The conceptual “smart home” features a multitude of sensors to monitor temperature, motion, humidity, etc. The prospect of wireless sensor powering via an ambient Wi-Fi field is enticing, but current methods to harvest this energy are too inefficient. The authors show that a simple, clever control scheme for indoor Wi-Fi reverberation can concentrate the waves on a harvesting device, drastically increasing energy collection. Also, the harvester’s natural re-emission of specific signals during collection can be exploited, for focusing without direct access to the device. Such indirect, unsolicited, blind feedback might be useful in other fields, too, such as telecommunications.

For acoustic waves, in general the source must be much larger than the wavelength to produce directional waves, yet porpoises have efficient biosonar systems that break this size rule. How? This study reveals that the porpoise’s forehead is a natural gradient-index material, and key to production and control of directional beams. The whale’s compression of the multiphase structure of its forehead effectively manipulates the view angle of the beam. Aside from advancing our knowledge of cetaceans, these results could inspire the development of human-made metamaterials for our own subwavelength applications.

Error correction is essential to quantum information processing, but the demanding performance requirements for $useful$ error correction make it a difficult proposition. This study shows that incorporating prior knowledge of physical error models can dramatically improve the efficacy of quantum error correction, in a small code. This progress significantly expands the range in which quantum error correction can be usefully applied, facilitating interesting experiments that use accurate device models to protect quantum memories.

The conceptual “smart home” features a multitude of sensors to monitor temperature, motion, humidity, etc. The prospect of wireless sensor powering via an ambient Wi-Fi field is enticing, but current methods to harvest this energy are too inefficient. The authors show that a simple, clever control scheme for indoor Wi-Fi reverberation can concentrate the waves on a harvesting device, drastically increasing energy collection. Also, the harvester’s natural re-emission of specific signals during collection can be exploited, for focusing without direct access to the device. Such indirect, unsolicited, blind feedback might be useful in other fields, too, such as telecommunications.

An essential task in infrared (IR) technology is to develop and exploit advanced photodetectors, to enable both fundamental research and applications. The authors utilize quantum Hall states and cyclotron-resonance absorption in a hybrid graphene-GaAs/Al${}_x$Ga${}_{1-x}$As system to implement a dual-band, tunable detector covering both mid- and far-IR wavelengths, with a log-periodic antenna to minimize chip size and significantly enhance the photoresponse. These results point toward a panoply of solutions for highly sensitive, tunable, multicolor IR detectors and imaging arrays.

For acoustic waves, in general the source must be much larger than the wavelength to produce directional waves, yet porpoises have efficient biosonar systems that break this size rule. How? This study reveals that the porpoise’s forehead is a natural gradient-index material, and key to production and control of directional beams. The whale’s compression of the multiphase structure of its forehead effectively manipulates the view angle of the beam. Aside from advancing our knowledge of cetaceans, these results could inspire the development of human-made metamaterials for our own subwavelength applications.

The knife cuts both ways: Rather than turning waste heat into electricity as usual, a thermoelectric material instead can use electrical current to remove heat, if you prefer. Due to the robustness of the Peltier effect, solid-state cooling devices have great potential for application. Considering engineered materials of graded composition, the authors analytically establish a criterion that allows drastic improvement of cooling performance, compared to an everyday, homogeneous material. Numerical modeling further allows quantitative investigation of the rate and sources of entropy production associated with such a device.

Error correction is essential to quantum information processing, but the demanding performance requirements for $useful$ error correction make it a difficult proposition. This study shows that incorporating prior knowledge of physical error models can dramatically improve the efficacy of quantum error correction, in a small code. This progress significantly expands the range in which quantum error correction can be usefully applied, facilitating interesting experiments that use accurate device models to protect quantum memories.

With their domain switching under electrical loading or mechanical stress, ferroelectric materials have been studied for applications in high-density nonvolatile memory. However, backswitching in such systems could cause significant data loss. In this work, experiments and stimulations are used to investigate the unique ferroelastic domain-switching kinetics of single-crystalline pillars of relaxor material. The electromechanical hysteresis loop shifts due to constraint of the pillars, resulting in various mechanically reversible $or$ $irreversible$ states. This behavior could be exploited to overcome the backswitching problem in advanced bit writing and reading.

One of the most intriguing features of spin-ice materials is that their elementary excitations behave as emergent magnetic monopoles. While the properties of spin ices suggest applications in magnetism and spintronics, $artificial$ spin ices—metamaterials built of interacting ferromagnetic nanowires—also exhibit unusual properties that could be utilized in reconfigurable magnetoresistive devices. The circuit model presented in this study provides a simple picture for understanding these systems, underscoring the many-body origin of their complex magnetotransport phenomena. This model also provides a guiding principle for designing devices based on these metamaterials.

A lack of compact, tunable sources of coherent THz-frequency electromagnetic signals is a fundamental problem in modern microwave/terahertz technology. The authors propose a THz signal generator based on the spin Hall effect in a thin film bilayer: a platinum layer driven by dc current, plus a layer of a canted dielectric antiferromagnet (AFM) with a small net magnetization. Precession of magnetization of the AFM caused by an external spin current produces THz electromagnetic radiation. The power of this signal can exceed 1µW, and increases with increasing frequency; however, the achievable density of the external spin current limits the maximum output frequency and power.

Electronic transitions, such as those induced by light, excite a molecule’s vibrations in a way that is very difficult to compute. The authors propose a superconducting circuit that can efficiently and robustly simulate these transitions and provide the distribution of vibronic excitations. This single-purpose quantum simulator would consist of microwave resonators, tunable inductors, and superconducting qubits, with realistic conditions for preparation, quench, and measurement. Using the nonlinearities of Josephson junctions, the simulator could also treat the anharmonic regime of vibronic transitions, which is an even harder computational problem.

Beam tracking in x-ray phase-contrast imaging provides a simple, effective method for multimodal analysis of specimens with micrometric resolution. Its tomographic implementation with nonmicrofocal laboratory sources (no synchrotron required) yields three-dimensional maps of the real and imaginary parts of a sample’s complex refractive index, and of its scattering power. These three independent signals significantly increase the information content obtained from an x-ray tomographic scanner, with strong potential for improved material discrimination in several fields of application, from materials science to biomedical research.

The authors describe a high-resolution microscopy technique for the detection and characterization of chiral samples less than 100 nm in scale. Conventional techniques require a considerable amount of sample, and are vulnerable to background noise. The proposed photoinduced-force microscopy is based on near-field effects that are isolated from the background. Circularly polarized photons induce force on a cantilever, transferring optical activity from a chiral sample to an achiral probe. This technology has the potential to advance the control of constructive interaction between drugs and receptors in the human body.

The phenomenon of injection locking, in which coupled oscillators become synchronized, is exciting from the points of view of both physics and technology, with relevance to many systems in physics, chemistry, and biology. This study explores injection locking in a nanoscale spintronic oscillator with feedback. Beyond integer and fractional locking, this oscillator is also a test bed for $sideband$ injection locking. For low driving fields, sideband locking can reduce the main peak’s linewidth. It also can considerably increase the locking range, providing a means to synchronize oscillators with a large difference in natural frequencies.

Tunable filters are crucial in networks for microwave signal processing, but are being stymied for millimeter-wave applications, due to a lack of materials with suitable properties. This study presents liquid-crystal-based filters with tunable passband resonances at 30, 50, and 85 GHz. These filter devices additionally provide an interesting method to characterize the dielectric properties of liquid crystals (and solids and liquids in general) in the 1–1000 GHz frequency range. Such a method promotes better engineering of planar microwave devices, and the scientific investigation of materials, in this frequency range.

A key element to realize secure, long-distance quantum communication is a device capable of storing and synchronizing quantum data without jeopardizing the security of the network. The size of and resources needed to build a quantum memory has held this technology back—until now. The authors send randomly polarized photons through a free-space channel, receive them with a portable quantum memory, store them, and finally read them out. They show that the data encoded in the photons remain fully protected throughout. This prototype quantum network using cost-efficient, room-temperature quantum memory could become the backbone of global quantum-communication protocols.

Further miniaturization of communication devices is bottlenecked by inductor-based oscillators, which potentially could be replaced by spin-torque nano-oscillators (STNOs). Progress is thwarted, however, by the low microwave power delivered by typical STNOs based on trilayer magnetic tunnel junctions. This work proposes to utilize the physics of double-barrier resonant tunneling in an STNO designed for significantly enhanced microwave power and conversion efficiency. This conceptual framework just might provide the spintronic cure for these technological ills.

Locally screening the effect of a global depletion gate offers an alternative method for realizing nanoelectronic circuits in a two-dimensional electron (or hole) system. This approach is relevant for future technologies based on the electric-field effect, aiming for complex or large-scale quantum circuits, or devices with a ring topology. This study experimentally explores the method’s feasibility for two devices, investigating their electrostatic, dynamic, and phase-coherent properties. The local-screening method promises important advantages, such as the possibility of complex circuits with fewer gates.

When subjected to an ac voltage, a dielectric elastomer (DE) exhibits complicated nonlinear vibration, implying significant applications for dynamical actuators. For a vibrational system, including such a DE system, the dynamical properties must be affected by its geometric size. In this article, a nonlinear dynamical model is deduced to investigate the geometrical effects on the dynamical properties of viscoelastic DEs. The results indicate that the deformation response, vibrational periodicity, resonance, and other properties of DEs may be tuned by varying the system’s size.

Artificial neural networks built around nanoelectronic components are a means to realizing compact, energy-efficient cognitive intelligence. The authors use the inherent device physics of nanomagnets to emulate the computational primitives of a neural network, reducing the energy and area requirements of the underlying hardware. They analyze the performance of stochastic neuromorphic computing platforms with magnets of different barrier heights, and show how the core network architecture must be modified as the magnets scale down to the superparamagnetic regime.

Dark excitons, which usually are optically forbidden, show great potential for implementing spin qubits with long coherence times in single quantum dots (QDs). Meanwhile, single QD photocurrent spectroscopy with resonant optical pumping is a proven, effective way to detect an excitonic qubit, and to initiate a spin qubit with high fidelity and resolution. The authors demonstrate that probing dark-exciton states with photocurrent spectroscopy in a magnetic field, at very high resolution due to the extremely narrow linewidth of the laser, could have applications in high-precision qubit detection for quantum information processing.

Two-dimensional materials continue to fascinate, for both their rich fundamental physics and their multifarious potential for applications. In such layered semiconductors, although the interlayer interactions are weak, they contribute significantly to overall electronic behavior. For example, this study shows that stable room-temperature ferromagnetism in bilayer SnO depends on the balance between hole doping and the interlayer interaction of lone-pair electrons. With its inherently flat structure and half-metallic, spin-polarized ground state, this compound is especially interesting for spintronic nanodevices.

Identifying high-performance thermoelectric materials for scavenging waste heat is an important but difficult problem, requiring a low electronic effective mass $m*$ for efficient conduction, yet a high effective mass for a large number of carriers. The authors predict that layered pnictogen dichalcogenides, with some elemental substitution, can satisfy both contradictory requirements. A peculiar band dispersion realized on a square lattice reconciles these two features, and can lead to remarkably high performance. This study offers a promising strategy for designing thermoelectric materials, and hopefully a short drive to devices.

What’s next in spintronics? Well, the $pseudospin$ is a discrete electronic degree of freedom that is tunable by an electric field, and suitable for application. The authors show that zigzag phosphorene nanoribbons support two incarnations of pseudospin: edge (where electron transport can be restricted to only one of the ribbon edges) and layer (transport through only one layer in a bilayer nanoribbon). The researchers describe a field-effect transistor that generates pseudospin-polarized current, and a pseudospin valve that selectively transmits only one pseudospin polarization. In particular, the edge-pseudospin valve is predicted to be remarkably robust.

Spin photodiodes pique interest for converting information carried by photon helicity into electrical current, or for solid-state information storage and readout by illuminating the device with circularly polarized light. To elucidate the influence of incident and azimuthal angles on helicity amplitude, the authors use a hybrid device for systematic measurements at room temperature, with no need for an external magnetic field. The results yield important insight on the angular dependence of spin-photodiode efficiency, indicating key parameters that must be optimized to use such systems for spin filters or polarization detectors in spin-optoelectronic applications.

Although electrical injection, transport, and detection of spins in silicon have all been achieved, the induced spin accumulation has been much smaller than expected and desired. Using nonlocal spin-transport devices with an $n$-type Si channel and Fe/MgO magnetic tunnel contacts, the authors demonstrate that it is possible to create a giant spin accumulation in Si, with the spin splitting reaching 13 meV. This result enables the development of Si spintronic devices with a large magnetic response.

Over the years, a compact, superconducting gravity gradiometer has been discussed, with the potential for important applications in planetary science, the measurement of gravity waves, and even early warning against earthquakes. Its test-mass dynamics, levitation, and successful operation still need to be verified, though. This study describes the design and construction of the instrument, and reports detailed results and the issues encountered along the way. The data suggest that this promising technology could deliver otherwise unobtainable gravity maps, which can be translated to planetary structural data.

Quantum annealing might be very useful for finding answers…but to which questions? A mechanistic understanding of how quantum annealers behave is crucial to identifying the tasks for which they are well suited. This study contrasts two annealers operating at different temperatures, to clarify the roles that thermal processes and analog errors play in their performance. Regrettably, the results show that present-day quantum annealers do $not$ function reliably as classical Boltzmann samplers. The insight from these experiments carries considerable implications for the design of future annealers, and the prospects for using them in machine learning and beyond.

In a network of ferromagnetic nanowires, a magnetic metamaterial known as $artificial$ $spin$ $ice$, the collective magnetization behavior strongly affects the dynamics and the magnetoresistive behavior. The authors use micromagnetic simulations to provide a microscopic picture of this unexpected response, and find that the sharp features in their experimental data are due to a collective change in the magnetization configuration. Given the geometric freedom enabled by modern lithography, these results offer the possibility to design innovative, reconfigurable microwave and magnetoresistive devices based on artificial spin ice, for $e.g.$ neuromorphic computing.

Increasing the surface area of a biomedical implant may improve its integration with bone tissue. Ion implantation can create surface structures of the right scale, but the underlying physical mechanisms are not well understood. The authors bombard biocompatible metals with 1-MeV gold ions, leading to self-organization of $\mu$m-sized ripples on target surfaces. Continuum modeling of this data elucidates synergy of surface mass redistribution and ion implantation, competing with an unstable sputtering yield, as the cause of pattern formation. This insight could facilitate the microstructuring of dental or orthopedic implants with prescribed roughness properties.

In quantum information processing, long qubit coherence times are crucial. In a superconducting qubit, quasiparticles set fundamental limits on coherence, and are generated not just predictably in the course of operation, but also sporadically by unknown sources. While prevent their random appearance is difficult, their number may be controlled and adverse effects mitigated by incorporating normal-metal islands (traps) into the qubit. For the technologically important transmon configuration, the authors provide strategies to optimize such traps by varying their number, size, and location, to evacuate quasiparticles rapidly, and suppress population fluctuations.

Gas-driven fracturing underlies both natural and industrial processes, such as volcanic degassing, methane venting, stimulated hydrocarbon extraction, and treatment of contaminated soil. The authors show how in such a complex system the capillary, frictional, and viscous interactions together produce a range of fracture patterns, with cracks separated by a characteristic length that varies based on the conditions. Discovering how material properties and injection rate affect these patterns helps to establish a physics framework for optimizing permeability and assessing risk in gas-driven fracturing of hydrocarbon reservoirs and remediation of polluted soil.

The rapid development of methods for scanning magnetic metrology at the nanoscale has greatly improved our ability to study the electric properties of low-dimensional materials and interfaces, including superconducting thin films and electronic devices. However, existing methods limit the imaging of electric current from magnetic images. This study provides a computational method that lifts these limitations, providing a more accurate solution of the inversion problem. This approach has the potential to enhance magnetic imaging of electrical transport in a variety of nanoscale two-dimensional systems.

The recent development of double-zero-index metamaterials (with both $ϵ$ and $\mu$ $\approx 0$) in photonics and acoustics has enabled exotic effects in wave manipulation, including tunneling, squeezing, and cloaking. This study presents the design, fabrication, and testing of an elastic waveguide with double-zero-index properties. This single-material design, featuring tapered locally resonant unit cells, is extremely simple to fabricate. With Dirac-like dispersion structures at the center of the Brillouin zone at the heart of its physics, this waveguide offers a means to control the flow of mechanical energy, such as vibrations and noise, in lightweight mechanical systems.

The quality factor $Q$ is a key figure of merit for ultrasensitive micro- and nanoscale mechanical resonators, and attaining very high quality factors at room temperature is a great challenge. This study provides a technique to enhance the dominant thermoelastic quality factor for micromechanical flexural resonators, allowing $Q$ > 250,000. This method could lead to a change in the overall design strategy for flexural-mode resonators, for microelectromechanical gyroscopes and other sensors.

Terahertz-frequency light is important for applications in mobile communication, imaging, and spectroscopy. Monolithic, tunable devices to generate circularly polarized THz radiation define the state of the art. Using simple structures made of truncated-edge square mesas of Bi${}_2$Sr${}_2$CaCu${}_2$O${}_8$ high-temperature superconductor, the authors succeed in electrically generating THz waves of over 99% circular polarization. Their work also verifies the applicability of antenna theory alongside existing electromagnetic simulation methods in this context.

In quantum computing based on semiconductor quantum dots, adding dots (or just electrons) allows a wider search for an optimal qubit-encoding scheme that is both controllable and coherent. This work reports experiments to realize a hybrid charge-spin qubit in a linear triple quantum dot with asymmetric tunnel couplings, in a multielectron charge configuration. This qubit’s energy splitting can be tuned conveniently over a wide range, and the authors attain qualitative understanding of the observations in terms of a three-electron system. This should stimulate further exploration of quantum coherent dynamics in the few-electron regime for semiconductor quantum processors.