A quantum network can inherit features of robustness and scalability from its classical counterparts, but because each node and link has quantum capability, the overall network can achieve things that are effectively impossible with conventional technologies. The authors comment on aspects of current research, including hybrid atom-light architectures and quantum memories.

Reimagining and miniaturizing the huge machines of the World War II era was a great step in the history of computing. Now quantum computers face a similar developmental step, on the way to tomorrow’s compact, scalable systems. In analogy with sockets in traditional electronics, the authors present a link for classical control hardware and solid-state qubits, with exceptional properties at microwave frequencies and cryogenic temperatures. This technology should bring us within a few years to a key milestone in quantum computing: quantum error-correction implemented for hundreds of physical qubits.

Low-frequency $1/f$ noise in magnetic flux is a dominant source of dephasing in superconducting qubits. Its origin has been a longstanding open question in condensed matter and device physics. The authors identify adsorbed O${}_2$ (paramagnetic, due to its spin triplet state) as the major contributor to magnetism and flux noise in superconducting thin-film devices, and show that improving a sample’s vacuum environment leads to significant noise reduction. These results open the door to improved superconducting sensors and qubits with enhanced coherence times.

Injection locking is a common technique to control the frequency of a conventional, macroscopic oscillator, but how does a tiny device operating in the quantum regime behave? To answer, the authors study external control of microscopic lasers governed by cavity quantum electrodynamics. Surprisingly, in such devices $both$ stationary oscillations synchronized to the external signal $and$ oscillations at the solitary frequency occur $simultaneously$. This “partial injection” phenomenon is unique to nonlinear oscillators excited with a few tens of quanta, and is relevant to applications in optomechanics, spintronics, and integrated photonics.

Low-frequency $1/f$ noise in magnetic flux is a dominant source of dephasing in superconducting qubits. Its origin has been a longstanding open question in condensed matter and device physics. The authors identify adsorbed O${}_2$ (paramagnetic, due to its spin triplet state) as the major contributor to magnetism and flux noise in superconducting thin-film devices, and show that improving a sample’s vacuum environment leads to significant noise reduction. These results open the door to improved superconducting sensors and qubits with enhanced coherence times.

“Too good to last” could be the motto of quantum information processing, where decoherence of qubit states is the ultimate limitation on any device’s speed and fidelity. The authors use advanced electron-spin-resonance techniques to directly measure the diffusion through a solid sample of the wave function of a physically stationary spin, a phosphorus donor in silicon (think $NV$ center in diamond). They observe a subtle, ubiquitous effect, “spin flip-flop”, in which two spins swap states—a process that obviously could spoil fidelity, were readout not quick enough.

In spintronics, the subfield of $spin-orbitronics$ concentrates on exploiting both the spin and orbital magnetic properties of materials in, say, the electrodes of a magnetic tunnel junction (MTJ). Here a remarkable tunnel magnetoresistance as large as 200% is attained in MTJs based on SmN, an intrinsic ferromagnetic semiconductor with a very small magnetic moment that is dominated by orbital contributions. Uncommonly, this magnetoresistance is largest at $high$ fields, and at intermediate fields the SmN layer exhibits twisted magnetization.

More is always better, right? No, not always. The authors show that in intermediate-band solar cells based on quantum dots, infrared photocurrent generation is strongly nonlinear according to dot density, but $opposite$ to expectations, and that only low dot densities provide enhanced up-conversion photocurrent. A high density of dots is counterproductive; low-density layers can greatly outperform high-density layers. This startling discovery is essential for overcoming the limits of present-day devices.

Bubbles generated from nanoparticle surfaces by ultrasound can significantly enhance drug delivery to tumors, yet much remains to be understood of the dynamic response to different acoustic frequencies and amplitudes. The authors present direct optical observations of inertial cavitation from nanoscale cups at 10 million frames per second, and use these observations to validate a recent crevice model for acoustic cavitation. Excitation frequency is found to have the greatest impact on bubble dynamics, as expected, but surprisingly pressure amplitude primarily affects the number of bubbles, rather than the violence of cavitation.

Magnetic biosensing, which uses functionalized magnetic particles to label and detect biological analytes, is attractive for future “point-of-care” medical diagnostic applications. The authors demonstrate detection of a wide range of magnetic particle types and sizes (from 6 nm to 4 $\mu$m) via their influence on geometrically nanoconfined ferromagnetic resonance modes. This approach provides a novel magnetic particle detection modality extending all the way down to the nanoscale.

The optical Kerr effect is the variation of a material’s refractive index in response to high-intensity light, and as such causes the spectrum of the light to change. The authors measure the spectral broadening of chirped laser pulses in graphene-covered silicon waveguides, and determine the sign of graphene’s Kerr index to be $negative$, contrary to common assumption. This finding suggests greatly extended applicability of graphene in nonlinear photonic devices.

Despite much study of the general mechanism of transport in organic transistors, their $polarization-dominated$ transport is still not well understood. Using a ferroelectric polymer for the dielectric layer in an organic field-effect transistor (OFET) allows tuning of polarization strength with temperature. Remarkably, at 200 K TIPS-pentacene OFETs with a ferroelectric dielectric exhibit a negative coefficient of mobility (bandlike transport, reminiscent of a metal, instead of the usual activated transport). The key to this behavior is the discrete nature of the trapping levels in TIPS-pentacene, and presumably other solution-processable $\pi$-conjugated conductors.

Integrating an optical cavity into a radio-frequency ion trap yields an ideal building block for quantum information networks or quantum computers. Unfortunately, it also jeopardizes the trap’s stability. The authors compare the advantages and disadvantages of known ion-trap geometries and cavity configurations, and identify general features of the geometries best suited to developing a scalable network. This comprehensive study is an essential step in the development of tomorrow’s secure quantum networks.

The visible luminescence related to crystalline defects in wide-band-gap semiconductors like zinc oxide could be employed in LEDs, but emission intensity saturates at high power, due to the filling of defect states with excited carriers. This is a critical problem for high-power applications of LEDs, including solid-state lighting. The author shows how to avoid this saturation and enhance emission by exploiting a plasmonic effect: When surface plasmons form in adjacent metal nanostructures, they facilitate carrier recombination in the semiconductor, which keeps the defect states from filling up.

Reimagining and miniaturizing the huge machines of the World War II era was a great step in the history of computing. Now quantum computers face a similar developmental step, on the way to tomorrow’s compact, scalable systems. In analogy with sockets in traditional electronics, the authors present a link for classical control hardware and solid-state qubits, with exceptional properties at microwave frequencies and cryogenic temperatures. This technology should bring us within a few years to a key milestone in quantum computing: quantum error-correction implemented for hundreds of physical qubits.

Amorphous oxide semiconductors are promising channel materials for transparent, flexible thin-film transistors, but actual devices suffer from instability in threshold voltage, due to structural defects. First-principles calculations reveal that undercoordinated cations in nonstoichiometric samples lead to localized subgap states that are the root of the instability. These results establish a physical understanding of the precise origin of these states, and offer guidance to control such defects for successful applications.

Spectroscopists have long sought to make fast, continuous measurements of spectral lineshapes, to directly monitor the dynamics of chemical, optical, or physical processes. This ability would have many applications in both industry and academia. The authors use a frequency comb to obtain extremely high-quality spectra, with frequency and time resolutions close to the limits imposed by the Uncertainty Principle. This allows them to follow in real time the evolution of processes like power broadening and radiation reabsorption, and to explain some rather surprising observations about atomic vapors.

Employing topological properties of matter to manipulate magnetism and spin is gaining attention in spintronics research. The authors seek to exploit the exotic coupling of charge and magnetization in a Weyl semimetal with broken time-reversal symmetry, and propose a multilayered device in which magnetization is switched by a voltage pulse, and spin current is generated by an alternating voltage. The effect does not require constant current, making Weyl semimetals a promising class of materials for low-power spintronic devices.

Chemically functionalized graphene quantum dots (GQDs) are promising for photonic and optoelectronic applications. The authors’ calculations show that shaping destructive interference of the quantum states of GQDs can yield dark states at the red end of the spectrum for efficient electron transfer, or highly coherent bright states for photonic applications, and that strain can induce closely spaced, brighter states. This study offers a guide for the rational design of GQDs to obtain desired optical properties.

The electricity from a light-water nuclear reactor ultimately comes from the heat emitted by fuel rods made of UO${}_2$. However, as fission products and defects build up in the urania, its effective thermal conductivity—and power output—changes, with potentially serious implications for reactor efficiency and safety. The authors develop a methodology to account for spin-phonon scattering, which is usually neglected in multiscale modeling of nuclear fuel. Their results are of interest both for reactor operation and for studying a much wider class of paramagnetic materials with antiferromagnetic transitions at low temperatures, including transition-metal oxides.

A promising route toward large-scale quantum information processing is to use as qubits the spin states of individual phosphorus atoms in silicon. One challenge in making this system scalable, though, is to fit control and readout structures—which are necessarily bigger than the single-atom qubits—into a device at the same length scale. Combining rf measurement techniques with ultraprecise lithography, this study presents a charge sensor that can be fabricated at the atomic scale, needs only one electrical lead, and is capable of the sensitivity needed for single-shot dispersive readout of a lone electron’s spin state.

In the quest for better solar panels, we continue to refine our understanding of the physics of photovoltaic absorbers, such as hybrid metal-organic perovskites. By scrutinizing photoluminescence transients, the authors explain how to determine fundamental semiconductor properties and estimate the potential open-circuit voltage of perovskite thin films. Additionally, they highlight the impacts of photon recycling, doping, and bulk and surface recombination of charge carriers on device performance.

Reduced emission efficiency (“efficiency droop”) at high current is a serious problem for solid-state lighting based on In${}_x$Ga${}_{1-x}$N LEDs. Auger recombination may be the culprit, yet fluctuations in alloy composition also seem to matter. The authors use a scanning near-field optical microscope to study the photoluminescence of blue- and green-emitting In-Ga-N quantum wells, and find that increasing excitation density causes carrier overflow from potential minima. The mechanism behind droop depends on emission color (indium content), and carrier-activated nonradiative recombination is key in green emitters.

Metasurfaces (metamaterials of near-vanishing thickness) are of interest for engineering subwavelength structures with complex responses. Typically a graphene metasurface exhibits just one or two resonances and absorbs light in a narrow frequency band, but this study demonstrates tunable broadband absorption due to hierarchical, fractal structuring. Over a very wide band (about 190% of the central frequency), the average absorption exceeds 20%, without any metallic mirror. This approach to broadband absorbers, and plasmonic components more generally, offers substantially improved performance at terahertz and optical frequencies.

Feedback control is an important means to keep nonlinear dynamical systems from wandering into undesirable—even dangerous—regimes of behavior. For example, premixed lean combustion is indispensable for reducing NO${}_x$ emissions from gas turbine engines, but is prone to causing acoustic oscillations in the gas that can lead to destruction of the entire engine system. The authors present a very simple method to stop these spontaneous gas oscillations, based on $self-feedback$ of pressure with time delay via a connecting tube of the right length.

Physical science is rooted in measurement, but even this simple idea becomes complicated at the nanoscale. For example, how does one actually measure the charge of a single nanoparticle? Traditional electrical methods are limited to micrometer-sized objects, yet nanoparticle charge is a crucial parameter in fields spanning astronomy, optics, chemistry, and manufacturing. The authors point out that by monitoring the transmission spectrum of a high-$Q$ whispering gallery mode resonator, surplus charge at very low electron density can be detected.

When its corners are switched from disconnected to connected, a metallic checkerboard exhibits a sort of insulator-metal transition in its electromagnetic response. To use this phenomenon for dynamic polarization control, which is essential for polarization-selective spectroscopy, the authors study anisotropy as a general means to artificially engineer phase transitions of checkerboardlike metasurfaces. Application of this approach to yield a dynamic terahertz polarizer is discussed.

Injection locking is a common technique to control the frequency of a conventional, macroscopic oscillator, but how does a tiny device operating in the quantum regime behave? To answer, the authors study external control of microscopic lasers governed by cavity quantum electrodynamics. Surprisingly, in such devices $both$ stationary oscillations synchronized to the external signal $and$ oscillations at the solitary frequency occur $simultaneously$. This “partial injection” phenomenon is unique to nonlinear oscillators excited with a few tens of quanta, and is relevant to applications in optomechanics, spintronics, and integrated photonics.