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.

Continuous-variable measurement-device-independent quantum key distribution (CV-MDI QKD) can fundamentally defend all attacks on the measurement devices. Consequently, higher requirements are put forward for the source of a CV-MDI QKD system. However, the imperfections of the actual source brings practical security risks to the CV-MDI QKD system. Therefore, the characteristics of the realistic source must be controlled in real time to guarantee the practical security of the CV-MDI QKD system. Here we propose a countermeasure for negative impact introduced by the actual source in the CV-MDI QKD system based on the one-time-calibration method, not only eliminating the loophole induced from the relative intensity noise (RIN), which is part of the source noise, but also modeling the source noise thus improves the performance. In particular, three cases in terms of whether the preparation noise of the practical sources are defined or not, where only one of the users or both of the two users operate monitoring on their respective source outputs, are investigated. The simulation results show that the estimated secret key rate without our proposed scheme is about 10.7 times higher than the realistic rate at 18 km transmission distance when the variance of RIN is only 0.4. What is worse, the difference becomes greater and greater with the increase of the variance of RIN. Thus, our proposed scheme makes sense in further completing the practical security of the CV-MDI QKD system. In other words, our work enables the CV-MDI QKD system not only to resist all attacks against detectors, but also to close the vulnerability caused by the actual source, thus making the scheme closer to practical security.

We investigate near-resonant ac-Stark shifts for optical atomic clocks, which can also be interpreted as a special class of line-pulling effects due to the Zeeman structure of atomic levels split in a dc magnetic field. This shift can arise due to residual ellipticity in the polarization of the probe field and uncertainty in the magnetic field orientation. Such a shift can have an arbitrary sign and, for some experimental conditions, can reach a fractional value of the order of ${10}^{-18}$–${10}^{-19}$, i.e., it is not negligible. Thus, it should be taken into account in the uncertainty budgets for modern ultraprecise atomic clocks. In addition, it is shown that when using hyper-Ramsey spectroscopy, this shift can be reduced to a level much lower than ${10}^{-19}$.

Small-size $\mathrm{Pb}\mathrm{S}$ colloidal quantum dots (CQDs) have achieved excellent performance in photoelectric conversion devices through the ligand-exchange method of mixed lead-halide passivation. However, with the increase of $\mathrm{Pb}\mathrm{S}$ CQD diameter, the proportion of (100) facets on the CQD surface increases and the original proportion of mixed lead-halide ligand cannot passivate (100) facets completely, which will introduce deep defects and deteriorate device performance. Here, we demonstrate an excessive ${\mathrm{Pb}\mathrm{Br}}_2$ concentration ligand strategy to sufficiently passivate large-size $\mathrm{Pb}\mathrm{S}$ CQDs with an absorption peak at 1300 nm. The first-principles calculation results suggest that ${\mathrm{Br}}^{-}$ can passivate (100) facets more efficiently compared with ${\mathrm{I}}^{-}$. With the increase of ${\mathrm{Pb}\mathrm{Br}}_2$ concentration (0–0.464 mmol/mL), both optical and electrical measurements imply that defects are effectively passivated, while carrier lifetime increases and dark-current density decreases. Finally, a device with specific detectivity of $5.22\times {10}^{12}$ Jones is obtained. This passivation strategy can also be used in other large-size $\mathrm{Pb}\mathrm{S}$ CQDs (diameter >4 nm) to realize a better device performance.

Two-dimensional (2D) materials provide a promising platform for exploring spintronic devices. However, the low Curie temperature of most 2D magnetic materials limits their development and application. Based on density-functional theory and the nonequilibrium-Green’s-function formalism, we present a systematic simulation study of room-temperature strained ${\mathrm{VSi}}_2{\mathrm{N}}_4$-based magnetic tunnel junctions (MTJs), using a high-accuracy Heyd-Scuseria-Ernzerhof (HSE) functional. In contrast to Perdew-Burke-Ernzerhof results, a spin-conductance match is observed in $\mathrm{Ag}/{\mathrm{VSi}}_2{\mathrm{N}}_4$-${\mathrm{VSi}}_2{\mathrm{N}}_4/\mathrm{Ag}$ MTJs at the HSE06 level. Thus, a 1200% tunnel-magnetoresistance (TMR) ratio and a perfect spin-injection efficiency are theoretically predicted in $\mathrm{Ag}/{\mathrm{VSi}}_2{\mathrm{N}}_4$-${\mathrm{VSi}}_2{\mathrm{N}}_4/\mathrm{Ag}$ MTJs. Interestingly, the coexistence of a Weyl semimetal and a half-metal state is found in 1.965% tensile-biaxial-strained monolayer ${\mathrm{VSi}}_2{\mathrm{N}}_4$. In addition, a slight compressive strain leads to a boost of the TMR ratio by 2 orders of magnitude, up to ${10}^5\mathrm{\%}$ in strained ${\mathrm{VSi}}_2{\mathrm{N}}_4$-based MTJs. Other MTJs based on ${\mathrm{VSi}}_2{\mathrm{P}}_4$, ${\mathrm{VSi}}_2{\mathrm{As}}_4$, and ${\mathrm{Nb}\mathrm{Si}}_2{\mathrm{N}}_4$ are investigated. Our results show that these spin-polarized magnetic silicide compounds, with Curie temperatures close to room temperature, are promising candidates for next-generation spintronic devices.

Nanoporous materials have attracted great interest because of their variety of applications in nanodevices, such as gas storage, low-density magnetic storage, energy storage, supercapacitors, catalysis, membranes, etc. The most common purpose of using nanoporous materials is to make a material much lighter while preserving or improving the high structural stability of these compounds. In this work, we propose a two-dimensional dodecagonal zinc oxide ($d$-$\mathrm{Zn}\mathrm{O}$) monolayer via first-principles calculations based on density-functional theory (DFT). Our extensive analysis shows that this semiconducting porous $d$-$\mathrm{Zn}\mathrm{O}$ material is mechanically, dynamically, and thermally stable and suitable for various applications, such as water membrane and gas detection at room temperature and above. We study the water permeability and ${\mathrm{Na}}^{+}$ and ${\mathrm{Cl}}^{-}$ ions’ rejection of $d$-$\mathrm{Zn}\mathrm{O}$ material via conducting DFT and molecular dynamics (MD) simulations. Our simulations show that the energy barrier of the water molecule and ${\mathrm{Na}}^{+}/{\mathrm{Cl}}^{-}$ ions passing through the porous $d$-$\mathrm{Zn}\mathrm{O}$ structure is low and high, respectively. In addition, MD calculations show that the water permeability performance of $d$-$\mathrm{Zn}\mathrm{O}$ material is high enough to use this material for water desalination applications. For further investigations, the detection of some selected gases ($\mathrm{CO}$, $\mathrm{SO}$, $\mathrm{NO}$, ${\mathrm{CO}}_2$, ${\mathrm{SO}}_2$, and ${\mathrm{NO}}_2$) are investigated on $d$-$\mathrm{Zn}\mathrm{O}$ and find that ${\mathrm{NO}}_2$, and ${\mathrm{SO}}_2$ would preferentially be detected on the $d$-$\mathrm{Zn}\mathrm{O}$ substrate due to their high adsorption energy values as compared to physisorption of $\mathrm{CO}$, $\mathrm{NO}$, $\mathrm{SO}$, and ${\mathrm{CO}}_2$ molecules on the $d$-$\mathrm{Zn}\mathrm{O}$ surface.

Micro-light-emitting diodes ($\text{μ}\mathrm{LEDs}$) based on $\mathrm{Ga}\mathrm{N}$ are a key component for next-generation displays, but serious surface $e$-$h$ recombination deteriorates the device efficiency. To gain microscopic insights into the surface recombination, we use hybrid functional first-principles calculations to investigate $\mathrm{Ga}$ ($V_{\mathrm{Ga}}$) and $\mathrm{N}$ ($V_{\mathrm{N}}$) vacancies on the $\mathrm{Ga}\mathrm{N}$ $m$ plane that can be created considerably during the production of one-dimensional $\mathrm{Ga}\mathrm{N}$ structures. We find that the surface $V_{\mathrm{Ga}}$ is not critical for nonradiative Shockley-Read-Hall (SRH) recombination in light of the large formation energy (>2 eV) and its shallow levels. In contrast, the surface $V_{\mathrm{N}}$ exhibits a low formation energy (<2 eV) and develops a deep defect state near the midgap, indicating the possibility of becoming useful SRH recombination centers. By constructing configuration-coordinate diagrams, we demonstrate that the energy barriers for electron and hole capture on the surface $V_{\mathrm{N}}$ are small enough to cause significant capture coefficients due to strong electron-phonon coupling.

We investigate facilitation-induced transparency (FIT) enabled by strong and long-range Rydberg atom interactions between two spatially separated optical channels. In this setting, the resonant two-photon excitation of Rydberg states in a target channel is conditioned by a single Rydberg excitation in a control channel. Through the contactless coupling enabled by the Rydberg interaction, the optical transparency of the target channel can be actively manipulated by steering the optical detuning in the control channel. By adopting a dressed-state picture, we identify two different interference pathways, in which one corresponds to Rydberg blockade and an emergent one results from facilitation. We show that the FIT is originated from the Rydberg interaction and the quantum interference effect between the two pathways, which is different from conventional electromagnetically induced transparency realized by single-body laser-atom coupling. We find that the FIT in such a dual-channel setting is rather robust, insensitive to changes of systemic parameters, and can be generalized to multichannel settings. Moreover, we demonstrate that such a FIT permits the realization of controllable single-photon switches, which also paves a route to detect Rydberg facilitation by using optical absorption spectra. Our study contributes to current efforts in probing correlated many-body dynamics and developing single-photon quantum devices based on Rydberg atom ensembles.

We propose and demonstrate an autofocusing beam, named the circular Pearcey Airy beam (CPAB). It is found that the CPAB exhibits a superior autofocusing property, leading to an enhanced trapping force within the autofocusing region. By thoroughly analyzing the power flow and the trapping-force distribution, we present a quantitative understanding of the autofocusing properties and the trapping capabilities of the CPAB. We also show how to tune the autofocusing and trapping properties of the CPAB by adjusting the physical parameters, such as transverse scale factor, spatial offset, and exponential decay factor. Moreover, the CPAB exhibits multiple focusing peaks along the propagation direction, capable of trapping multiple particles longitudinally and periodically. These results indicate that the CPAB is promising for optical trapping and manipulations, which may be useful in various fields, including biological research and atomic physics.

The brightness of electron beams emitted from photocathode sources plays a critical role in determining the performance of x-ray free-electron lasers and ultrafast electron-diffraction applications. In order to achieve the maximum brightness, the electrons need to be emitted from a photocathode with the lowest-possible mean transverse energy (MTE). Recent investigations have shown that capping a $\mathrm{Cu}$(110) photocathode with a monolayer of graphene can protect the quantum efficiency (QE) from long-term exposure to varying vacuum conditions. However, there have been no studies that investigate the effects that a monolayer of graphene has on the MTE. Here, we report on measurements of a graphene-coated $\mathrm{Cu}$(110) single crystal near the photoemission threshold for room and liquid-nitrogen temperatures. At room temperature, a minimum MTE of 25 meV is measured at 295 nm. At liquid-nitrogen temperatures, a minimum MTE of 9 meV is measured at the photoemission threshold of 290 nm.

High-fidelity two-qubit gates in quantum computers are often hampered by fluctuating experimental parameters. The effects of time-varying parameter fluctuations lead to coherent noise on the qubits, which can be suppressed by designing control signals with appropriate filter functions. Here, we develop filter functions for Mølmer-Sørensen gates of trapped-ion quantum computers that accurately predict the change in gate error due to small parameter fluctuations at any frequency. We then design the filter functions of frequency-modulated laser pulses, and compare this method with pulses that are robust to static offsets of the motional-mode frequencies. Experimentally, we measure the noise spectrum of the motional modes and use it for designing the filter functions, which improves the gate fidelity from 99.23(7)% to 99.55(7)% in a five-ion chain.

The reciprocity between thermal emission and absorption in materials that satisfy the Lorentz reciprocity places a fundamental constraint on photonic energy conversion and thermal management. For approaching the ultimate thermodynamic limits in various photonic energy conversions and achieving nonreciprocal radiative thermal management, broadband nonreciprocal thermal emission is desired. However, existing designs of nonreciprocal emitters are narrowband. Here, we introduce a gradient epsilon-near-zero magneto-optical metamaterial for achieving broadband nonreciprocal thermal emission. We start by analyzing the nonreciprocal thermal emission and absorption in a thin layer of epsilon-near-zero magneto-optical material atop a substrate. We use temporal coupled-mode theory to elucidate the mechanism of nonreciprocal emission in the thin-film emitter. We then introduce a general approach for achieving broadband nonreciprocal emission by using a gradient epsilon-near-zero magneto-optical metamaterial. We numerically demonstrate broadband nonreciprocal emission in gradient-doped semiconductor multilayer, as well as in a magnetic Weyl semimetal multilayer with gradient chemical potential. Our approach for achieving broadband nonreciprocal emitters is useful for developing broadband nonreciprocal devices for energy conversion and thermal management.

With the development of technologies taking advantage of emerging quantum phenomena under extreme conditions of dimensionality and temperature, the search for alternative materials and heterostructure engineering has opened up on several fronts. Here, we report the magnetotransport properties of topological-insulator/two-dimensional-ferromagnet (TI/2D-FM) heterostructures composed of ${\mathrm{Cr}}_{1+\delta }{\mathrm{Te}}_2/{\mathrm{Bi}}_2{\mathrm{Te}}_3$ stacks grown by molecular-beam epitaxy. The electrical transport measurements reveal high levels of fieldlike effective torques, up to 115 mT at a current density of ${10}^7\mathrm{A}/{\mathrm{cm}}^2$; the occurrence of interfacial magnetoresistance effects, such as the anisotropic interfacial magnetoresistance; and anomalies in the anomalous Hall effect. Furthermore, we report on complementary characterization with scanning tunneling microscopy, angle-resolved photoemission spectroscopy, and superconducting quantum interference device measurements. Finally, magnetization reversal induced by current pulses is also reported. The reported results make the relevance of the TI/2D-FM interface evident and indicate the preservation of polarized surface states at the interface.