After thousands of years of practice, we may not $need$ to know physics to wash our clothes, but (as usual) it might really help. In the cleaning of fabric, a hierarchical porous material, it is understood that soil particles are first detached by detergent, then washed away by fluid flow during rinsing. A common belief is that dirt in the fabric’s pores is removed by flow advection, but small pores do not admit significant flow, leaving the role of advection dubious. The authors show that the detergent concentration gradient arising during rinsing with fresh water, though an unintended consequence, can be an effective route to enhancing soil removal from deep pores in fabric.

Feedback is a main component of many algorithms for quantum computing and communication. A key requirement for any quantum feedback scheme is that the $latency$ of the feedback loop ($i.e.$ the time between beginning to measure a state and the end of feedback action on the state) must be significantly shorter than the coherence time of the system. In this work a superconducting qubit is initialized in its ground state by active feedback, using a field-programmable gate array (FPGA) with very short latency. This in-depth discussion of the FPGA-based processing unit provides a useful reference for future development of feedback electronics for quantum systems.

Techniques for dynamic control of the polarization of light are well established, with many applications in photonics, including fiber-based telecommunication. Quantum technologies, however, often demand polarization purities and modulation bandwidths beyond the reach of existing technology. The authors demonstrate an advanced technique to obtain arbitrary polarization states, without resetting and with a 1-µs response time and 99.99% purity—about a 100-fold improvement over established technology. Further improvement by two more orders of magnitude is also envisioned.

In cutting-edge technology for optical data transmission, the two degrees of freedom in the polarization of light are utilized to double the amount of information that can be carried; different types of information, such as images and sound, are encoded in orthogonally polarized light, and then sent at once. The phenomenon discussed in this paper—namely, that the polarization dependence of the optical response of a carbon nanotube is completely reversed by charge doping—is considered to be of immediate interest to various lines of research, as a 90° polarization switch. This is expected to have an impact on engineering solutions for information handling in highly miniaturized structures.

The molecular machines known as DNA nanowalkers turn fuel into motion with unrivaled efficiency, serving as artificial parallels to naturally evolved motor proteins, but their study is hampered by a lack of quantitative, experimentally useful models. Using three-dimensional molecular modeling, the authors visualize the ability of a nanowalker’s foot to accept or reject a fuel molecule, based on whether it is pulled forward or backward respectively. This lets a two-footed nanowalker step forward instead of backward along a suitable track, despite its feet being chemically identical. Optimization predicts that the shorter this nanowalker’s body is, the more efficient it will be.

The breakdown of polymers by enzymes is of key importance in chemical engineering, biotechnology, and nature. The process involves a localized, moving front of degradation within the polymer, but little is known about its physical character. This analysis shows that the front’s width and propagation speed depend on the physical properties (size, structure, and reactivity) of both enzyme and polymer. Based on competition between reaction and transport of enzymes, simple scaling laws are obtained. These results will facilitate applications of such systems in tissue regeneration, targeted drug delivery, and biomass processing for energy generation.

An ordered stream of single photons is fundamentally different from conventional light, which features bunches of random numbers of photons. Single-photon sources are essential for emerging technologies in $e.g.$ quantum cryptography and computing, but widespread use of bright quantum-dot sources has been thwarted by the need for complex optical setups. Thus the authors present a $fiber-integrated$ source of high-quality single photons. This marriage with conventional optical-fiber technology will not only promote broad use in quantum photonics, but also may enable fundamental studies in fields from microscopy to quantum metrology, by significantly simplifying experiments.

The Sm${}_2$Fe${}_{17}{X}_3$ compounds exhibit all the necessary magnetic properties for use as rare-earth-lean permanent magnets, which have been identified as key to energy technology. However, the thermodynamic instability of these materials thwarts their adoption in applications. The authors’ calculations show that doping these compounds with La or Ce generally maintains or enhances their magnetic properties, and substantially increases their stability against $X$ vacancy formation. This points experimentalists in the right direction to find practical magnets with performance rivaling that of the champion, Nd${}_2$Fe${}_{14}$B.

With quantum information processing in mind, homodyne optical amplification tremendously enhances the sensitivity of spin noise spectroscopy applied to delicate few- and single-spin systems. In realizing the allied experimental techniques, the authors are able to boost the low-frequency spin noise signal of a few remaining impurities in an isotopically enriched Rb vapor above the electronic noise level dictated by the photodetector system. This proof-of-principle experiment advances semiconductor spin research and facilitates higher-order spin noise measurements on semiconductor qubits, such as single (In,Ga)As quantum dots.

Beyond traditional charged-based devices, logic operations can also be performed with the spins in an array of magnetic nanoislands. Unfortunately, even though such nanomagnetic logic (NML) gates yield correct output individually, they could produce erroneous output when integrated into circuits. Using a section of a kagome lattice, where the so-called “spin ice rules” apply, the authors realize a class of NML gates, taking advantage of the system’s geometrical frustration and degenerate ground states. The gate discussed in this study is $balanced$ ($i.e.$ each desired output state is at the same energy), allowing a bottom-up approach for the design of large NML circuits.

For transmissive wave-shaping applications of metasurfaces, Huygens sources have been the meta-atoms of choice to date. The authors reveal that the principle behind reflectionless transmission with wide phase coverage is a combination of symmetric and antisymmetric scattering by a meta-atom, with a reflection zero synthesized by destructive interference. As an alternative to Huygens metasurfaces, a tilted-electric-dipole meta-atom can synthesize dispersionless transmission with 360° phase coverage. The physical principle and design examples for microwave and optical frequencies shown here enable an innovative class of high-efficiency transmissive metasurfaces.

Particles smaller than 10 nm are increasingly important for applications ranging from quantum-confinement optoelectronics to drug delivery and diagnostics. The properties of any single nanoparticle can differ significantly from an ensemble average, so it is important to develop sensitive methods to characterize individuals. The authors offer a simple, high-speed, noncontact, wide-field technique for measuring the optical-extinction cross sections of hundreds of individual nanoparticles simultaneously. This approach works for any type of nanoparticle (including dielectric and semiconducting), using a standard microscope, and could attain single-molecule absorption sensitivity.

An object’s magnetic signature plays an essential role in everyday life, being used as a material fingerprint in applications from medical imaging to credit cards to security scanners. Can this signature be suppressed and substituted with a different one, in a magnetic illusion? Yes, and this work shows how. Theory is validated by experimental demonstration of transforming a ferromagnet into a superconductor, to all appearance. The essential ingredient in the illusion is negative magnetostatic permeability, which is realized with tailored sets of currents. These results open possibilities in the control of magnetic fields.

GaAs/(Al,Ga)As heterostructures are used to realize solid-state qubits and numerous other mesoscopic devices. Unfortunately, these structures are frequently afflicted by charge noise, which degrades device performance and prevents stable operation. In this study of a series of wafers with different doping densities, quantum point contacts are used as charge sensors to characterize the level of charge noise in each wafer. Both charge noise and conductance drift are reduced by decreasing doping density. This result will enable optimized heterostructures with minimal charge noise, which can serve as a robust, stable platform for spin-qubit based quantum computing.

Manipulating the polarization state of light is important in numerous applications in photonics and electromagnetism. Among many possible approaches, plasmonic polarizers have attracted widespread attention, due to their flexibility in structural design and convenience in on-chip integration. The authors demonstrate a VO${}_2$-based composite plasmonic nanostructure that can dynamically modulate the polarization of reflected light, via the thermally induced insulator-metal transition of the oxide. The composite structure can also be applied to realize switchable infrared imaging.

Transparent, conductive thin films are essential to modern optoelectronic devices such as displays, touch screens, and solar cells. This study shows how networks of carbon nanotube can be used as such films, and how the main technical challenge regarding the high sheet resistance of present-day thin films can be solved. Using density functional theory plus the Green’s function method for their transport calculations, the authors find that nitrate moieties from nitric acid adsorbed on the nanotubes yield $p$-type doping, and remarkably improve conductance both within and between tubes.

Feedback is a main component of many algorithms for quantum computing and communication. A key requirement for any quantum feedback scheme is that the $latency$ of the feedback loop ($i.e.$ the time between beginning to measure a state and the end of feedback action on the state) must be significantly shorter than the coherence time of the system. In this work a superconducting qubit is initialized in its ground state by active feedback, using a field-programmable gate array (FPGA) with very short latency. This in-depth discussion of the FPGA-based processing unit provides a useful reference for future development of feedback electronics for quantum systems.

After thousands of years of practice, we may not $need$ to know physics to wash our clothes, but (as usual) it might really help. In the cleaning of fabric, a hierarchical porous material, it is understood that soil particles are first detached by detergent, then washed away by fluid flow during rinsing. A common belief is that dirt in the fabric’s pores is removed by flow advection, but small pores do not admit significant flow, leaving the role of advection dubious. The authors show that the detergent concentration gradient arising during rinsing with fresh water, though an unintended consequence, can be an effective route to enhancing soil removal from deep pores in fabric.

Accurate control of Rayleigh waves (which feature phase-shifted longitudinal and transverse components) is essential to improving devices based on surface acoustic waves (SAWs). The authors propose an approach for efficient manipulation of Rayleigh waves, using multilayered phononic pillars to provide high-$Q$ cavity modes to interact with the SAWs. Interaction with these strongly confined modes can give rise to Fano-like resonances, as well as an acoustic analog of electromagnetically induced transparency (EIT). This approach could open the way to SAW-based solutions in high-performance sensing applications, optomechanics, and phonon circuitry.

The authors discuss a thermal router, based on magnetically excited solitons in a temperature-biased long Josephson junction. Their theoretical exploration of the coherent thermal transport in such a junction reveals the enhancement of both heat current and temperature, as induced by solitons (solitary waves, here “fluxons”, each bearing a quantum of magnetic flux). Finally, they describe a multiterminal device that allows the distribution of heat among several reservoirs at will, the desired terminal being selected by shifting a soliton along the junction via the bias current. This could provide a platform for alternative logic processing based on heat, or on fluxons.

Techniques for dynamic control of the polarization of light are well established, with many applications in photonics, including fiber-based telecommunication. Quantum technologies, however, often demand polarization purities and modulation bandwidths beyond the reach of existing technology. The authors demonstrate an advanced technique to obtain arbitrary polarization states, without resetting and with a 1-µs response time and 99.99% purity—about a 100-fold improvement over established technology. Further improvement by two more orders of magnitude is also envisioned.

The recent push toward THz sensing, transduction, and memory has created a significant need for materials that operate above ferromagnetic resonance frequencies, which are typically in the low GHz range. Antiferromagnetic materials with THz resonances seem promising, but their lack of a net magnetic moment makes manipulating them difficult. Here a fully coupled magnetomechanical model is developed, showing that antiferromagnetic single domains are controllable via strain coupling. The results indicate that near-THz device response is possible with ultralow power consumption.

In cutting-edge technology for optical data transmission, the two degrees of freedom in the polarization of light are utilized to double the amount of information that can be carried; different types of information, such as images and sound, are encoded in orthogonally polarized light, and then sent at once. The phenomenon discussed in this paper—namely, that the polarization dependence of the optical response of a carbon nanotube is completely reversed by charge doping—is considered to be of immediate interest to various lines of research, as a 90° polarization switch. This is expected to have an impact on engineering solutions for information handling in highly miniaturized structures.

Scandium nitride holds great promise for applications in several fields, based on its thermoelectric, piezoelectric, spintronic, and optoelectronic properties. Applications particularly in the last areas are strongly associated with this compound’s point defects. The authors comprehensively investigate ScN in terms of the properties of its point defects (native defects, unintentional impurities, and $p$-type dopants), electronic structure, and chemical stability. The physical understanding offered by these results lights the path to applications relying on this interesting semiconductor.

Despite great progress in photovoltaic technology, actually measuring how easily current flows in a working thin-film solar cell remains difficult, due to the complex, disordered nature of the absorber material. Understanding how well charge carriers conduct is important, because (in broad terms) devices with high conductivity are efficient and produce low-cost electricity. The authors propose a simple method to determine carrier mobility, using only common measurement techniques. This work is expected to speed the development of third-generation solar cells, by helping researchers to better understand device performance.

Can one size really fit all? The authors demonstrate the versatility of a graphene-based metasurface that is capable of actively controlling anomalous beam steering, focusing, cloaking, and illusion optics. These various functionalities are usually discussed in disparate fashion, but this work shows that they can all be described within a general framework for arbitrary surface morphology. This unified, simplified approach seems quite appealing for device design.

Semiconductor defects allowing efficient interaction between spins and photons can serve as building blocks for scalable quantum networks. The silicon vacancy ($V_{\text{Si}}$) in SiC possesses controllable, long-lived ground-state spins, for adjustable fluorescence properties. However, its broad distribution of emitted-photon energies at room temperature means $V_{\text{Si}}$’s feasibility needs to be checked at liquid-helium temperature, where phonon coupling is suppressed. This study finds a long spin-coherence time, a doubling in fluorescence intensity by spin control, and 40% photon emission into the zero-phonon line, indicating that $V_{\text{Si}}$ in SiC truly is promising for spin-based quantum technology.

Size control of nanocrystals is becoming more crucial in growing, for example, granular ferromagnetic thin films for heat-assisted magnetic recording, or self-assembled quantum dots for quantum computing. This study identifies granular patterns with surprisingly “quantized” feature sizes in Fe-Pt thin film grown on lattice-mismatched MgO. This arises from the periodic nature of the misfit-strain energy in such a system; island lengths that are integer multiples of the misfit dislocation period are energetically favored. These results point to synthesizing nanostructured materials with well-controlled grain size by tuning the interlayer lattice mismatch.

The performance of sensors based on nanoelectromechanical systems (NEMS) can be further enhanced by utilizing more modes of the resonator than just the fundamental. Often, though, it is challenging to detect higher-order mechanical modes, as their frequencies are too high, or they couple weakly to output transducers. The authors develop a technique for detection through intermodal coupling: When higher-order modes are excited, they induce a shift in the fundamental mode’s frequency, which can be detected by a sensitive phase-locked loop circuit. Thus the spectrum of a mechanical structure can be obtained, even if some modes cannot be detected conventionally.

The molecular machines known as DNA nanowalkers turn fuel into motion with unrivaled efficiency, serving as artificial parallels to naturally evolved motor proteins, but their study is hampered by a lack of quantitative, experimentally useful models. Using three-dimensional molecular modeling, the authors visualize the ability of a nanowalker’s foot to accept or reject a fuel molecule, based on whether it is pulled forward or backward respectively. This lets a two-footed nanowalker step forward instead of backward along a suitable track, despite its feet being chemically identical. Optimization predicts that the shorter this nanowalker’s body is, the more efficient it will be.

Currently, at the basis of all piezoelectric applications are just a handful of materials. Among these AlN is important, even though its piezoelectric response is not particularly large. To enhance the response of wurtzite AlN, we could alloy it with a rocksalt nitride, and density functional theory indicates that the piezoelectricity of (Cr,Al)N is very sensitive to composition. Experiments show that the wurtzite-to-rocksalt transition occurs at 30% Cr, where the alloy’s response is about four times that of pure AlN. This viable, tunable system offers significant improvements in piezoelectric applications, such as resonators and acoustic-wave generators.

Optical diffraction tomography is a quantitative imaging technique that can provide three-dimensional, high-resolution images of biological samples, without staining. Multiple scattering of photons renders the relationship between measurements and refractive-index distribution of the sample nonlinear, which makes the inversion process very difficult. This paper assesses a nonlinear forward model combined with an iterative reconstruction algorithm for imaging spherical and cylindrical objects, using Mie theory as ground truth. This work can provide useful engineering insight for nonlinear inversion problems in physics.

In spin caloritronics, thermomagnetic effects in nanostructure devices are interesting for reading magnetic states in information-storage applications, but this approach is limited by a lack of knowledge of heat transfer in such devices. Combining the 3$\omega$ method with harmonic Hall voltage measurement, this study closely correlates the measured electrical signal with the underlying heat-transfer behavior in Hall crosses. This protocol provides key ingredients for the design of thermomagnetic devices, from a thermal point of view.

A type of Boolean logic that does not distinguish between a device’s input and output terminals—self-organizing, “terminal-agnostic” logic—has recently been proposed. Borrowing concepts from high-energy physics and applying them to the dynamical systems that describe these unusual logic gates, the authors investigate the process by which these gates self-organize to a logically consistent solution. They also show explicitly that these gates are robust against noise and perturbations. With this improved understanding of their dynamics, these gates can be optimized to tackle important problems, like Boolean satisfiability and optimization.

Controlling spin orientation in magnetic nanostructures is one of the crucial issues in modern spintronics. The authors show a mechanism for steering the spin direction of a ferromagnet that is coupled to FeRh alloy. In Co/FeRh epitaxial bilayers, when the FeRh undergoes its antiferromagnetic-to-ferromagnetic transition, spins in the Co film reversibly switch between orthogonal in-plane directions. This phenomenon, which goes beyond the popular idea of heat-assisted magnetic recording (HAMR), provides a means of writing information $purely$ by a temperature change, which could be due to localized laser illumination, for example.

Nanomechanical resonators can couple to a large variety of degrees of freedom, and may be easily designed to transduce electromagnetic signals of very different wavelengths, which can be important in quantum information processing. The authors present a hybrid optoelectromechanical transducer based on a Nb-metallized SiN nanomembrane, which is able to detect very weak rf signals with shot-noise-limited optical detection. The sensitivity-bandwidth tradeoff of the device is characterized, and it is found that the transduction bandwidth can be significantly increased by properly engineering the interference between the transduction pathways in a multimode mechanical system.

Acoustic analogues of electronic topological insulators allow a wealth of opportunities for manipulating sound with unconventional acoustic edge modes that are immune to backscattering. However, acoustic devices based on topological edge states still lag, due to a lack of tunability and adaptability to functional needs. This study proposes a path to remote control and structural improvement, to achieve broadband, reconfigurable topological systems. In particular, it takes advantage of reflection-free sound propagation to engineer robust phase-delay detours. This approach could be used in various functions, including signal buffering and pulse processing.

In controlling sound, it is not trivial to engineer a device that can strongly break the reciprocity (symmetry under time reversal) of acoustic wave propagation. Despite significant progress, it is still difficult to obtain a large contrast ratio between the allowed (passing) and forbidden propagation directions, over a broad frequency range, without distortion or attenuation of the signal in the passing direction. The authors propose an approach, based on the acousto-electric effect in piezoelectric semiconductors, that could achieve all of these goals for acoustic waves in elastic solids, to the benefit of noise control, energy harvesting, and transducer technology in general.

A surface acoustic wave (SAW) device, consisting of a pair of transducers on a piezoelectric substrate, works as a high-frequency band-pass filter. Usually the substrate is nonmagnetic, and the transmission strength and frequency cannot be controlled by an external field. This study reports a SAW device based instead on multiferroic BiFeO${}_3$; in this case, the amplitude and phase of the SAW signal $can$ be modulated by an external magnetic field, thanks to the coupled magnetoelastic response of the ferrite. This approach extends the functionality of such SAW devices, which are indispensable in communication technology (your mobile phone uses one, for example).

Directional emission of sound waves is critical in imaging and communication, yet is held back by the inefficient emission of a small source. This study designs a hybrid resonant structure that converts a monopole emission to a dipole field, improving efficiency while maintaining subwavelength dimensions for the source. The design scheme also enables the flexibility of tuning the working frequency by adjusting the structure’s geometrical parameters. This work offers a practical path toward miniaturization in applications that demand efficient emission, such as sonar, loudspeakers, or ultrasound transducers.

The states of π electrons localized in zigzag edges of graphene flakes exhibit a variety of electronic and magnetic properties. The authors review these edge-state spins, highlighting their use as probes in molecular sensors made of nanographene-based microporous carbon.