In optical spectroscopy it is standard to split a beam of light, for sensitive reference measurements; not so for molecular beams, unfortunately. The authors fill this lacuna, combining 3D printing with electroplating to produce a $macroscopic$ beam splitter for polar neutral molecules, with clearly separated beams at high densities. This study offers not only an important piece of technology to change beam work, but also proof of principle for creating $metallic$ scientific apparatus with shapes that cannot be produced by traditional means—with high quality, at low cost.

Compared to conventional absorption-based methods, phase-contrast imaging provides superior x-ray images for $e.g.$ medical diagnosis, but its application in normal laboratories (without access to synchrotron radiation) has been very limited, due to long acquisition times, complex setup, or high doses of radiation. The authors present a method that greatly simplifies the procedure, allows for low-dose imaging, and enables acquisition times of a few minutes for computed tomography. Their approach is a crucial step toward applying this technique to real-world problems.

The usual Hall effect in a semiconductor leads to a voltage perpendicular to an applied static magnetic field. The authors significantly extend their recent work and demonstrate $experimentally$ that not only the sign but also the direction of the Hall field can be tailored by a metamaterial’s microstructure. They show that, with judicious engineering, the Hall voltage can be $parallel$ to the applied field, enabling $e.g.$ detection of local magnetic vortices.

A ring-shaped magnetic logic device offers two vortex states (clockwise and counterclockwise) to encode bits, with relative stability against external magnetic fields. The dynamics of magnetization switching in such structures, though, still need unraveling. The authors present direct experimental visualization of reproducible, robust switching in magnetic rings via domain-wall automotion, which does not require an applied field. Simulations reveal that annihilation of domain walls through automotion always occurs, with the detailed topology of the walls only influencing the dynamics locally, in line with the experimental results.

Sure, oil and water don’t mix, but there is more to it than just an old adage. The authors show how to reliably trap pockets of either oil or water by sequentially injecting immiscible liquids over photopatterned obstacles in a microchannel. According to their geometric model, the amount of liquid trapped may be controlled by varying the wetting and shape of an obstacle. These findings seem likely to interest researchers working on two-phase flows, microfluidics for lab-on-a-chip applications, or the wetting of fibers and fabrics, for example.

In state-of-the-art quantum processors, the time spent tuning gates is a significant portion of experimental run time, and could continue to grow, as number of qubits and achievable fidelities both rise. The authors devise a tune-up protocol that realizes a tenfold speed-up over traditional methods, by constructing a cost function using real-time correlations of the outcomes of nondemolition measurements. This method can be readily generalized to other tune-up problems, including two-qubit gates and measurement operations, and can be parallelized for multi-qubit settings.

For 15 years, the rapid degradation of luminance over time has rendered blue phosphorescent light emitting diodes (PHOLEDs) too short-lived for many practical applications. To date, studies have concentrated on the degradation of the emitting materials, yet not the many other parts of these devices. The authors develop a method to measure charge and exciton leakage directly over time, and find that these effects are $not$ responsible for PHOLED degradation—an important revelation that shifts our attention toward the actual sources of failure.

The usual Hall effect in a semiconductor leads to a voltage perpendicular to an applied static magnetic field. The authors significantly extend their recent work and demonstrate $experimentally$ that not only the sign but also the direction of the Hall field can be tailored by a metamaterial’s microstructure. They show that, with judicious engineering, the Hall voltage can be $parallel$ to the applied field, enabling $e.g.$ detection of local magnetic vortices.

Quantum cryptography can provably secure communications against eavesdropping, by distributing quantum states of light. A laser pulse comprises a random number of photons, but certain quantum protocols demand an exact number every time—and not always a single photon. By engineering the geometry of a nanoresonator and waveguides coupled to an InAs quantum dot, the authors tune Fano interference to interferometrically cancel out the unwanted photons from a pulse. This architecture should enable on-chip generation of multiphoton quantum states of high purity, to improve cryptographic performance by orders of magnitude.

Under a perpendicular temperature gradient, a heterostructure of ferromagnetic insulator plus normal metal yields a voltage via the inverse spin Hall effect (ISHE). When a $ferromagnetic$ metal is used instead, though, it becomes tricky to evaluate the ISHE voltage. In systems with large spin-orbit interaction, the authors observe significantly enhanced ISHE signals, suggesting that exchange-bias structures can be effective for detecting spin currents. This study provides insight into engineering hybrid spintronic devices to generate, manipulate, and detect pure spin current.

With low magnetic damping and strong spin-orbit torques, thin-film heterostructures of ferromagnetic insulator plus normal metal are at the frontier of spintronics research—but how can we directly study the local, short-time response of the insulator? This capability is crucial to the science and engineering of active spintronic technology. Thus the authors develop a highly sensitive magnetic microscope that uses spin-heat interactions to monitor the magnetic dynamics in a very thin layer of ferromagnetic insulator, with temporal resolution better than 100 ps and spatial resolution better than 700 nm.

Spin-valve nanopillars are under extensive development to realize spintronic nonvolatile memory. Here highly efficient spin-transfer-torque switching is required, but cannot be enhanced by simply adjusting the first-order uniaxial magnetic anisotropy. The authors find that, by introducing second-order anisotropy and manipulating the ratio of first- and second-order quantities, efficiency can be raised dramatically, or the thermal stability against accidental switching can be maximized. These results provide an indispensable guide for the design of magnetoresistive random-access memory.

As Moore’s Law winds down, we look to do more with less—more computing with fewer components, that is. One approach is “chaos computing”, in which nonlinear dynamics is exploited to create any type of logic gate on the fly, almost instantly, from a single generic setup. Starting from super-stable initial conditions robust to noise, the authors combine a conventional digital circuit with an analog nonlinear circuit of just three transistors, to implement arbitrary Boolean functions $exponentially$ faster than in prior architectures. This improves the reliability of chaos-based systems, and expands the scope of their potential applications.

Nickel agglomeration in the fuel electrode of a solid oxide fuel cell spoils its performance, so the ability to understand, predict, and minimize this effect is of great interest. The authors present an analytical model of Ni nucleation, revealing the time evolution of average particle size based on the electrode’s initial microstructure and the cell’s operating conditions. Their approach applies to all such two-component porous systems where only one component agglomerates, and could be extended for atmospheres with more than two reactants, as in Co electrolysis, or for systems in which both solid phases tend to clump.

Developing micro- and nanomechanical systems with ultralow dissipation is a key avenue for exploring quantum nanomechanics. Exploiting a superfluid Helmholtz resonance in liquid helium, the authors build a resonator with a quality factor of $9\times {10}^5$, with the potential to reach ${10}^8$. Its frictionless flow, low dielectric losses, large thermal conductivity, and micrometer-scale confinement make this mesoscopic system a promising tool to study mechanics and optomechanics at the quantum-to-classical crossover.

A ring-shaped magnetic logic device offers two vortex states (clockwise and counterclockwise) to encode bits, with relative stability against external magnetic fields. The dynamics of magnetization switching in such structures, though, still need unraveling. The authors present direct experimental visualization of reproducible, robust switching in magnetic rings via domain-wall automotion, which does not require an applied field. Simulations reveal that annihilation of domain walls through automotion always occurs, with the detailed topology of the walls only influencing the dynamics locally, in line with the experimental results.

For secure communication, quantum key distribution using high-dimensional quantum states—$qudits$, rather than qubits—can overcome many experimental nonidealities, and achieve higher secure-key rates than with qubits. However, high-dimensional systems are difficult to implement, especially when eavesdropping is monitored via frequency, as the devices required to measure these frequency states are complex. Using commercially available gear, the authors build a “tree of interferometers” with the potential to achieve record-breaking secure-key rates.

As an avenue to spintronics, controlling spin-polarized electronic states in low-dimensional heterostructures is at the frontier of physics and nanotechnology. Manipulating these states via ferroelectric polarization seems promising, but actually making high-quality interfaces, to realize states with large spin splitting upon a ferroelectric substrate, remains tricky. The authors rise to the challenge, directly demonstrating metallic states with large spin splitting in the wide band gap of BaTiO${}_3$, with evidence for considerable coupling between overlayer and substrate.

In metals, polymers, and even some rocks, dynamic structural failure is often due to shear localization (adiabatic shear banding), which is linked to the dynamic recrystallization of nanoscale grains. While it is well known that reducing grain size leads to higher yield strength, the effect of introducing nanograins into a coarsely grained matrix remains obscure. Systematic experiments reveal that the embedded nanograins both increase the material’s yield strength and decrease its hardening capabilities. These results allow physically informed modeling of this failure mechanism, in safety engineering and beyond.

Sure, oil and water don’t mix, but there is more to it than just an old adage. The authors show how to reliably trap pockets of either oil or water by sequentially injecting immiscible liquids over photopatterned obstacles in a microchannel. According to their geometric model, the amount of liquid trapped may be controlled by varying the wetting and shape of an obstacle. These findings seem likely to interest researchers working on two-phase flows, microfluidics for lab-on-a-chip applications, or the wetting of fibers and fabrics, for example.

Nanowires of group-$III$ nitride semiconductors might be the avenue to cost-effective, phosphorless white LEDs for displays and solid-state lighting. The authors investigate the optoelectronic properties of N-polar (In,Ga)N/GaN nanowire LEDs, and compare the main features of LEDs based on nanowires versus conventional planar layers. Their detailed insight on the consequences of strain relaxation in nanowires for LED emission, and the role of crystal polarity in the recombination mechanisms of planar and nanowire systems, will facilitate the design of tomorrow’s devices.

Tunable inductors are important for $in$ $situ$ performance optimization of electronics. Compared to traditional components, voltage-controlled magnetoelectric inductors based on ferromagnetic-ferroelectric composites are efficient and highly integrated. Previous efforts have exploited the linear piezoelectric effect, a volatile behavior, but here the authors use the large residual strain induced by ferroelastic domain switching to realize a $nonvolatile$ inductor with 250% tunability. This work expands the horizons of multiferroic devices, power electronics, and rf/microwave systems, for academia and industry.

Efficiencies of many solar-cell technologies steadily approach their thermodynamic limits, and consistent, thermodynamically correct methods are needed to address the remaining fundamental loss mechanisms. Unfortunately, the communities working on different technologies use different standards to determine the band gap of a solar cell, which in turn is used to quantify voltage losses. Motivated by the idealized efficiency limit of Shockley and Queisser, the authors offer an analytic perspective on the band gap that applies to $all$ photovoltaic technologies, allowing meaningful comparison of losses.

Organic bulk heterojunction solar cells suffer from a huge, nonmonotonic loss of open-circuit voltage $V_{oc}$ at low temperatures, which thwarts their practical application. The authors incorporate energetic disorder into a drift-diffusion model to study the $V_{oc}-T$ relations under various working conditions. In reproducing the empirical observations, they find that disorder-suppressed carrier mobility is the culprit. This work provides a comprehensive picture from the charge-dynamics perspective that will facilitate the engineering of this emerging class of devices.

The next step in building quantum computers is to develop hardware architectures that allow scalability without sacrificing coherence. Future designs seem headed toward multilayered circuits with embedded three-dimensional structures and superconducting coatings. The authors demonstrate a multilayered integrated quantum circuit, combining a micromachined microwave cavity resonator with a superconducting qubit. They address the mechanism of coupling and its control, engineering considerations that affect coherence times, and challenges in fabrication such as superconductor bonding.

The ever-decreasing size of nanoscale emitters necessitates a shift away from using planar theories of electron emission to analyze experimental results. Returning to the problem’s roots, the authors develop a comprehensive theory of field and thermal emission from grounded spheres as small as 2 nm in radius. Precise expressions for the emitted current show that the planar theory overestimates it by several orders of magnitude. These results are applicable to dusty plasmas and many industrial processes involving aerosols and powders, as well as those producing soot and other pollutants.

While optical pass-band and stop-band filters are widely used, angle-selective transmission and reflection filtering remain less explored, even though it is uniquely promising for creating complex patterns of thermal radiation in a potentially lossless fashion. The authors develop a concept for reflection filters that exploit resonant couplings to tune angle-sensitive dips in transmission, for arbitrary directional selectivity. These structures may find many applications, including daytime radiative cooling, sensitive detectors for infrared telescopes, and high-fidelity thermoluminescent spectroscopy.

In $caloritronics$, nanoscale heat flow can be not merely managed, but exploited. For example, the authors find that as the amount of quantized flux passing through a SQUID’s loop changes, the temperature in one of its branches suddenly jumps, so that in sweeping the magnetic field back and forth, a hysteretic path results. This device can be easily implemented using standard nanofabrication techniques, and, according to its hysteretic switching behavior, could promptly find applications as a temperature-based superconducting memory element, a quantum sensor, or an energy harvester to support nanocircuitry.

In optical spectroscopy it is standard to split a beam of light, for sensitive reference measurements; not so for molecular beams, unfortunately. The authors fill this lacuna, combining 3D printing with electroplating to produce a $macroscopic$ beam splitter for polar neutral molecules, with clearly separated beams at high densities. This study offers not only an important piece of technology to change beam work, but also proof of principle for creating $metallic$ scientific apparatus with shapes that cannot be produced by traditional means—with high quality, at low cost.

Sometimes, as Aristotle noted, “the whole is more than the sum of its parts”. In a dual-stage ballistic rectifier, the output voltage usually is the sum of the voltages of the single stages. However, the authors demonstrate that for stage separation smaller than a critical length, the addition of output voltages switches to addition of $input$ $currents$, inducing a synergetic enhancement of the output voltage by as much as a factor of two. Small output voltages have stymied applications so far, but the effect identified here may open the door for the adoption of such rectifiers in nanoscale electronics.

$Emergent$ $phenomena$ result from cooperative behavior that cannot be observed in a system’s individual components. Interest in understanding and harnessing such phenomena cuts across many fields of research. Here the authors demonstrate that, in an array of miniband semiconductor superlattices connected only via their substrate, self-organized synchronization of current oscillations can dramatically boost the microwave power output. Their experiments and theory suggest a solution to the longstanding problem of boosting the power output of solid-state generators in the subterahertz-to-terahertz regime.

Multifunctional perovskite thin films offer a plethora of fascinating physical effects and technological applications. The oxygen vacancies typically present in these films alter their intrinsic structural, magnetic, and ionic-transport properties. State-of-the-art density functional theory (DFT) reveals that thermal lattice excitations are crucial to the strain dependence of O-vacancy formation, as they can fully $reverse$ the energy trends deduced at zero temperature (as in standard DFT). This work enables accurate prediction of the actual properties of these important nanomaterials, with exciting prospects for device engineering.

“Squeezed” states of light are the fundamental building blocks for continuous-variable quantum optics, so a quantum network for optical communication would seem to require a compact, efficient source of such states. Combining clever design with recent advances in waveguide materials engineering, the authors produce a device that yields very high levels of squeezing, compared to other integrated architectures. Their innovative approach allows for rarely seen continuous-wave squeezing, paving the way for more advanced on-chip functions.

Thermoacoustic instability is of interest to applied physicists and engineers as a significant, widely observed phenomenon in various types of engines. This study shows how to use a pragmatic, online detection methodology, based on the theory of complex networks, to control unstable combustion in a turbulent thermoacoustic system. This combination of network physics, nonlinear dynamics, and engineering helps to avoid blowout conditions and engine shutdown.

Traditional plasmons in a metal enable light to be focused beyond the diffraction limit, but the large dissipative losses at optical frequencies seriously limit practical applications. Taking advantage of the structural dispersion of waveguide modes below the cutoff frequency, the authors experimentally realize propagation of effective surface plasmons using conventional $dielectrics$, for greatly suppressed dissipation. This work enables low-frequency “designer” surface plasmons that could find applications in compact microwave or terahertz devices.

Compared to conventional absorption-based methods, phase-contrast imaging provides superior x-ray images for $e.g.$ medical diagnosis, but its application in normal laboratories (without access to synchrotron radiation) has been very limited, due to long acquisition times, complex setup, or high doses of radiation. The authors present a method that greatly simplifies the procedure, allows for low-dose imaging, and enables acquisition times of a few minutes for computed tomography. Their approach is a crucial step toward applying this technique to real-world problems.

Two-dimensional electronic materials are a dream come true for creating thin-film transistors (TFTs), and here monolayer transition-metal dichalcogenides can be even more appealing than graphene, as they present a natural band gap. An understanding of the physics behind observed device behavior is still needed, though. The authors calculate the $I-V$ characteristics of a system with high charge density near the channel interface, then build an actual MoS${}_2$ TFT to compare. Strong agreement with experiment shows that their model can quantify the densities of trap sites and trapped charge based on simple measurements of transfer curves.

Though superconductivity and ferromagnetism are traditionally seen as competing phenomena, when representative materials are artificially juxtaposed, a trove of unusual physics is found at the interface. For example, in a Nb/Er bilayer, adjusting the magnetic state of the Er at remanence changes the shape of the resistivity transition in the Nb, controlling its superconducting $T_c$. Thus the Nb “remembers” the field applied to the Er, which could be the basis of energy-efficient data storage in cryogenic $superconducting$ $spintronics$.

Nitrogen-vacancy centers in diamond continue to attract much attention as a basis for hybrid quantum information processing. The authors develop the coherent manipulation and readout of the spin states of a small ensemble of centers, using tailored microwave pulse sequences to reduce background photocurrent and dramatically improve the signal to noise ratio. This technique is a major step toward single-spin quantum hardware featuring compact designs, easy on-chip integration, and operation at room temperature.

Devices for manipulating spin-polarized light in $transmission$ (not reflection) mode are highly desired in photonics research. To this end, the authors craft a Pancharatnam-Berry metasurface of subwavelength thickness $\lambda /8$ with a 91% efficient photonic spin Hall effect, at microwave frequencies. Key to this result is the magnetic response of the constituent meta-atoms. These findings open up the control of spin-polarized light using spin-dependent metaholograms, polarization modulators, or chirality-controlled surface-plasmon couplers, for example.