Radio-frequency (rf) coils are used in all clinical and research magnetic-resonance-imaging (MRI) systems to excite nuclear spins and to receive signals from them. The quality of imaging depends strongly on the signal-to-noise ratio (SNR) and the transmit efficiency of the coils. The birdcage volume coils used in most MRI scanners for homogeneous imaging of a sample are typically shielded from the external systems of the MRI scanner, i.e., the gradient coils, to confine the rf field within the region of interest. However, the near magnetic field of a conventional copper rf shield surrounding a birdcage coil interferes destructively with the primary field of the coil in the sample, which significantly limits the SNR and the transmit efficiency. In the work presented here, we theoretically study and experimentally demonstrate the possibility of creating an artificial magnetic rf shield for a birdcage coil with constructive interference in a sample. This effect is similar to the in-phase reflection of antenna far fields from a magnetic shield but affects the near field in MRI. We build an analytical model of a birdcage coil shielded with a cylindrical impedance boundary and analyze the conditions for increasing the efficiency of the coil by means of the shield. We conclude that by replacing a copper shield with an artificial magnetic one, it is possible to reduce the dissipative intrinsic losses of the coil and increase the power absorbed by the sample, which improves the efficiency. To demonstrate the effect, we perform a detailed numerical simulation and an experiment with a small birdcage in a 7-T ${}^{19}\mathrm{F}$ MRI system with a magnetic shield implemented as a periodic cylindrical metal structure with corrugations filled with a ceramic.

Vertical transistors with conductive-network electrodes composed of carbon- or metal-based nanowires or meshes are attractive because of their high current density, low operational voltage, and high degree of integration. However, the devices lack concise physical images to understand the operations and explicit design rules to achieve the necessary performance, such as sharp subthreshold swing and a large on:off ratio. Here, we develop a device theory with concise physical images, which are generally applicable for devices with organic or inorganic semiconductors. The simplified solution of Poisson’s equation reveals that the electrostatic potential at the semiconductor-dielectric interface is controlled by both the gate and drain field, behaving like a plucked string. The spacing between electrodes and the capacitance ratio between semiconductors and dielectrics are critical for achieving strong gate tunability of the interfacial potential, and such gate tunability can be maximized to achieve a sharp turn-on property toward the Boltzmann limit in the subthreshold regime. Above the threshold, the conduction channels in devices with Schottky contacts can change from the “L type” to “I type”, or vice versa, during scanning and the current-voltage relations can be well described by modifying classical transistor equations. The derived theories and equations agree well with the numerically simulated devices and reported experiments, revealing the physical images and providing explicit rules for designing, fabricating, and characterizing such transistors.

Teleportation is a fundamental concept of quantum mechanics with an important application in extending the range of quantum communication channels via quantum relay nodes. To be compatible with real-world technology such as secure quantum key distribution over fiber networks, such a relay node should ideally operate at gigahertz clock rates and accept time-bin-encoded qubits in the low-loss telecom band around 1550 nm. Here, we show that $\mathrm{In}\mathrm{As}$-$\mathrm{In}\mathrm{P}$ droplet-epitaxy quantum dots, with their sub-Poissonian emission near 1550 nm, are ideally suited for the realization of this technology. To create the necessary on-demand photon emission at gigahertz clock rates, we develop a flexible-pulsed optical-excitation scheme and demonstrate that the fast driving conditions are compatible with a low multiphoton emission rate. We show further that, even under these driving conditions, photon pairs obtained from the biexciton cascade show an entanglement fidelity close to 90%, comparable to the value obtained under continuous-wave excitation. Using asymmetric Mach-Zehnder interferometers and our photon source, we finally construct a time-bin qubit quantum relay able to receive and send time-bin-encoded photons and demonstrate mean teleportation fidelities of $0.82\pm 0.01$, exceeding the classical limit by more than ten standard deviations.

Valleytronics is an emerging field of research utilizing the valley-pseudospin degree of freedom. In this work, we propose to exploit spin-valley locking in two-dimensional materials, such as silicene, germanene, stanene, and 1T′ transition-metal dichalcogenides, to realize the logic devices with multiple voltage-controlled gate contacts. These materials possess space-inversion and time-reversal symmetries and have two valleys at the non-time-reversal invariant momenta, K and K′, related by time-reversal symmetry. Due to these properties, the valley-spin polarization in these materials can be switched by electric field, and the device conductance and the output voltage can be controlled by the polarity of the input gate voltage. Based on the explicit quantum-transport calculations, we demonstrate the realization of seven logic gates, namely not, xnor, xor, and, nand, or, and nor.

The thermalization of nonequilibrium charge carriers is at the heart of thermoelectric energy conversion. In nanoscale systems, the equilibration length can be on the order of the system size, leading to a situation where thermoelectric effects need to be considered as spatially distributed, rather than localized at junctions. The energy exchange between charge carriers and phonons is of fundamental scientific and technological interest, but their assessment poses significant experimental challenges. We address these challenges by imaging the temperature change induced by Peltier effects in crystal phase engineered $\mathrm{In}\mathrm{As}$ nanowire (NW) devices. Using high-resolution scanning thermal microscopy (SThM), we study current-carrying $\mathrm{In}\mathrm{As}$ NWs, which feature a barrier segment of wurtzite (WZ) of varying length in a NW of otherwise zincblende (ZB) crystal phase. The energy barrier acts as a filter for electron transport around the Fermi energy, giving rise to a thermoelectric effect. We find that thermalization through electron-phonon heat exchange extends over the entire device. We analyze the temperature profile along a nanowire by comparing it to spatially dependent heat diffusion and electron thermalization models. We are able to extract the governing properties of the system, including the electron thermalization length of $223\pm 9\mathrm{nm}$, Peltier coefficient and Seebeck coefficient introduced by the barrier of $39\pm 7\mathrm{mV}$ and $89\pm 21\mu \mathrm{V}/\mathrm{K}$, respectively, and a thermal conductivity along the wire axis of $8.9\pm 0.5$ W/m K. Finally, we compare two ways to extract the elusive thermal boundary conductance between the NW and underlying substrate.

We demonstrate mutual synchronization of Josephson oscillations in multiple stacks of intrinsic Josephson junctions of the cuprate superconductor ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Ca}\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$. Detailed analysis of the full polarization parameters allows the determination of a phase correlation between the stacks: a simultaneous emission state is described by a linear combination of individual emission states with a phase retardation. This proves that the stacks are coupled via a Josephson plasma in a superconducting substrate and that the coupling matrices can be extracted from polarization analyses. Our findings suggest a route towards the realization of high-power terahertz sources based on the synchronization of a large number of intrinsic Josephson junctions.

Experimentally quantifying entanglement and coherence are extremely important for quantum resource theory. However, because the quantum state tomography requires exponentially growing measurements with the number of qubits, it is hard to quantify entanglement and coherence based on the full information of the experimentally realized multipartite states. Fortunately, other methods have been found to directly measure the fidelity of experimental states without quantum state tomography. Here we present a fidelity-based method to derive experimentally accessible lower bounds for measures of genuine multipartite entanglement and coherence. On the one hand, the method works for genuine multipartite entanglement measures including the convex-roof extended negativity, the concurrence, the G-concurrence, and the geometric measure for genuine multipartite entanglement. On the other hand, the method also delivers observable lower bounds for the convex roof of the $l_1$-norm of coherence, the geometric measure of coherence, and the coherence of formation. Furthermore, all the lower bounds are based on the fidelity between the chosen pure state and the target state, and we obtain the lower bounds of several real experimental states as examples of our results.

While quantum dots are at the forefront of quantum-device technology, the tuning of multidot systems requires a lengthy experimental process as multiple parameters need to be accurately controlled. This process becomes increasingly time-consuming and difficult to perform manually as the devices become more complex and the number of tuning parameters grows. In this work, we present a crucial step toward automated tuning of quantum-dot qubits. We introduce an algorithm driven by machine learning that uses a small number of coarse-grained measurements as its input and tunes the quantum-dot system into a preselected charge state. We train and test our algorithm on a $\mathrm{Ga}\mathrm{As}$ double-quantum-dot device and we consistently arrive at the desired state or its immediate neighborhood.

Defining quantum dots in semiconductor-based heterostructures is an essential step in initializing solid-state qubits. With growing device complexity and increasing number of functional devices required for measurements, a manual approach to finding suitable gate voltages to confine electrons electrostatically is impractical. Here, we implement a two-stage device characterization and dot-tuning process, which first determines whether devices are functional and then attempts to tune the functional devices to the single or double quantum-dot regime. We show that automating well-established manual-tuning procedures and replacing the experimenter’s decisions by supervised machine learning is sufficient to tune double quantum dots in multiple devices without premeasured input or manual intervention. The quality of measurement results and charge states are assessed by four binary classifiers trained with experimental data, reflecting real device behavior. We compare and optimize eight models and different data preprocessing techniques for each of the classifiers to achieve reliable autonomous tuning, an essential step towards scalable quantum systems in quantum-dot-based qubit architectures.

We demonstrate that magnetic skyrmions with a mean diameter around 60 nm can be stabilized at room temperature and zero external magnetic field in an exchange-biased $\mathrm{Pt}$/$\mathrm{Co}$/${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$/${\mathrm{Ir}}_{20}{\mathrm{Mn}}_{80}$ multilayer stack. This is achieved through an advanced optimization of the multilayer-stack composition in order to balance the different magnetic energies controlling the skyrmion size and stability. Magnetic imaging is performed both with magnetic force microscopy and scanning nitrogen-vacancy magnetometry, the latter providing unambiguous measurements at zero external magnetic field. In such samples, we show that exchange bias provides an immunity of the skyrmion spin texture to moderate external-magnetic-field perturbations, in the tens-of-millitesla range, which is an important feature for applications such as memory devices. These results establish exchange-biased multilayer stacks as a promising platform toward the effective realization of memory and logic devices based on magnetic skyrmions.

Contactless manipulation of particles and cells using sound radiation forces that can be tuned and adjusted in real time has become important in various applications. These applications include display technology, biomedical sensors, imaging devices, and diagnostic tools. Phononic crystals have many properties that could be beneficial for tunable manipulation in a microfluidic channel. We use phononic crystals to tune sound fields in a microfluidic channel for controllable manipulation of microparticles and cells. An arbitrary stop-and-go motion of particles and cells along a predefined path in the channel is experimentally demonstrated. Analytical and computational modeling reveals how the resonances of phononic crystals tune the sound fields and radiation forces for the desired manipulations. These concepts and realizations of dynamic manipulation of particles in the microfluidic channel advance the development of the microfluidic channel for dynamic acoustic manipulation technologies, particularly benefiting tunable cell analysis.

In this manuscript, we develop a generalized theory for the scattering process produced by interface roughness on charge carriers that is suitable for any semiconductor heterostructure. By exploiting our experimental insights into the three-dimensional atomic landscape of $\mathrm{Ge}/\mathrm{Ge}\text{−}\mathrm{Si}$ heterointerfaces obtained by atom probe tomography, we are able to define the full set of interface parameters relevant to the scattering potential, including both the in-plane and axial correlation inside real diffuse interfaces. Our experimental findings indicate a partial coherence of the interface roughness along the growth direction within the interfaces. We show that it is necessary to include this feature, previously neglected by theoretical models, when heterointerfaces characterized by finite interface widths are taken into consideration. To show the relevance of our generalized scattering model in the physics of semiconductor devices, we implement it in a nonequilibrium Green’s function simulation platform to assess the performance of a $\mathrm{Ge}/\mathrm{Si}\text{−}\mathrm{Ge}$-based terahertz quantum cascade laser.

Artificial spin ices are ensembles of geometrically arranged interacting nanomagnets that have shown promising potential for the realization of reconfigurable magnonic crystals. Such systems allow for the manipulation of spin waves on the nanoscale and their potential use as information carriers. However, there are presently two general obstacles to the realization of artificial spin-ice-based magnonic crystals: the magnetic state of artificial spin ices is difficult to reconfigure and the magnetostatic interactions between the nanoislands are often weak, preventing mode coupling. We demonstrate, using micromagnetic modeling, that coupling a reconfigurable artificial spin-ice geometry made of weakly interacting nanomagnets to a soft magnetic underlayer creates a complex system exhibiting dynamically coupled modes. These give rise to spin-wave channels in the underlayer at well-defined frequencies, based on the artificial spin-ice magnetic state, which can be reconfigured. These findings open the door to the realization of reconfigurable magnonic crystals with potential applications for data transport and processing in magnonic-based logic architectures.

Unidirectional wave propagation can introduce strong direction-dependent energy responses, and thus, greatly benefits the construction of switching and logic devices. Such an effect has drawn much attention in acoustics, but has not been able to practically address the essential issue of insufficient transmission efficiency in the forward direction. Here, we present a one-way structure that experimentally realizes near-total forward acoustic wave transportation, while still maintaining a broadband high contrast ratio and single-mode property. The experimentally observed rectification is achieved with a comprehensive design of combining acoustic wave intermode transitions, selective-mode filtering, and cavity resonance for strong wave-structure interactions. The demonstrated simplicity and flexibility make this material an appealing integration solution, enabling an alternative control methodology in biomedical ultrasonography, high-performance acoustic sensors, and noise control.

Nonreciprocity of signal transmission enhances the capacity of communication channels and protects the transmission quality against possible signal instabilities, thus becoming an important component for ensuring coherent information processing. However, nonreciprocal transmission requires breaking time-reversal symmetry (TRS), which poses challenges of both practical and fundamental character that hinder progress. Here we report an alternative scheme for achieving broadband nonreciprocity using a specially engineered hybrid microwave cavity. The TRS breaking is realized via strong coherent coupling between a selected chiral mode in the microwave cavity and a single collective spin excitation (magnon) in a ferromagnetic yttrium iron garnet sphere. The nonreciprocity in transmission is observed to span nearly a 0.5-GHz frequency band, which outperforms by two orders of magnitude the previously achieved bandwidths. Our findings suggest a promising direction for robust coherent information processing in a broad range of systems in both the classical and the quantum regime.

Frequency-dependent linear-permittivity measurements are commonplace in the literature, providing key insights into the structure of dielectric materials. These measurements describe a material’s dynamic response to a small applied electric field. However, nonlinear dielectric materials are widely used for their responses to large applied fields, including switching in ferroelectric materials, and field tuning of the permittivity in paraelectric materials. These behaviors are described by nonlinear permittivity. Nonlinear-permittivity measurements are fraught with technical challenges because of the complex electrical coupling between a sample and its environment. Here, we describe a technique for measuring the complex nonlinear permittivity that circumvents many of the difficulties associated with other approaches. We validate this technique by measuring the nonlinear permittivity of a tunable ${\mathrm{Ba}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{Ti}\mathrm{O}}_3$ thin film up to 40 GHz and comparing our results with a phenomenological model. These measurements provide insight into the dynamics of nonlinear dielectric materials down to picosecond timescales.

Sensing vector magnetic fields is critical to many applications in fundamental physics, bioimaging, and material science. Magnetic field sensors exploiting nitrogen-vacancy (N-$V$) centers are particularly compelling as they offer high sensitivity and spatial resolution even at the nanoscale. Achieving vector magnetometry, however, often requires applying microwaves sequentially or simultaneously, limiting the sensors’ applications under cryogenic temperature. Here, we propose and demonstrate a microwave-free vector magnetometer that simultaneously measures all Cartesian components of a magnetic field using N-$V$ ensembles in diamond. In particular, the present magnetometer leverages the level anticrossing in the triplet ground state at 102.4 mT, allowing the measurement of both longitudinal and transverse fields with a wide bandwidth from zero to the megahertz range. Full vector sensing capability is proffered by modulating fields along the preferential N-$V$ axis and in the transverse plane and subsequent demodulation of the signal. This sensor exhibits a root-mean-square noise floor that approximately equals 300 $\mathrm{pT}/\sqrt{\mathrm{Hz}}$ in all directions. The present technique is broadly applicable to both ensemble sensors and potentially also to single-N-$V$ sensors, extending the vector capability to nanoscale measurements under ambient temperatures.

The construction of a photon-photon quantum phase gate based on photonic nonlinearity has long been a fundamental issue, and is vital for deterministic and scalable photonic quantum-information processing. It requires not only strong nonlinear interaction at the single-photon level but also suppressed phase noise and spectral entanglement for high gate fidelity. In this paper, we propose that a high-quality-factor microcavity with strong ${\chi }^{(2)}$ nonlinearity can be quantized to anharmonic energy levels and can be effectively treated as an artificial atom. Such an artificial atom has a size much larger than the photon wavelength, which enables passive and active ultrastrong coupling to traveling photons. A high-fidelity quantum control phase gate is realized by our mediating the phase between photons with an intermediate artificial atom in a photonic molecule structure. The scheme avoids two-photon emission and thus eliminates spectral entanglement and quantum phase noise. Experimental realization of the artificial atom can be envisioned on an integrated photonic chip and holds great potential for single-emitter-free, room-temperature quantum-information processing.

The current practice of manually tuning quantum dots (QDs) for qubit operation is a relatively time-consuming procedure that is inherently impractical for scaling up and applications. In this work, we report on the in situ implementation of a recently proposed autotuning protocol that combines machine learning (ML) with an optimization routine to navigate the parameter space. In particular, we show that a ML algorithm trained using exclusively simulated data to quantitatively classify the state of a double-QD device can be used to replace human heuristics in the tuning of gate voltages in real devices. We demonstrate active feedback of a functional double-dot device operated at millikelvin temperatures and discuss success rates as a function of the initial conditions and the device performance. Modifications to the training network, fitness function, and optimizer are discussed as a path toward further improvement in the success rate when starting both near and far detuned from the target double-dot range.

Receiver protectors (RPs) shield sensitive electronics from high-power incoming signals that might damage them. Typical RP schemes range from simple fusing and PIN diodes to superconducting circuits and plasma cells—each having a variety of drawbacks ranging from unacceptable system downtime and self-destruction to significant insertion losses and power consumption. Here, we theoretically propose and experimentally demonstrate a unique self-shielding RP based on a coupled-resonator microwave waveguide, with a topological defect being inductively coupled to a diode. This RP utilizes a charge-conjugation- ($\mathcal{C}$) symmetric resonant-defect mode that is robust against disorder and demonstrates high transmittance at low incident powers. When the incident power exceeds a critical value, a self-induced resonant trapping effect occurs, leading to a dramatic suppression of transmittance and a simultaneous increase of the reflectance close to unity. The proposed RP device is self-protected from overheating and electrical breakdown and can be utilized in radars, reflection altimeters, and a broad range of communication systems.