Engineering better batteries for mobile electronics is a major endeavor in energy research. Silicon is potentially a great anode material for lithium-ion systems, but unfortunately it undergoes dramatic volume changes during lithiation and delithiation, leading to rapidly fading ability to hold a charge. The authors show that lithiation of amorphous silicon nanoclusters is a two-stage process governed by Coulomb repulsion, and that at high loading the Li atoms switch where they like to reside in the clusters, causing expansion. Knowing the physics of this threshold allows for optimized battery design.

Reservoir computers have received much attention recently, for state-of-the-art performance on benchmark tasks and very high data rates. Their major drawback has been offline readout, which decouples the output signal from the system. The authors address this issue with a digital readout layer implemented on a fast field-programmable gate array chip, which can compute the output in real time and feed it back into the photonic reservoir. The optoelectronic delay system yields a simple artificial neural network with output feedback, capable of autonomously generating periodic and chaotic time series—a significant step forward.

If general quantum information processing and communication using superconducting qubits is to be successful, it is crucial to realize fast, single-shot readout with fidelity approaching 100%. Starting from the physics of measurement, the authors explore current engineering limitations and find a way to halve the measurement time for single-shot dispersive readout, without sacrificing fidelity. This theoretical and experimental insight may bring superconducting quantum technology even closer to the demanding thresholds of quantum computing.

The authors study coupled resonator optical waveguides (CROWs) to extend control of light propagation in photonic devices. Exploiting parity-time ($\mathcal{P}\mathcal{T}$) symmetry of the waveguide allows selection of the group velocity and its dispersion for cavity modes. The researchers describe a scalable, controllable CROW with $\mathcal{P}\mathcal{T}$ symmetry, which is induced by periodic and balanced amplification and absorption, and analyze its potential for switching from slow to fast light transport. With suitable tweaking, even superluminal propagation could be within reach.

When excited by a short laser pulse, many semiconductors naturally emit terahertz radiation through charge-carrier acceleration. The authors present an experimental and modeling study of this phenomenon for bare $III-V$ surfaces, to obtain a detailed physical picture of the dynamics of photogenerated electron-hole ensembles. Understanding the behavior of “hot carriers” is very important for applications in microphotonics, photovoltaics, and laser processing, and the authors’ approach offers critical advantages over other time-resolved methods, including freedom from a band-to-band recombination process.

How do we get in sync with each other? Through various sorts of feedback. Swarms of mobile pulse-coupled agents (such as robots) exhibit distinct synchronization regimes, which can be controlled by tuning the velocity of the agents, or their interaction rules, as the authors show. Between slow and fast speeds lurks an intermediate zone where lock-step behavior can be impeded—possibly on purpose, depending on an application’s goals.

Chiroptical effects, characterized by different optical responses for light with left- and right-handed polarizations, are extremely weak in natural materials. Strong effects have been limited to complex three-dimensional metamaterials, but now the authors find a huge effect using just a monolayer of planar chiral metamaterial, thanks to the physics of multimode interference. The planar design greatly facilitates fabrication and can be used in a wide range of applications, such as polarization-resolve infrared detectors or chemical or biological sensors.

Functional films and heterostructures with customizable electronic, magnetic, and optical properties are indispensable in science and applications, but we must know how their properties develop as we grow them. The authors offer proof of principle for the spin-sensitive technique of $in$ $situ$ polarized neutron reflectometry, with acquisition time short enough to watch both structure and magnetization evolve during epitaxial growth of sputtered Fe films as one atomic layer after another is deposited. With modification, this neutron technique could be used with other deposition methods, too, such as molecular beam epitaxy or pulsed laser deposition.

In ferromagnet/antiferromagnet multilayers for spintronic devices, most work on optimizing exchange bias has focused on the ferromagnetic layer. Taking a different approach, the authors study an alternative structure and explore an unusual exchange bias that scales linearly with the number of repetitions $n$ in a (Co/Pt)${}_n$/Co multilayer. Considering the difficulty of increasing perpendicular exchange bias and its importance in applications, this work would seem to be a breakthrough.

The authors describe a $coiling-up-space$ approach to harvesting energy from ubiquitous, ambient low-frequency sound. They design multilateral metasurfaces composed of a labyrinthine series, for acoustic focusing and energy confinement. Control of the first reflection, coupling effects, and multiple reflections between and among the metasurfaces are discussed.

A detailed understanding of spin-transfer torque is crucial for the development of a number of applications, including STT-MRAM and spin-torque oscillators. The authors show how the different components of the spin torque, the $dampinglike$ and $fieldlike$ parts, depend on the system parameters of a magnetic multilayer. The fieldlike component is often neglected as insignificant, but in fact it strongly depends on certain material parameters, can overshadow the dampinglike component, and can even switch signs.

Storage and manipulation of quantum information requires (nearly) perfect timing—or precisely timed pulse sequences, anyhow. In quantum memory based on photon echoes, for example, the appearance time of the echo must be determined with great accuracy. This study shows how to control photon echoes in a quantum-dot optical amplifier operating at room temperature. As the amplifier is a distributed device, propagation effects are dominant in the generation of the echo pulse, and shaping the excitation pulse serves to control the echo’s appearance time and strength.

Nanoscale sensing experiments based on defects in diamond are at the forefront of quantum-technology research. Nanoscale NMR and MRI employ microwave pulses to decouple the quantum sensor from sources of decoherence, and to enhance weak signals from a single nuclear spin. Here finite pulse duration has been largely considered a source of experimental error, but the authors see it differently: A quantum model shows that seemingly spurious signals due to nonzero pulse width are predictable, allowing experimental protocols for either their full suppression, or their exploitation to increase resolution.

Color centers in nanodiamonds draw interest as an appealing class of fluorescent reporters (fluorophores) for biological labeling. The authors show that two-photon excitation of the silicon-vacancy center in diamond offers a powerful combination of narrow, bright, and nonbleaching emission that seems quite promising for deep-tissue imaging applications.

Is your phone’s screen cracked, or your car’s windshield? Making tougher glass is an important challenge for applications. It is known that modifying the structure of an amorphous solid at high pressure and temperature yields properties unattainable under normal conditions, but deeper physical insight is crucial for designing damage-resistant glasses. In studying modifier-free quaternary glasses, the authors establish a unified picture of the role of atomic structure in opposing densification, demonstrating that crack resistance can be controlled at the atomic scale.

Engineering better batteries for mobile electronics is a major endeavor in energy research. Silicon is potentially a great anode material for lithium-ion systems, but unfortunately it undergoes dramatic volume changes during lithiation and delithiation, leading to rapidly fading ability to hold a charge. The authors show that lithiation of amorphous silicon nanoclusters is a two-stage process governed by Coulomb repulsion, and that at high loading the Li atoms switch where they like to reside in the clusters, causing expansion. Knowing the physics of this threshold allows for optimized battery design.

Photoionizing a beam of ultracold atoms can provide a high-brightness ion beam, to improve resolution in focused-ion-beam nanofabrication. With this aim, the authors use laser cooling and compression to give a ${}^{85}$Rb atomic beam an equivalent brightness better than the current standard for the semiconductor industry. This could represent an important step toward smaller feature sizes in integrated circuits.

Reservoir computers have received much attention recently, for state-of-the-art performance on benchmark tasks and very high data rates. Their major drawback has been offline readout, which decouples the output signal from the system. The authors address this issue with a digital readout layer implemented on a fast field-programmable gate array chip, which can compute the output in real time and feed it back into the photonic reservoir. The optoelectronic delay system yields a simple artificial neural network with output feedback, capable of autonomously generating periodic and chaotic time series—a significant step forward.

The electrical contacts needed for conventional probing of a graphene-based device can mask the very properties to be measured. The authors demonstrate a contactless measurement scheme that capacitively couples a device to a superconducting microwave resonator. Both quantum capacitance and charge-relaxation resistance can be inferred from a single measurement, and residual doping and the Fermi velocity can be quantitatively deduced. This technique offers fast, sensitive, noninvasive measurement of graphene nanocircuits, or other systems for which ohmic contacts are difficult to obtain.

Domain walls can act as energy-efficient spin waveguides in magnonic devices that are considered promising for CMOS technologies, yet cannot be readily imprinted into magnetic nanostructures, especially in bent geometries. The authors develop a full set of protocols for writing strip-domain walls into bent magnetic nanowires by virtue of spin Hall torque. Along the way, they also discover unusual domain-wall dynamics. These results point the way to practical spintronic memory and logic devices.

In the context of quantum information processing, carbon nanotubes allow accurate control of electronic charge, spin, and valley degrees of freedom, in an atomically perfect and isotopically pure material. Using rf reflectometry to measure quantum capacitance, the authors study a carbon nanotube charge qubit with the information encoded in an electron’s position. By manipulating qubit states with microwaves, they are able to directly measure charge coherence in the qubit. Their technique allows operation at a sweet spot where the device is first-order insensitive to charge noise, for much longer coherence times.

Generating certifiably random bit sequences using quantum devices is a fundamental challenge for cryptography, particularly as applied to secure communication. The device-independent approach offers the best security, but is impractical. This study presents a quantum random-number generator (QRNG) that is simple and achieves high bit rates, yet requires only mild assumptions (almost no trust) regarding the setup. This protocol offers a promising solution for the next generation of QRNGs, combining ease of implementation, commercially viable rates, and strong security.

Spin Hall oscillators are a promising basis for engineering spin-wave devices. The authors use a micromagnet’s inhomogeneous dipolar field to confine localized spin-wave modes (magnons), and to control their response to spin currents. This offers continuously variable spatial mode dimensions and systematic tunability of the magnon spectrum, for the study of spin-orbit-torque oscillators and the development of electrically actuated spintronic circuitry.

If general quantum information processing and communication using superconducting qubits is to be successful, it is crucial to realize fast, single-shot readout with fidelity approaching 100%. Starting from the physics of measurement, the authors explore current engineering limitations and find a way to halve the measurement time for single-shot dispersive readout, without sacrificing fidelity. This theoretical and experimental insight may bring superconducting quantum technology even closer to the demanding thresholds of quantum computing.

To paraphrase Nietzsche, when you look into the abyss, it also looks into you—especially regarding mesoscopic physics: In the $inverse$ $proximity$ $effect$, electrons from a normal material worm their way into an adjacent superconductor and spoil its ordering. This and overheating are detrimental to device performance. The authors give experimental evidence of the interplay of these two effects, and their efficient suppression by a weak magnetic field. Understanding this interplay in the context of quasiparticle overheating promotes the improvement of superconducting nanoelectronics for photon detection and quantum computation and metrology.

In quantum computing, robustness against noise is a primary concern. Meanwhile, geometric (topological) phases in physical systems are determined by global properties that are insensitive to the details of evolution, and thus offer some built-in noise resilience. Marrying these ideas for superconducting qubits is not easy, but this study details a scalable scheme for fast holonomic quantum computation based on superconducting circuits with all-resonant microwave control. The main difficulties for physical implementation in this context are overcome, with both single-qubit and nontrivial two-qubit gates being described.

The authors study coupled resonator optical waveguides (CROWs) to extend control of light propagation in photonic devices. Exploiting parity-time ($\mathcal{P}\mathcal{T}$) symmetry of the waveguide allows selection of the group velocity and its dispersion for cavity modes. The researchers describe a scalable, controllable CROW with $\mathcal{P}\mathcal{T}$ symmetry, which is induced by periodic and balanced amplification and absorption, and analyze its potential for switching from slow to fast light transport. With suitable tweaking, even superluminal propagation could be within reach.

Impedance spectroscopy, the measurement of electrical current in response to excitation voltage at various frequencies, is a ubiquitous analytical technique, particularly in device physics and biophysics. The authors apply adaptive filtering (in a sense, the simplest version of artificial intelligence) for noise reduction in time-domain impedance measurements, with outstanding results compared to standard Fourier-transform techniques. This approach can be used in any context, including portable biosensors. As a demonstration, the authors detect $in$ $vitro$ a single living cell in the presence of high ambient noise.

Superconducting circuits offer great flexibility in designing quantum hardware. The authors present a multimodal “trimon” circuit implementing three qubits, and exploit the strong all-to-all longitudinal coupling to demonstrate high-fidelity multiqubit gates. Their design is a marked departure from conventional ones that combine individual, decoupled qubits. Nonetheless, this scheme allows fairly straightforward scale-up using established multiqubit architectures. The trimon mimics natural molecules, as used in NMR techniques, and shows significant potential for applications in quantum information processing.

For lab-on-a-chip applications, an attractive mode of controlling fluid transport is piezoelectric actuation of acoustic pressure waves. In this study, optimal acoustophoretic frequencies are identified using mechanical macroscopic indicators, which allows one to bypass examination of the pressure field inside a microchannel. These macroscopic indicators are linked to the system’s electromechanical characteristics, so electrical measurements can also be used to detect the optimal frequencies. These results may greatly simplify microfluidic devices, promoting this important biomedical technology.

The quest to harness dynamic collective excitations, such as spin waves or dissipative solitons, as a basis for spintronics continues. This study shows that thermal fluctuations have a significant effect on the dynamics of magnetic excitations induced by spin-transfer torque (STT), and provide a pathway to controlling spin-wave condensation, propagation, stochasticity, and stability. As it turns out, droplet solitons nucleate for lower current densities at $lower$ temperatures, in contrast to typical STT-induced switching between static magnetic states.