Cryogenic systems in laboratories and hospitals worldwide require liquid helium, a precious natural resource that must be carefully conserved. This means recycling, on a large scale, but even traces of H${}_2$ can clog the capillary plumbing in a recovery plant—a common, chronic problem that is very time-consuming, expensive, and disruptive to supplies. The authors propose a microscopic mechanism to understand plugging due to the presence of non-solid molecular hydrogen in liquid He. They also describe the development and testing of purification solutions that have proved highly effective in producing “clean helium” at considerably reduced operating costs.

Generating microdroplets of viscous fluids is crucial to “drop-on-demand” applications ranging from inkjet printing to additive manufacturing, but is dogged by the clogging of microscale nozzles. In this study a flow-focusing phenomenon, resulting from a laser-induced shock wave, is used to produce monodisperse viscous droplets smaller than the nozzle from which they fly. The authors prove that this technique can even be used to print a biologically relevant sample without spoiling its functionality.

For a quantum computer, switching a signal carried by single microwave photons is desirable in scaling up superconducting circuitry. The authors design and build such a switch, integrate it on a chip with a single-photon source, and prove that it works with nonclassical microwave input. Their device offers negligible heating, relatively large bandwidth, low nonlinearity, and single-pole double-throw switching in mere nanoseconds. It is well suited for applications where signal routing must be controlled by real-time feedback, as in multiplexing qubit control and readout, or distributing entanglement in quantum networks.

Computer hardware is moving away from volatile DRAM and SRAM toward nonvolatile universal memory (NVRAM). In next-generation NVRAM technologies, binary information is typically stored using one of three quantities: the direction of magnetization or electric polarization, or the magnitude of resistance. The authors prove experimentally that the sign of the magnetoelectric coefficient α of a multiferroic material can also be used effectively for binary encoding. The virtues of this kind of memory include high speed, low power consumption, parallel reading, a simple structure for easy fabrication, and a diversity of available materials.

Arguably the most crucial challenge in developing organic solar cells is to optimize the collection of photogenerated charges, and therefore the cell’s $fill$ $factor$, a dimensionless parameter directly proportional to its power-conversion efficiency. The authors provide a complete physical picture of the fill factor, with a detailed discussion of the model’s assumptions, easy ways to spot deviations from the model, and a simple method of analysis to quantify a material’s electronic quality, without needing further equipment. This analysis can also be applied retrospectively to the deep literature on solar cells made of many semiconducting polymers.

Some things never change, including questions at the root of empirical science, such as, “What is this stuff made of?” The authors introduce a method for $nondestructive$ elemental analysis, combining particle-induced broadband x-ray emission (PIXE) with detection based on superconducting microcalorimeter arrays. This technique is especially promising for tricky samples containing trace concentrations of many elements with similar signatures, or for which additional chemical information is desired.

Generating microdroplets of viscous fluids is crucial to “drop-on-demand” applications ranging from inkjet printing to additive manufacturing, but is dogged by the clogging of microscale nozzles. In this study a flow-focusing phenomenon, resulting from a laser-induced shock wave, is used to produce monodisperse viscous droplets smaller than the nozzle from which they fly. The authors prove that this technique can even be used to print a biologically relevant sample without spoiling its functionality.

In $collective$ $operations$, units interact to generate synergistic effects beyond what the individual, noninteracting units could yield. A quantum collective with no classical analogue is even more intriguing. In the context of quantum thermodynamics, the authors use the concept of extracting and injecting $coherence$ to envision a collective heat machine of the same efficiency as its individual engines, yet drawing more heat from thermal baths and producing more work, its power scaling quadratically with the number of engines. Surprisingly, the collective machine can be more reversible than its most reversible element.

For very small, very cold objects, merely knowing how cold they actually are is a challenge. The authors present a “Josephson bolometer” that detects weak electrical fluctuations and indicates their strength directly via frequency shift in a microwave resonator, without relying on power-spectrum analysis by external electronics. This combined experimental and theoretical study advances nanoscale thermometry and the detection of weak electrical signals, offering prospects not just for high-resolution calorimetry, but also for manipulating nonclassical microwave fields, and microwave amplification.

Charge recombination is a bugaboo of solar-cell efficiency, and much effort has been devoted to understanding and preventing it. There has been debate about whether the important hybrid organic-inorganic perovskite solar absorbers are ferroelectric; if so, their polarization could be exploited to boost charge separation, $i.e.$ reduce recombination. The authors show that CH${}_3$NH${}_3$PbI${}_3$ does in fact present both ferroelectric and semiconducting behaviors, adding a new angle to optimizing perovskite solar cells.

The structural integrity of essentially everything around us depends crucially on the resistance to crack propagation of its constituent materials—their $toughness$. Predicting the toughness of noncrystalline materials is a challenge with far-reaching theoretical and technological implications. The authors develop a unified theory of the fracture toughness of glasses, addressing its dependence on a wide range of physical parameters. The emerging picture offers a path to deeper understanding that, for example, may finally allow us to realize the great potential of bulk metallic glasses for structural engineering.

For a quantum computer, switching a signal carried by single microwave photons is desirable in scaling up superconducting circuitry. The authors design and build such a switch, integrate it on a chip with a single-photon source, and prove that it works with nonclassical microwave input. Their device offers negligible heating, relatively large bandwidth, low nonlinearity, and single-pole double-throw switching in mere nanoseconds. It is well suited for applications where signal routing must be controlled by real-time feedback, as in multiplexing qubit control and readout, or distributing entanglement in quantum networks.

Much attention has been paid to the nitrogen-vacancy ($NV$) center in diamond as a promising emitter for applications in quantum information science and photonics, but so far even the best quantum emitters in nanodiamonds have suffered from spectral diffusion and broad zero-phonon lines. The authors show that $silicon-vacancy$ ($SiV$) defects in diamond exhibit a nearly lifetime-limited linewidth, do not suffer from blinking, and are optically stable even above saturation. These results point the way to progress in $e.g.$ hybrid quantum photonics and quantum sensing.

The two-dimensional electron gas (2DEG) at the LaAlO${}_3$/SrTiO${}_3$ interface continues to be of interest, in terms of basic physics as well as applications in oxide electronics. However, inhomogeneity can be a major spoiler of the 2DEG’s special transport properties. In a quest to make a more homogeneous 2DEG, the authors find that a cuprate cap on nanostructures in the gas helps matters—a lot. Furthermore, below 0.2 K they find superconductivity, showing that dissipationless transport is not out of the question.

The speed of light is medium-dependent, and when the properties of the medium can be chosen—as in a photonic crystal—light can be slowed to the point where it can be “stored” or handily manipulated. The authors use a $magnetophotonic$ crystal to harness the Faraday effect, and realize $time-resolved$ rotation of the polarization of a single femtosecond laser pulse. This achievement bodes well for photonic applications such as spatial light modulators, and holographic memory for optical computing.

Many applications, particularly wind turbines and electric motors for tomorrow’s vehicles, require the strongest permanent magnets, which contain rare-earth metals. Issues with rare-earth supply and production have led to a quest for powerful magnets without these metals. Exploiting the physics of a solid-solid phase transition under tensile stress, the authors obtain a magnetically hard and anisotropic material made only of abundant, cheap iron and nitrogen. In doing so, they directly observe the coupling between tensile strain and the martensitic transition.

Under negative bias, a semiconductor’s $radiant$ temperature can drop significantly below its $thermodynamic$ temperature, and it can be used to cool another body via radiative exchange. The technique has never taken off, though, due to limited power density, and the weak performance of nonideal real-world materials. This study presents a near-field refrigeration system with high cooling power density and efficiency, and supporting fast temperature modulation. Ideally, this device can operate close to the Carnot limit—and it continues to perform well even in the presence of strong nonidealities.

As feature size of electronics decreases below 20 nm, magnetoresistive random-access memory (MRAM) is a promising successor to DRAM technology, offering good downsize scalability, functionally infinite endurance, and invulnerability against ionizing radiation ($e.g.$ in space). The authors discuss a stacked thermally assisted MRAM, in which information is encoded via two degrees of freedom in the position of a magnetic vortex’s core. This yields high-density memory with at least five bits per cell.

We often consider bulk metamaterials with engineered electromagnetic properties, or even metasurfaces, but what about an individual $metamolecule$? The authors study the magnetization configuration of an isolated, micrometer-sized metamolecule, designed and cleverly fabricated to break both space-inversion and time-reversal symmetry. Such a system could be used for a one-way mirror controlled by magnetic field, or as an artificial multiferroic material operating at convenient fields and temperatures.

Cryogenic systems in laboratories and hospitals worldwide require liquid helium, a precious natural resource that must be carefully conserved. This means recycling, on a large scale, but even traces of H${}_2$ can clog the capillary plumbing in a recovery plant—a common, chronic problem that is very time-consuming, expensive, and disruptive to supplies. The authors propose a microscopic mechanism to understand plugging due to the presence of non-solid molecular hydrogen in liquid He. They also describe the development and testing of purification solutions that have proved highly effective in producing “clean helium” at considerably reduced operating costs.

Devices with multiple insulating ($I$), ferromagnetic ($F$), and superconducting ($S$) layers in various configurations have been studied for some time, both in basic research and to exploit quantum effects for improved electronics, and lately have enjoyed renewed interest. This systematic study addresses Cooper-pair tunneling out of equilibrium in a hybrid $SISFIFS$ sandwich that acts as a transistor. The results of the analytical model agree well with experiment, providing an important tool for designing tomorrow’s ultralow-power computing hardware.

Nuclear magnetic resonance (NMR) with single-molecule sensitivity and magnetic resonance imaging (MRI) with atomic-scale resolution are the prime challenges in magnetic microscopy. The authors advance leading-edge quantum sensing by developing a scheme for MRI with angstrom-level resolution. This would determine the position and conformation of a single isotopically labeled molecule, using the response of an $NV$ center in diamond to the characteristic wave-function signatures of target nuclear spins. This approach differs from typical multidimensional NMR, and beats the resolution limit set by frequency gradients in conventional MRI.

Traditional optical techniques suffer from the diffraction limit of about $\lambda /2$ in resolution, which $e.g.$ prevents using a visible-light microscope to see truly nanoscale objects. Beating this limit, especially in the optical far field, is tricky. The authors propose a method for observing a $moving$ object illuminated by multiple plane waves, and analyzing the far-field measurements to reconstruct an image super-resolved to an amazing $\lambda /100$. This method could have a strong impact across disciplines related to high-resolution microscopy, including the inspection of integrated circuits, nanoparticles, biomolecules, or structures within living cells.

Hybrid quantum systems marrying the best aspects of different platforms are keenly sought for applications in quantum information processing. The authors use a superconducting resonator at millikelvin temperatures to measure the angle-resolved electron spin resonance spectra of gadolinium ions implanted in sapphire, the results agreeing well with predictions. This optical two-level system with long coherence times in its electronic transitions is interesting as a basis for a microwave quantum memory, and ion implantation seems a promising avenue to its controllable, scalable implementation in the solid state.

As quantum processors become increasingly complex, their control infrastructure needs to move from lab to application scale. This study shows how superconducting single-flux-quantum logic can be applied in a way that meets the stringent precision requirements of quantum computing at high speed, using a strongly modified version of quantum optimal-control theory. These results bring us one step closer to a fast, high-fidelity, solid-state quantum computer.

Power transformers, key components of an electrical grid, have cores made of iron-silicon “electrical steel” alloys. The structures of magnetic domains in these highly anisotropic, grain-oriented laminates chiefly determine a core’s key properties, and thus its performance. The authors use neutron grating interferometry to observe​ these domain structures and their response to $static$ external magnetic fields. This direct access to a sample’s magnetic structure will enable physicists to improve our understanding of macroscopic magnetic phenomena in bulk ferromagnets, and will help engineers optimize these alloys and systems.

The core of a power transformer is subject to $time-varying$ magnetic fields. In this study the authors visualize in detail the interplay between the frequency and amplitude of magnetic excitation, to study the frequency-induced freezing of bulk magnetic domain walls in electrical steel. A basic understanding of this freezing is needed to develop macromagnetic models, leading to transformers with improved performance and efficiency.

Common sense dictates that particles in an ultrasonic standing wave should experience a nonzero radiation force, and be moved. However, the authors predict that layered particles can be engineered to ignore this force—akin to “invisibility cloaking” for sound. Establishing how to choose the material and thickness of the coating for an acoustically neutral object, this study opens possibilities for acoustophoretic methods of particle trapping, separation, and sorting in microfluidic devices.

Research on hybrid quantum systems for sensing and information processing often focuses on cantilevers containing single-photon emitters. Rather than a vibrating beam, here the authors design, fabricate, and characterize an architecture based on a sheet of high-quality diamond. Implanted $NV$ centers are seen to be effective nanosensors of residual stress in the membrane, and its motion under static pressure and resonant vibration, in both dc and ac regimes. This platform is promising for piezometry, vibrometry, and optomechanical cavities.