For a frequency-tunable two-qubit system, a controlled-Z (cz) gate can be realized by adiabatic driving of the qubit system through an avoided level crossing between an auxiliary state and computational levels. Here we theoretically propose a fast cz gate using a shortcut to adiabaticity (STA). Experimentally, the STA cz gate is implemented with a 52-ns control pulse for two coupled superconducting Xmon qubits. The measured fidelity of the STA cz gate is higher than 96.0% in both quantum-process tomography and randomized benchmarking. The protocol allows a flexible design of the evolution time and control waveform. We suggest that this “fast adiabatic” cz gate can be directly applied to other multiqubit quantum systems.

Waveguide-based spin-photon interfaces on the $\mathrm{Ga}\mathrm{As}$ platform have emerged as a promising system for a variety of quantum information applications directly integrated into planar photonic circuits. The coherent control of spin states in a quantum dot can be achieved by applying circularly polarized laser pulses that may be coupled into the planar waveguide vertically through radiation modes. However, proper control of the laser polarization is challenging since the polarization is modified through the transformation from the far field to the exact position of the quantum dot in the nanostructure. Here, we demonstrate polarization-controlled excitation of a quantum-dot electron spin and use that to perform coherent control in a Ramsey interferometry experiment. The Ramsey interference reveals an inhomogeneous dephasing time of $2.2\pm 0.1$ ns, which is comparable to the values so far only obtained in bulk media. We analyze the experimental limitations in spin initialization fidelity and Ramsey contrast and identify the underlying mechanisms.

Magnetic microscopy of malarial hemozoin nanocrystals is performed by optically detected magnetic resonance imaging of near-surface diamond nitrogen-vacancy centers. Hemozoin crystals are extracted from Plasmodium falciparum–infected human blood cells and studied alongside synthetic hemozoin crystals. The stray magnetic fields produced by individual crystals are imaged at room temperature as a function of the applied field up to 350 mT. More than 100 nanocrystals are analyzed, revealing the distribution of their magnetic properties. Most crystals ($96\mathrm{\%}$) exhibit a linear dependence of the stray-field magnitude on the applied field, confirming hemozoin’s paramagnetic nature. A volume magnetic susceptibility of $3.4\times {10}^{-4}$ is inferred with use of a magnetostatic model informed by correlated scanning-electron-microscopy measurements of crystal dimensions. A small fraction of nanoparticles (4/82 for Plasmodium falciparum–produced nanoparticles and 1/41 for synthetic nanoparticles) exhibit a saturation behavior consistent with superparamagnetism. Translation of this platform to the study of living Plasmodium-infected cells may shed new light on hemozoin formation dynamics and their interaction with antimalarial drugs.

Magnetostrictive ${\mathrm{Co}}_{77}{\mathrm{Fe}}_{23}$ films are fully suspended to produce free-standing, clamped-clamped, microbeam resonators. A negative or positive shift in the resonant frequency is observed for magnetic fields applied parallel or perpendicular to the length of the beam, respectively, confirming the magnetoelastic nature of the shift. Notably, the resonance shifts linearly with higher-bias fields oriented perpendicular to the beam’s length. Domain imaging elucidates the distinction in the reversal processes along the easy and hard axes. Together, these results suggest that through modification of the magnetic anisotropy, the frequency shift and angular dependence can be tuned, producing highly magnetic-field-sensitive resonators.

Directional transmission or amplification of microwave signals is indispensable in various applications involving sensitive measurements. In this work we show experimentally how to use a generic cavity optomechanical setup to nonreciprocally amplify microwave signals above 3 GHz in one direction by 9 dB and simultaneously attenuate the transmission in the opposite direction by 21 dB. We use a device including two on-chip superconducting resonators and two metallic drumhead mechanical oscillators. Application of four microwave pump-tone frequencies allows the design of constructive or destructive interference for a signal tone depending on the propagation direction. The device can also be configured as an isolator with lossless nonreciprocal transmission and 18 dB of isolation.

The lithium niobate integrated photonic platform has recently shown great promise in nonlinear optics on a chip scale. Here, we report second-harmonic generation in a high-$Q$ lithium niobate microring resonator through modal phase matching, with a conversion efficiency of $1500\mathrm{\%}{\mathrm{W}}^{-1}$. Our device also allows us to observe difference-frequency generation in the telecom band. Our work demonstrates the great potential of the lithium niobate integrated platform for nonlinear wavelength conversion with high efficiencies.

Scientists must overcome fundamental measurement problems if microfluidic devices are to become reliable and commercially viable. In particular, microfluidic devices require precise control over operating conditions such as flow rate, $v_v$, which is difficult to measure continuously and in situ. Given the small scales involved, state-of-the-art approaches generally require accurate models to infer $v_v$ on the basis of indirect measurements. However, such methods necessarily introduce model-form errors that dominate at the nl/min scale being targeted by the community. To address these problems, we develop a robust and largely assumption free scaling method that relates the fluorescence efficiency $\mathcal{I}$ of fluorophores to $v_v$ through a dosage parameter $\xi$, which depends on the flow rate and laser power. Notably, we show that this scaling relationship emerges as a universal feature from a general class of partial differential equations (PDEs) describing the experimental setup, which consists of an excitation beam and fluorescence detector. As a result, our approach avoids uncertainties associated with most modeling assumptions, e.g., the exact system geometry, the flow profile, the physics of fluorescence, etc. Moreover, the corresponding measurements remain valid down to the scale of 10 nl/min, with some devices potentially capable of reaching 1 nl/min. As an added benefit, the measurement procedure is mathematically simple, requiring a few trivial computations, as opposed to the full solution of a PDE. To support these claims, we discuss and quantify uncertainties associated with our method and present experimental results that confirm its validity.

Cobalt/poly(3-hexylthiophene,2,5-diyl) ($\mathrm{Co}$/$\mathrm{P}3\mathrm{HT}$) hybrid films with different $\mathrm{Co}$ thicknesses are prepared by magnetron sputtering combined with solution process. An anomalous magnetoresistance (MR) crossover phenomenon changing from the intrinsic negative sign of $\mathrm{Co}$ to positive with decreased $\mathrm{Co}$ thickness is observed. The electric transport measurement indicates that the weak localization (WL) effect gradually dominants the $\mathrm{Co}/\mathrm{P}3\mathrm{HT}$ at low temperature with decreased $\mathrm{Co}$ thickness. The MR test in different magnetic-field directions reveals the MR crossover phenomenon that results from the $\mathrm{Co}/\mathrm{P}3\mathrm{HT}$ contact due to the $\mathrm{Co}$ penetration and the formation of the reverse barrier. These results show the significant influence of the metal/organic semiconductor interface on the MR effect.

We propose and evaluate a new type of optical force microscope based on a standing-wave optical trap. Our microscope, calibrated in situ and operating in a dynamic mode, is able to trap, without heating, a single metallic nanoparticle of 150 nm that acts as a highly sensitive probe for external radiation pressure. An Allan-deviation-based stability analysis of the setup yields an optimal $0.1$-Hz measurement bandwidth over which the microscope is thermally limited. Over this bandwidth, and with a genuine sine-wave external drive, we demonstrate an optical force resolution down to $3$ fN in water at room temperature with a dynamical range for force detection that covers almost $2$ orders of magnitude. This resolution is reached in both the confined regime and the freely diffusing regime of the optical trap. In the latter, we measure induced displacements of ${10}^{-11}$ m on the trapped nanoparticle spatially confined within less than 25 nm along the optical axis.

The anisotropic magneto–Peltier effect (AMPE) and the anomalous Ettingshausen effect (AEE) are investigated in U-shaped $\mathrm{Ni}$ thin films of differing thickness and substrate by means of the lock-in-thermography (LIT) method. We establish a procedure to extract pure AMPE and AEE contributions, separated from other thermoelectric effects, for ferromagnetic thin films. The measurements of the magnetic-field-angle ${\theta }_H$ dependence of the LIT images clearly show that the temperature modulation induced by the AMPE (AEE) in the $\mathrm{Ni}$ films varies with a $\cos 2{\theta }_H$ $(\cos {\theta }_H)$ pattern, confirming the symmetry of the AMPE (AEE). The systematic LIT measurements with various substrates show that the AMPE-induced temperature modulation decreases with the increase in thermal conductivity of the substrates, whereas the AEE-induced temperature modulation is almost independent of the thermal conductivity, indicating that the heat loss into the substrates plays an important role in determining the magnitude of the AMPE-induced temperature modulation in thin films. Our experimental results are reproduced by numerical calculations based on a two-dimensional finite-element method. These findings provide a platform for investigating the AMPE and AEE in thin-film devices.

Quantum reservoir computing provides a framework for exploiting the natural dynamics of quantum systems as a computational resource. It can implement real-time signal processing and solve temporal machine-learning problems in general, which requires memory and nonlinear mapping of the recent input stream using the quantum dynamics in the computational supremacy region, where the classical simulation of the system is intractable. A nuclear-magnetic-resonance spin-ensemble system is one of the realistic candidates for such physical implementations, which is currently available in laboratories. In this paper, considering these realistic experimental constraints for implementing the framework, we introduce a scheme, which we call a spatial multiplexing technique, to effectively boost the computational power of the platform. This technique exploits disjoint dynamics, which originate from multiple different quantum systems driven by common input streams in parallel. Accordingly, unlike designing a single large quantum system to increase the number of qubits for computational nodes, it is possible to prepare a huge number of qubits from multiple but small quantum systems, which are operationally easy to handle in laboratory experiments. We numerically demonstrate the effectiveness of the technique using several benchmark tasks and quantitatively investigate its specifications, range of validity, and limitations in detail.

We experimentally and numerically investigate the breakup of a droplet at the entrance to a microfluidic bypass channel. The polydimethylsiloxane (PDMS) microchannels are manufactured using standard photolithography techniques and we employ a high-speed camera to visualize the interfacial dynamics of the droplets. Three-dimensional (3D) numerical simulations are carried out for conditions causing both droplet breakup and nonbreakup. The droplets are generated upstream of the bypass channel using a standard T junction and the subsequent interaction with the entrance to the bypass channel is studied. In particular, the effects of capillary number (Ca) and the relative width of the bypass channel with respect to the main channel $(\beta )$ are examined. The regimes of breakup and nonbreakup are experimentally plotted on the Ca−$\beta$ plane with the volume of the daughter droplets formed. The finger length in the bypass channel is also plotted on this regime map. There exists a critical value of Ca at constant $\beta$ and a critical value of $\beta$ at constant Ca at which droplet breakup occurs. Combining measurements with numerical simulations, we show that the droplet breakup occurs if the hydrodynamic pressure drops across the droplet and average shear stress overcomes the differential Laplace pressure. We also find that droplet breakup at the entrance to the bypass channel can exhibit squeezing and dripping regimes, which are equivalent to those occurring in droplet generation at a T junction.

P type doping is often difficult in nitrides due to the high electronegativity of nitrogen. Beryllium ($\mathrm{Be}$) is isovalent but more electronegative than $\mathrm{Mg}$, so it is hoped that if $\mathrm{Mg}$ is replaced by $\mathrm{Be}$ as the dopant in nitride semiconductors, the acceptor level would be shallower. However, surprisingly, it is observed that the $\mathrm{Be}$ acceptor level is actually much deeper than that of $\mathrm{Mg}$ in nitride semiconductors. Based on the density functional theory (DFT) calculations with a hybrid functional, we systematically investigate the doping behaviors of $\mathrm{Be}$ and $\mathrm{Mg}$ acceptors in $\mathrm{Ga}\mathrm{N}$ and $\mathrm{Al}\mathrm{N}$. We reveal that the large local structural distortion around the $\mathrm{Be}$ dopant, as well as the large covalency of the $\mathrm{Be}$—$\mathrm{N}$ bonds, are responsible for the deeper acceptor-transition-energy level of $\mathrm{Be}$ than that of $\mathrm{Mg}$ acceptors in $\mathrm{Ga}\mathrm{N}$. Our results, therefore, provide a comprehensive understanding of chemical trends of p type doping in nitride semiconductors.

The behaviors of spin-orbit-torque- (SOT) induced magnetization switching under an in-plane assisting magnetic field with varied directions are investigated in heavy metal ($\mathrm{Pt}$, $\mathrm{Ta})/{\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}/\mathrm{Mg}\mathrm{O}$ perpendicularly magnetized multilayers. When the assisting field direction is not collinear with that of the electric current, asymmetric SOT-induced switching between up-to-down and down-to-up magnetized states is observed and this is more prominent for the $\mathrm{Ta}$ sample. Such asymmetry is attributed to the transverse SOT effective field $H_T$, which is −566 (278) Oe per ${10}^8{\mathrm{A}/\mathrm{cm}}^2$ for the $\mathrm{Ta}$ ($\mathrm{Pt}$) sample as characterized by the harmonic Hall voltage measurements. The atomistic simulations using the Landau--Lifshitz--Gilbert equation with inclusion of the SOT terms adequately explain the asymmetric SOT switching. By exploring the SOT-induced magnetization switching in more general scenarios with arbitrary direction of the in-plane assisting field, our results have important implications for optimizing the functionality and efficiency of SOT devices.

We utilize an effective Hamiltonian formalism, within the Floquet scattering framework, to design a class of driving-induced nonreciprocal components with minimal complexity. In the high-driving-frequency limit, where our scheme is formally applicable, these designs demonstrate a leading-order nonreciprocal performance that is inversely proportional to the driving frequency. Surprisingly, the optimal nonreciprocal behavior persists also in the slow driving regime. Our approach highlights the importance of physical loops in the design of these driven nonreciprocal components.

The quantum-mechanical tunneling passing through the sub-2-nm-thick oxide in $\mathrm{Ge}/a$-$\mathrm{Ge}{\mathrm{O}}_2/\mathrm{Ge}$ structures is presented, using the first-principles scattering-state method, where $a$ stands for the amorphous phase. The suboxide interface layer (IL) between $\mathrm{Ge}$ and the dioxide region (DOX) does not play a critical role in blocking tunneling due to the presence of $\mathrm{Ge}$—$\mathrm{Ge}$ bonds in it. The thickness of DOX, where all the $\mathrm{Ge}$ has four $\mathrm{Ge}$—$\mathrm{O}$ bonds, is the effective tunneling-blocking thickness and the thickness for the thinnest usable $a$-$\mathrm{Ge}{\mathrm{O}}_2$ is approximately 0.85 nm. The width and magnitude of the band offset differently affect the tunneling in the sub-2-nm-thick oxide. The valence-band offset is larger and thicker than the conduction-band offset for all the structures, resulting in the smaller tunneling current of the holes than of the electrons. It is also found that the effect of the hydrogen passivation at the IL on tunneling is not evident in $\mathrm{semiconductor}/a$-oxide. The crystallographic orientation of $\mathrm{Ge}$ has no distinct effect on the band-gap alignment and the tunneling current in $\mathrm{Ge}/a$-$\mathrm{Ge}{\mathrm{O}}_2/\mathrm{Ge}$ structures, consistent with the experimental results about the effect of the $\mathrm{Ge}$ orientation on the interface properties.

Stochastic units based on magnetic tunnel junctions have shown a high energy-efficient pathway to perform neuromorphic computing. We propose a voltage-controlled spintronic stochastic device based on a magnetic tunnel junction by introducing perpendicular anisotropy into the free layer. We observe a stochastic behavior at low bias and demonstrate that this behavior can be used to mimic the artificial neurons of an artificial neural network to recognize the handwritten digits in the Mixed National Institute of Standards and Technology (MNIST) database. Furthermore, the stochastic behavior can be modulated by a bias voltage owing to the voltage-controlled magnetic anisotropy effect. The voltage-controlled stochastic behavior is theoretically and experimentally studied, which indicates that it can be used to perform as an adaptive neuron. These results provide a way for building energy-efficient spintronic neuromorphic computing systems.

We report on the implementation and detailed modeling of a Josephson parametric amplifier (JPA) made from an array of eighty superconducting quantum interference devices (SQUIDs), forming a nonlinear quarter-wave resonator. This device is fabricated using a very simple single-step fabrication process. It shows a large bandwidth (45 MHz), an operating frequency tunable between 5.9 and 6.8 GHz, and a large input saturation power ($-117\mathrm{dBm}$) when biased to obtain 20 dB of gain. Despite the length of the SQUID array being comparable to the wavelength, we present a model based on an effective nonlinear $LC$ series resonator that quantitatively describes these figures of merit without fitting parameters. Our work illustrates the advantage of using array-based JPA since a single-SQUID device showing the same bandwidth and resonant frequency would display a saturation power 15 dB lower.

Acoustic imaging is crucial in diverse applications, such as target tracking, clinical treatment, and structural health monitoring. Current approaches mainly use complex and large transducer arrays. A space-coiling anisotropic metamaterial with stochastic structural parameters is proposed to realize a single-detector acoustic camera. Analytical and numerical results demonstrate that the high anisotropy based on the space-coiling method enables the metamaterial to produce an orthogonal directional modulation for input acoustic waves. Thus, the information of sound sources can be reconstructed from the modulated mixed signals by means of a compressive sensing framework. Experimental results show that the localization and imaging of multiple broadband sources can be achieved in azimuthal and planar geometries. The metamaterial-based single-detector acoustic camera expands the capabilities of single-detector acoustic imaging, thereby opening avenues for designing an alternative class of acoustic sensing systems.

We propose a fast spin-initialization method for a single electron trapped in an electrostatic quantum dot. The dot is created in a nanodevice composed of a catalytically grown indium antimonide ($\mathrm{In}\mathrm{Sb}$) nanowire and nearby gates to which control voltages are applied. Initially, we insert a single electron of arbitrary spin into the wire. Operations on spin are performed using the Rashba spin-orbit interaction induced by an electric field. First, a single pulse of voltages applied to lateral gates is used to split the electron wave packet into two parts with opposite spin orientations. Next, another voltage pulse applied to the remaining gates rotates the spins of both parts in opposite directions by $\pi /2$. In this way, initially opposite-spin parts eventually point in the same direction, along the axis of the quantum wire. We thus set spin in a predefined direction regardless of its initial orientation. This is achieved in time less than $60\mathrm{ps}$, without the use of microwaves, photons, or external magnetic fields.