Imaging the surface topography and elasticity of a living cell can provide new insights into the roles played by its volumetric and elastic properties in its functions and disease. Atomic force microscopy (AFM) would be a natural means to this end, but is designed for operation in air and does not function well in a liquid biological environment. The authors develop a technique for noncontact viscoelastic imaging of live cells using AFM with a long glass probe. This method opens the door to studying a wide range of interesting problems in the nanomechanics of soft matter and the mechanobiology of living cells and tissues.

Traumatic brain injury (TBI), particularly in athletes, has finally come to public attention as a widespread medical problem. To advance TBI research, the authors develop a high-frame-rate ultrasound technique that can track motion at subcellular scale, offering a unique combination of scanning speed, accuracy, and penetration. Their slow-motion movies reveal that at accelerations routinely observed in sports, shear waves easily develop into even worse shear $shock$ $waves$ within the brain. These observations closely match theoretical predictions, and could direct $e.g.$ the design of helmets that dampen frequencies likely to generate shear shocks.

Nature has been working on single-photon technology for a lot longer than we have, so why not reap the benefits? Taking advantage of the well-studied capability of the human retina for single-photon detection, this project proposes ultrasecure biometric identification based on the perception of weak flashes of light. The authors turn the physics of photon statistics and the related detection by the eye into a quantum parameter-estimation problem, leading to a biometric quantifier. The security of this “fingerprint”, based on subject-specific optical-loss parameters, can be understood and quantified—and guaranteed—by the physics of quantum measurement.

A cavity optomechanical system enables optical readout and control of the phonons trapped in a nanomechanical resonator. For a network that uses mechanical waves to distribute quantum information, we must find ways to move phonons in and out of cavities, without altering the other properties of the system. This study solves the problem by judiciously breaking specific symmetries of the structure, to generate controllable phonon leakage out of a resonator on a chip without modifying the optical properties. Interestingly, disorder is observed to cause fluctuations in the leakage rate, which counterintuitively could $reduce$ losses.

Cation diffusion in perovskite oxides is important to the performance and long-term stability of cathodes in solid-oxide fuel cells. Unfortunately, current models of this migration are significantly inaccurate. Thus the authors conduct a first-principles investigation of $A$- and $B$-site cation migration pathways involving various defect complexes in a representative manganite. Combining bulk defect chemistry and cation transport kinetics, their theoretical framework provides not only insight into the factors governing cation transport in such oxides, but also a foundation for understanding the related material degradation, which is essential for applications.

The study of metallic nanoclusters growing on insulating substrates comprises a wide research domain, driven by the desire to tailor specific electronic, magnetic, optical, and chemical properties for applications. The authors show how to deposit gold on AlN(0001) to yield nanoislands just one atom high. A combination of experiment and theory reveals the atomic structure of these islands and their chemical bonds to the reconstructed surface, pointing to strategies for growing $e.g.$ metallic electrodes on nitride heterostructures, for high-power or high-frequency electronics.

As superconducting-qubit architectures progress to more complex layouts, frequency crowding in qubit arrays increasingly hinders computing performance. Frequency-tunable qubits can assuage this problem, but are subject to the magnetic-flux noise that is ubiquitous in such circuitry. The authors exploit asymmetric Josephson junctions in transmon qubits to reduce sensitivity to flux noise, while preserving sufficient tunability to avoid frequency collisions. In their most asymmetric qubit, dephasing due to flux noise is reduced to the point where its rate is independent of the flux bias.

Rotation sensors based on magnetic domain walls offer nonvolatility and low power consumption, but currently can sense just a few turns—the same number as the windings of the device’s nanospiral. In a clever twist, the authors envision a different sort of setup featuring several closed-loop structures and a special “siphon” to direct traffic through intersections. This allows the counting of a coprime number of turns, which scales exponentially with device size—into the millions. Extensive simulations verify the feasibility of manufacturing this design.

High-entropy alloys (HEAs) with three to five nearly equimolar components have garnered interest in the past few years, due to their great material strength and tremendously tunable composition. Most work so far has focused on mechanical and structural properties rather than electrical transport, but recent studies indicate that HEAs possess unexpected superconductivity and magnetism. The authors’ experiments demonstrate that amorphous Ta-Nb-Hf-Zr-Ti can provide a sizable spin Hall effect, and supports magnetization switching via the induced spin-orbit torque, making it an interesting candidate for spintronic memory devices.

Voltage control of perpendicular magnetic anisotropy (PMA) is a promising approach for high-density, lightweight, energy-efficient information storage. Electric-field regulation of PMA in conventional multiferroic laminates is difficult, though, since the voltage-induced magnetic anisotropy is relatively small. This study combines ferromagnetic resonance and the magneto-optical Kerr effect for significant electric-field manipulation of PMA in multiferroic multilayers containing cobalt. Along the way, the authors also gain interesting insight about the instability of Co orbital moments near a critical transition.

Understanding the behavior of oxygen vacancies in an oxide multilayer in an electric field continues to be challenging, due to their complex transport between layers. In this study, a cycle of electronic phase transitions in a SrCoO${}_{3-x}$/La${}_{0.45}$Sr${}_{0.55}$MnO${}_{3-y}$ heterostructure is realized by sweeping an applied voltage, confirming that field-induced formation and annihilation of O vacancies is chiefly determined by their formation energies and differences in Gibbs free energies. These results yield important insight into control mechanisms for oxide electronics, including multiferroic tunnel junctions and solid-oxide fuel cells.

The emerging field of $superconducting$ $spintronics$ offers a radically different approach to information processing and storage in the superconducting state, with minimal Joule heating. Many proposed devices would benefit from materials that naturally maximize the total spin of the superconducting state, and in this respect the reliable use of half-metallic ferromagnets is seen as a game-changer. Here the mixed-valence manganite La${}_{2/3}$Ca${}_{1/3}$MnO${}_3$ is shown to be ideal, as it crucially offers not only 100% spin polarization, but also chemical stability. Using this material, creation of spin-polarized Cooper pairs can be controlled in magnetic fields of just a few mT.

The Sanchez-Lacombe equation of state (or lattice fluid model) is heavily used in polymer science and engineering, due in part to its simplicity. However, as Einstein might have quipped, everything should be made as simple as possible, $but$ $not$ $simpler$. This model is thermodynamically inconsistent, and recent experiments on foams have shown it to fail badly. Eliminating the lattice and arbitrary rules from its derivation, the authors improve the theory and verify its predictive power against data for a mixture quite relevant to industry. Their equation is approximately valid even if the polymer has not been characterized.

Imaging the surface topography and elasticity of a living cell can provide new insights into the roles played by its volumetric and elastic properties in its functions and disease. Atomic force microscopy (AFM) would be a natural means to this end, but is designed for operation in air and does not function well in a liquid biological environment. The authors develop a technique for noncontact viscoelastic imaging of live cells using AFM with a long glass probe. This method opens the door to studying a wide range of interesting problems in the nanomechanics of soft matter and the mechanobiology of living cells and tissues.

Epitaxial growth of high-quality graphene upon SiC is one of the most promising routes towards commercialization of this multipurpose wonder material. Bringing a technological perspective to the intense debate on improving this process, the authors correlate the chemical atmosphere in the growth chamber with the quality and physical properties of the product. Their experiments reveal surprisingly rich chemistry that must be taken into account to optimize the process. These results will benefit the engineering of industrial furnaces for the production of electronics-grade graphene.

Nature has been working on single-photon technology for a lot longer than we have, so why not reap the benefits? Taking advantage of the well-studied capability of the human retina for single-photon detection, this project proposes ultrasecure biometric identification based on the perception of weak flashes of light. The authors turn the physics of photon statistics and the related detection by the eye into a quantum parameter-estimation problem, leading to a biometric quantifier. The security of this “fingerprint”, based on subject-specific optical-loss parameters, can be understood and quantified—and guaranteed—by the physics of quantum measurement.

Graphene has a big head start, but other two-dimensional (2D) materials continue to pique interest—and not just for electronics. These systems also might be good, for example, in thermoelectric devices to scavenge wasted energy, and many basic questions need to be addressed. The authors use density functional theory plus semiclassical Boltzmann transport to study As and Sb monolayers, and find that both show excellent thermoelectric response. With low phonon group velocities and vibrational frequencies reducing lattice thermal conductivity, they outperform all known 2D materials, and are serious competition even for well-established thermoelectrics.

BaZrS${}_3$ belongs to an emerging class of ionic semiconductors that are stable and nontoxic, and offer a direct band gap, strong light absorption, and potential defect tolerance—great for photovoltaic applications. The band gap of the title compound is too high, though, for an optimized single-junction solar cell. High-pressure spectroscopy plus first-principles calculations reveal that this perovskite’s structure remains stable when squeezed, while its band gap decreases. These results suggest the use of equivalent $chemical$ pressure, $e.g.$ by cation alloying with Ti, to tweak the band gap of BaZrS${}_3$ for efficient energy harvesting.

Thermometry with both high sensitivity and high spatial resolution is essential in many areas of cutting-edge science and technology. The authors demonstrate very sensitive infrared thermometry based on optically detected magnetic resonance of spins in $4H$-SiC divacancies. These “spin sensors” are robust against magnetic noise, due to intrinsic self-protection by the transverse strain that is natural to this sort of defect, and provide a promising platform for high-quality temperature sensing in noisy real-world environments, especially in biology and microelectronics.

Ultrafast deposition of laser energy into noble-metal films yields irreversible mass redistribution and resolidified nanostructures with hidden features, which can strongly affect the optical response that is harnessed for applications in plasmonics and nonlinear optics. A combination of three surface probes reveals the three-dimensional mass distributions induced by the laser pulse, and simulations show the spatiotemporal dynamics of the nanoscale hydrodynamic flows. This approach allows quantitative prediction of film thickness, and realistic forecasting of how more complex nanostructures might be made via laser processing.

Even with quantum key distribution for guaranteed secure communication, errors can creep in. Information reconciliation is used to fix any mismatched bits in the secret keys of conversationalists Alice and Bob, rendering their keys identical (and thus useful) once again. In particular, $blind$ $reconciliation$ is appealing for key correction, as it does not require an $a$ $priori$ estimate of the quantum bit-error rate. The authors show that introducing symmetry into the blind-reconciliation scheme significantly improves the procedure’s efficiency, providing a path to higher throughput of keys—and secure data.

Sometimes you need a steady drip, not a torrent—as in certain optics experiments, where the precise number of photons in each “drip” of a faint beam is important. The authors use an intensified charge-coupled device as a photon-number-resolving detector with spatial resolution, to study at once the joint photocount distributions and spatial correlations of weak twin beams of light. Compared to the usual analysis of photocount distributions, their spatially resolved analysis provides additional information about the statistical properties of the beams, and allows noise reduction.

Magnetic signatures are an important diagnostic tool for understanding biological processes. Typically the biomagnetic field produced by these processes is probed outside the human body, but signal strength and spatial resolution could be improved by close-up $internal$ detection. The authors exploit nitrogen-vacancy centers in diamond to create a compact magnetic-field sensor that can be used to measure biological signals endoscopically. Their sensor’s size and great sensitivity make it a promising basis for imaging in biology and medicine.

Intrinsic defects in solid-state devices are generally seen as detrimental to coherent amplification of phonons (phonon lasing), and thus fabricating devices with as few defects as possible has been key in optomechanics and nanoelectromechanical systems. However, the authors show that by surpassing an exceptional point in the parameter space of the non-Hermitian system, mechanical gain can be enhanced $despite$ increasing defect-related losses. This counterintuitive effect is analogous to loss-induced optical lasing, and offers a remarkable way to operate and utilize quantum acoustic devices.

Single-electron pumps based on quantum dots (QDs) with tunable barriers are promising candidates for the emerging quantum standard of electrical current, but accuracy at elevated reservoir temperatures can be challenging. In this study, a planar silicon QD achieves accurate, high-speed single-electron pumping in the thermal regime, where reservoir temperature could rise to a few kelvin. This device operates without the demanding conditions of previous experiments, indicating that it could deliver a cost-effective and easily distributed standard of current—and, furthermore, could be useful for long-range transfer of quantum information.

Graphene field-effect transistors are attractive for sensing applications, offering high charge-carrier mobility and relatively inexpensive device fabrication. However, integrating high-quality analyte-selective layers on graphene without spoiling its properties remains a challenge. The authors solve the problem for thin layers of metal oxide on large-area high-mobility graphene, which respond to acidity. These structures operate at the limit of quantum capacitance and Nernstian response, achieving a detection limit of 0.0001 $p$H units. The same approach could be extended to sensing species other than the proton, by substituting appropriate overlayers.

Quiet, efficient, coolant-free magnetic refrigeration could be the technology of the future, but for now it still presents challenges, even at the level of plain physics. For example, understanding the kinetics of first-order phase transitions in magnetocaloric compounds is key to improving the operating frequency of magnetic cooling devices. The authors measure these transitions and analyze them, in terms of nonequilibrium thermodynamics and the heterogeneous nucleation and pinning of magnetic phases at crystallographic defects. Their insight should be helpful in selecting optimal working materials for such applications.

Traumatic brain injury (TBI), particularly in athletes, has finally come to public attention as a widespread medical problem. To advance TBI research, the authors develop a high-frame-rate ultrasound technique that can track motion at subcellular scale, offering a unique combination of scanning speed, accuracy, and penetration. Their slow-motion movies reveal that at accelerations routinely observed in sports, shear waves easily develop into even worse shear $shock$ $waves$ within the brain. These observations closely match theoretical predictions, and could direct $e.g.$ the design of helmets that dampen frequencies likely to generate shear shocks.

Determining the environment that a nuclear reactor’s spent fuel has experienced is a challenging, critical task for reactor monitoring—to verify nonproliferation, for example. Certain isotopic ratios are reliable diagnostics for the total neutron exposure and cooling time of a sample, but many fail for very low burnup and very long cooling times. The authors derive new diagnostic tools for these scenarios and test them on archived samples by radiochemical analysis. The results show that multiple diagnostics can be used to identify self-consistency and systematic errors, such as fractionation, to guide future efforts in reactor forensics.

Efficient collection of energy from electromagnetic radiation is important to emerging technologies for wireless power transfer, but is challenging in dynamic environments, because performance generally depends on the light’s polarization direction. The authors show that engineering the chirality of a subwavelength structure can enable uniform collection of energy from all orientations of an incident polarization state. Such structures also enjoy enhanced performance when the chirality of the incident field matches that of the structure. This approach could enable miniaturization and more robust wireless powering of moving devices.