A century ago, Samuel Barnett magnetized a large body by mechanical rotation, and Albert Einstein and Wander de Haas observed rotation of a solid produced by a change in its magnetization. These effects are weak when the rotation is slow, but local mechanical rotations produced by high-frequency surface acoustic waves (SAWs) are very fast. The authors show that one can reverse the moment of a nanomagnet on a solid’s surface via SAWs generated by short voltage pulses, for logic and memory applications.

In an antiferromagnet, spins arrange in opposing sublattices with mutually compensating magnetization, and in real systems there can be different domains. Identifying a specific antiferromagnetic domain is notoriously difficult, but the authors’ tabletop setup allows just that, for magnetoelectric antiferromagnets such as chromia, which rotate the polarization of transmitted light in response to an electric field. In chromia, domain states can be not just $detected$, but also $selected$—a key property for ultralow-power spintronic memory and logic devices.

A century ago, Samuel Barnett magnetized a large body by mechanical rotation, and Albert Einstein and Wander de Haas observed rotation of a solid produced by a change in its magnetization. These effects are weak when the rotation is slow, but local mechanical rotations produced by high-frequency surface acoustic waves (SAWs) are very fast. The authors show that one can reverse the moment of a nanomagnet on a solid’s surface via SAWs generated by short voltage pulses, for logic and memory applications.

The emerging field of $atomtronics$ uses cold-atom systems to mimic or complement conventional electronic systems. Taking advantage of the flexibility and tunability of cold atoms, the authors present a mechanism for quantum memory effects based on geometrical transformations in noninteracting systems. They illustrate two specific applications: an accelerometer, and a nonvolatile atomic memory for data storage. These principles and designs can be extended to other condensed-matter systems, offering exciting opportunities for quantum-memory-effect devices.

Despite the increasing prevalence of mechanical resonators as sensors, timing references, and frequency filters, fundamental aspects of energy dissipation in these structures are still not well understood. The authors derive an expression for energy loss that provides a robust upper bound on the quality factor of a dielectric or semiconductor resonator. This analytical result reveals that vibrational mode shape plays a key role in determining the quality-factor limit.

Even in this day and age, precise absolute measurement of electric fields is not trivial. The authors extend quantum sensing via cold atoms and electromagnetically induced transparency to the strong-field regime. In this regime, existing techniques are insufficient due to the complex and highly nonlinear response of the atoms. This is an essential step toward calibration-free rf electric-field sensors for a wide range of applications that include antenna calibration, metamaterial characterization, and subwavelength imaging.

In the Casimir effect, virtual photons can exert a net force between uncharged nanoscale bodies. Typically this force is attractive, but under special circumstances it can be made repulsive. This enables levitation of carefully engineered systems, with potential applications in passive-suspension and frictionless devices. This study predicts that stable quantum levitation may be achieved using periodic gratings made of dissimilar materials. Furthermore, the levitation position is insensitive to temperature variations, and can be tuned by adjusting the relative lateral displacement of the gratings.

Bulk hexagonal boron nitride (hBN) has recently attracted attention due to its unique directionally dependent optical properties. Usually a metamaterial must be meticulously designed and fabricated, but it happens that hBN is a $naturally$ $occurring$ $hyperbolic$ $metamaterial$. The authors report room-temperature emission of single photons from bulk hBN, and the associated photodynamics. These results point to using hBN as a platform for a broad range of applications in nanophotonics and quantum optics.

One-way propagation of acoustic waves has been achieved using acoustic superlattices or nonlinear acoustic diodes, but these structures are much larger than the acoustic wavelength. This makes them impractical for applications such as low-frequency underwater signal transmission. The authors show that unidirectional acoustic wave propagation is possible with a device much smaller than the wavelength. Their design offers high rectification and broadband performance, and does not require an external power source.

Quantum computers can perform certain tasks much faster than classical computers. An example is $quantum$ $annealing$, an operation that permits the exploration of energy landscapes to find global minima $i.e.$, optimal solutions) for problems that are classically intractable. This permits simultaneous exploration of enormous computational spaces, as is necessary for complex problems in economics, network design, and nonlinear control. The authors show how suitably engineered quantum annealers can efficiently guide and focus the quantum wave function towards the solutions of constrained optimization problems.

The Hall effect would be a powerful probe for studying delocalized (bandlike) transport in semiconductors, because localized carriers do not contribute significantly to the effect. Unfortunately, the Hall voltage is typically very small in organic semiconductors. The authors develop a technique for high-resolution measurements of this effect in organic field-effect transistors (OFETs), utilizing a small ac magnetic field ($<$0.25 T). This method extends studies of intrinsic charge transport to systems with very low carrier mobilities, providing a significant step forward that should improve the design and optimization of OFETs, and other devices.

Photonic crystals may be engineered to steer light as desired, but surface effects must be carefully managed in real devices, adding complexity to their design and manufacture. The authors find topologically induced interface states that do not require any surface decoration, and can be predicted solely from the properties of the bulk photonic crystal. Without the need for surface decorations, designing and fabricating such systems will be much simpler than for today’s structures, showing a way forward to next-generation optical wave-guiding applications.

Mechanical resonators can couple to a wide variety of qubits to yield hybrid quantum systems, and thus are of keen interest for quantum information science and technology. The authors use high-quality nanofabrication to realize nitrogen-vacancy ($NV$) centers in nanoscale diamond cantilevers. The strain-mediated interaction of $NV$ electron spin qubits with cantilever motion, $i.e.$ spin-phonon coupling, is significantly enhanced, making this an important engineering milestone towards the strong-coupling regime.

Electrical readout of spin qubits requires fast, sensitive measurements, which are hindered by poor impedance matching in a practical device. The authors demonstrate perfect impedance matching in an rf readout circuit, measure the absolute sensitivity to quantum capacitance changes, and establish a clear requirement for single-shot readout. This work will aid research not just in quantum computing, but also in nanomechanics and ac quantum transport.

To realize useful, exotic properties, acoustic metamaterials for airborne sound traditionally have relied on materials that are much harder than the surrounding fluid. Soft acoustic metamaterials utilizing mesoporous silica aerogel structures offer an alternative means to tune the effective properties of the resulting metamaterial, while simultaneously providing better coupling to the acoustic environment. This study presents systems demonstrating negative density, density near zero, and nonresonant, broadband “slow sound” propagation.

A room-temperature (and thus cryogen-free) source of highly sought terahertz radiation relies on difference-frequency generation inside a dual-wavelength midinfrared quantum cascade laser (QCL). The authors develop a nonperturbative transport model to give fresh insight into this system’s physical mechanisms of terahertz generation, including strategies to yield frequencies below 2 THz. Their analysis reveals significant contributions that were previously underappreciated. This approach may also address interesting physics in other QCL-based devices, including terahertz and midinfrared frequency combs.

Many soft materials respond neither homogeneously nor linearly to an applied shear stress. The authors describe local oscillatory rheology from echography (LORE), a technique based on ultrafast ultrasonic imaging that provides access to spatially resolved viscoelastic moduli under an oscillatory shear. LORE could be used to study a great range of phenomena and systems spanning fundamental and practical interests, such as wall slip, strain hardening, shear thickening, and heterogeneous materials like hardening concrete, foods, and biocomposites.