On-chip electronic thermometry is an essential tool at the cutting edge of low-temperature research. However, very few primary thermometers are available for temperatures under 10 mK, and they usually entail a complex structure or experimental setup. The authors demonstrate a temperature reading down to 7.3 mK with a single NIS tunnel junction, support their measurements with thermal analysis, and show that the temperatures down to 1 mK are within reach using the present design.

Patterning magnetic materials at the micro- and nanoscale is of growing research interest for potential applications in sensors and media for high-density data storage. The authors demonstrate the influence of scaling down pattern size on domain-wall formation and magnetization-reversal processes in magnetically soft stripes by structural and quantitative Lorentz transmission electron microscopy. Their results could have wide implications for additional systems where different magnetic phases interact laterally at an interface.

Using an ensemble of ions to register one qubit each, the so-called trapped ion quantum computer could serve as a universal computing machine, with the advantages in speed and power that a quantum device could bring. However, actually making a suitably large trap is far from trivial. The authors microfabricate a ring-shaped ion trap and demonstrate its storage of 400 evenly spaced calcium ions. Its circularly symmetric trapping potential furthermore enables fundamental experiments on e.g. simulated Hawking radiation or the Aharonov-Bohm effect.

On-chip electronic thermometry is an essential tool at the cutting edge of low-temperature research. However, very few primary thermometers are available for temperatures under 10 mK, and they usually entail a complex structure or experimental setup. The authors demonstrate a temperature reading down to 7.3 mK with a single NIS tunnel junction, support their measurements with thermal analysis, and show that the temperatures down to 1 mK are within reach using the present design.

Electrical circulators enforce the unidirectional flow of microwave signals through junctions in transmission lines, but off-the-shelf components create large magnetic fields that would interfere with nearby superconducting circuits in quantum-computing hardware. The authors design a circulator based on active modulation of its components, a design that uses no large magnets and may be integrated on the same chip as superconducting circuitry. An analogous problem exists in integrated optical networks, for both quantum and classical information applications, and this approach could be adapted to that context as well.

Metamaterials allow us to manipulate all aspects of a propagating wave (of light or sound) as it passes through a material, including its speed, direction, and amount of energy reflected or transmitted. The authors demonstrate a sonic-crystal lens that focuses underwater sound with control over these three aspects. This lens is acoustically transparent over a broad bandwidth, proving that high sound speeds and matched acoustic impedance can be simultaneously achieved in such a design, which offers progress in underwater and ultrasound applications that have been held back by material limitations.

Acoustic tweezers allow for contactless manipulation of tiny objects, such as cells or droplets. The cornerstone of current technology is a corkscrewlike acoustic vortex, typically generated using handmade assemblies of individual transducers, which limits system size and accuracy and causes fabrication costs to skyrocket. The authors instead generate a similar two-dimensional acoustic field with an integrated array of miniaturized transducers, which should enable large-scale industrial fabrication of acoustic tweezers on a chip, for applications in microfluidics, microelectromechanical actuation, and nanotechnology.

A reusable battery must provide sustained performance over many cycles of charging and discharging, a key aspect for technological improvement. The authors investigate the uptake and release of lithium from tin oxide, a promising anode material for next-generation lithium-ion cells. They develop a microscopic model that explains experimental observations and shows why a layered design should yield better batteries—and may give insights into other systems as well.

Solar cells based on organic polymers could serve as a cheap, renewable energy source, but their adoption has been hampered by poor performance due to the disorder inherent to their constituent materials. The authors present a simple one-dimensional model to investigate the deleterious effects of structural disorder on bulk heterojunction solar cells. Little work has been done on this aspect of photovoltaic design, and this study is bound to influence not just device engineering, but also more sophisticated models for finer assessment.

Adiabatic superconductor logic, in which bit-switching energy can be reduced significantly by changing the potential energy landscape adiabatically, is an attractive candidate for next-generation energy-efficient computing. The authors’ simulations of the effect of thermal noise on energy dissipation in this logic scheme show that there is no lower bound on the energy of switching operations, indicating that so-called reversible computing is achievable even at finite temperature, and thus ushering the technique toward practical use.

Layered two-dimensional materials present an appealing platform for integrating optoelectronic devices with nanoscale lasers. The authors exploit the anisotropic optical properties and unique optical-selection rules of gallium selenide to generate linearly polarized luminescence at the remote edges of slabs of 200 or more layers. These results reveal the carrier spin dynamics in GaSe, and nanoscale slabs of this layered, polar semiconductor are compatible with standard fabrication processes—two requirements for spintronic technology.

Conventional phononic crystals have already been used to control acoustic waves, but would be much more versatile if one could conveniently tune their response during operation, without having to stop and manually swap out or rearrange their building blocks. The authors present a tunable phononic crystal composed of the ferroelectric ceramic Ba${}_{0.7}$Sr${}_{0.3}$TiO${}_3$, which near room temperature undergoes a phase transition that can induce a 20% shift in the frequency of transmitted waves. This system is promising for “smart” versions of acoustic cloaks, isolators, waveguides, sensors, and filters.

Efficient generation of spins in nonmagnetic semiconductors is key to realizing all-electrical spintronic devices. The authors show that in symmetric double quantum wells of a narrow-gap semiconductor like (In,Ga)As, the interband Rashba effect provides an explicit mechanism for blocking only “down” spins (for example), in a parallel magnetic field that breaks time-reversal symmetry. Current-induced spin polarization should be enhanced dramatically in the proposed device, which is compatible with mature top-down fabrication techniques and thus offers a clear pathway to practical spintronics applications.

A scanning tunneling microscope (STM) reveals a sample’s electronic structure at the atomic scale. Knowing the charge of the carriers that are actually tunneling is an important clue to the origin of a conductance peak, i.e. the phenomenon actually being observed. The authors use a specialized STM with simultaneous microwave irradiation at very low temperatures to determine that charge, which in their test case is the 2e of a Cooper pair of electrons in superconducting niobium. The agreement of their data with theory is striking, and this technique promises to be a great boon to superconductivity research, including for example the hunt for Majorana fermions.

Patterning magnetic materials at the micro- and nanoscale is of growing research interest for potential applications in sensors and media for high-density data storage. The authors demonstrate the influence of scaling down pattern size on domain-wall formation and magnetization-reversal processes in magnetically soft stripes by structural and quantitative Lorentz transmission electron microscopy. Their results could have wide implications for additional systems where different magnetic phases interact laterally at an interface.

Spies and wizards, take note: The authors provide an overview of the recent efforts in metamaterial technology devoted to cloaking, as well as a perspective on the future of this rapidly changing field. The potential of passive cloaking techniques is outlined, as is the possibility of overcoming their limitations using active or nonlinear cloaks.