The recent development of arrays of quantum dots in semiconductor nanostructures highlights the progress of quantum devices toward a large scale. However, how to realize such arrays on a scalable platform such as silicon is still an open question. One of the main challenges lies in the detection of charges within the array. It is a prerequisite to initialize a desired charge state and read out spins through spin-to-charge conversion mechanisms. In this work, we use two methods based on either a single-lead charge detector or a reprogrammable single-electron transistor. By these methods, we study the charge dynamics and sensitivity by performing single-shot detection of the charge. Finally, we can probe the charge stability at any node of a linear array and assess the Coulomb disorder in the structure. We find an electrochemical potential fluctuation induced by charge noise comparable to that reported in other silicon quantum dots.

The combination of machine learning and quantum computing has emerged as a promising approach for addressing previously untenable problems. Reservoir computing is an efficient learning paradigm that utilizes nonlinear dynamical systems for temporal information processing, i.e., processing of input sequences to produce output sequences. Here we propose quantum reservoir computing that harnesses complex dissipative quantum dynamics. Our class of quantum reservoirs is universal, in that any nonlinear fading memory map can be approximated arbitrarily closely and uniformly over all inputs by a quantum reservoir from this class. We describe a subclass of the universal class that is readily implementable using quantum gates native to current noisy gate-model quantum computers. Proof-of-principle experiments on remotely accessed cloud-based superconducting quantum computers demonstrate that small and noisy quantum reservoirs can tackle high-order nonlinear temporal tasks. Our theoretical and experimental results pave the path for attractive temporal processing applications of near-term gate-model quantum computers of increasing fidelity but without quantum error correction, signifying the potential of these devices for wider applications including neural modeling, speech recognition, and natural language processing, going beyond static classification and regression tasks.

We report a first-principles density-functional study of electron-phonon interactions in and thermoelectric transport properties of the full Heusler compounds ${\mathrm{Sr}}_2\mathrm{Bi}\mathrm{Au}$ and ${\mathrm{Sr}}_2\mathrm{Sb}\mathrm{Au}$. Our results show that ultrahigh intrinsic bulk thermoelectric performance across a wide range of temperatures is physically possible and point to the presence of multiply degenerate and highly dispersive carrier pockets as the key factor for achieving this. ${\mathrm{Sr}}_2\mathrm{Bi}\mathrm{Au}$, which features ten energy-aligned low-effective-mass pockets (six along $\mathrm{\Gamma }\text{−}X$ and four at $L$), is predicted to deliver $n$-type $zT=0.4$–$4.9$ at $T=100$–$700$ K. Comparison with the previously investigated compound ${\mathrm{Ba}}_2\mathrm{Bi}\mathrm{Au}$ shows that the additional $L$ pockets in ${\mathrm{Sr}}_2\mathrm{Bi}\mathrm{Au}$ significantly increase its low-temperature power factor to a maximum value of $12$ mW ${\mathrm{m}}^{-1}$ ${\mathrm{K}}^{-2}$ near $T=300$ K. However, at high temperatures the power factor of ${\mathrm{Sr}}_2\mathrm{Bi}\mathrm{Au}$ drops below that of ${\mathrm{Ba}}_2\mathrm{Bi}\mathrm{Au}$ because the $L$ states are heavier and subject to strong scattering by phonon deformation, as opposed to the lighter $\mathrm{\Gamma }\text{−}X$ states, which are limited by polar-optical scattering. ${\mathrm{Sr}}_2\mathrm{Sb}\mathrm{Au}$ is predicted to deliver a lower $n$-type $zT=3.4$ at $T=750$ K due to appreciable misalignment between the $L$ and $\mathrm{\Gamma }\text{−}X$ carrier pockets, generally heavier scattering, and a slightly higher lattice thermal conductivity. Soft acoustic modes, which are responsible for the low lattice thermal conductivity, also increase the vibrational entropy and high-temperature stability of these Heusler compounds, suggesting that their experimental synthesis may be feasible. The dominant intrinsic defects are found to be $\mathrm{Au}$ vacancies, which drive the Fermi level towards the conduction band and work in favor of $n$-doping.

Spin waves, the collective excitations of the magnetic order parameter, and magnons, the associated quasiparticles, are envisioned as possible data carriers in future wave-based computing architectures. On the road toward spin-wave computing, the development of a diodelike device capable of transmitting spin waves in only one direction, thus allowing controlled signal routing, is an essential step. Here we report on the design and experimental realization of a microstructured magnonic diode in a ferromagnetic bilayer system. Effective unidirectional propagation of spin waves is achieved by taking advantage of nonreciprocities produced by dynamic dipolar interactions in transversally magnetized media, which lack symmetry about their horizontal midplane. More specifically, dipolar-induced nonreciprocities are used to engineer the spin-wave dispersion relation of the bilayer system so that the group velocity is reduced to very low values for one direction of propagation and not for the other, thus producing unidirectional slow spin waves. Brillouin light scattering and propagating-spin-wave spectroscopy are used to demonstrate the diodelike behavior of the device, the composition of which is first optimized through micromagnetic simulations.

Coherent anti-Stokes Raman scattering (CARS) offers many advantages for nonlinear bioimaging, thanks to its subcellular spatial resolution and unique chemical specificity. Its working principle requires two incident pulsed-laser beams with distinct frequencies to be focused in space and time, where focus quality, however, rapidly deteriorates when propagating at large depths in biological tissues. The depth limits of CARS and the capability of wavefront correction to overcome these limits are currently unknown. In this work we exploit the spectral correlation properties of the transmission matrix of a scattering medium in a pulsed regime, to recover coherent focusing for two distant incident CARS wavelengths, where propagation is initially uncorrelated. Using wavefront shaping with a single spatial-light modulator, we recover CARS generation through thick mice spinal-cord tissues where initially no signal is measurable due to scattering, and demonstrate point scanning over large field of views of tens of micrometers.

Optical absorption is one of the most fundamental processes in light-matter interactions. The ability to achieve and control high absorption is crucial for a broad range of modern photonic technologies. In nanomaterials of length scales much smaller than a wavelength, optical absorption is typically a weak perturbation. To achieve high absorption, exquisite techniques and structures have been developed, such as coherent interference of multiple laser beams and plasmonic metasurfaces. Here, we show that a robust critical-coupling condition exists to allow perfect absorption of light by a subnanometer-thick two-dimensional semiconductor, when the radiative-decay rate of the exciton resonance balances with its loss rate. We measure an absorption up to $99.6\mathrm{\%}$ in a monomolecular ${\mathrm{Mo}\mathrm{Se}}_2$ crystal placed in front of a flat mirror. We furthermore demonstrate control of the perfect absorption by tuning the exciton-phonon, exciton-exciton, and exciton-photon interactions with temperature, pulsed laser excitation, and a movable mirror, respectively. Our work suggests a mechanism to achieve and control critical coupling in two-dimensional excitonic systems, enabling photonic applications including ultrafast low-power light modulators and sensitive optical sensing.

The ideal diode is a theoretical concept that completely conducts the electric current under forward bias without any loss and that behaves like a perfect insulator under reverse bias. However, real diodes have a junction barrier that electrons have to overcome and thus they have a threshold voltage $V_T$, which must be supplied to the diode to turn it on. This threshold voltage gives rise to power dissipation in the form of heat and hence is an undesirable feature. In this work, based on half-metallic magnets (HMMs) and spin-gapless semiconductors (SGSs) we propose a diode concept that does not have a junction barrier and the operation principle of which relies on the spin-dependent transport properties of the HMM and SGS materials. We show that the HMM and SGS materials form an Ohmic contact under any finite forward bias, while for a reverse bias the current is blocked due to spin-dependent filtering of the electrons. Thus, the HMM-SGS junctions act as a diode with zero threshold voltage $V_T$ and linear current-voltage ($I$-$V$) characteristics as well as an infinite on:off ratio at zero temperature. However, at finite temperatures, non-spin-flip thermally excited high-energy electrons as well as low-energy spin-flip excitations can give rise to a leakage current and thus reduce the on:off ratio under a reverse bias. Furthermore, a zero threshold voltage allows one to detect extremely weak signals and due to the Ohmic HMM-SGS contact, the proposed diode has a much higher current drive capability and low resistance, which is advantageous compared to conventional semiconductor diodes. We employ the nonequilibrium Green’s function method combined with density-functional theory to demonstrate the linear $I$-$V$ characteristics of the proposed diode based on two-dimensional half-metallic $\mathrm{Fe}/{\mathrm{Mo}\mathrm{S}}_2$ and spin-gapless semiconducting ${\mathrm{VS}}_2$ planar heterojunctions.

We present experimental and theoretical studies of spin-wave mode dynamics in artificial kagome spin ice vertices made of three identical 15-nm thick elongated ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ nanoislands (macrospins). We consider several possible configurations, from completely disjointed macrospins (full dipolar interelement interactions) to fully jointed macrospins (full dipolar-exchange interactions). Using angular-resolved magnetic field dependent broadband ferromagnetic resonance (FMR), we demonstrate the occurrence of a mode localized in the vertex region as indicated by the distinct behavior of the FMR spectra at different angles and configurations. Theoretical calculations using micromagnetic simulations support the existence, origin, and behavior of this mode by interpreting it as a localized, quasi-uniform Kittel mode. Our findings pave the way for designing the most appropriate network consisting of ferromagnetic nanomagnets for specific application purposes in magnonics.

Superconducting circuits are at the forefront of quantum-computing technology because of the unparalleled combination of good coherence, fast gates, and flexibility in design parameters. The majority of experiments demonstrating small quantum algorithms in the superconducting architecture have used transmon qubits, which are coupled via capacitors or microwave cavities. Apart from coherence and gate fidelity, two important factors that currently limit the performance of a superconducting processor are nearest-neighbor interqubit coupling (in one or two dimensions) and being restricted to two-qubit entangling gates only. In this work, we present a programmable three-qubit processor circuit, nicknamed “trimon,” with strong all-to-all coupling and access to native three-qubit gates. We benchmark our processor by implementing a three-qubit version of various algorithms, namely Deutsch-Jozsa, Bernstein-Vazirani, Grover’s search and the quantum Fourier transform. In particular, we note that the native three-qubit controlled operations enable our ancilla-free implementation of Grover’s algorithm to outperform previous demonstrations. Our results clearly show the advantage of having native three-qubit gates, and that can play a crucial role in improving the performance of larger systems having the trimon as a building block.

A general concept is presented for increasing the in-slice spatial resolution in computed tomography in a flexible manner and with relatively large source focal spots and detector pixels. The flexibility is a result of probing the sample with an array of thin beamlets shaped by a mask. Provided that the mask apertures are smaller than the combined blur of the x-ray source and detector and that no significant overlap between beamlets occurs, this introduces higher spatial frequencies into the image-formation process. The application of a cycloidal acquisition scheme, by which the sample is simultaneously rotated and translated, facilitates an efficient exploitation of these frequencies and their reconstruction into high-resolution tomographic images. Additionally, a preliminary study of the signal-to-noise ratio versus the delivered dose reveals significant dose-saving potential.

We present a method of optical magnetometry with parts-per-billion resolution that is able to detect biomagnetic signals generated from the human brain and heart in Earth’s ambient environment. Our magnetically silent sensors measure the total magnetic field by detecting the free-precession frequency in a highly spin-polarized alkali-metal vapor. A first-order gradiometer is formed from two magnetometers that are separated by a 3-cm baseline. Our gradiometer operates from a laptop consuming 5 W over a USB port, enabled by state-of-the-art microfabricated alkali-vapor cells, advanced thermal insulation, custom electronics, and compact lasers within the sensor head. The gradiometer has a sensitivity of 16 ${\mathrm{fT}/\mathrm{cm}/\mathrm{Hz}}^{1/2}$ outdoors, which we use to detect neuronal electrical currents and magnetic cardiography signals. Recording of neuronal magnetic fields is one of a few available methods for noninvasive functional brain imaging that usually requires extensive magnetic shielding and other infrastructure. This work demonstrates the possibility of a dense array of portable biomagnetic sensors that are deployable in a variety of natural environments.

$\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ solar cells are investigated by luminescence measurements. We construct the current versus internal voltage characteristics of these devices from the luminescence intensity at different voltage and light bias conditions. A comparison of these characteristics to electrically measured current versus voltage curves unveils an internal resistance loss that is strongly dependent on voltage bias and illumination. In particular, we find significant residual luminescence for the device under short-circuit conditions. Numerical device simulations reveal that this effect is caused by a drop of the electron quasi-Fermi-level within the space charge region of the absorber material. We use a modified equivalent circuit model to describe the observed behavior in terms of simple equations. We show that such a voltage-dependent series resistance leads to a violation of a linear-network theorem, which under standard circumstances provides a useful method for the determination of the photocurrent collection efficiency. An analysis of resistive and recombination losses in the devices demonstrates that the internal voltage-dependent series resistance causes an efficiency loss of about 1.3% (absolute) for a device with an efficiency 13.4%. Finally, we show that the observed behavior is a general feature of charge-carrier separation in solar cells with finite charge-carrier mobility and that the intensity of the residual short-circuit luminescence provides valuable information on the efficiency of this process.

We present the concept, theoretical model, and experimental implementation of a full-duplex nonreciprocal-beam-steering transmissive phase-gradient metasurface. Such a metasurface is realized by exploiting the unique properties of the frequency-phase transition in time-modulated twin unit cells. The metasurface may be placed on top of a source antenna to transform the radiation pattern of the source antenna, and introduce different radiation patterns for the transmit and receive states. In contrast to the recently proposed applications of time modulation, here the incident and transmitted waves share the same frequency. The metasurface is endowed with directive, diverse, and asymmetric transmission and reception radiation beams, and tunable beam shapes. Furthermore, these beams can be steered by simply changing the modulation phase. The proposed twin unit cells inherently suppress undesired time harmonics, leading to a high conversion efficiency, which is of paramount importance for practical applications such as point-to-point full-duplex communications.

The effect of magnetic interactions is a key issue for the performance of nanoparticles in magnetic fluid hyperthermia. There are reports informing on beneficial or detrimental effects in terms of the specific power absorption depending on the intrinsic magnetic properties and the spatial arrangement of the nanoparticles. To understand this effect, our model treats a simple system: an ensemble of identical nanoparticles arranged in an ideal chain with the easy axis of the effective uniaxial anisotropy of each particle aligned parallel to the chain. We study the magnetic relaxation of linear chains with low anisotropy in magnetic-fluid-hyperthermia experiments, a system that yields a larger hysteresis area than the noninteracting case (i.e., improved specific power absorption) for all orientations of the chain (even in the perpendicular configuration and the randomly oriented case). The most-favorable case is the chain parallel to the external field; however, we show that the incorporation of a dipolar-field component perpendicular to the external field is necessary for the correct modeling of chains nearly in the perpendicular configuration, which is not always done. The mechanism involved in the hysteresis-area increase can be interpreted as a shift between the local field and the applied field.

We derive upper bounds to free-space concentration of electromagnetic waves, mapping out the limits to the maximal intensity for any spot size and optical-beam-shaping device. For sub-diffraction-limited optical beams, our bounds suggest the possibility for orders-of-magnitude intensity enhancements compared with existing demonstrations, and we use inverse design to discover metasurfaces operating near these new limits. We also demonstrate that our bounds may surprisingly describe maximal concentration defined by a wide variety of metrics. Our bounds require no assumptions about symmetry, scalar waves, or weak scattering, instead relying primarily on the transformation of a quadratic program via orthogonal-projection methods. The bounds and inverse-designed structures presented here can be useful for applications from imaging to three-dimensional printing.

Polarization in ferroelectrics can be switched in the direction of an applied electric field by dipole reorientation, enabling numerous applications and fundamental phenomena. Here, we demonstrate that, in the van der Waals (vdW) layered ferrielectric ${\mathrm{Cu}\mathrm{In}\mathrm{P}}_2{\mathrm{S}}_6$, a unique mechanism exists where polarization aligns against the direction of the applied electric field, seemingly in violation of the fundamental properties of a dipolar solid. The mechanism is the result of the electric field driving the $\mathrm{Cu}$ atoms unidirectionally across the vdW gaps, which is distinctively different from dipole reorientation. The crossing of $\mathrm{Cu}$ atoms is the fundamental process of ionic conductivity, yet it is compatible with the existence of polarization. These phenomena are confirmed by nanoscale imaging and spectroscopy of ferroelectric capacitors, coupled with dynamic density-functional-theory simulations. The symbiotic relationship of ferroelectric and ionic phenomena enables alternative approaches to control polarization and necessitates a change in perspective on nucleation, domain-wall dynamics, and other ferroelectric and electromechanical characteristics in material systems where ionic and ferroelectric phenomena manifest.

We immerse a two-dimensional physical wave-propagation experiment in a virtual (simulated) environment in real time. This enables broadband hybrid wave experimentation in physical media that may be governed by unknown physics, embedded seamlessly within virtual media with either modeled or measured physics. Using the theory of immersive boundary conditions and a recently presented control system [Becker et al., Immersive Wave Propagation Experimentation: Physical Implementation and One-Dimensional Acoustic Results, Phys. Rev. X 8, 031011 (2018)], we convert a circular two-dimensional acoustic waveguide into a larger physicovirtual square waveguide. Real-time wave-field extrapolation allows waves to propagate seamlessly between physical and virtual media, producing correct wave-field interactions between them, including nonlinear effects. We show that the laboratory can physically measure the response of nonphysical energy-gain materials in real time. The physicovirtual laboratory thus allows previously inaccessible wave phenomena to be investigated experimentally and constitutes an alternative approach for wave-physics investigations by connecting experimental and numerical approaches.

Ferromagnetic metals and insulators are widely used for generation, control, and detection of magnon spin signals. Most magnonic structures are based primarily on either magnetic insulators or ferromagnetic metals, while heterostructures integrating both of them are less explored. Here, by introducing a $\mathrm{Pt}$/yttrium iron garnet ($\mathrm{YIG}$)/permalloy ($\mathrm{Py}$) hybrid structure grown on a $\mathrm{Si}$ substrate, we study the magnetic coupling and magnon transmission across the interface of the two magnetic layers. We find that within this structure, $\mathrm{Py}$ and $\mathrm{YIG}$ exhibit an antiferromagnetic coupling field as strong as 150 mT, as evidenced by both magnetometry and polarized neutron reflectometry measurements. By controlling individual layer thicknesses and external fields, we realize parallel and antiparallel magnetization configurations, which are further utilized to control the magnon current transmission. We show that a magnon spin valve with an on:off ratio of approximately 130% can be realized out of this multilayer structure at room temperature through both spin pumping and spin-Seebeck-effect experiments. Owing to the efficient control of magnon current and the compatibility with $\mathrm{Si}$ technology, the $\mathrm{Pt}$/$\mathrm{YIG}$/$\mathrm{Py}$ hybrid structure could potentially find applications in magnon-based logic and memory devices.

Levitated nanoparticles and microparticles are excellent candidates for the realization of extremely isolated mechanical systems, with a huge potential impact in sensing applications and in quantum physics. Magnetic levitation based on static fields is a particularly interesting approach, owing to the unique property of being completely passive and compatible with low temperatures. Here, we show experimentally that micromagnets levitated above type-I superconductors feature very low damping at low frequency and low temperature. In our experiment, we detect five out of six rigid body mechanical modes of a levitated ferromagnetic microsphere, using a dc superconducting quantum interference device with a single pick-up coil. The measured frequencies are in agreement with a finite-element simulation based on an ideal Meissner effect. For two specific modes, we find further substantial agreement with analytical predictions based on the image method. We measure damping times $\tau$ exceeding ${10}^4\mathrm{s}$ and quality factors $Q$ beyond ${10}^7$, an improvement of 2–3 orders of magnitude over previous experiments based on the same principle. We investigate the possible residual loss mechanisms besides gas collisions, and argue that a much longer damping time can be achieved with further effort and optimization. Our results open the way towards the development of ultrasensitive magnetomechanical sensors with potential applications to magnetometry and gravimetry, as well as to fundamental and quantum physics.

Generating and distributing encryption keys via chaos synchronization of optical systems is a promising method for achieving secure and fast real-time encryption of large data volumes. However, current state-of-the-art systems based on lasers subjected to time-delayed feedback have exhibited serious limitations in regard to stability and speed due to their inherent phase sensitivity and their fundamental bandwidth limitation by the laser relaxation oscillation frequency. Here, we propose, simulate, and build a fundamentally different concept based on optoelectronic oscillators (OEOs) to generate and distribute encryption keys. We demonstrate both theoretically and experimentally that by injecting chaotic OEOs with a common external chaotic signal, chaos synchronization can be achieved. The key distribution signal shared between sender and receiver is in the optical domain and can thus be transmitted over a standard fiber-optic network. As the optoelectronic feedback is phase insensitive, stable synchronization becomes possible without any need for active stabilization, resulting in a compact and inexpensive setup. From the chaotic time traces, identical random bit sequences can be generated on the sender’s and receiver’s sides at rates of up to 6 GBit/s, which is comparable to current state-of-the-art systems of the same bandwidth. Since OEOs are not bandwidth limited by relaxation oscillations, the key generation can be further increased compared to laser-based systems by increasing the OEO bandwidth, thus presenting a promising platform for achieving fast and secure real-time encryption.