No talking! The authors solve the critical scalability issue of multiqubit crosstalk in quantum processors, by harnessing destructive interference. Their architecture involves linking two superconducting qubits with both a bus cavity and a tunable coupler; the tunability enables simultaneous suppression of crosstalk and realization of high-fidelity two-qubit gates. This result paves the way for the next generation of crosstalk-free multiqubit systems.

Coherently controlled quantum systems have lately emerged as precision sensors, especially on small length scales. In this study the authors present a sensing protocol that enables direct detection of time-dependent magnetic fields, without the need to reconstruct the signal, effectively turning their quantum sensor into a sort of oscilloscope. This scheme will be useful for investigating $e.g.$ the nanoscale dynamics of photocurrents or magnetic domain walls. While it is demonstrated using N-$V$ centers in diamond, the authors’ method is applicable to any qubitlike sensor.

Understanding how information encoded in spins is transported in antiferromagnets is important for the development of spintronics. In metallic antiferromagnets, spin information is carried by both electrons and magnons, and discriminating between these two contributions remains a challenge. Combining experiment and theory, the authors are able to separate the magnonic and electronic contributions. Remarkably, magnons are far more efficient than electrons for conveying information, even in disordered antiferromagnets. This result suggests that technologically relevant sputtered metallic antiferromagnets are suitable for use as interconnects in devices.

No strings attached… Robustness and high efficiency in systems for wireless power transfer are in high demand, and are crucial for broad commercialization of wireless charging technologies. This study presents a simple, reliable path to this goal, by utilizing the principle of self-oscillating wireless power generation, which combines robust operation with pulsed oscillations that yield high efficiency. This approach can be applied to many wireless charging scenarios, including dynamic wireless power transfer systems.

A long-term goal of phononic communication is controlled transport of elastic wave signals with improved robustness and enhanced information capacity, but existing approaches suffer from unwanted backscattering by defects and disorder that may substantially reduce the transmission rate, or even disable a data channel. This study uses the robust edge states along the interfaces between distinct topological classes to make progress. These fault-tolerant edge channels are protected jointly by both pseudospin and valley degrees of freedom, naturally providing doubled information carriers within every channel, and may serve as a building block for large-scale phononic circuits and networks.

A $geometric$ $qubit$, consisting of $e.g.$ a spin-triplet electron in a nitrogen-vacancy center in diamond, is crucial as an interface to integrate quantum communication, computing, and sensing, owing to its structural similarity to a photonic polarization qubit. Implementation is held back because such a system’s holonomic gate operations unexpectedly induce slow depopulation from the qubit space. This study demonstrates robust dynamical decoupling, by applying intentional detuning to the qubit’s operations, to suppress the depopulation. This approach should have an impact on engineering a robust quantum gate sequence to facilitate large-scale quantum information processing.

Spin Hall metals with a giant spin Hall ratio ${\theta }_{\text{SH}}$ and relatively low resistivity are key to developing spin-orbit-torque-based technologies ($e.g.$ memory, logic, and oscillators) that require high energy efficiency, high endurance, and low impedance. The limit to which ${\theta }_{\text{SH}}$ can be enhanced in practice, and how, remain to be seen. This study establishes that the intrinsic spin Hall conductivity of Pt is robust against strain and moderate crystal disorder, but diminishes with carrier lifetime, which sets a practical limit for ${\theta }_{\text{SH}}$. Finally, an actual Pt/Ti multilayer near this limit is identified.

Thin superconducting films are important components of modern low-temperature electronics, and their performance can degrade with decreasing thickness, owing to various factors that are often difficult to disentangle. The authors analyze material properties, electron-phonon coupling, and transport in high-quality epitaxial TiN films down to 3 nm in thickness. The thinnest films show a reduction of the superconducting transition temperature by almost a factor of 3, which is associated with a minute amount of magnetic disorder. In thin films the magnetic scatterers occur predominantly on the surface, and can originate from O vacancies in a naturally oxidized surface layer.

Separation and fractionation of microscale and smaller particles are of ever-growing importance in both research and industrial applications, including chemical and biological research and medical diagnostics. Here a proposed technique separates particles of a precisely defined size by exploiting the highly counterintuitive phenomenon of absolute negative mobility induced by thermal fluctuations. Moreover, the separation process may be controlled solely by temperature, $i.e.$ by changing it, one is able to separate particles of different desired sizes.

The efficiency of a solar cell depends on many factors, including the amount of light that is reflected from the solar-cell stack, and hence immediately lost for energy conversion. Arrays of dielectric nanoparticles have been proposed as antireflective coatings, their low profile being particularly suited to ultrathin solar cells, where traditional etching is impractical. This work studies the physics of back-reflection from the point of view of symmetries and conservation laws, and ties antireflection performance to two conditions: a high enough degree of discrete rotational symmetry of the array, and the suppression of crosstalk between the two helicities of the electromagnetic field.

Achieving multiwavelength nonreciprocal optical devices, which can realize nonreciprocal (perfect one-way) transmission for two or more wavelengths at the same time, without magnetic materials is challenging, but is highly desirable for densely integrated multiwavelength photonic components. The authors experimentally realize multiwavelength magnetic-free optical nonreciprocity using optical pumping of warm ${}^{85}$Rb atoms, with the assistance of Doppler effect. This scheme may have advantages in decreasing the number of optical isolators, and thus can reduce the complexity of an integrated multiwavelength system.

Nonlinear-optical image recovery is very important for underwater, atmospheric and biological imaging, but such techniques are held back in practice because dephasing of temporally and spatially incoherent white light spoils most nonlinear optical effects. The authors demonstrate the nonlinear method of $dynamical$ $stochastic$ $resonance$, based on seeded modulation instability. This study shows that the signal modes of the entire temporal spectrum carry different gain characteristics, and collectively contribute to the resonance. The work boosts the application of nonlinear optics in white-light imaging with natural or artificial illumination.

Time-domain Brillouin scattering, in which propagating coherent acoustic phonons locally change a material’s optical properties, is an important tool for nanoscale imaging. This study shows the effect of doping on Brillouin oscillations in GaAs, for which the amplitude of oscillations is found to be very sensitive to the doping level, for probe energies near the band gap. This insight on the energy dependence of Brillouin oscillations with respect to doping is expected to have an impact on metrology applications for semiconductor wafers.

Intersubband transitions in ZnO-based materials are predicted to be a promising basis for infrared and terahertz optoelectronic devices, particularly quantum cascade lasers. Utilizing a nonpolar ZnO substrate and optimizing growth conditions, the authors observe strong coupling between asymmetric ZnO/Mg${}_x$Zn${}_{1-x}$O quantum wells. Intersubband coupling due to the high doping level in the heterostructures leads to formation of multisubband plasmons. The results are an important step toward the realization of ultrafast ZnO/MgxZn1-xO devices exploiting these transitions.

From radio to optical frequencies, phased-array antennas are ubiquitous in modern technology and find wide application in communication and radar systems, imaging, sensing, and radio astronomy, among many other endeavors. These devices provide identical responses in transmission and reception, due to the constraints imposed by time-reversal symmetry. Here, however, the authors present the concept of $nonreciprocal$ phased-array antennas, and demonstrate that such structures significantly extend the functionalities of common arrays by efficiently enabling independent, dynamic control of transmitted and received radiation patterns at the same operating frequency.

Bulk relocation of coflowing streams in an acoustic field occurs when there is a mismatch in acoustic impedance between the fluids. In separation of red blood cells from whole blood, for example, this relocation is prevented by ensuring that the central buffer stream has the higher impedance. This study shows that for dense suspensions like blood, in addition to typical full-relocation/zero-relocation regimes, an intermediate partial relocation may occur when the buffer’s impedance is not high enough. This establishes the influence of particles in acoustic relocation, and provides a way to split any dense suspension into a concentrated central stream and a dilute side stream.

Understanding how small particles can be precisely manipulated using only fluid flow is critically important for enabling new studies in soft materials, colloidal science, and biophysics. This study characterizes the underlying flow topologies and performance of a Stokes trap, using multiplexed particle-trapping experiments and numerical simulations. Unexpectedly, the authors find that optimal control of two particles involves the occurrence of zero or one stagnation points— not two distinct points. The results will facilitate studies of soft matter involving particle-particle interactions, such as controlled adhesion between vesicles and biological cells.

Here a composed metasurface is designed for multi-image hiding and seeking in the terahertz band, based on polarization multiplexing. Manipulation of linear and circular polarization states simultaneously is demonstrated, using a combination of rod and C-shaped slot antennas. Furthermore, there is interference between the waves from the two types of antennas, since the linear and circular bases are not orthogonal, leading to the unique properties of the designed device. This work may show a fresh avenue for optical image encoding and optical information security.

To realize an “acoustic laser”, enhanced emission of a source of plane waves of sound is important, but because such sources possess a large surface area and thus high reflection loss, amplifying emission is difficult. With this in mind, the authors design and fabricate a high-$Q$ acoustic surface that incorporates a sound-plane-wave source, and with appropriate structural parameters yields strongly enhanced emission. This work suggests an inventive method to create high-intensity collimated sound beams, which should see application in sound manipulation and nonlinear acoustics.

Can encrypted communication be secure at practical speeds? The authors propose a $rateless$ $reconciliation$ $protocol$ to overcome the technical difficulties for efficient error correction at different signal-to-noise ratios (SNRs), and therefore to significantly improve the performance of continuous-variable quantum key distribution (CV-QKD). Using this method, highly efficient key extraction can be maintained even at ultralow SNR. This method remarkably reduces the complexity of reconciliation, improves the robustness of practical systems, and can significantly improve the post-processing performance of CV-QKD, bringing secure quantum communication one step closer to everyday use.

An ongoing research theme is identifying and exploiting similarities in exotic behaviors of light and sound waves. The titular device is the acoustic analogue of a metal-insulator-metal plasmonic waveguide. This study experimentally analyzes and validates the properties of the proposed waveguide, in terms of its dispersion and modal properties. Similar to a plasmonic waveguide, the proposed acoustic waveguide, and its supported symmetric and antisymmetric modes and group and phase velocities, can be largely controlled by changes in geometrical parameters, opening the possibility of applying this technology in acoustic tunable delay lines, modulators, and sensors.

The control of highly coherent qubits is crucial to building scalable superconducting quantum processors; in particular, the performance of entangling gates relies on the coherence times of individual qubits. Parametrically activated entangling gates between two coupled qubits are realized through rf flux modulation of one qubit. Here the authors discuss modulation amplitudes at which the qubit is first-order insensitive to $1/f$ flux noise: “sweet spots” in the ac flux. By properly filtering the white flux noise from the control electronics, one can realize high-fidelity two-qubit gates at a sweet spot. Recently this protection from flux noise has also been seen experimentally.

Being able to tune magnetic anisotropy (MA) by an electric field is crucial for the development of low-power magnetoelectric devices. The authors present an approach for tuning the MA of a manganite film by modulating the crystal phase of a cobaltite cap layer. By driving the cap layer to switch between SrCoO${}_{2.5}$ and SrCoO${}_{3-\delta }$ structures, reversible tuning of MA between two crystal directions was achieved for the manganite layer, accompanying a change in Mn $3d$ orbital occupancy. This work demonstrates indirect tuning of the magnetic easy axis of this highly spin-polarized manganite, providing guidance for constructing electrically tunable magnetoelectric devices.

This study demonstrates controlled maneuvering of polystyrene particles in a simple microfluidic device by exploiting the underlying electrokinetic phenomena, primarily dielectrophoresis (DEP). The device mainly uses negative DEP force to trap micrometer-sized particles near the positive electrode in a low-conductivity suspension, but also exploits positive DEP force if the conductivity increases. This device furthermore shows flexibility in selective yet localized trapping with low electric field requirements, laying a foundation for flexible preconcentrators in low-cost bioanalytical detection systems for $e.g.$ medical diagnostics.

Highly confined surface plasmon polaritons (HC-SPPs) can significantly enhance light-matter interaction and dramatically shrink electronic or photonic systems, including x-ray sources. However, due to the short wavelength and low phase velocity of HC-SPPs, traditional approaches have a hard time exciting them. Here electron-beam excitation of hot carriers plus the quantum Cherenkov effect in graphene are predicted to yield monochromatic HC-SPPs at 1/300 of the vacuum wavelength, with the Fermi energy of graphene dynamically tuning the frequency. Thus one can realize a compact x-ray source without artificial periodic structures, pointing ultimately to tabletop x-ray systems.

Generation of photons in controlled temporal modes is a key part of connecting distant nodes of a quantum network built with atomic and cavity systems. For efficient coupling, such systems require narrowband photons with specific spectral and temporal wavefunctions. To date, this has only been possible in bulk optical setups, and mostly for broadband light. Here researchers present a chip-scale technique based on controlling the density of states of a silicon whispering-gallery-mode resonator, using Rayleigh scattering. The cavity generates entangled photons of sub-GHz bandwidth with four-wave mixing, in temporal modes that can be controlled via the scattering process.

Implanted medical devices often transmit wireless signals to communicate with external equipment, but the efficiency of signal transmission is limited by internal reflection at the surface of the body. This work presents an approach to greatly enhance transmission, through the design of diffractive patterns capable of converting evanescent waves on the body’s surface into waves propagating into the surrounding space. When printed on clothing and worn on the body, these patterns can increase the wireless signal’s strength by nearly an order of magnitude, which may pave the way for longer-lasting and more functional bioelectronics.

On-chip light sources are essential components for integrated photonic circuits and quantum information processing chips. High directionality, high collection efficiency, and ultrasmall feature size are the most significant features for a light source in the wavelength range for optical communication, around 1550 nm. The authors use nanomanipulation to create an ultrasmall, unidirectional on-chip light source based on PbS quantum dots and an epsilon-near-zero (ENZ) material. This work not only shows the way to integrated photonic devices based on ENZ materials, but also provides an advanced method for the precise assembly of composite functional nanostructures.

As nanoscale magnetic devices such as sensors continue to shrink, in conjunction with being driven using higher current densities, the inherently nonlinear nature of magnetization dynamics may emerge during device operation. To optimize these nanodevices or choose the most favorable operating point, one needs a characterization protocol tailored to nonlinear measurements in the presence of dynamical effects. With this in mind, the authors develop such a protocol, which reveals a magnetic sensor’s response at fractional frequencies of its natural ferromagnetic resonance modes. These resonances are then identified via micromagnetic modeling.

No talking! The authors solve the critical scalability issue of multiqubit crosstalk in quantum processors, by harnessing destructive interference. Their architecture involves linking two superconducting qubits with both a bus cavity and a tunable coupler; the tunability enables simultaneous suppression of crosstalk and realization of high-fidelity two-qubit gates. This result paves the way for the next generation of crosstalk-free multiqubit systems.

High-power, few-cycle midinfrared pulses at wavelengths longer than a few micrometers are important for applications in a variety of fields, but are difficult to produce using conventional methods. This study shows that one may efficiently generate relativistic, near-single-cycle vortex pulses of the 100-mJ,TW class via photon deceleration in plasma. The resulting pulse frequency is tunable over a broad spectral range of up to 18 $\mu$m covering the “molecular fingerprint” region, and its carrier-envelope phase is also adjustable. Such pulses provide unique capabilities in controlling light-matter interactions at these wavelengths.

Nanoscale magnetic resonance spectroscopy allows study of physical, chemical, and biophysical phenomena in minute samples, but its sensitivity and spatial resolution are limited by the probe-target distance, and a chip-integrated quantum sensor is still in demand. To address both challenges, this work proposes a quantum sensor based on a valley-spin qubit created from a deformed carbon nanotube. The system achieves high sensitivity because of the large valley $g$-factor and tiny diameter of a single-walled carbon nanotube, and offers all-electric coherent control and efficient readout, without optical elements, which facilitates integration of a sensor array on a chip.

Drowning in data: Autonomous classification of skyrmion dynamics is important for creating next-generation data storage, spintronics, and quantum technologies. The extremely large amount of experimental data in this field cannot be processed manually; there is a need for automation. This study uses an artificial recurrent neural network to classify various dynamical processes exhibited by skyrmions (topologically protected spin structures) under the influence of a picosecond magnetic pulse. It is hoped that this machine-learning approach will speed the development of devices that use skyrmions.

Efficient terahertz emission (a phenomenon of keen interest) was recently demonstrated in magnetic multilayers, based on either the inverse spin Hall effect or inverse Rashba-Edelstein effect. This study reports the generation of THz waves from a $single$ ferromagnetic layer, based on the anomalous Hall effect. Due to the asymmetry in reflection of nonthermal electrons from the top and bottom interfaces, upon laser excitation a longitudinal spin-polarized current is generated and converted to a transverse charge current, thereby leading to THz emission. The emission efficiency can be significantly enhanced by introducing a vertical composition gradient in the ferromagnetic layer.

Coherently controlled quantum systems have lately emerged as precision sensors, especially on small length scales. In this study the authors present a sensing protocol that enables direct detection of time-dependent magnetic fields, without the need to reconstruct the signal, effectively turning their quantum sensor into a sort of oscilloscope. This scheme will be useful for investigating $e.g.$ the nanoscale dynamics of photocurrents or magnetic domain walls. While it is demonstrated using N-$V$ centers in diamond, the authors’ method is applicable to any qubitlike sensor.

High-visibility quantum interference (indistinguishability) among single photons is the key to scalable, high-fidelity linear optical quantum gates. Measuring indistinguishability by interference is laborious and time-consuming, though, and thus not scalable. The authors find that the faster, simpler second-order correlation functions provide results equivalent to the indistinguishability, and could be useful in rapid-prototyping source design of large-scale photonic circuits. Also, for mature guided-wave integrated optics such as silicon photonics, the high-visibility bottleneck is due to the physics of a process that tends to produce single photons in multiple spectral modes.

Understanding how information encoded in spins is transported in antiferromagnets is important for the development of spintronics. In metallic antiferromagnets, spin information is carried by both electrons and magnons, and discriminating between these two contributions remains a challenge. Combining experiment and theory, the authors are able to separate the magnonic and electronic contributions. Remarkably, magnons are far more efficient than electrons for conveying information, even in disordered antiferromagnets. This result suggests that technologically relevant sputtered metallic antiferromagnets are suitable for use as interconnects in devices.

While coupling of spin waves and mechanical excitations could be used for quantum information processing, creating chip-integrable magnon resonators which at the same time are mechanical oscillators at suitable frequencies has seemed futile—until now. The authors develop a process that allows them to fabricate three-dimensional, freestanding yttrium iron garnet (YIG) magnon resonators of various shapes. These devices exhibit surprisingly low Gilbert damping, rivaling that of high-quality YIG thin films. At the same time, their dimensions and material properties promise mechanical resonances in the upper MHz or even GHz regime.

Ferromagnetic skyrmions are promising information carriers for nonvolatile, energy-efficient, ultradense memory and logic devices of the future. However, their integration in these applications is hampered by the undesirable skyrmion Hall effect (SkHE), a motion transverse to the direction of current flow. This study uses the theory of emergent electrodynamics to derive the effective emergent magnetic fields in skyrmionic systems in the presence of spin-orbit interaction. The authors propose a clever, easy way to tune the SkHE by modulating the system’s spin-orbit-interaction strength. This approach opens alternative avenues to overcome the SkHE and enable spintronic applications.

Breaking of reciprocity in acoustic propagation can be exploited in various applications, such as energy trapping and harvesting, acoustic logic gates, or noise control. Nevertheless, the conventional methods for nonreciprocal (one-way) acoustic propagation are limited by narrow bandwidth or inefficiency. The authors instead propose a special form of nonlocal boundary control to achieve nonreciprocal acoustic propagation. Despite some practical limitations at this early stage of design, their approach demonstrates the capability of achieving unidirectional acoustic waves in a much broader sense than with traditional techniques.

Quantum key distribution (QKD) can provide secure communication even when an eavesdropper (the villainous “Eve”) completely controls the channel. In practice, though, side-channel effects exist due to device imperfections, and Eve can still intercept information via a side channel. Thus the authors devise a QKD scheme that is both free of side channels in the source state and measurement-device-independent. While some other protocols can also achieve side-channel-free security, this one is based on mature technology without any demand for local detection efficiency, and works for distances greater than 200 km, even though the misalignment error rate may be as large as 20%.

Recent experiments have offered clues about exotic Majorana zero modes (MZMs, neutral quasiparticle excitations) possibly lurking in the vortices of iron-based superconductors such as FeTe${}_x$Se${}_{1-x}$, which therefore could be an effective two-dimensional platform for topological quantum computing. The authors present a theoretical proposal for reading out the quantum information encoded in MZMs within the vortex cores in a topological-superconductor island. Their work also demonstrates non-Abelian statistics for MZMs in vortices, pointing the way to advanced quantum information processing.

Two-dimensional $p-n$ junction diodes could be components in all future nanoelectronic devices. This study uses density functional theory to investigate the possible rectification of electric current in monolayer boron phosphide via adsorbed organic molecules. For the $p$- and $n$-type semiconductors, the authors consider a junction based on simultaneous adsorption of two different organic molecules, with each molecule providing localized states in the energy gap of the two-dimensional BP monolayer. The insight obtained here motivates further exploration of $p-n$ junctions based on van der Waals heterostructures.

Now you see me… This article reports a switchable liquid-crystal smart window that can be independently operated in two voltage-modulated modes. In the first mode, for privacy control, the window is switched between a transparent state and a scattering state by a low-frequency voltage, based on flexoelectric interaction; in the second, for energy-flow control, it is switched between transparent and absorbing states by a high-frequency voltage, based on dielectric interaction. This technology ought to be very useful for both architectural and automobile windows.

To understand the behavior of a semiconductor device, you need to know how the material’s $excitons$ (weakly bound electron-hole pairs) behave. The exciton diffusion length in GaN is often determined from the size of dark areas around dislocation outcroppings in cathodoluminescence maps. The present work shows that, contrary to common belief, the spatial extent of these areas is only weakly affected by diffusion, being set primarily by exciton dissociation in the piezoelectric field around the dislocation. In contrast, the local band-gap variation in the dislocation’s strain field $does$ depend sensitively on exciton diffusion length, and hence enables its experimental determination.

Creating the conditions for frequency locking is essential for coupled light sources in a photonic system. Here researchers show that the nature of the coupling mechanism (dispersive versus dissipative) plays a crucial role in the frequency synchronization of lasers. In particular, dispersive synchronization is found to be a “hard” transition accompanied by bistability, while dissipative synchronization is a “soft” process that yields a single stable state. These results suggest that engineering the coupling mechanism can provide exciting opportunities in designing frequency-locked arrays of light sources.

No strings attached… Robustness and high efficiency in systems for wireless power transfer are in high demand, and are crucial for broad commercialization of wireless charging technologies. This study presents a simple, reliable path to this goal, by utilizing the principle of self-oscillating wireless power generation, which combines robust operation with pulsed oscillations that yield high efficiency. This approach can be applied to many wireless charging scenarios, including dynamic wireless power transfer systems.

A long-term goal of phononic communication is controlled transport of elastic wave signals with improved robustness and enhanced information capacity, but existing approaches suffer from unwanted backscattering by defects and disorder that may substantially reduce the transmission rate, or even disable a data channel. This study uses the robust edge states along the interfaces between distinct topological classes to make progress. These fault-tolerant edge channels are protected jointly by both pseudospin and valley degrees of freedom, naturally providing doubled information carriers within every channel, and may serve as a building block for large-scale phononic circuits and networks.

The simple quantum setup of the Fabry-Perot interferometer from optics is found to exhibit very strong directional preference for impinging particle beams. Two types of resonances are identified, the different mechanisms of which are demonstrated through the spatial distribution of wave-function magnitude and probability current. These findings offer additional degrees of freedom when designing architectures for a broad range of quantum engineering applications involving beam tunneling, from e-beam steering to matter-wave filtering and quantum sensing. They also provide a set of limits for the angular selectivity that can be achieved with the considered geometry.

Long-range propagation of a laser is critical for applications such as free-space optical communication and power transmission, and is often limited by beam distortions caused by atmospheric turbulence. Through the use of high-power laser pulses, this study illustrates how a nonlinear self-channeling beam can resist such distortions. This approach yields an increase in power delivery by a factor of 7 compared to linear propagation, even in the strongest turbulence produced by a nearly 1-km-long generator built for this study. These results will have an immediate impact on potential solutions to the limitations of optical propagation in realistic atmospheric turbulence.

Multilayers of insulating magnetic oxides are important for potential spintronic devices operated by pure spin current without Ohmic heating. How interfaces in such multilayers affect magnetic damping (dissipation of dynamically excited spins) is not well understood, though. This study of highly crystalline MgAl${}_{1/2}$Fe${}_{3/2}$O${}_4$/CoCr${}_2$O${}_4$ bilayers as well-characterized model systems elucidates the impact of the interface on damping. The results reveal that damping in magnetic oxides is extremely sensitive to interfacial disorder, even if it makes up only a small fraction of the total magnetic thickness, and that more precise growth is likely required for ultralow-loss devices.

This study describes a chip-integrated optical biosensor based on dual coupled racetrack resonators. A physical analysis of the device is carried out using conformal transformation and coupled-mode theory. The intrinsic limit of detection is improved by a factor of 100, compared to a conventional single-resonator system, with better dynamic range as well. The proposed sensor could be a promising candidate for analyzing different components of blood samples in medical diagnostics, which is especially important for the early detection of critical cancers, such as prostate cancer.

Optical isolators are $nonreciprocal$ devices, transmitting forward-propagating light while blocking backward propagation. They are as important in optical and microwave applications as diodes are in electrical circuits. Common problems with free-space isolators are an inherently small aperture and the failure to block backward propagation at oblique incidence. This paper presents a thin free-space isolator offering virtually unlimited aperture and broadband rejection of light incident from behind, regardless of the exact direction. The key design elements are verified at microwave frequencies, and the same physical idea can also be applied up to the midinfrared.

Graph and hypergraph states are of wide interest in quantum information processing as well as fundamental physics, and efficient verification of these states is key to various applications. The authors propose a simple recipe for verifying hypergraph states that requires only two distinct Pauli measurements for each party, and is dramatically more efficient than conventional protocols based on local measurements. This approach enables verification of hypergraph states and genuine multipartite entanglement of thousands of qubits, even in an adversarial scenario.

Angle-dependent asymmetric acoustic absorption attracts growing interest, due to its great importance in various applications where directional responses are required, such as acoustic antennas, sensing, and angle-encoded steganography (concealed messaging). However, studies of angular-asymmetric absorbing structures remain scarce in acoustics. This work demonstrates a planar acoustic metamirror that can realize asymmetric sound absorption by controlling the surface impedance profile. The proposed structure enriches the toolbox for acoustic wave manipulation, and the systematic design method may also be used for $e.g.$ acoustic focusing, acoustic carpet cloaking, and sound steering.

Exploring the phases of matter is important in condensed matter physics and materials discovery, and here Raman spectroscopy gives a material’s structural fingerprint. The authors develop a highly correlated kernel model based on a set of Raman-scattering spectra, to deduce thermally induced transitions among three phases in a model ferroelectric crystal, by mining and learning the phonon behaviors in the crystal lattice. The underlying physical mechanism behind these behaviors, and just how the model “learns” these rules to make precise predictions, are clarified. This insight paves the way to applying the generic approach for prediction of unexplored structures and materials.

In semiconductor physics for electronics, conventional impurity doping presents a major obstacle to further miniaturization for ultralarge-scale integration. The authors discuss a phenomenon occurring at deep nanoscale Si volumes with a large surface-to-volume ratio: The electronic structure of Si in such nanocrystals is massively shifted by coating with a few monolayers of SiO${}_2$ or Si${}_3$N${}_4$, pushing the Si electronic structure toward or away from the vacuum level respectively. This energy offset of about 0.9 eV allows one to create $n$-type (SiO${}_2$-coated) or $p$-type (Si${}_3$N${}_4$-coated) in deep nanoscale Si, mimicking the familiar impurity doping of bulk Si.

Engineering higher-order nonlinear interactions is vital in autonomous protection of quantum systems against errors. Such interactions are often not directly available, though, or are slow compared to error rates of the system. The authors present a nonlinear eight-wave mixing process that exchanges four photons of a harmonic oscillator with two excitations of a transmon-qubit mode and two pump photons, by combining more accessible lower-order interactions via a sort of Raman transition. Surprisingly, this technique produces a stronger interaction than a six-wave mixing process in the same system. This eight-wave mixing process is expected to become a key component of autonomous continuous-variable quantum error correction.

There’s more than one way to do it: Concealing an object need not rely on transformation optics. If detection is based on the signature of reflected light, as in radar, then distorting that signature into something unrecognizable can be sufficient. Here the authors design a time-varying metasurface that can act as spectral camouflage, by converting an incident narrowband radar signal into broadband scattered waves that look like white noise. This study provides practical insight for the development of broadband time-varying metasurfaces, and their applications.

Smooth control of the intrinsic orbital angular momentum (OAM) carried by a beam of light is important for $e.g.$ optical tweezers, communication, and quantum information processing, while control of extrinsic OAM is useful for $e.g.$ super-resolution microscopy and light-matter interaction with atoms, molecules, and condensates. This study presents a technique to control intrinsic and extrinsic OAM in a single-path configuration that is free from mechanical errors. By managing the relative intensity and Pancharatnam-Berry phase difference between two orthogonal spatial modes with orthogonal polarizations, one may tune the net transverse linear momentum to yield variable extrinsic OAM.

Manipulation of photonic quantum states in the frequency domain can be a valuable physical resource in quantum information processing. Here researchers demonstrate that two essential components for frequency-domain quantum photonics, quantum light generation and quantum frequency conversion, can be realized on a common platform based on integrated nonlinear nanophotonics. The authors realize the tunable quantum beat of single photons, a signature of controlled quantum interference in which single photons are precisely tuned into spectral alignment via quantum frequency conversion.

Optical smoothing techniques are crucial for achieving well-controlled experiments at large-scale laser facilities dedicated to high-energy-density physics. Due to detrimental effects in the laser chain, however, those techniques can convert phase modulation into potentially large amplitude modulation, reducing laser performance at the target and causing laser damage. This study uses innovative smoothing paradigms to address this key issue. In particular, one simple solution is to distribute the laser bandwidth over the beams, a result that might have considerable impact on engineering solutions to such problems.

Fluctuations in nanomagnets are interesting for application in areas such as probabilistic computing, stochastic optimization, statistical sampling, and cryptography, as these fields rely on random numbers. The authors show theoretically and numerically that superparamagnetic nanomagnets can be used to generate random numbers on a subnanosecond time scale. The order parameters of both thermally excited easy-plane and antiferromagnetically coupled nanomagnets are found to possess similar dynamics. This insight could have an impact on emerging devices such as stochastic magnetic tunnel junctions, where such magnets could be used in the free layer to speed operation by orders of magnitude.

Femtosecond electron pulses are increasingly used as $e.g.$ probes of ultrafast dynamics. The information obtained from these experiments currently is limited by the source’s brightness. This study explores the potential of bullseye plasmonic lenses as ultrafast nanoemitters. This design offers many promising qualities, including emission from a flat nanoscale region and tunability of pulse duration and charge via geometric parameters. Such structures could also be utilized in high-brightness rf guns to enable more signal-intensive experiments, such as detailed spatiotemporal mapping of dynamics in heterogeneous systems, or high-fidelity measurements of weakly scattering systems.

Topological band structures in exciton-polariton systems are of interest for realizing chiral edge states that offer robust transport in the presence of disorder. However, their application in polariton spintronics has been limited, as edge states appear in counterpropagating pairs and correspond to impure spin states. By considering a lattice with spatially modulated on-site energy, the authors design a scheme wherein a fully spin-polarized edge state undergoes $unidirectional$ propagation, with no backscattering and no opposite edge state. Remarkably, such states can also exist in thin lattice strips.

The phenomenon known as $superscattering$ maximizes interaction between incident waves and subwavelength structures, enabling a myriad of applications, including sensing and energy harvesting. It is difficult to extend optical superscattering (based on core-shell multilayered structures) to its acoustic counterpart, though, due to limited material availability. This study demonstrates acoustic superscattering by means of subwavelength scatterers, each consisting of a rigid cylinder decorated with deeply subwavelength resonators. The results provide a critical understanding of superscattering, in both optics and acoustics, that could strongly boost sensing technology.

Research on magnetic skyrmions (topologically protected spin textures) as potential information carriers in spintronics has steadily expanded in recent years. The authors report the control of skyrmion chirality at room temperature in Mn-Ni-Ga, a centrosymmetric alloy with uniaxial magnetic anisotropy. The ability to manipulate the collective behavior of spin-chirality reversal under small external fields is promising for applications in nonvolatile spintronic devices with reduced power dissipation.

One of the major challenges in surface acoustic wave (SAW) technology, which is widely used in modern signal processing, is the absence of wave nonreciprocity (one-way propagation). To tackle this problem, the authors propose an approach based on magnetoelastic coupling of SAWs to spin waves possessing a suitable spectrum. This approach allows the creation of nonreciprocal SAW devices with almost no limitations on the working-frequency band (in contrast to the severe band limitations of traditional approaches), to realize the full potential of magnetoelastically induced SAW nonreciprocity. These results could become a breakthrough step toward nonreciprocal SAW signal processing.

The integration of memory and logic functions in a single device is required to develop advanced computing systems beyond the von Neumann architecture. The $memtranstor$, a two-terminal circuit element based on the relation between charge and magnetic flux via nonlinear magnetoelectric effects, provides an alternate path toward logic-in-memory devices. This work demonstrates that all 16 Boolean logic functions can be implemented in a single memtranstor, using a lock-in technique. This approach would greatly reduce computational complexity while retaining functional completeness.

Frequency standards find wide use in navigation and timing for industry, civilian, and defense applications. Unfortunately, the very best frequency standards are bulky, power-hungry, and confined to well-controlled laboratory environments. Here the authors aim to deliver a compact, high-performance standard, the key challenge being the need to produce a strong, narrow atomic transition. They produce a superstrong signal by driving a two-photon transition in ${}^{87}$Rb vapor that exploits a fortuitous third atomic level to greatly enhance the effective strength of the atom-light interaction, allowing short-term clock performance comparable to today’s best commercial frequency standards.

This study demonstrates the relevance of active electroacoustic resonators to controlling the performance of subwavelength inclusions in an acoustic metamaterial, which is a major difficulty. A feedback control scheme for loudspeakers (used as acoustic scatterers) is proposed, involving passband-current control via real-time sensing and processing of the pressure signal by a field-programmable gate array. This allows external, independent tuning of the absorption (including levels near zero and near unity), bandwidth, and resonance frequency of subwavelength acoustic resonators. The work offers a viable way past the current limitations of metamaterials, to enable practical applications.

Modulation of magnetism by electric-field-driven ion migration has huge potential for tuning of magnetic properties and inducing magnetic functionalities. This article reports the use of a solid-electrolyte Li${}^{+}$ redox capacitor for reversible modulation of magnetic anisotropy and magnetization in thin films of Co${}_{40}$Fe${}_{40}$B${}_{20}$, an alloy much studied for spintronics. The technique is a means to control the magnetic properties of metallic thin films via solid-state ionics, by marrying the physics of lithium-ion batteries and magnetic tunnel junctions. Strain engineering via the lithiation-delithiation cycle is also a possibility.