This work shows that, counterintuitively, adding extremely small ($<10$ nm in dimension) fully absorbing metal nanoparticles to a material can notably enhance its light emission. It also shows that the mystery behind the unexpected large enhancements in surface-enhanced Raman spectroscopy, even greater than a factor of ${10}^{10}$, is the tunneling out of photons from the strongly absorbing metal surface. Dynamics in a strongly coupled emitter-matter quantum system demands such behavior in the case of limiting small metal nanoparticles and proximal metal surfaces. This effect can be exploited further in light generation, optical sensing, and radiative heat transfer.

Deterministic switching of polarization vortices in ferroelectrics by a uniaxial mechanical load is challenging. Here, we demonstrate, by using phase field simulations and ferroelectric instability analyses, a deterministic switching of polarization vortices in a compositionally graded ferroelectric (CGFE) nanoplate under a compressive stress. The obtained results suggest that the rational distribution of material constituents in the CGFE nanoplate tailors the distributions of electromechanical coupling and total energy densities of the nanoplate, which modifies the stress-induced vortex multiplication and annihilation behaviors, distinguished from those in a homogeneous ferroelectric nanoplate. The switching of polarization vortices by compressive stress is verified through instability analyses for the CGFE nanoplate, which not only confirms when the polarization switching occurs, but also demonstrates how the polarization responds at the onset of the switching. In addition, the effects of various factors on the feasibility of stress-induced vortex switching, including material gradient, temperature, and width-to-height ratio of the CGFE nanoplate, are investigated and summarized in phase diagrams.

Quantum dots attached to BCS superconducting leads exhibit a $0-\pi$ impurity quantum phase transition, which can be experimentally controlled either by the gate voltage or by the superconducting phase difference. For the pertinent superconducting single-impurity Anderson model, we present two simple analytical formulae describing the position of the phase boundary in parameter space for the weakly correlated and Kondo regime, respectively. Furthermore, we show that the two-level approximation provides an excellent description of the low-temperature physics of superconducting quantum dots near the phase transition. We discuss reliability and mutual agreement of available finite-temperature numerical methods (numerical renormalization group and quantum Monte Carlo) and suggest an alternative approach for efficient determination of the quantum phase boundary from measured finite-temperature data. Our results enable fast and efficient, yet reliable characterization and design of such nanoscopic tunable Josephson-junction devices.

Spintronic neurons that emit sharp voltage spikes are required for the realization of hardware neural networks enabling fast data processing with low power consumption. In many neuroscience and computer science models, neurons are abstracted as nonlinear oscillators. Magnetic nano-oscillators called spin-torque nano-oscillators are interesting candidates for imitating neurons at the nanoscale. These oscillators, however, emit sinusoidal waveforms without spiking while biological neurons are relaxation oscillators that emit sharp voltage spikes. Here, we propose a simple way to imitate neuron spiking in high-magnetoresistance nanoscale spin valves, where both magnetic layers are free and thin enough to be switched by spin torque. Our numerical-simulation results show that the windmill motion induced by spin torque in the proposed spintronic relaxation oscillator gives rise to spikes whose shape and frequency, set by the charging and discharging times, can be tuned through the amplitude of injected dc current. We also find that these devices can exhibit chaotic oscillations. Chaoticlike neuron dynamics has been observed in the brain, and it is desirable in some neuromorphic computing applications, whereas it should be avoided in others. We demonstrate that the degree of chaos can be tuned in a wide range by engineering the magnetic stack and anisotropies and by changing the dc current. The proposed spintronic relaxation oscillator is a promising building block for hardware neuromorphic chips, leveraging nonlinear dynamics for computing.

A key requirement to perform simulations of large quantum systems on near-term quantum hardware is the design of quantum algorithms with a short circuit depth that finish within the available coherence time. A way to stay within the limits of coherence is to reduce the number of gates by implementing a gate set that matches the requirements of the specific algorithm of interest directly in the hardware. Here, we show that exchange-type gates are a promising choice for simulating molecular eigenstates on near-term quantum devices since these gates preserve the number of excitations in the system. We report on the experimental implementation of a variational algorithm on a superconducting qubit platform to compute the eigenstate energies of molecular hydrogen. We utilize a parametrically driven tunable coupler to realize exchange-type gates that are configurable in amplitude and phase on two fixed-frequency superconducting qubits. With gate fidelities around 95%, we are able to compute the eigenstates to within an accuracy of 50 mHa (milliHartree) on average, a limit set by the coherence time of the tunable coupler.

Due to their easy, fast, and noninvasive natures, acoustic wave methods have been widely used in the physical property measurements of liquids, such as shear waves for viscosity and ultrasound for density. We investigate the physical interaction between approximately GHz longitudinal acoustic waves and simple Newtonian fluids in micro- and nanoscales, and report a longitudinal-acoustic-wave microsensor for microfluidic bulk-modulus measurement. We also verify that the microsensor is insensitive to viscosity, in contrast to shear-wave sensors commonly used for viscosity measurements. With its small size and nanometer wavelengths, the microsensor in nonreflective regimes can be integrated into microfluidics and can make fast measurements even under harsh conditions, such as in limited (15∼200 µm) channel spaces and sample volumes (approximately µl). The longitudinal-acoustic-waves measuring theory is demonstrated by the Mason model, a finite element analytical model, and experiments on glycerol-water mixture samples. The microsensor is originally designed for microfluidic use, nevertheless, it can be used under diverse conditions in which miniaturization matters, namely, in biomedical, chemistry, industry, environmental, healthcare, energy applications, and so on.

Magnetic vortices are characterized by the senses of in-plane magnetization chirality and by the polarity of the vortex core. The electrical control of vortex polarity and chirality is highly demanded not only for fundamental understanding of spin dynamics in nanodisks under different circumstances, but also for technological applications, such as magnetic nonvolatile memories and spin torque oscillators for neuromorphic computing. We report an alternative approach that enables one to electrically control both the vortex chirality and polarity with low energy consumption. Thorough micromagnetic simulations, we show that in thin nanodisks of diameters larger than 160 nm, with the presence of a current-induced Oersted field, the dynamic transformation of the edge solitons is able to efficiently switch both vortex chirality and polarity with low current under certain circumstances. We then develop an approach to directly write any of the four vortex states by electrical current pulses from a random state. We further investigate the switching phase diagram as a function of disk diameters. The results show that the switching process is highly size dependent. As the disk diameter becomes smaller than 160 nm, the switch of vortex core (VC) chirality and polarity always take place at the same time, resulting in an unchanged handedness before and after switching. Furthermore, the critical switch current can be as low as $3\times {10}^6\text{A/c}{\text{m}}^2$, indicating a possible way for low current switch of vortex chirality in small disks.

We theoretically propose scattering cancellation-based cloaks for heat waves that obey the Maxwell-Cattaneo equation. The proposed cloaks possess carefully tailored diffusivity to cancel the dipole scattering from the object that they surround, and thus can render a small object invisible in the near and far fields, as demonstrated by full-wave finite-element simulations. Mantle heat cloaking is further analyzed and proposed to simplify the design and bring this cloaking technology one step closer to its practical implementation, with promising applications in nanoelectronics and defense-related applications.

A metasurface based on defect lattices and an alternative physical mechanism, multiwave destructive interference (MWDI), is proposed for ultrawideband radar cross-section (RCS) reduction. The bandwidth of RCS reduction $({\sigma }_R)$ Is greatly expanded by second destructive interference. The metasurface is composed of 16 basic defect lattices. First, the defect lattice can generate primary destructive interference with the capacity of RCS reduction and amplitude-phase manipulation, which consists of an aperiodic array of square rings with an embedded cross. Second, the interference between multiple backscattered waves produced by the defect lattices at multiple frequencies sampled in an ultrawide band is simultaneously manipulated and optimized by the principle of superposition of waves and particle swarm optimization (PSO) to obtain second destructive interference. The metasurface enables a 10-dB RCS reduction over an ultrawide frequency band ranging from 6.16 to 41.63 GHz with a ratio bandwidth $(f_H/f_L)$ of 6.76:1 under normal incidence for both polarizations. The estimated, simulated, and measured results are in good agreement and prove that the proposed metasurface is of great significance for bandwidth expansion of RCS reduction.

The variational quantum eigensolver (VQE) is an attractive possible application of near-term quantum computers. Originally, the aim of the VQE is to find a ground state for a given specific Hamiltonian. It is achieved by minimizing the expectation value of the Hamiltonian with respect to an ansatz state by tuning parameters $\mathit{\theta }$ on a quantum circuit, which constructs the ansatz. Here we consider an extended problem of the VQE, namely, our objective in this work is to “generalize” the optimized output of the VQE just like machine learning. We aim to find ground states for a given set of Hamiltonians $\{H(\mathbf{x})\}$, where $\mathbf{x}$ is a parameter that specifies the quantum system under consideration, such as geometries of atoms of a molecule. Our approach is to train the circuit on the small number of $\mathbf{x}$’s. Specifically, we employ the interpolation of the optimal circuit parameter determined at different $\mathbf{x}$’s, assuming that the circuit parameter $\mathit{\theta }$ has simple dependency on a hidden parameter $\mathbf{x}$ as $\mathit{\theta }(\mathbf{x})$. We show by numerical simulations that, using an ansatz, which we call the Hamiltonian-alternating ansatz, the optimal circuit parameters can be interpolated to give near-optimal ground states in between the trained $\mathbf{x}$’s. The proposed method can greatly reduce, on a rough estimation by a few orders of magnitude, the time required to obtain ground states for different Hamiltonians by the VQE. Once generalized, the ansatz circuit can predict the ground state without optimizing the circuit parameter $\mathit{\theta }$ in a certain range of $\mathbf{x}$.

Exploration of topology in photonic and sonic crystals has inspired a number of intriguing phenomena, such as backscattering-immune and defect-insensitive transport at domain walls. Instead of introducing topological gauge flux, the topological insulator realized by the pseudospin Hall effect exhibits extraordinary advantages in acoustics, in which the noise caused by flow is avoided. Here, we report on the experimental observation of the topological transport of sound in a sonic crystal with a Kekulé lattice. With different designs of the domain walls, pseudospin-Hall or valley-Hall edge states emerge around either $\mathrm{\Gamma }$ or $K$ point of the Brillouin zone. Through spatial scanning the sound field, the parity and the angular selectivity of the edge states are visualized. Our work provides an alternative way towards the design of directional antennas for sound.

Transmission radiography using megaelectronvolt radiation is a powerful nondestructive method for determining elemental composition. Radiography is typically performed with either neutrons or photons, but neither of these probes is universally applicable. We experimentally demonstrate that a significant increase in the contrast for small elemental variations in object composition can be realized by combining the multiple-monoenergetic-neutron and multiple-monoenergetic-photon transmission radiography techniques. The multimodal source is based on deuteron-driven low-energy nuclear reactions that produce both neutrons and photons at discrete energies. The neutron time-of-flight technique is used to measure the transmission over a broad range of neutron energies and is combined with spectroscopic photon transmission radiography. This work demonstrates the use of a single, multiparticle, multiple-monoenergetic-radiation source and a single type of radiation detector to simultaneously perform neutron and photon spectroscopic radiography. Four different material-identification metrics are used, which show a factor-of-3-or-greater increase in sensitivity to changes in material composition when compared with traditional dual-energy photon radiography, and are in agreement with simulations that establish a direct correspondence to known photon and neutron interaction cross sections. Furthermore, the ability to infer the presence of objects consisting of impure elements, layers of different elements, or non-natural isotope concentrations is demonstrated.

We measure the temperature dependence of the two-photon absorption and optical Kerr nonlinearity of a silicon waveguide over a range of temperatures from 5.5 to 300 K. Measurements are taken at a wavelength of 1.55 $\mu \mathrm{m}$ in the technologically important telecom $\mathrm{C}$ band. We observe a near halving (45% reduction) of the two-photon absorption coefficient at low temperature, whereas a smaller reduction in the Kerr nonlinearity of 25% is found. The increased ratio of Kerr to absorptive nonlinearity at low temperatures indicates an improved operation of integrated photonic devices that make use of a nonlinear phase shift, such as optical switches or parametric photon-pair sources. As an example, we examine how the heralding efficiency of a photon-pair source will change at low temperatures and predict a modest improvement in source performance. In addition, the modeling and experimental techniques developed can readily be extended to other wavelengths or materials of interest.

We investigate alternative annealing schedules on the current generation of quantum-annealing hardware (the D-Wave 2000Q), which includes the use of forward and reverse annealing with an intermediate pause. This work provides new insights into the inner workings of these devices (and quantum devices in general), particularly into how thermal effects govern the system dynamics. We show that a pause midway through the anneal can cause a dramatic change in the output distribution, and we provide evidence suggesting thermalization is indeed occurring during such a pause. We demonstrate that upon pausing the system in a narrow region shortly after the minimum gap, the probability of successfully finding the ground state of the problem Hamiltonian can be increased over an order of magnitude. We relate this effect to relaxation (i.e., thermalization) after diabatic and thermal excitations that occur in the region near to the minimum gap. For a set of large-scale problems of up to 500 qubits, we demonstrate that the distribution returned from the annealer very closely matches a (classical) Boltzmann distribution of the problem Hamiltonian, albeit one with a temperature at least 1.5 times higher than the (effective) temperature of the annealer. Moreover, we show that larger problems are more likely to thermalize to a classical Boltzmann distribution.

By scanning with a $30$-$\mathrm{keV}$ focused $\mathrm{He}$ ion beam ($\mathrm{He}$-FIB) across ${\mathrm{Y}\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ (YBCO) thin-film microbridges, we create Josephson barriers with critical current density $j_c$ adjustable by irradiation dose $D$. The dependence $j_c(D)$ yields an exponential decay. At $4.2\mathrm{K}$, a transition from flux-flow to Josephson behavior occurs when $j_c$ decreases below approximately $2\mathrm{MA}/\mathrm{c}{\mathrm{m}}^2$. The Josephson junctions exhibit current-voltage characteristics (IVCs) that are well described by the resistively and capacitively shunted junction model, without excess current for characteristic voltages $V_c\lesssim 1\mathrm{mV}$. Devices on $\mathrm{Mg}\mathrm{O}$ and LSAT substrates show nonhysteretic IVCs, while devices on ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$ show a small hysteresis. For all junctions, an approximate scaling $V_c\propto j_c^{1/2}$ is found. $\mathrm{He}$-FIB irradiation with a high dose produces barriers with $j_c=0$ and high resistances of $10\mathrm{k}\mathrm{\Omega }$ to $1\mathrm{G}\mathrm{\Omega }$. This provides the possibility to write highly resistive walls or areas into YBCO using a He-FIB. Transmission electron microscopy reveals an amorphous phase within the walls, whereas for lower doses the YBCO stays crystalline. We have also “drawn” superconducting quantum-interference devices (SQUIDs) by using a $\mathrm{He}$-FIB for the definition of the SQUID hole and the junctions. The SQUIDs show high performance, with flux noise $<500\mathrm{n}{\mathrm{\Phi }}_0/\mathrm{H}{\mathrm{z}}^{1/2}$ in the thermal white-noise limit for a device with $19\mathrm{pH}$ inductance.

Nonlinear phenomena are a frequent occurrence in nature and inherent to many optical systems. Nevertheless, nonlinearities are often avoided or suppressed in optical resonators. Here we demonstrate the application of nonlinear phenomena to pump-probe spectroscopy by using period doubling in a fiber-feedback optical parametric oscillator. On the basis of an analysis of the optical single-pulse spectra and successive second-harmonic spectra, we introduce a method to enhance the pulse-energy modulation depth to the level of conventional high-frequency modulators. We find that the modulator is suitable for high-speed, low-noise stimulated Raman scattering imaging with performance equal to that of acousto-optic modulation. The concept removes the need for electronic synchronization, reduces the detection effort and artifacts, and has great potential in terms of scalability of the modulation frequency.

A dynamical quantum phase transition can occur during time evolution of sudden quenched quantum systems across a phase transition. It corresponds to the nonanalytic behavior at a critical time of the rate function of the quantum-state return amplitude, analogous to nonanalyticity of the free-energy density at the critical temperature in macroscopic systems. A variety of many-body systems can be represented in momentum space as a spin-1/2 state evolving on the Bloch sphere, where each momentum mode is decoupled and thus can be simulated independently by a single qubit. Here, we report the observation of a dynamical quantum phase transition in a superconducting qubit simulation of the quantum-quench dynamics of many-body systems. We take the Ising model with a transverse field as an example for demonstration. In our experiment, the spin state, which is initially polarized longitudinally, evolves based on a Hamiltonian with adjustable parameters depending on the momentum and strength of the transverse magnetic field. The time-evolving quantum state is read out by state tomography. Evidence of dynamical quantum phase transitions, such as paths of time-evolution states on the Bloch sphere, nonanalytic behavior of the dynamical free energy, and the emergence of Skyrmion lattice in momentum-time space, is observed. The experimental data agrees well with theoretical and numerical calculations. The experiment demonstrates explicitly the topological invariant, both topologically trivial and nontrivial, for dynamical quantum phase transitions. Our results show that the quantum phase transitions of this class of many-body system can be simulated successfully with a single qubit by varying certain control parameters over the corresponding momentum range.

The measurement and control of the temperature in microscopic systems, which are increasingly required in diverse applications, are fundamentally important. Yet, the measurement of the three-dimensional (3D) temperature distribution in microscopic systems has not been demonstrated. We propose and experimentally demonstrate the measurement of the 3D temperature distribution by exploiting the temperature dependency of the refractive index (RI). Measurement of the RI distribution of water makes it possible to quantitatively obtain its 3D temperature distribution above a glass substrate coated with gold nanorods with submicrometer resolution in a temperature range of 100 °C and with a sensitivity of 2.88 °C. The 3D temperature distributions that are obtained enable various thermodynamic properties including the maximum temperature, heat flux, and thermal conductivity to be extracted and analyzed quantitatively.

We propose a threshold detector for Lévy-distributed fluctuations based on a Josephson junction. The Lévy-noise current added to a linearly ramped bias current results in clear changes in the distribution of switching currents out of the zero-voltage state of the junction. We observe that the analysis of the cumulative distribution function of the switching currents supplies information on both the characteristics’ shape parameter $\alpha$ of the Lévy statistics. Moreover, we discuss a theoretical model, which allows characteristic features of the Lévy fluctuations to be extracted from a measured distribution of switching currents. In view of these results, this system can effectively find an application as a detector for a Lévy signal embedded in a noisy background.

The valley Chern effect is theoretically demonstrated with an alternating current circuitry, where closed-loop LC resonators sitting at the nodes of a honeycomb lattice are inductively coupled along the bonds. This enables us to generate a dynamical matrix, which copies identically the Hamiltonian driving the electrons in graphene. The valley Chern effect is generated by splitting the inversion symmetry of the lattice. After a detailed study of the Berry-curvature landscape and of the localization of the interface modes, we derive an optimal configuration of the circuit. Furthermore, we show that $Q$ factors as high as ${10}^4$ can be achieved with reasonable materials and configurations.