We provide a theoretical description of light scattering by a spherical particle the permittivity of which is modulated in time at twice the frequency of the incident light. Such a particle acts as a finite-sized photonic time crystal and, despite its subwavelength spatial extent, can host optical parametric amplification. Conditions of parametric Mie resonances in the sphere are derived. We show that time-modulated materials provide a route to tailor directional light amplification, qualitatively different from that in scatterers made from a gain media. We design two characteristic time-modulated spheres that simultaneously exhibit light amplification and desired radiation patterns, including those with zero backward and/or vanishing forward scattering. The latter sphere provides an opportunity for creating shadow-free detectors of incident light.

Some voltage sources are transient, lasting only for a moment of time, such as the voltage generated by converting a human motion into electricity. Such sources moreover tend to have a degree of randomness as well as internal resistance. We investigate how to put a number to how valuable a given transient source is. We derive several candidate measures via a systematic approach. We establish an interconvertibility hierarchy between such sources, where interconversion means adding passive interface circuits to the sources. Resistors at the ambient temperature are at the bottom of this hierarchy and sources with low internal resistance and high internal voltages are at the top. We provide three possible measures for a given source that assign a number to the source respecting this hierarchy. One measure captures how much “$\mathrm{unitdc}$” the source contains, meaning $1$ V dc with $1\mathrm{\Omega }$ internal resistance for $1$ s. Another measure relates to the signal-to-noise ratio of the voltage time series, whereas a third is based on the relative entropy between the voltage probability distribution and a thermal noise resistor. We argue that the $\mathrm{unitdc}$ measure is particularly useful by virtue of its operational interpretation in terms of the number of unit dc sources that one needs to combine to create the source or that can be distilled from the source.

Electromagnetic scattering bounds on subwavelength structures play an important role in estimating performance of antennas, radio frequency identification tags, and other wireless communication devices. An appealing approach to increase a scattering cross section is accommodating several spectrally overlapping resonances within a structure. However, numerous fundamental and practical restrictions have been found and led to the formulation of Chu-Harrington, Geyi, and other limits, which provide an upper bound to scattering efficiencies. Here we introduce a two-dimensional array of near-field coupled split-ring resonators and optimize its scattering performance with the aid of a genetic algorithm operating in 19-dimensional space. Experimental realization of the device is demonstrated to surpass the theoretical single-channel limit by a factor of >2, motivating the development of tighter bounds of scattering performance. A superradiant criterion is suggested to compare maximal scattering cross sections with the single-channel dipolar limit multiplied by the number of elements within the array. This empirical criterion, which aims to address performance of subwavelength arrays formed by near-field coupled elements, is found to be rather accurate in application to the superscatterer, reported here. Furthermore, the superradiant bound is empirically verified with a Monte Carlo simulation, collecting statistics on scattering cross sections of a large set of randomly distributed dipoles. The demonstrated flat superscatterer can find use as a passive electromagnetic beacon, making miniature airborne and terrestrial targets radar visible.

We discuss the effect of the dielectric environment (substrate/bottom oxide, gate insulator, and metal gates) on electronic transport in two-dimensional (2D) transition metal dichalcogenides (TMD) monolayers. We employ well-known ab initio methods to calculate the low-field carrier mobility in free-standing layers and use the dielectric continuum approximation to extend our study to layers in double-gate structures, including the effects of dielectric screening of the electron-phonon interaction caused by the bottom oxide and the gate insulator, and of scattering with hybrid interface optical-phonon/plasmon excitations (“remote phonon scattering”). We find that the presence of insulators with a high dielectric constant may improve significantly the carrier mobility. However, scattering with the interface hybrid excitations negates this gain and degrades the mobility significantly below its free-standing value. We find that this process is dominated by long-wavelength interactions that, for the carrier sheet density of interest, are strongly affected by the coupling with the 2D plasmons. Considering 2D layers in a double-gate geometry with ${\mathrm{Si}\mathrm{O}}_2$ as bottom oxide and various top-gate insulators, we find that the mobility decreases as the top-insulator dielectric constant increases (from hBN to ${\mathrm{Zr}\mathrm{O}}_2$), as expected. However, we observe two main deviations from this trend: high mobility is predicted in the case of the weakly polar hBN, and much lower than expected mobility is calculated in the case of gate-insulator/TMD/bottom-oxide stacks in which two or more polar materials have optical phonons with similar resonating frequencies. We also find that the effect of screening by metal gates is noticeable but not particularly strong. Finally, we discuss the effect of the TMD dielectric constant, of the free-carrier density, and of temperature on the transport properties of TMD monolayers.

Quantum transduction refers to the coherent conversion between microwave and optical states, which can be achieved by quantum teleportation if given high-fidelity microwave-optical entanglement, namely entanglement-based quantum transduction. Reliable microwave-optical entanglement can be generated using various platforms. In this paper we base the discussion on a piezo-optomechanical system and make the teleportation induced conversion scheme more concrete in the framework of quantum channel theory. By comparing the quantum capacity between the entanglement-based conversion channel and the traditional direct quantum transduction channel, we show that the entanglement-based scheme indeed admits a positive transduction rate when the direct quantum transduction has zero quantum capacity. Given two piezo-optomechanical systems, we also investigate the generation of microwave-microwave entanglement from entanglement swapping within continuous-variable and discrete-variable settings, showing the potentials of directly connecting microwave quantum processors by microwave-microwave quantum teleportation.

Ghost imaging with thermal light is a well-known imaging technique in which the light beam used for image reconstruction does not necessarily interact with the object. However, ghost imaging is generally time-consuming due to data acquisition and complex algorithm. Thus a fundamental question arises as to whether we can realize object identification without the need of reconstructing the full ghost images. Here we present a protocol of thermal-light-based object identification in a nonlocal manner, i.e., ghost identification, to answer this question positively. We employ a holography-type method to add an auxiliary planar phase together with the conjugate Fourier spectrum of the test object to the thermal light source, which revives the coherent transfer of complex spectrum from thermal light modulation to the second-order correlation function, regardless of no entanglement. In our experiment, we succeed in demonstrating the fairly good performance of ghost identification for both simple geometric objects and quick response codes, and particularly for fingerprints. Our protocol can find direct applications in secure biosensing and covert detection where very low light illumination levels are needed.

We propose a scheme to obtain highly sensitive oscillators in a coupled-resonator system with an exceptional point of degeneracy (EPD) and a small instability. The oscillator with the exceptional degeneracy is realized by using two coupled resonators with an almost balanced small-signal gain and loss, that saturates due to nonlinear effects of the active component, resulting in an oscillation frequency that is very sensitive to a perturbation of the circuit. Two cases are investigated, with two parallel $LC$ resonators with balanced small-signal gain and loss that are either coupled wirelessly by mutual inductance or coupled wired by a capacitor. This paper demonstrates theoretically and experimentally the conditions to obtain a second-order EPD oscillator and analyzes the ultrasensitivity of the oscillation frequency to components’ perturbation, including the case of asymmetric perturbation that breaks $PT$ symmetry. We discuss the effects of nonlinearity on the performance of the oscillator and how the proposed scheme improves the sensing’s sensitivity of perturbations. In contrast to previous methods, our proposed degenerate oscillator can sense either positive or negative changes of a circuit component. The degenerate oscillator circuit may find applications in various areas, such as ultrasensitive sensors, tunable oscillators, and modulators.

We investigate numerical and experimental nonadiabatic relocation of a topological interface. We construct a reconfigurable electroacoustic Su-Schreiffer-Heeger lattice using negative-capacitance (NC) shunted piezoelectric patches attached to a host beam. Tuning the NC circuits allows us to create a topological-interface state by varying the effective elastic modulus along the beam. We then use a microcontroller to switch relays placed in the NC shunt circuits to move the interface by one unit cell. We present the results for continuous and pulse inputs and show the shift of energy localization in the experiment. Further, to distinguish topological protection, we simulate the nonadiabatic shifting of a trivial defect in a discrete lattice. Numerical simulations indicate higher mean energy at the interface after shifting a topological state as compared to a defect state. Topological interfaces as shown in our experiment may have interesting applications in acoustic communication, such as source switching and signal encoding.

Antireflection coatings with a broad relative bandwidth (RBW) and reduced electrical length are desirable for a variety of applications in the microwave frequencies. Here, we propose the design principle of bilayer metasurfaces with 100% RBW and subwavelength thickness based on the traditional Chebyshev transformer, with excellent agreement with our experimental validation for both transverse electric and magnetic polarizations. We utilize the multiple-beam interference theory to reveal the underlying mechanism of the preferred choices for nonresonant or weakly resonant metasurface unit cells. Being able to manipulate the transmissions and reflections within the bilayer structure, we further demonstrate the universality and sensitivity of the designs capable of providing near-unity microwave transmission from air into the full range of natural media, with prescribed RBW and reduced electrical length.

Prior work showed the efficacy of pausing midanneal: such a pause improved the probability of success by orders of magnitude in a class of native problem instances and improved the time to solution in a class of embedded problem instances. A physics-based picture provides qualitative suggestions for where pausing midanneal is effective, for the interplay between annealing schedule parameters and other annealing properties and parameters such as embedding size and strength of the ferromagnetic coupling $|J_F|$, and for the conditions under which pausing can improve the time to solution. Here, through demonstrations on an updated annealing architecture that has higher connectivity than previous annealers, and on multiple embedded problem classes, we are able to confirm various aspects of this picture. We demonstrate the robustness of the optimal pause parameters across platforms and problem classes, explore how to set $|J_F|$ to optimize performance in different scenarios, and provide empirical evidence that short pauses trump longer overall annealing times in time to solution. We also identify the number of different coefficients in a problem as a predictor of problem hardness, and explore its interplay with the optimal $|J_F|$ and embedding size. Based on these results we are able to present qualitative guidelines for parameter setting in quantum annealers.

With the advent of quantum technologies, control issues are becoming increasingly important. In this paper, we address control in phase space under a global constraint provided by a minimal energylike cost function and a local (in Fourier space) constraint resulting from a robustness criterion. The inverse-engineering Lagrangian formalism developed here generalizes the formalism commonly used to describe the superoscillation phenomenon. It is applied to both nondissipative and dissipative quantum mechanics, and extended to stochastic thermodynamics. Interestingly, our approach also allows one to improve sensing capabilities by appropriate control of the system.

The best performance from compact, portable, and repeatably manufactured electromagnetic (EM) oscillators has for decades been built upon crystalline quartz resonators operating at radio frequencies. Such a source of frequency-stable EM waves has enabled a wide range of applications of significant technical and societal benefit, such as navigation and communication systems, distributed sensing, geodesy, and fundamental physics. However, meeting demands for increased measurement sensitivity requires pushing beyond a fractional frequency stability of ${10}^{-13}$, typical of the best ovenized quartz. To date, such performance has been available in the microwave domain only with the use of complex cryogenic systems, or in the optical domain at the expense of larger volume and weight. In either case, the size, weight, and power of state-of-the-art EM oscillators has relegated their use to laboratory experiments. Here we demonstrate the validity of a manufacturable, compact Fabry-Perot reference that improves upon all other centimeter-scale oscillators by more than a factor of 10, including those based on solid-state optical resonators as well as quartz. In a specific implementation, we show an 8 ml bonded assembly containing three vacuum-gap Fabry-Perot optical cavities with mirrors lithographically fabricated on a single substrate, each with quality factor exceeding 10 billion. A laser stabilized to one of the cavities exhibits thermal-noise-limited phase noise for offset frequencies ranging from 1 Hz to nearly 1 kHz, and fractional frequency stability of $7\times {10}^{-15}$ at 1 s. Our advance with lithographically defined and microfabricated mirrors is validated by rigorous models, which highlight a path to oscillators with size comparable to ovenized quartz, but with significantly improved performance. For example, we predict sub-${10}^{-14}$ performance is readily attainable with a Fabry-Perot cavity volume less than 2 ml.

Bell states are the most fundamental resource in realizing quantum information tasks and have a very unique position in quantum mechanics. Utilizing orbital angular momentum (OAM) to encode single-photon Bell states enables the realization of high-dimensional Hilbert space, which is crucial for the field of quantum information. In this paper, we design a single-photon OAM Bell-state evolution device based on the Sagnac interferometer, which can make one-to-one correspondence between the input Bell states and the output states. Moreover, we also develop a single-photon single-pixel imaging (SPI) technique, which improves spatial resolution and decreases acquisition time in the meantime, to capture interference images of output states. The results show that single-photon OAM Bell states can be fully recognized by comparing the differences of interference images. We innovatively utilize the SPI technique for single-photon OAM Bell-state recognition. This indicates that the SPI technique effectively promotes the study of quantum information based on OAM, while the quantum information based on OAM provides a clear application scenario for the SPI technique.

Materials with a negative and tunable Poisson ratio have great potential applications in fracture resistance and energy absorption. However, one material usually exhibits one specific Poisson ratio and a systematic tuning is highly desired. In this study, we design disordered networks with a wide tunable range of the Poisson ratio based on triangular analysis. By distorting the equilateral triangle lattice, the Poisson ratio can be systematically decreased due to the large area change in highly nonequilateral triangles. Analysis further shows that in a nonequilateral triangle, bonds with different lengths play distinct roles: the removal of short bonds can decrease the Poisson ratio, while the removal of long bonds does the opposite. Thus a negative Poisson ratio can be achieved by removing a small amount of short bonds. Moreover, such networks can serve as unit cells to build larger systems with a similar performance. Such auxetic networks are further experimentally realized in both spring and three-dimensionally printed systems, demonstrating the general validity of our analysis and its possible applications in building practical systems.

Metamaterials composed of asymmetric elements have been shown to break reciprocity; permitting the propagation of waves in one direction. The configuration of these metamaterials typically limits the propagation of the wave to a single direction, speed, or occurrence with little or no tunability. Here we present—theoretically, numerically, and experimentally—a simple design approach for reprogrammable metamaterials, which allows for all of these parameters to be tuned, enabled, disabled, or reset. We present a map of different geometries that allow for reciprocal and nonreciprocal wave propagation and attenuation. We show how a single design can be programmed on demand to support wave propagation and attenuation in both directions (reciprocal) or wave propagation in only one direction (nonreciprocal). In addition, we demonstrate real-time control of the wave velocity, both in amplitude and direction, as it propagates through the metamaterial. We show speeding up, slowing down, or complete reversal of the propagating wave direction. Furthermore, we show that the waves maintain a constant velocity regardless of the impulse magnitude, and even along a curved path. Our metamaterial could open exciting possibilities for designing nonlinear materials with exotic topological properties.

Photon-efficient imaging with the single-photon light detection and ranging captures the three-dimensional structure of a scene by only a few detected signal photons per pixel. However, the existing computational methods for photon-efficient imaging are pretuned on a restricted scenario or trained on simulated datasets. When applied to realistic scenarios whose signal-to-background ratios and other hardware-specific properties differ from those of the original task, the model performance often significantly deteriorates. In this paper, we present a domain adversarial adaptation design to alleviate this domain shift problem by exploiting unlabeled real-world data, with significant resource savings. This method demonstrates superior performance on simulated and real-world experiments using our home-built up-conversion single-photon imaging system, which provides an efficient approach to bypass the lack of ground-truth depth information in implementing computational imaging algorithms for realistic applications.

Speckle patterns generated in a disordered medium carry a lot of information despite the apparent complete randomness in the intensity pattern. When the medium possesses ${\chi }^{(2)}$ nonlinearity, the speckle is sensitive to the phase of the incident fundamental light, as well as the light generated within. Here, we examine the speckle decorrelation in the fundamental and second-harmonic transmitted light as a function of the varying power in the fundamental beam. At low incident powers, the speckle patterns produced by successive pulses exhibit strong correlations, which decrease with increasing power. The average correlation in the second-harmonic speckle decays faster than in the fundamental speckle. Next, we construct a theoretical model, backed up by numerical computations, to obtain deeper physical insights into the faster decorrelations in the second-harmonic light. While providing excellent qualitative agreement with the experiments, the model sheds light on the contribution of two effects in the correlations, namely, the generation of second-harmonic light and the propagation thereof.

Spin density waves (SDWs) are antiferromagnetic ground states characterized by real-space spin modulation. When an electronic system undergoes a paramagnetic-SDW transition, the translational symmetry is spontaneously broken and energy gaps are developed near the Fermi level, which offers potential for constructing various SDW components. Here we report a prototype resistive memory device based on a prototypical SDW metal, antiferromagnetic chromium. Transport and magnetic measurements show that the paramagnetic-SDW transition, i.e., the SDW antiferromagnetism, can be effectively suppressed by the electric-field-generated piezoelectric strain in epitaxial $\mathrm{Cr}/0.7\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_3$–$0.3{\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_3$ (PMN-PT) heterostructures. This enables a large electroresistance effect for metallic systems as the SDW band gaps can be intentionally controlled to vanish or develop. Combining this electroresistance effect with the different remanent piezoelectric strain of PMN-PT after poling by electric-field pulses of opposite polarity, we obtain two nonvolatile resistance states differing by about 1.8% and stable against a magnetic field of 3 T at room temperature. Our work unveils the electric-field controllability of the SDW transitions in thin films and the consequent wide application prospects of SDW materials.