decay of ${}^7\mathrm{Be}$ to ${}^7\mathrm{Li}$ inside cryogenic $\mathrm{Ta}$-based sensors. To understand the material effects at the required level, we use density-functional theory to model the electronic structure of lithium atoms in different atomic environments of the polycrystalline $\mathrm{Ta}$ absorber film. The calculations reveal that the $\mathrm{Li}$ 1s binding energies can vary by more than 2 eV due to insertion at different lattice sites, at grain boundaries, in disordered $\mathrm{Ta}$, and in the vicinity of various impurities. However, the total range of $\mathrm{Li}$ 1s shifts does not exceed 4 eV, even for extreme amorphous disorder. Furthermore, when investigating the effects on the $\mathrm{Li}$ 2s levels, we find broadening of more than 5 eV due to hybridization with the $\mathrm{Ta}$ band structure. Material effects are shown to contribute significantly to peak broadening in $\mathrm{Ta}$-based sensors that are used to search for physics beyond the standard model in the EC decay of ${}^7\mathrm{Be}$, but they do not explain the full extent of observed broadening. Understanding these in-medium effects will be required for current- and future-generation experiments that observe low-energy radiation from the EC decay of implanted isotopes to evaluate potential limitations on the measurement sensitivity.

Phononic frequency combs represent the mechanical analog of optical frequency combs. Several independent experimental studies have demonstrated the onset and evolution frequency comb response in a variety of micro- and nanoelectromechanical devices in recent years. A theoretical basis for exploring and understanding the conditions for comb generation and evolution with varying driving parameters is essential to enable future practical applications. Here, we present the comparison between modeling and experimental results on the generation and evolution mechanism of phononic frequency combs in a nonlinear micromechanical resonator from the perspective of coherent energy transfer between two mechanical modes. Phononic frequency combs emerge in a strong coupling regime involving nonlinear resonances when the amplitudes and phases of two coupled mechanical modes are modulated via coherent energy transfer. The spacing and number of comb teeth can be analytically estimated based on modeling the nature of the interaction of the coupled modes under the specified driving conditions. As the driving conditions are varied, the phononic frequency comb evolves into different forms and a phenomenological model for the system is established to accurately predict the evolution of phononic frequency combs. The alignment between experiment and model provides a basis for the engineering of this phenomenon in future device applications.

Detecting a signal at an unknown frequency is a common task, arising in settings from dark-matter detection to magnetometry. For any detection protocol, the precision achieved depends on the frequency of the signal and can be quantified by the quantum Fisher information (QFI). To study limitations in broadband sensing, we introduce the integrated quantum Fisher information and derive inequality bounds that embody fundamental trade-offs in any sensing protocol. Our inequalities show that sensitivity in one frequency range must come at the cost of reduced sensitivity elsewhere. For many protocols, including those with small phase accumulation and those consisting of $\pi$ pulses, we find that the integrated quantum Fisher information scales linearly with $T$. We also find protocols with substantial phase accumulation that can have integrated QFI that grows quadratically with $T$ and prove that this scaling is asymptotically optimal. These protocols may allow the very rapid detection of a signal with unknown frequency over a very wide bandwidth. We discuss the implications of these results for a wide variety of contexts, including dark-matter searches and dynamical decoupling. Thus we establish fundamental limitations on the broadband detection of signals and highlight their consequences.

Oxides lacking a center of symmetry are highly desired, as they usually have fascinating physical properties. However, there are limited numbers of noncentrosymmetric media in nature and most of them do not have multiple functionalities integrated. Here, using the bulk photovoltaic effect as the probing technique, we demonstrate that, at interfaces with a wide range of oxides, strontium titanate (${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$) is polar, i.e., with broken inversion symmetry in the quantum paraelectric phase. Studies comprising conductivity and the bulk photovoltaic effect on ${\mathrm{La}\mathrm{Al}\mathrm{O}}_3/{\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$ thin films further show that excessive electronic band bending screens the induced polarity, revealing that appropriate band bending at the interface is the key parameter to control symmetry breaking. Inheriting high carrier mobility from ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$, the polar interface under illumination at low temperatures is conductive or metallic, permitting multifunctionality coupling between the oxides and ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$. Our studies show that significant photovoltaic effects can be generated at the polar interfaces, especially in the quantum-paraelectric phase of ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$, thus giving practical design strategies for multifunctional devices.

We present a continuous monitoring system for intermediate-scale quantum processors that allows extracting estimates of noisy native gate and read-out measurements based on the set of executed quantum circuits and resulting measurement outcomes. In contrast to standard approaches for calibration and benchmarking quantum processors, the executed circuits, which are input to the monitoring system, are assumed to be out of any control. We provide the results of applying our system to the synthetically generated data obtained from a quantum emulator, as well as to the experimental data collected from a publicly accessible cloud-based quantum processor. In both cases, we demonstrate that the developed approach provides valuable results about inherent noises of emulators and processors. Considering that our approach uses only already accessible data from implemented circuits without the need to run additional algorithms, the monitoring system can complement existing approaches. We expect that our monitoring system can become a useful tool for various quantum computers in the near-term horizon, including publicly accessible cloud-based platforms, and reduce resources that are required for their benchmarking and calibration.

Large anomalous Nernst effect (ANE) in magnetic topological materials (MTMs) has attracted various attentions because of its potential thermoelectric applications. Recently, MTMs with the kagome lattice have become a focus of research to investigate enhanced ANE. In this work, the ANE ($S_{yx}^A$) at room temperature in a kagome ferromagnet ${\mathrm{Fe}}_3{\mathrm{Sn}}_2$ with massive Dirac bands is greatly enhanced to $5.44\mu {\mathrm{VK}}^{-1}$ when its Fermi level is gradually tuned to Dirac nodal points. Then the intrinsic contributions to the enhanced $S_{yx}^A$ is further investigated. The anomalous transverse Peltier conductivity ${\alpha }_{yx}^A$ shows the linear temperature-dependent behavior, which is consistent with Berry-curvature-contributed ${\alpha }_{yx}^A$, and this behavior is different from temperature-dependent variation of magnetization. Combined with theoretic calculations, the ${\alpha }_{yx}^A$ and $|{\alpha }_{yx}^A/{\sigma }_{xy}^A|$ ratio increases significantly when approaching the massive Dirac cone, indicating that the massive Dirac dispersion and intrinsic contributions play a role in enhanced ANE instead of the side-jump mechanism. Our work demonstrates an effective universal approach to enhance ANE in such MTMs.

Electric field sensors based on Rydberg atoms offer unique capabilities, relative to traditional sensors, for detecting radio-frequency (rf) signals. In this work, we demonstrate simultaneous demodulation and detection of five rf tones spanning nearly two decades (six octaves), from 1.7 GHz to 116 GHz. We show continuous recovery of the phase and amplitude of each tone and report on the sensitivity of the system and its bandwidth capabilities for multiband detection. We use these capabilities to demonstrate a digital communication protocol, simultaneously receiving on-off-keyed binary data from four bands spanning nearly one decade of frequency.

The existence of multimodes is a unique characteristic of various elastic wave systems, including elastic metasurfaces. Nevertheless, most of the previous research on elastic metasurfaces has focused on a single mode only, so the related physics is largely unknown and noise is generated by the undesired incident wave mode. Here, we present a mode-selective elastic metasurface that can tailor the target wave mode while filtering out the undesired wave mode. To this end, analytical investigation is carried out for an elastic metasurface with multimode incidence such that the metasurface can operate for both longitudinal and shear wave modes. After that, the mode-selective elastic metasurface is designed and validated numerically and experimentally. With our research, it is now possible to explicitly design an elastic metasurface for the multimode incidence case. Furthermore, since the noise caused by the undesired wave mode is barely generated with the proposed mode-selective metasurface, various elastic wave devices and physical findings are expected from the current research.