Because of its superior coherent and optical properties at room temperature, the nitrogen-vacancy ($\mathrm{N}$-$V$) center in diamond has become a promising quantum probe for nanoscale quantum sensing. However, the application of $\mathrm{N}$-$V$-containing nanodiamonds to quantum sensing suffers from their relatively short spin coherence times. Here we demonstrate energy-efficient protection of $\mathrm{N}$-$V$ spin coherence in nanodiamonds using concatenated continuous dynamical decoupling, which exhibits excellent performance with a less-stringent microwave-power requirement. When this is applied to nanodiamonds in living cells, we are able to extend the spin coherence time by an order of magnitude to the $T_1$ limit of $30\mu \mathrm{s}$. Further analysis demonstrates concomitant improvements of sensing performance, which shows that our results provide an important step toward in vivo quantum sensing using $\mathrm{N}$-$V$ centers in nanodiamond.

Gate leakage current is an issue for the reliability of modern high-$\kappa$ MOSFETs. Although various physical models describing both direct tunneling and trap-assisted contribution of leakage current have been presented in the literature, many of them treats traps in the dielectric as a continuum distribution in energy and position, and trap-to-trap transport of electrons has so far been mostly neglected or not treated three dimensionally. In this work, we present a mechanistic model for calculation of gate leakage current in high-$\kappa$ MOSFET multilayer stacks based on multiphonon trap-assisted tunneling theory, taking into account the intrinsic three-dimensional (3D) discreteness of traps in the dielectric. Our model can to a good approximation reproduce the experimental results at different dielectric thicknesses, gate voltages, temperatures, and different gate materials. We find that in realistic devices, the 3D trap-to-trap transport of electrons contributes a non-negligible part to the gate leakage current. This contribution is more pronounced at low-voltage device operations, which is important for low-power applications. We calculate the intrinsic fluctuation of gate leakage current due to positional and energetic disorder of traps in the dielectric, and conclude that positional disorder is more important than energetic disorder for realistic material parameters. The calculated gate leakage current depends sensitively on temperature, trap energy, and trap density. We provide a computationally efficient 3D master equation approach that enables 3D mechanistic simulation of ${10}^3$ traps on the order of minutes on a standard desktop computer.

Silicon spin qubits show great promise as a scalable qubit platform for fault-tolerant quantum computing. However, fast high-fidelity readout of charge and spin states, which is required for quantum error correction, has remained elusive. Radio-frequency reflectometry enables rapid high-fidelity readout of $\mathrm{Ga}\mathrm{As}$ spin qubits, but the large capacitances between accumulation gates and the underlying two-dimensional electron gas in accumulation-mode $\mathrm{Si}$ quantum-dot devices, as well as the relatively low two-dimensional electron gas mobilities, have made radio-frequency reflectometry challenging in these platforms. In this work, we implement radio-frequency reflectometry in a $\mathrm{Si}$/$\mathrm{Si}\text{−}\mathrm{Ge}$ quantum-dot device with overlapping gates by making minor device-level changes that eliminate these challenges. We demonstrate charge-state readout with a fidelity above $99.9\mathrm{\%}$ in an integration time of $300\mathrm{ns}$. We measure the singlet and triplet states of a double quantum dot via both conventional Pauli spin blockade and a charge latching mechanism, and we achieve maximum fidelities of $82.9$ and $99.0\mathrm{\%}$ in $2.08$- and $1.6$-$\mu \mathrm{s}$ integration times, respectively. We also use radio-frequency reflectometry to perform single-shot readout of single-spin states via spin-selective tunneling in microsecond-scale integration times.

We directly connect specific magnetic domain behaviors in magnetically modulated thin-film magnetoelectric composite sensors to the effective noise levels exhibited. Through simultaneous magnetoelectric response and time-resolved-magneto-optical-microscopy measurements, as well as additional complementary noise and limit of detection measurements, different regimes of magnetic noise with distinct magnetic domain activities are identified. Transitions between magnetic domain states with differing domain-wall densities and local effective magnetic anisotropy perturbations directly influence the magnetoelectric sensor signal and the effective noise level. By this, the limit of detection of the sensor deteriorates by orders of magnitude, depending on the discrete magnetic domain characteristics and interconnecting magnetic losses. We show that the performance of magnetic field sensors is dominated by the physical micromagnetic processes revealed. The underlying physical mechanisms should affect all magnetic-layer-based field-sensing devices.

High-density communication through optical fiber is made possible by wavelength-division multiplexing (WDM), which is the simultaneous transmission of many discrete signals at different optical frequencies. Vast quantities of data may be transmitted without interference using this scheme but flexible routing of these signals requires an electronic middle step, which carries a cost in latency. We present a technique for frequency conversion across the entire WDM spectrum with a single device, which removes this latency cost. Using an optical waveguide in lithium niobate and two infrared pump beams, we show how to maximize the conversion efficiency between arbitrary frequencies by analyzing the role of dispersion in cascaded nonlinear processes. The technique is presented generally and may be applied to any suitable nonlinear material or platform and to classical or quantum optical signals.

Determining the state of a qubit on a time scale much shorter than its relaxation time is an essential requirement for quantum information processing. With the aid of a nondegenerate parametric amplifier, we demonstrate the continuous detection of quantum jumps of a transmon qubit with $90\mathrm{\%}$ fidelity of state discrimination. Entirely fabricated by standard two-step optical-lithography techniques, this type of parametric amplifier consists of a dispersion-engineered Josephson-junction (JJ) array. By using long arrays, containing ${10}^3\mathrm{JJs}$, we can obtain amplification in multiple eigenmodes with frequencies below $10\mathrm{GHz}$, which is the typical range for qubit readout. Moreover, if a moderate flux tunability of each mode is introduced, employing superconducting-quantum-interference-device junctions, a single amplifier device could potentially cover the entire frequency band between 1 and 10 GHz.

We introduce a near-quantum-limited amplifier with a large tunable bandwidth and high dynamic range—the Josephson array-mode parametric amplifier (JAMPA). The signal and idler modes involved in the amplification process are realized by the array modes of a chain of 1000 flux-tunable Josephson-junction-based nonlinear elements. The frequency spacing between the array modes is comparable to the flux tunability of the modes, ensuring that any desired frequency can be occupied by a resonant mode, which can further be pumped to produce high gain. We experimentally demonstrate that the device can be operated as a nearly quantum-limited parametric amplifier with 20 dB of gain at almost any frequency within a 4–12-GHz band. On average, it has a 3-dB bandwidth of 11 MHz and an input 1-dB compression power of $-108\mathrm{dBm}$, which can go as high as $-93\mathrm{dBm}$. We envision the application of such a device to the time- and frequency-multiplexed readout of multiple qubits, as well as to the generation of continuous-variable cluster states.

To obtain a complete description of a quantum system, one usually employs standard quantum state tomography, which, however, requires an exponential number of measurements to be performed and hence is impractical when the system’s size grows large. In this paper, we introduce a self-learning tomographic scheme based on the variational hybrid quantum-classical method. The key part of the scheme is a learning procedure, in which we learn a control sequence capable of driving the unknown target state coherently to a simple fiducial state, so that the target state can be directly reconstructed by applying the control sequence in reverse. In this manner, the state tomography problem is converted to a state-to-state transfer problem. To solve the latter problem, we use a closed-loop-learning control approach. Our scheme is experimentally tested using techniques of four-qubit nuclear magnetic resonance. The experimental results indicate that the proposed tomographic scheme can handle a broad class of states, including entangled states in the field of quantum information, as well as dynamical states of quantum many-body systems common to condensed-matter physics.

The guided and rectified motion of magnetic flux quanta are important effects governing the magnetoresistive response of nanostructured superconductors. While at low ac frequencies these effects are rather well understood, their manifestations at higher ac frequencies remain poorly investigated. Here, we explore the upper frequency limits for guided and rectified net motion of superconducting vortices in epitaxial $\mathrm{Nb}$ films decorated with ferromagnetic nanostripes. By combining broadband electrical spectroscopy with resistance measurements, we reveal that the rectified voltage vanishes at a geometrically defined frequency of about 700 MHz. By contrast, a vortex-guiding-related low-ac-loss response persists up to about 2 GHz. This value corresponds to the depinning frequency $f_d^s$ associated with the washboard pinning potential induced by the nanostripes, which exhibits peaks for commensurate vortex-lattice configurations. By applying a sum of dc and microwave ac currents at an angle $\alpha$ with respect to the nanostripes, the angle dependence of $f_d^s(\alpha )$ is found to be correlated with the angle dependence of the depinning current. In all, our findings suggest that superconductors with higher $f_d^s$ should be favorable for efficient vortex manipulation in the gigahertz ac frequency range.

The photon count rate (PCR) of superconducting single-photon detectors made of ${\mathrm{Mo}}_x{\mathrm{Si}}_{1\text{−}x}$ films shaped as a 2-$\mu \mathrm{m}$-wide strip and a 115-nm-wide meander strip line is studied experimentally as a function of the dc biasing current at different values of the perpendicular magnetic field. For the wide strip, a crossover current $I_{\mathrm{cross}}$ is observed, below which the PCR increases with an increasing magnetic field and above which it decreases. This behavior contrasts with the narrow ${\mathrm{Mo}}_x{\mathrm{Si}}_{1\text{−}x}$ meander, for which no crossover current is observed, thus suggesting different photon-detection mechanisms in the wide and narrow strips. Namely, we argue that in the wide strip the absorbed photon destroys superconductivity locally via the vortex-antivortex mechanism for the emergence of resistance, while in the narrow meander superconductivity is destroyed across the whole strip line, forming a hot belt. Accordingly, the different photon-detection mechanisms associated with vortices and the hot belt determine the qualitative difference in the dependence of the PCR on the magnetic field.

Transverse mode locking is receiving renewed attention due to its combination with the formation of optical vortices. In previous investigations, transverse mode locking is focused only on the phase locking of modes in different orders to research the beam pattern dynamics or between frequency-degenerate transverse modes to form a spatially stationary beam pattern with optical vortices. It is experimentally shown that both phase and frequency locking between transverse modes in nonfrequency-degenerate families can be obtained by microchip cavities with high nonlinearity. It is also pointed out that, for a spatiotemporally stationary composed beam pattern formed by locking of transverse modes in different orders, not only should the frequency. but also the propagation parameters. of the beam be coupled.

We study the spin-orbit (SO) field in ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$(Permalloy; Py)/$\mathrm{W}/\mathrm{Pt}$ trilayer by means of spin-torque ferromagnetic resonance, and demonstrate that the $\mathrm{W}/\mathrm{Pt}$ interface generates an extra SO field that acts on the Py layer. This unprecedented field originates from the following three processes: (1) spin accumulation at the $\mathrm{W}/\mathrm{Pt}$ interface via the Rashba–Edelstein effect, (2) diffusive spin transport in the W layer, and (3) spin absorption into the Py layer through accumulation at the Py/$\mathrm{W}$ interface. Our results mean that we can create an extra SO field away from the ferromagnet/metal interface and control its strength by a combination of two different metals.

The performance of many control tasks with Rydberg atoms can be improved via suppression of the motion-induced dephasing between the ground state and Rydberg states of neutral atoms. This dephasing often occurs during the gap time when the atom is shelved in a Rydberg state before its deexcitation. This paper presents two theories about how to suppress this dephasing. First, by using laser fields to induce a specific extra phase change in the Rydberg state during the gap time, it is possible to faithfully transfer the Rydberg state back to the ground state after the gap. Although the Rydberg state transitions back and forth between different eigenstates during the gap time, it preserves the blockade interaction between the atom of interest and a nearby Rydberg excitation. This simple method of suppressing the motional dephasing of a flying Rydberg atom can be used in a broad range of methods of quantum control of neutral atoms. Second, we find that the motional dephasing can also be suppressed by using a transition in a “V”-type dual-rail configuration. The left (right) arm of this “V” represents a transition to a Rydberg state $|r_{1(2)}⟩$ with a Rabi frequency $\mathrm{\Omega }{e}^{ikz}(\mathrm{\Omega }{e}^{-ikz})$, where $z$ is frozen without atomic drift, but changes linearly in each experimental cycle. Such a configuration is equivalent to a transition between the ground state and a hybrid time-dependent Rydberg state with a Rabi frequency $\sqrt{2}\mathrm{\Omega }$, such that there is no phase error whenever the state returns to the ground state. We study two applications of the second theory: (i) it is possible to faithfully transfer the atomic state between a hyperfine ground state and Rydberg states $|r_{1(2)}⟩$ with no gap time between the excitation and deexcitation; and (ii) if infrared laser fields are added to induce a transition between $|r_{1(2)}⟩$ and a nearby Rydberg state $|r_3⟩$ via a greatly detuned low-lying intermediate state during the gap time, the atom can keep its internal state in the Rydberg level as well as adjust the population branching in $|r_{1(2)}⟩$ during the gap time. This allows an almost perfect Rydberg deexcitation after the gap time, making it possible to recover a high fidelity in a Rydberg blockade gate. These theories pave the way for high-fidelity quantum control over neutral Rydberg atoms without cooling qubits to motional ground states in optical traps.

Spin-dependent charge transport in magnetic tunnel junctions (MTJs) can be manipulated by a temperature gradient, which can be utilized for spintronic and spin caloritronic applications. Evaluation of the thermally induced phenomena requires knowledge of the temperature differences across the oxide tunnel barrier adjacent to the ferromagnetic (FM) leads. However, it is challenging to accurately measure thermal properties of an oxide tunnel barrier consisting of only a few atomic layers. In this work, we experimentally interrogate the temperature evolutions in $\mathrm{Ru}/\mathrm{oxide}/\mathrm{FM}/\mathrm{seed}/\mathrm{Mg}\mathrm{O}$ (oxide = $\mathrm{Mg}\mathrm{O}$, ${\mathrm{Mg}\mathrm{Al}}_2{\mathrm{O}}_4$; FM = $\mathrm{Co}$, ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$; seed = $\mathrm{Pt}$, $\mathrm{Ta}$) structures having perpendicular magnetic anisotropy using ultrafast thermometry. The $\mathrm{Ru}$ layer is optically thick and heated by ultrafast laser pulses; the subsequent temperature changes are monitored using the thermoreflectance of $\mathrm{Ru}$ and magneto-optic Kerr effect (MOKE) of the FM layers. We independently measure the response times of $\mathrm{Co}$ and $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ magnetism using quadratic MOKE and obtain ${\tau }_{e-m}=0.2$ ps for $\mathrm{Co}$ and 2 ps for $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$. These time scales are much shorter than the time scale of heat transport through the oxide tunnel barrier, which occurs at 10–3000 ps. We determine effective thermal conductivities of $\mathrm{Mg}\mathrm{O}$ and ${\mathrm{Mg}\mathrm{Al}}_2{\mathrm{O}}_4$ tunnel barriers in the range of 0.4–0.6 W ${\mathrm{m}}^{-1}$ ${\mathrm{K}}^{-1}$, comparable with an estimate of the series thermal conductance of the $\mathrm{Ru}/\mathrm{oxide}$ and oxide/FM interfaces and an order of magnitude smaller than the thermal conductivity of $\mathrm{Mg}\mathrm{O}$ thin films. We find that the electron-phonon thermal conductance near the tunnel barrier is only a factor of 5–12 larger than the thermal conductance of the oxide tunnel barrier. Therefore, the drop in the electronic temperature is approximately 20%–30% larger than the drop in the phonon temperature across the tunnel barrier.

Excited-state charge-transfer complexes (exciplexes) are actively exploited in organic optoelectronics to understand their nature in the solid state and improve device performance. However, the diffusion of exciplexes is not actively studied, despite its ever-growing importance in such devices due to the lack of apparent charge-transfer (CT) absorption. Here, we report, based on time-resolved photoluminescence spectroscopy, that energy transfer (ET) takes place directly from exciplexes to exciplexes; this is called exciplex-exciplex ET. Recent reports and our own observation of the subband-gap CT absorption in exciplex-forming systems support the direct exciplex-exciplex ET. The ET takes place dominantly via the Dexter-type exchange mechanism, as revealed by the exponential decrease of the ET rate constant with separation between the energy-donating exciplex and the energy-accepting exciplex.

A phononic crystal formed in a suspended membrane provides full confinement of hypersonic waves and thus realizes a range of chip-scale manipulations. In this paper, we demonstrate mode-resolved real-space characterization of mechanical-vibration properties in cavities and waveguide systems. Multiple resonant modes in various designed cavities are independently characterized, and wavelength-scale high-$Q$ resonances up to $Q=4200$ under atmospheric conditions are confirmed. This also reveals that the waveguide allows us to observe mode-resolved wave transmission and thereby drive evanescently coupled cavities. The methods offer a significant tool with which to build compact and low-power microwave phononic circuitry for applications to signal processing and hybrid quantum systems.

We report on an experimental study of current-induced switching in perpendicular magnetic random-access memory (MRAM) cells with variable resistance-area products (RAs). Our results show that in addition to spin-transfer torque (STT), current-induced self-heating and voltage-controlled magnetic anisotropy also contribute to switching and can explain the RA dependencies of the switching-current density and STT efficiency. Our findings suggest that thermal optimization of perpendicular MRAM cells can result in significant reduction of switching currents.

Identically sized $\mathrm{Co}$ quantum dots (QDs) are constructed on monolayer tungsten disulfide (${\mathrm{WS}}_2$), forming coupled heterostructures. Topographical images investigated by means of in situ scanning tunneling microscopy (STM) show a bias-dependent feature. First-principles-calculated binding energies combined with the simulated STM images identify that the $\mathrm{Co}$ QDs possess a unique magic number, with a tetrahedral ${\mathrm{Co}}_4$ configuration. Numerical differential conductance measured by means of scanning tunneling spectroscopy indicates a p-type doping and an Ohmic contact property for the ${\mathrm{Co}}_4/{\mathrm{WS}}_2$ system. The mechanism of the transport and conduction properties of the coupled heterostructures is further revealed by analyzing the work functions and interfacial interaction. Our findings offer some references for the controlled fabrication of identically sized zero-dimensional and two-dimensional heterostructures, and we propose a feasible strategy for Ohmic interface contact in nanodevices.

Transformation theory is a powerful tool for material design to control waves in electromagnetics, acoustics, and elastic mechanics. Deformable materials are demonstrated to guide the propagation of ultrasonic waves freely. Toothed whales, such as dolphins and porpoises, may manipulate directional beams by simply deforming their foreheads. However, existing materials face great challenges in realizing their bioacoustic function. Here, we propose a bioinspired semianalytical conformal acoustics that can predict the evolution of acoustic functions strictly. Based on this method, we design a series of acoustic steering and collimation models. The sound-speed distribution functions can be determined by conformal mappings, and the acoustic-beam steering and expansion are numerically confirmed. We further experimentally fabricate acoustic steering and collimation devices by using metamaterials. The experimental results show good agreement with theoretical predictions. The proposed bioinspired conformal acoustics may bridge the gap between an animal’s biosonar and artificial materials, which show potential application value in underwater acoustics, medical ultrasonography, and other related applications.