Mice, it turns out, are dielectric. The authors use this property to couple intrinsic resonant modes of a mouse’s body to a wireless transmitter, and so power an implanted miniature electronic device with improved efficiency. This approach will enable behavioral and neuroscience experiments without wires, harnesses or tethers.

Manipulating sound waves is key in applications such as ultrasound imaging and nondestructive testing. To this end, the authors present an acoustic phased array using a metascreen that transmits sound energy from a single source and steers the outgoing wavefront in the desired direction. Significantly, this metascreen does not itself contain any source of sound, unlike a conventional phased array with many individual sources. This passive array is therefore notably appealing for its simplicity, low cost, and good acoustic performance.

Single-walled carbon nanotubes (SWNTs) present very appealing electronic properties, but are susceptible to uncontrolled effects that cause significant device-to-device variability—a critical issue for practical technology. The authors’ calculations show that fixed charges on the gate-oxide surface are responsible for the variation in threshold voltage of SWNT transistors, and also limit their turn-on sharpness. This predictive understanding offers guidance for improving nanotube devices.

Silicon photonics is a promising platform to realize the dense and scalable integration required for quantum computing, communication, and sensing. The authors demonstrate a key building block for this platform, a simple, single source of entangled photons, and use it to observe quantum interference on the same chip. This removes the need for clumsy interfacing of multiple photon sources, as in previous studies, and provides a basis for highly scalable photonic circuits that achieve multi-qubit entanglement.

Mice, it turns out, are dielectric. The authors use this property to couple intrinsic resonant modes of a mouse’s body to a wireless transmitter, and so power an implanted miniature electronic device with improved efficiency. This approach will enable behavioral and neuroscience experiments without wires, harnesses or tethers.

Physical, hardware-based random-number generators (RNGs) are keenly sought for numerous applications, including secure communication. The authors show how a Josephson junction or superconducting nanowire can act as a coin with tunable probability, generating a random sequence of bits by remaining in the zero-voltage superconducting state (“heads”) or switching to the normal state (“tails”) in response to current pulses. This solid-state RNG is simple and compact for straightforward on-chip integration, and with optimization a bit rate above 1 Gb/s is anticipated.

Manipulating sound waves is key in applications such as ultrasound imaging and nondestructive testing. To this end, the authors present an acoustic phased array using a metascreen that transmits sound energy from a single source and steers the outgoing wavefront in the desired direction. Significantly, this metascreen does not itself contain any source of sound, unlike a conventional phased array with many individual sources. This passive array is therefore notably appealing for its simplicity, low cost, and good acoustic performance.

Nuclear magnetic resonance (NMR) spectroscopy has become a prime means of determining chemical structure. Currently nanoscale NMR has the sensitivity for single-spin detection, yet its resolution is too poor. The authors using diamond NV centers to improve spectral resolution by orders of magnitude, to the subkilohertz regime, which is sufficient to resolve chemical shifts in magnetic fields of a few tesla. This could be used in identification of the structure of an individual molecule, with major implications for drug design and chemical (especially biochemical) analysis.

Droplet-based microfluidic devices can use electric fields to actively drive the fusion of tiny fluid aliquots. The authors numerically study the electrohydrodynamics of two drops confined in a microchannel under ac electric field, revealing five coalescence regimes for various combinations of initial separation and field intensity. This physical insight can be used to predict droplet fusion behavior, and thus to advance lab-on-a-chip designs.

Liquid helium is a limited resource that can be difficult to obtain, yet leading-edge research and applications rely on it to reach the very low temperatures needed for, say, quantum computing or magnetic resonance imaging. The authors offer a qualitative advance in the helium-free cooling of macroscopic objects by engineering a robust, thermally isolated stage chilled by superconducting tunnel junctions with an electromechanical heat switch. The cooling power of this macroscopic quantum refrigerator is increased by connecting multiple junction structures in parallel, including structures on completely different silicon substrates.

A wish list for a chemical sensor would include sensitivity, selectivity, and speed. The authors present a sensor design in which sample molecules adsorbed on graphene modify its conductivity, which in turn modifies the reflectance of a coupled terahertz resonator. By reading the resulting change in reflectance optically, the authors show that such a sensor can hit all three marks on the list, for example producing a sensitivity around 1 ppm for various molecules while detecting within a subwavelength layer.

E’ centers in amorphous silicon dioxide are paramagnetic, highly localized dangling-bond states with long spin-relaxation times. The authors study the E’ center for use as a readout probe in force-detected single-spin tunneling force microscopy with atomic resolution, which is of interest for spin-based quantum information processing and spintronics. The observed spin dynamics suggest that this defect would be an excellent probe of individual spins, even at room temperature.

Scintillator materials convert high-energy radiation into visible or near-ultraviolet photons, and are used in medical diagnostics, high-energy physics, and security applications. Often a scintillation response on the nanosecond time scale is required because afterglow interferes with readings of interest, and so fast scintillators are the key to progress. The authors elucidate the microscopic mechanism that allows elimination of afterglow in mixed-crystal scintillators based on Lu and Gd ions, showing a way forward in terms of both basic physics and a strategy for controlled materials preparation.

Nanoscale ferromagnetic structures are of great interest for magnetic random-access memory (MRAM), but how stable are their bits? The authors investigate how the energy barrier to unwanted thermally induced transitions scales with size, in thin disks with large perpendicular magnetic anisotropy. They predict a critical diameter above which the barrier depends not on the disk’s volume, but on its width. Their analytical model is easily applied to interpreting experiments and guiding design, to ensure that stored data will last.

Nanoscale materials can be extremely strong compared to their bulk forms, but this strength can be accompanied by significant sample-to-sample fluctuations. The authors combine simulations and analytical theory to describe the size-dependent distribution of fracture strength in defective graphene flakes, highlighting their differences from systems of macroscopic extent. This understanding may be important for controlling thermally activated failure in nano- and micromechanical systems.

To achieve high-fidelity, coherent control of quantum systems via imperfect classical hardware, the authors provide a general framework for designing control sequences that account for distortions—even those that are nonlinear and noninvertible. These sequences are robust to both a distribution of device Hamiltonians (such as for spatially varying fields) and uncertainties in the description of the control hardware itself, greatly extending the bounds of quantum device operation.

Electromagnetically induced transparency (EIT) has been extensively investigated in atomic systems, to control transmission through optical components and the velocities of “slow light” packets. The authors realize EIT using a metamaterial that turns transparent with the incidence of an auxiliary electromagnetic wave, in the same manner as from the original, atomic EIT effect. This approach could lead to applications including the storage of electromagnetic waves (the ultimate slowing of light) across a wide frequency range for optical computing.

Since the 1980s, superconducting traveling-wave parametric amplifiers (TWPA) have promised high gain, wide bandwidth, and low noise—the ideal for astronomical detectors, or qubit readers. Actual TWPAs, though, have been plagued by insufficient gain and excessive noise, largely due to poor phase matching. The authors propose to solve these problems with an amplifier made of a chain of superconducting quantum interference devices (SQUIDs), the nonlinear behavior of which is tunable and thus allows phase matching, for high-gain amplification over a wide bandwidth.

The performance of both large linear electron accelerators and smaller-scale ultrafast electron-diffraction setups is limited by the source of electrons, the photocathode. This study shows how a nonuniform surface of the source affects the energies of emitted electrons, using fundamental physics principles to guide the engineering of photocathodes, and thus giving insight to improve potentially all such applications.

An optical cavity pumped hard enough—into the nonlinear dynamical regime—begins to oscillate. This study explores the quantum limits of those oscillations, and their applications to photonic computing. The authors quantitatively analyze amplification behavior below the oscillatory bifurcation threshold and phase diffusion above the threshold, and propose using these effects in an all-optical XOR gate, and in an Ising machine that should be both orders of magnitude faster and less power-hungry than a supercomputer.

Transient absorption (TA) spectroscopy is a powerful probe of optical excitations in e.g. conducting polymers, but is limited because the unknown time-dependent excitation density is needed to find the absorption cross section. Leaving the Beer-Lambert regime to exploit the dependence of the TA signal on probe-pulse intensity, the authors decouple the absorption cross section from the excitation density, allowing quantitative spectroscopy. This technique could prove especially useful in analyzing systems with branching processes (such as exciton fission and annihilation) in their photophysics.

Understanding the sequestration and transport of natural gas in nanometer-scale pores is important for efficient extraction from unconventional reservoirs, which supply an increasing portion of total gas production worldwide. The authors use high-field NMR to study the adsorption characteristics of supercritical methane in Vycor nanoporous glass, as a model system. They measure the gas storage potential in the nanopores over a wide pressure range and show how free and adsorbed gas can be separately quantified—vital input for recovery strategies for real deposits.

As metamaterials research continues to evolve, we may consider effects and systems that go beyond the electric-dipole approximation. The authors fabricate an optical metasurface in which nanoscale artificial metamolecules exhibit a dominant electric-current quadrupolar response at visible wavelengths, and the optical reflection and absorption change considerably when the illumination direction is reversed. This bifacial metasurface could be applied in interferometric devices and coatings, solar cells, or directional optical elements such as light sources and detectors.

Organic solar cells have many merits, including low cost and the ability to cover large and flexible substrates, yet there is much room to improve them. To maximize their efficiency, this study investigates the effects of surface recombination in a model system, in which the work function of electrode contacts is varied via oxygen-plasma treatment. The authors explain how changing the properties of the contact has a profound effect not only on device efficiency, but also on the interpretation of optoelectronic measurements of these devices.

Superconducting quantum interference devices (SQUIDs) are ultrasensitive to magnetic fields, and are used in diverse applications such as MRI scanners, voltmeters and amplifiers, and quantum-computing hardware. Some can even detect the spin of a single electron. The authors study the temperature- and field-dependence of the retrapping current $I_r$ of such a nano-bridge SQUID, present a simple analytical expression for $I_r$, and show how their analysis can be used to avoid output hysteresis and thus optimize device performance.

Single-walled carbon nanotubes (SWNTs) present very appealing electronic properties, but are susceptible to uncontrolled effects that cause significant device-to-device variability—a critical issue for practical technology. The authors’ calculations show that fixed charges on the gate-oxide surface are responsible for the variation in threshold voltage of SWNT transistors, and also limit their turn-on sharpness. This predictive understanding offers guidance for improving nanotube devices.