We introduce a quantum control protocol that produces smooth, experimentally implementable control sequences optimized to combat temporally correlated noise for single-qubit systems. The control ansatz is specifically chosen to be a functional expansion of discrete prolate spheroidal sequences, a discrete time basis known to be optimally concentrated in time and frequency, and quite attractive when faced with experimental control hardware constraints. We leverage the filter function formalism to transform the control problem into a filter design problem, and show that the frequency response of a quantum system can be carefully tailored to avoid the most relevant dynamical contributions of noise processes. Using gradient ascent, we obtain optimized filter functions and exploit them to elucidate important details about the relationship between filter function design, control bandwidth, and noise characteristics. In particular, we identify regimes of optimal noise suppression and, in turn, optimal control bandwidth directly proportional to the size of the frequency bands where the noise power is large. In addition to providing guiding principles for filter design, our approach enables the development of controls that simultaneously yield robust noise filtering and high-fidelity single-qubit logic operations in a wide variety of complex noise environments.

Impedance spectroscopy (IS) is a straightforward experimental technique that is commonly used to obtain information about the physical and chemical characteristics of photovoltaic devices. However, the nonstandard physical behavior of perovskite solar cells (PSCs), which are heavily influenced by the motion of mobile ion vacancies, has hindered efforts to obtain a consistent theory of PSC impedance. This work rectifies this omission by deriving a simple analytic model of the impedance response of a PSC from the underlying drift-diffusion model of charge-carrier dynamics and ion-vacancy motion, via an intermediate model that shows extremely good agreement with the drift-diffusion model in the relevant parameter regimes. Excellent agreement is demonstrated between the analytic impedance model and the much more complex drift-diffusion model for applied biases (including both open circuit and the maximum power point at 0.1 and 1 Sun) close to the cell’s built-in voltage $V_{\mathrm{bi}}$. Both models show good qualitative agreement to experimental IS data in the literature and predict many of the observed anomalous features found in impedance measurements on PSCs. The analytic model provides a practical and useful tool with which to interpret PSC impedance data and extract physical parameters from IS experiments. We define a physical parameter, $n_{\mathrm{el}}$ (the electronic ideality factor), that is of particular significance to PSC physics, since, in contrast to the apparent ideality factor, the value of $n_{\mathrm{el}}$ can be used to identify the dominant source of recombination in the cell independent of its ionic properties.

This paper provides a feasible way to suppress flame flickering under normal gravity and atmospheric pressure without changing the fuel mass burning rate, thus promoting steady-state combustion. By periodically reciprocating one burner horizontally in a dual burner system, a death mode (namely, amplitude death of flame height oscillation) is generated around the critical burner separation distance, which is a transition point between in-phase and antiphase flickering modes of the two flames. The criterion to obtain a complete death mode region in a general dual burner system (with horizontal movement function) is defined by two dimensionless parameters, which respectively quantify the burner configuration and its dynamic performance.

Optomechanical Brillouin nonlinearities—arising from the coupling between traveling photons and phonons—have become the basis for a range of powerful optical signal processing and sensing technologies. The dynamics of such interactions are largely set and limited by the host material’s elastic, optical, and photoelastic properties, which are generally considered intrinsic and static. Here we propose and theoretically show that it is feasible to dynamically reconfigure the Brillouin nonlinear susceptibility in transparent semiconductors through acoustoelectric phonon-electron coupling. Acoustoelectric interactions permit a wide range of tunability of the phonon dissipation rate and velocity, perhaps the most influential parameters in the effective optomechanical cooperativity and Brillouin nonlinear susceptibility. We develop a Hamiltonian-based analysis that yields self-consistent dynamical equations and noise coupling, allowing us to explore the physics of such acoustoelectrically enhanced Brillouin (AEB) interactions and show that they give rise to a significant enhancement of the performance of Brillouin-based photonic technologies. Moreover, we show that these AEB effects can drive systems into regimes of fully coherent scattering that resemble the dynamics of optical parametric processes, significantly different than the incoherent traditional Brillouin limit. We propose and computationally explore a particular semiconductor heterostructure in which the acoustoelectric interaction arises from a piezoelectric phonon-electron coupling. We find that this system provides the necessary piezoelectric and carrier response ($k^2\approx 6\mathrm{\%}$), favorable semiconductor materials properties, and large optomechanical confinement and coupling [$|g_0|\approx 8000$ (rad/s)$\sqrt{\mathrm{m}}$] sufficient to demonstrate acoustoelectrically enhanced optomechanical interactions.

We propose a feasible scenario for directly polarizing a relativistic electron beam and obtain an overall polarization in various directions through a filter mechanism for single-shot collisions between an ultrarelativistic unpolarized electron beam and an ultraintense circularly polarized laser pulse. The electrons are scattered over a large angular range of several degrees, and the polarization states of the electrons are connected with their spatial positions after the collision. Therefore, we can employ a filter to filter out a part of the scattered electrons based on their position, and obtain a high degree of overall polarization for the filtered beam. Through Monte Carlo simulations with consideration of spin, polarizations with a degree of up to 62% in arbitrary transverse directions and a longitudinal polarization of up to 10% are obtained for filtered beams with currently achievable laser intensities. We analyze theoretically the formation of the distribution of the scattered electrons and investigate the influence of different initial parameters through simulations to demonstrate the robustness of our scheme. This scenario provides a simple and flexible way to produce relativistic polarized electron beams with various polarization directions.

Nanoscale amplitude and phase sensing of a rf electromagnetic field can benefit broad frontiers in science and technology. The imaging applications in these research areas demand a nanoscale sensor able to detect rf electric fields over a wide frequency range under ambient conditions, which remains a challenge. Here, we present a rf electrometry based on the forbidden magnetic dipole transition of a single shallow nitrogen-vacancy center in diamond and experimentally demonstrate the detection of rf electric fields at frequencies ranging from 13.85 MHz to 2.02 GHz. A sensitivity of $265{\mathrm{V}\mathrm{cm}}^{-1}{\mathrm{Hz}}^{-1/2}$ for amplitude measurement and a standard deviation of ${0.2}^{\circ }$ for phase measurement are achieved. The potential applications of nanoscale sensing and imaging of electromagnetic field are discussed.

Hydrodynamic lubrication is studied for both shear thinning and viscoelastic polymer solutions. We find that elasticity, notably strong normal stresses, does not change the friction significantly for the range of parameters tested in this manuscript. Shear-thinning properties, on the other hand, do change the formation of the lubricating layer thickness and the dependence of friction on velocity relative to Newtonian fluids. A hydrodynamic model that includes shear thinning is developed and compared to experimental data. The model describes the dependence on lubrication parameters well, but underestimates the lubricating layer thickness by a constant factor of roughly 1.5. The theory allows us to define a Hersey-like number for shear-thinning fluids that describes the lubricating layer thickness as a result of the balance between normal load and viscous force. For each tested liquid it succeeds in collapsing friction measurements onto the same curve. The friction analysis for both lubrication theory and experiments then reveals that shear thinning mainly changes the layer thickness, which is the main determinant of the friction coefficient.

Magnetic resonance imaging (MRI) enables noninvasive three-dimensional thermometry, which has potential applications in biological tissues and engineering systems. In biological tissues, where MRI is routinely used to monitor temperature during thermal therapies, $T_1$ or $T_2$ contrast in water are relatively insensitive to temperature, and techniques with greater temperature sensitivity, such as chemical shift or diffusion imaging, suffer from motional artifacts and long scan times. MR thermometry is not well developed for nonbiological or engineering systems. We describe an approach for highly sensitive and high-throughput MR thermometry that is not susceptible to motional artifacts and could be applied to various biological systems and engineering fluids. We use superparamagnetic iron-oxide nanoparticles (SPIONs) to spoil $T_2$ of water protons. Motional narrowing results in proportionality between $T_2$ and the diffusion constant, dependent only on the temperature in a specific environment. Our results show, for pure water, the nuclear magnetic resonance linewidth and $T_2$ follow the same temperature dependence as the self-diffusion constant of water. Thus, a $T_2$ mapping is a diffusion mapping in the presence of SPIONs, and $T_2$ is a thermometer. For pure water, a $T_2$ mapping of a 64 × 64 image (voxel size = 0.5 mm × 0.5 mm × 3 mm) in a 9.4 T MRI scanner resulted in a temperature resolution of 0.5 K for a scan time of 2 min. This indicates a highly sensitive and high-throughput MR thermometry technique that potentially has a range of applications from thermal management fluids to biological tissues.

Second-order memristors are considered as ideal synaptic emulators for their capability of exhibiting ${\mathrm{Ca}}^{2+}$-like dynamics. Recently, ferroelectric second-order memristors were developed, but whether their temporal conductance evolution is related to polarization dynamics remains unclear owing to the difficulty in directly measuring polarization in these devices. This issue is addressed here by using a ferroelectric diode (FD) that shows both second-order memristive behavior and well-shaped polarization-voltage hysteresis loops. It is demonstrated that the resistance-state change in the FD is triggered by polarization switching, arising from polarization-controlled Schottky emission. Moreover, concurrent conductance decay and polarization relaxation are observed, and their correlation is quantitatively evidenced, suggesting that conductance decay is caused by the polarization-relaxation-induced increase in the Schottky barrier height. Using polarization relaxation as an internal timing mechanism, our FD-based second-order memristor faithfully emulates various synaptic functions, where short-term plasticity is indispensable, including excitatory postsynaptic current, paired-pulse facilitation, the transition from short-term plasticity to long-term plasticity, learning experience, and associative learning. Our study not only reveals a polarization-dominated internal timing mechanism in the FD-based second-order memristor, but also demonstrates that such a device is a promising building block for biorealistic neuromorphic systems.

Acoustic manipulation is extensively employed due to its versatility and noncontact capability. Although significant developments have been made in this field, the dynamic process of the acoustic trap position in the manipulation of acoustically levitated objects has not been fully investigated, hindering the development of acoustic manipulation technology. Herein, we propose the gravity-acoustic trap morphology ($G$-ATM) method to elucidate the dynamic process in single-axis levitators. First, the acoustic trap step is defined to describe the dynamic process. Then, based on the $G$-ATM method, the study is carried out for single-axis levitators from the radial and axial points of view. In the radial direction, the stage concept is proposed to explain the impact of gravity, the impact of acoustic trap morphology is expressed by radial acoustic radiation force distribution, and transitional levitation states are observed and explained. In the axial direction, the impact of gravity is analyzed from the kinetic energy perspective, the impact of acoustic trap morphology is presented by multiple adjacent acoustic traps, and the principle of proximity followed by the motion of levitated objects is proposed. Finally, all analysis results are verified by simulations and experiments. The work will help to improve the rapidity and stability of acoustic manipulation and promote the development of acoustic manipulation technology.

We introduce a fast and highly efficient physically realizable bit swap. Employing readily available and scalable Josephson junction microtechnology, the design implements the recently introduced paradigm of momentum computing. Its nanosecond speeds and sub-Landauer thermodynamic efficiency arise from dynamically storing memory in momentum degrees of freedom. As such, during the swap, the microstate distribution is never near equilibrium and the memory-state dynamics fall far outside of stochastic thermodynamics that assumes detailed-balanced Markovian dynamics. The device implements a bit-swap operation—a fundamental operation necessary to build reversible universal computing. Extensive, physically calibrated simulations demonstrate that device performance is robust and that momentum computing can support thermodynamically efficient, high-speed, large-scale general-purpose computing that circumvents Landauer’s bound.

To achieve secure quantum key distribution, all imperfections in the source unit must be incorporated in a security proof and measured in the lab. Here we perform a proof-of-principle demonstration of the experimental techniques for characterizing the source phase and intensity fluctuation in commercial quantum key distribution systems. When we apply the measured phase-fluctuation intervals to the security proof that takes into account fluctuations in the state preparation, it predicts a key distribution distance of over $100\mathrm{km}$ of fiber. The measured intensity fluctuation intervals are, however, so large that the proof predicts zero key, indicating a source improvement may be needed. Our characterization methods pave the way for a future certification standard.

The problem of inverse design is rooted to the classical problem of inverse scattering. In general, these highly nonlinear and often ill-posed problems are solved via extensive optimization techniques. In this paper, we suggest an optimization-free method for the inverse design of a one-dimensional medium with spatially inhomogeneous dielectric constant $ϵ(z)$. In addition, we derive the governing equation of an analog problem—a time-dependent homogeneous medium—and use the same technique for inverse design of the temporal profile $ϵ(t)$ of a spatially homogeneous medium that is required to achieve a desired frequency response in $k$-space. Lastly, we use this optimization-free inversion approach to demonstrate the design of the reflection response such that the reflected wave undergoes a desired filtering, such as of Tchebychev type, differentiator, and a matched filter for chirp signal detection over additive Gaussian noise, which is of high potential significance in chirp radar and sonar applications.

Acoustic separation of suspension cells can be achieved by altering the properties of the cell medium. Cells of different types can then migrate in opposing directions in the sound field due to differences in their relative density and compressibility with respect to the surrounding fluid. For near-zero acoustic contrast the acoustic radiation force on cells becomes negligible, and it is the primary objective of this paper to study the interplay of acoustic radiation, acoustic streaming, and buoyancy in this regime and how it may affect the separation outcome. We study the three-dimensional acoustophoretic motion of suspension cells in homogeneous suspending media and link this to the underlying acoustofluidic mechanisms. Cell trajectories are measured by a defocused-image tracking approach, and we assess the technique's applicability for tracking cells by determining the associated error when measuring the out-of-image-plane component of the tracks. For cells at near-zero acoustic contrast, we observe strong effects of buoyancy and acoustic streaming and that small distributions of cell properties within a population leads to large differences in the cell motion patterns. A neural network was developed to classify experimental cell trajectories according to their acoustic contrast in different suspending media. Further, we compare the experimentally measured trajectories to a numerical model by generating simulated trajectories of cells.

Orbital-angular-momentum- (OAM) based photonic nonlinear interfaces, such as the nonlinear manipulation of a single-photon OAM state between the communication wavelength for entanglement distribution and the near-visible wavelength for quantum memories. Here, we experimentally demonstrate this kind of quantum state transformation for simultaneously adjusting the weight of the spatial OAM modes and operating the wavelength of single photons by effectively cascading the frequency conversion and electro-optical (EO) polarization coupling in a periodically poled lithium niobate crystal. The fidelity of the OAM state is measured to be higher than 95% before and after the frequency conversion. The results show that the OAM state of single photons can be modulated on demand by applying a transverse electric field across the nonlinear crystal. Our work demonstrates the integration and high performance of our proposed transformation interface, and paves a route toward connecting different remote nodes based on quantum memories in a large-scale OAM-encoded quantum network.

$(\mathrm{In},\mathrm{Ga})\mathrm{N}$-based light-emitting diodes (LEDs) are known to suffer from low electron-hole wave-function overlap due to a high piezoelectric field. Staggered $(\mathrm{In},\mathrm{Ga})\mathrm{N}$ quantum wells (QWs) are proposed to increase the wave-function overlap and improve the efficiency of LEDs, especially for long-wavelength emitters. In this work, we show evidence that the growth of staggered QWs by plasma-assisted molecular beam epitaxy has another beneficial effect, as it allows a reduction in the formation of defects, responsible for nonradiative Shockley-Read-Hall recombination, at the bottom interface of the QW. Staggered QWs comprised an $(\mathrm{In},\mathrm{Ga})\mathrm{N}$ layer of an intermediate $\mathrm{In}$ content between the barrier and the QW. We show that insertion of such a layer results in a significant increase of the luminescence intensity, even if the calculated wave-function overlap drops. We study the dependence of the thickness of such an intermediate-$\mathrm{In}$-content layer on photoluminescence intensity behavior. Staggered QWs exhibit increased cathodoluminescence homogeneity that is a fingerprint of a lower density of defects, in contrast to standard QWs for which a high density of dark spots is observed in QW-emission mapping. Transmission electron microscopy of standard QWs reveals the formation of basal-plane stacking faults and voids that can result from vacancy aggregation. A stepwise increase of the $\mathrm{In}$ content in staggered QWs prevents the formation of point defects and results in an increased luminescence efficiency. The $\mathrm{In}$-composition difference between the barrier and the well is, therefore, a key parameter to control the formation of point defects in the high-$\mathrm{In}$-content QWs, influencing the luminescence efficiency. Characteristics of the cyan laser diode (LD) utilizing staggered QWs are presented.