A challenge for constructing large circuits of superconducting qubits is to balance addressability, coherence, and coupling strength. High coherence can be attained by building circuits from fixed-frequency qubits; however, leading techniques cannot couple qubits that are far detuned. Here, we introduce a method based on a tunable bus which allows for the coupling of two fixed-frequency qubits even at large detunings. By parametrically oscillating the bus at the qubit-qubit detuning we enable a resonant exchange ($XX+YY$) interaction. We use this interaction to implement a 183-ns two-qubit iswap gate between qubits separated in frequency by 854 MHz, with a measured average fidelity of 0.9823(4) from interleaved randomized benchmarking. This gate may be an enabling technology for surface-code circuits and for analog quantum simulation.

Using full 3D finite-element simulation and underlining Hamiltonian models, we demonstrate reconfigurable photonic analogues of topological insulators on a regular lattice of tunable posts in a reentrant 3D lumped element-type system. The tunability allows a dynamical in situ change of media chirality and other properties via the alteration of the same parameter for all posts, and as a result, great flexibility in the choice of bulk-edge configurations. Additionally, one-way photon transport without an external magnetic field is demonstrated. The ideas are illustrated by using both full finite-element simulation as well as simplified harmonic oscillator models. Dynamical reconfigurability of the proposed systems paves the way to a class of systems that can be employed for random access, topological signal processing, and sensing.

Materials with designed microscale architectures, like microlattices, can achieve extreme mechanical properties. Most studies of microlattices focus on their quasistatic response, but their structural dimensions naturally prime them for ultrasonic applications. Here we report that microlattices constitute a class of acoustic metamaterials that exploit elastoacoustic hybridization to tailor ultrasonic wave propagation. Selecting the microlattice geometry allows the formation of hybridization band gaps that effectively attenuate (by $>2$ orders of magnitude) acoustic signals. The hybridization gaps stem from the interaction of pressure waves in a surrounding medium (e.g., water) with localized bending modes of the trusses in the microlattice. Outside these band gaps, the microlattices are highly transmissive ($>80\%$) because their acoustic impedance is close to that of water. Our work can have important implications in the design of acoustic metamaterial applications in biomedical imaging, cell-based assay technology, and acoustic isolators in microelectromechanical systems.

We introduce an modular fixture designed for stress-controlled rheometers to perform simultaneous rheological and electrical measurements on strongly conductive complex fluids under shear. By means of a nontoxic liquid metal at room temperature, the electrical connection to the rotating shaft is completed with minimal additional mechanical friction, allowing for simultaneous stress measurements at values as low as 1 Pa. Motivated by applications such as flow batteries, we use the capabilities of this design to perform an extensive set of rheoelectric experiments on gels formulated from attractive carbon-black particles, at concentrations ranging from 4 to 15 wt %. First, experiments on gels at rest prepared with different shear histories show a robust power-law scaling between the elastic modulus $G_0^{\prime }$ and the conductivity ${\sigma }_0$ of the gels—i.e., $G_0^{\prime }\sim {\sigma }_0^{\alpha }$, with $\alpha =1.65\pm 0.04$, regardless of the gel concentration. Second, we report conductivity measurements performed simultaneously with creep experiments. Changes in conductivity in the early stage of the experiments, also known as the Andrade-creep regime, reveal for the first time that plastic events take place in the bulk, while the shear rate $\dot{\gamma }$ decreases as a weak power law of time. The subsequent evolution of the conductivity and the shear rate allows us to propose a local yielding scenario that is in agreement with previous velocimetry measurements. Finally, to establish a set of benchmark data, we determine the constitutive rheological and electrical behavior of carbon-black gels. Corrections first introduced for mechanical measurements regarding shear inhomogeneity and wall slip are carefully extended to electrical measurements to accurately distinguish between bulk and surface contributions to the conductivity. As an illustrative example, we examine the constitutive rheoelectric properties of five different grades of carbon-black gels and we demonstrate the relevance of this rheoelectric apparatus as a versatile characterization tool for strongly conductive complex fluids and their applications.

Over the past decade, spiking neural networks (SNNs) have emerged as one of the popular architectures to emulate the brain. In SNNs, information is temporally encoded and communication between neurons is accomplished by means of spikes. In such networks, spike-timing-dependent plasticity mechanisms require the online programing of synapses based on the temporal information of spikes transmitted by spiking neurons. In this work, we propose a spintronic synapse with decoupled spike-transmission and programing-current paths. The spintronic synapse consists of a ferromagnet–heavy-metal heterostructure where the programing current through the heavy metal generates spin-orbit torque to modulate the device conductance. Low programing energy and fast programing times demonstrate the efficacy of the proposed device as a nanoelectronic synapse. We perform a simulation study based on an experimentally benchmarked device-simulation framework to demonstrate the interfacing of such spintronic synapses with CMOS neurons and learning circuits operating in the transistor subthreshold region to form a network of spiking neurons that can be utilized for pattern-recognition problems.

We present a systematic study of properties of common native point defects in GaN based on hybrid density-functional calculations. These defects include vacancies, interstitials, antisites, and common complexes. Using configuration coordinate diagrams, we estimate the likelihood of defects to be radiative or nonradiative. Our results show that gallium vacancies exhibit a large magnetic moment in the neutral charge state and are most likely nonradiative. This is in contrast to nitrogen vacancies, which are probable sources of the experimentally observed green-luminescence band peaking at 2.35 eV in undoped GaN. We also show that infrared photoluminescence (PL) bands that are created by 2.5-MeV electron irradiation in GaN can be explained by the formation of native defects. Namely, the interstitial gallium is likely to be responsible for the narrow infrared PL band centered around 0.85 eV, with a phonon fine structure at 0.88 eV; the gallium-nitrogen divacancies are possible sources of the broad PL band with a peak at 0.95 eV.

We investigate the photoinduced spin dynamics of single nitrogen-vacancy ($\mathrm{N}\text{−}V$) centers in diamond near the electronic ground-state level anticrossing (GSLAC), which occurs at an axial magnetic field around 1024 G. Using optically detected magnetic resonance spectroscopy, we first find that the electron-spin transition frequency can be tuned down to 100 kHz for the ${}^{14}\mathrm{N}\text{−}V$ center, while, for the ${}^{15}\mathrm{N}\text{−}V$ center, the transition strength vanishes for frequencies below about 2 MHz owing to the GSLAC structure. Using optical pulses to prepare and read out the spin state, we observe coherent spin oscillations at 1024 G for the ${}^{14}\mathrm{N}\text{−}V$ center which originate from spin mixing induced by residual transverse magnetic fields. This effect is responsible for limiting the smallest observable transition frequency, which can span 2 orders of magnitude ranging from 100 kHz to tens of megahertz, depending on the local magnetic noise. A similar feature is observed for the ${}^{15}\mathrm{N}\text{−}V$ center at 1024 G. As an application of these findings, we demonstrate all-optical detection and spectroscopy of externally generated fluctuating magnetic fields at frequencies ranging from 8 MHz down to 500 kHz using a ${}^{14}\mathrm{N}\text{−}V$ center. Since the Larmor frequency of most nuclear-spin species lies within this frequency range near the GSLAC, these results pave the way towards all-optical, nanoscale nuclear magnetic resonance spectroscopy, using longitudinal spin cross-relaxation.

Organic field-effect transistors (FETs) with high-field-effect mobility (${\mu }_{\mathrm{FET}}$) and on:off ratio are attractive for flexible electronics. In this paper, we propose a design principle for manipulating transistor operations to in situ improve both the ${\mu }_{\mathrm{FET}}$ and on:off ratio of polymer transistors using nonuniformly distributed charges along an insulator electret layer between semiconductor and dielectric layers. Such nonuniform electrets obtained via applying gate-voltage stress with specific source-drain voltages provide a nonuniform electric field across the semiconductor layer and, thus, serve as a “nonuniform floating gate”. As compared with transistors without electrets or with uniform electrets, nonuniform electrets lead to a different device operation; therefore, a much narrower gate-voltage range can be sufficient to switch the transistor between the on and off states representing a higher switching speed. The apparent ${\mu }_{\mathrm{FET}}$ of poly(3-hexylthiophene) semiconductor film is significantly improved by 1 order to higher than $1{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, with simultaneously improving the on:off ratio up to ${10}^8$.

We propose and demonstrate the use of a pair of detuned acoustic resonators to efficiently absorb narrow-band sound waves in a terminated waveguide. The suggested configuration is relatively simple and advantageous for usage at low frequencies, since the dimensions of the resonators are very small compared to the wavelength. We present a theoretical description based on lumped parameters to calculate the absorption coefficient, which agrees very well with experimental data. The experimental results verify that the anechoic (reflection approximately $-38\mathrm{dB}$) narrow-band ($\mathrm{\Delta }f/f\sim 0.1$) termination with deeply subwavelength ($<\lambda /10$) sizes can be realized at a target frequency, suggesting thereby applications for noise control and sensing. As an illustration of possible applications for sound absorption in a room, we demonstrate by use of numerical simulations that a given axial resonant excitation in a room can be practically eliminated. Thus, a reduction of approximately 24 dB in the average acoustic energy is achieved in the room when using only four Helmholtz resonators. We also discuss various scenarios of noise control in rooms.

The development of high-resolution Fabry-Pérot interferometers has enabled a wide range of scientific and technological advances—ranging from the characterization of material properties to the more fundamental studies of quasiparticles in condensed matter. Spectral contrast is key to measuring weak signals and can reach a ${10}^3$ peak-to-background ratio in a single-pass assembly. At its heart, this limit is a consequence of an unbalanced field amplitude across multiple interfering paths, with an ensuing reduced fringe visibility. Using a high-resolution, high-throughput virtually imaged phased array spectrometer, we demonstrate an intensity-equalization method to achieve an unprecedented 1000-fold increase in spectral contrast in a single-stage, single-pass configuration. To validate the system, we obtain the Brillouin spectrum of water at high scattering concentrations where, unlike with the standard scheme, the inelastic peaks are highly resolved. Our method brings the interferometer close to its ultimate limits and allows rapid high-resolution spectral analysis in a wide range of fields, including Brillouin spectroscopy, mechanical imaging, and molecular fingerprinting.

We perform microwave imaging using a dynamically reconfigurable aperture based on a tunable disordered cavity. The electrically large cavity is cubic with a spherical deformation and supports a multitude of distinct electromagnetic modes that vary as a function of excitation frequency. With a set of irises introduced into one wall of the cavity, the cavity modes couple to spatially distinct radiative modes that vary as a function of the driving frequency. To increase the diversity of the radiated fields, we replace one of the cavity walls with a variable impedance surface consisting of a set of varactor-populated mushroom structures grouped into pixels. The reflection phase of each pixel is independently changed with application of a voltage bias, effectively altering the surface impedance. We demonstrate high-fidelity imaging and examine the role of the impedance-tunable boundary condition, revealing superior performance in comparison with just frequency-diverse measurements. We also demonstrate single-frequency imaging, which can significantly reduce the demands on the required microwave source. The dynamic cavity imager may find relevance in security screening, through-wall imaging, biomedical diagnostics, and radar applications.

Photoinduced atomic structural transitions of negative-$U$ defects: neutral oxygen vacancies (${V_{\mathrm{O}}}^0$), accompanied by lattice relaxation, can form ionized $1+$ and $2+$ vacancy defects in ZnO materials, giving rise to an optoelectronic phenomenon named “persistent photoconductivity,” thereby limiting the applications of ZnO materials in optoelectronic fields. Nevertheless, very little is known about the kinetics of the separation-recombination interactions between an electron and an ionized oxygen vacancy, constituting a photoexcited charge pair, in nanoscale ZnO material systems, especially when considering the effect of electric fields. In this report, we describe the charge-separation kinetics of photoexcited ${V_{\mathrm{O}}}^0$ defects in ZnO nanowire (NW) field-effect transistor (FET) systems, examined through modulation of the surface electric field of the ZnO NW. We apply oxygen plasma treatment to tailor the doping concentration within the ZnO NWs with the goal of modulating the electric field within their surface space-charge layers. X-ray photoelectron spectroscopy and low-frequency current-noise spectroscopy are applied to identify the change in the density of oxygen-vacancy defects near the NW surface after oxygen plasma treatment. A model describing the initial stage of the photoconductance responses associated with the formation of the photoinduced ionized $1+$ state of the oxygen-vacancy defects (${V_{\mathrm{O}}}^{+}$) in the fully depleted ZnO NW FETs in the low-photoconductance regime upon UV excitation is proposed to extract the charge-separation probabilities of the photoexcited $\text{electron}/{V_{\mathrm{O}}}^{+}$ pair. Accordingly, the charge-separation probability increases from approximately 0.0012 to 0.042 upon increasing the electric field at the NW surface from approximately $7.5\times {10}^6$ to $5.0\times {10}^7\mathrm{V}{\mathrm{m}}^{-1}$. Moreover, we employ modified Braun empirical theory to model the effect of the electric field on the charge-separation behavior of photoexcited $\text{electron}/{V_{\mathrm{O}}}^{+}$ pairs in ZnO NWs, obtaining a reactivity parameter of $4\times {10}^4\mathrm{m}{\mathrm{s}}^{-1}$. In addition, the mechanism of thermally activated charge capture associated with the recovery of photoexcited oxygen vacancies ${V_{\mathrm{O}}}^{2+}$ in ZnO NW FETs is also investigated, obtaining a thermally activated barrier and a capture cross-section prefactor of approximately 100 meV and approximately $5\times {10}^{-27}{\mathrm{cm}}^2$, respectively, for the transition of ionized oxygen vacancies from charged $2+$ to $1+$ oxygen-vacancy states. This report provides insight into the fundamental physics behind the effect of the electric field on the charge separation and recombination kinetics of photoexcited $\text{electron}/{V_{\mathrm{O}}}^{+}$ pairs and physical information regarding the mechanism of the persistent-photoconductivity recovery in ZnO NW FET systems, potentially benefiting the future applications of nanoscale ZnO materials in optoelectronic devices, phototransistors, and photodetectors.

We report on the fabrication of single-electron transistors using InAs nanowires with epitaxial aluminum with fixed tunnel barriers made of aluminum oxide. The devices exhibit a hard superconducting gap induced by the proximized aluminum cover shell, and they behave as metallic single-electron transistors. In contrast to the typical few-channel contacts in semiconducting devices, our approach forms opaque multichannel contacts to a semiconducting wire and, thus, provides a complementary way to study them. In addition, we confirm that unwanted extra quantum dots can appear at the surface of the nanowire. Their presence is prevented in our devices and also by inserting a protective layer of GaAs between the InAs and Al, the latter being suitable for standard measurement methods.

We combine the fields of heuristic optimization and optimal stopping. We propose a strategy for benchmarking randomized optimization algorithms that minimizes the expected total cost for obtaining a good solution with an optimal number of calls to the solver. To do so, rather than letting the objective function alone define a cost to be minimized, we introduce a further cost-per-call of the algorithm. We show that this problem can be formulated using optimal stopping theory. The expected cost is a flexible figure of merit for benchmarking probabilistic solvers that can be computed when the optimal solution is not known and that avoids the biases and arbitrariness that affect other measures. The optimal stopping formulation of benchmarking directly leads to a real-time optimal-utilization strategy for probabilistic optimizers with practical impact. We apply our formulation to benchmark simulated annealing on a class of maximum-2-satisfiability (MAX2SAT) problems. We also compare the performance of a D-Wave 2X quantum annealer to the Hamze-Freitas-Selby (HFS) solver, a specialized classical heuristic algorithm designed for low-tree-width graphs. On a set of frustrated-loop instances with planted solutions defined on up to $N=1098$ variables, the D-Wave device is 2 orders of magnitude faster than the HFS solver, and, modulo known caveats related to suboptimal annealing times, exhibits identical scaling with problem size.

We report on a systematic study of contact-induced spin relaxation in gated graphene nonlocal spin valves. We demonstrate the enhancement of the nonlocal magnetoresistance ($\mathrm{\Delta }{R}_{\mathrm{NL}}$) as the $\mathrm{Co}/\mathrm{Al}{\mathrm{O}}_x/\text{graphene}$ interface resistance increases relative to the graphene spin resistance. We measure Hanle precession at many gate voltages on 14 separate spin-valve devices fabricated from graphene grown by chemical vapor deposition (CVD). These measurements are compared by normalizing $\mathrm{\Delta }{R}_{\mathrm{NL}}$ to the ideal limit of large contact resistance, and the result is shown to be consistent with isotropic contact-induced spin relaxation caused by spin current flowing from the graphene into the Co contacts. After accounting for this source of spin relaxation, we extract spin lifetimes of up to 600 ps in CVD graphene with a gate-voltage dependence which can be described by a combination of both Elliott-Yafet and D’yakonov-Perel’ spin-relaxation mechanisms.

The design of a mesoscopic self-oscillating heat engine that works thanks to purely quantum effects is presented. The proposed scheme is amenable to experimental implementation with current state-of-the-art nanotechnology and materials. One of the main features of the structure is its versatility: The engine can deliver work to a generic load without galvanic contact. This versatility makes it a promising building block for low-temperature on-chip energy-management applications. The heat engine consists of a circuit featuring a thermoelectric element based on a ferromagnetic insulator-superconductor tunnel junction and a Josephson weak link that realizes a purely quantum dc-ac converter. This makeup enables the contactless transfer of work to the load (a generic RL circuit). The performance of the heat engine is investigated as a function of the thermal gradient applied to the thermoelectric junction. Power up to 1 pW can be delivered to a load $R_L=10\mathrm{\Omega }$.

We demonstrate a 12-quantum-dot device fabricated on an undoped Si/SiGe heterostructure as a proof of concept for a scalable, linear gate architecture for semiconductor quantum dots. The device consists of nine quantum dots in a linear array and three single-quantum-dot charge sensors. We show reproducible single-quantum-dot charging and orbital energies, with standard deviations less than 20% relative to the mean across the nine-dot array. The single-quantum-dot charge sensors have a charge sensitivity of $8.2\times {10}^{-4}e/\sqrt{\mathrm{Hz}}$ and allow for the investigation of real-time charge dynamics. As a demonstration of the versatility of this device, we use single-shot readout to measure the spin-relaxation time $T_1=170\mathrm{ms}$ at a magnetic field $B=1\mathrm{T}$. By reconfiguring the device, we form two capacitively coupled double quantum dots and extract a mutual charging energy of $200\mu \mathrm{eV}$, which indicates that 50-GHz two-qubit gate-operation speeds are feasible.

Nuclear magnetic resonance (NMR) has enormous applications. Two-dimensional NMR is an essential technique to characterize correlations between nuclei and, hence, molecule structures. Towards the ultimate goal of single-molecule NMR, dynamical-decoupling- (DD) enhanced diamond quantum sensing enables the detection of single nuclear spins and nanoscale NMR. However, there is still the lack of a standard method in DD-based quantum sensing to characterize correlations between nuclear spins in single molecules. Here we present a scheme of two-dimensional DD-based quantum sensing, as a universal method for correlation spectroscopy of single molecules. We design two-dimensional DD sequences composed of two sets of periodic DD sequences with different periods, which can be independently set to match two different transition frequencies for resonant DD. We find that under the resonant DD condition the sensor coherence patterns, as functions of the two independent pulse numbers of DD subsequences, can fully determine different types of correlations between nuclear spin transitions. This work offers a systematic approach to correlation spectroscopy for single-molecule NMR.

Microrefrigerators that operate in the subkelvin regime are key devices in quantum technology. A well-studied candidate, an electronic cooler using normal-metal–insulator–superconductor ($N\text{−}I\text{−}S$) tunnel junctions, offers substantial performance and power. However, its superconducting electrodes are severely overheated due to exponential suppression of their thermal conductance towards low temperatures, and the cooler performs unsatisfactorily—especially in powerful devices needed for practical applications. We employ a second $N\text{−}I\text{−}S$ cooling stage to thermalize the hot superconductor at the backside of the main $N\text{−}I\text{−}S$ cooler. Not only providing a lower bath temperature, the second-stage cooler actively evacuates quasiparticles out of the hot superconductor, especially in the low-temperature limit. We demonstrate the apparent advantage of our approach. This cascade design can also be employed to manage excess heat in other cryoelectronic devices.

Optical microcavities are a powerful tool for enhancing the fluorescence of individual quantum emitters. However, the broad emission spectra encountered in the solid state at room temperature limit the influence of a cavity, calling for an ultrasmall mode volume. We demonstrate Purcell-enhanced single-photon emission from nitrogen-vacancy centers in nanodiamonds coupled to a tunable fiber-based microcavity with a mode volume down to $1.0{\lambda }^3$. We record cavity-enhanced fluorescence images and study several single emitters with one cavity. The Purcell effect is evidenced by enhanced fluorescence collection and tunable lifetime modification, and we infer an effective Purcell factor of up to 2. Furthermore, we show an alternative regime for light confinement, where a Fabry-Perot mode is combined with additional mode confinement by the nanocrystal itself. Simulations predict effective Purcell factors of up to 11 for nitrogen-vacancy centers and 63 for silicon-vacancy centers, holding promise for bright single-photon sources and efficient spin readout under ambient conditions.