Reliable, simple, inexpensive methods for sequencing proteins, RNA, or DNA are of obvious interest across biology and medicine. The authors theoretically investigate the conditions under which the monomers of a biopolymer will retain their sequential order against the randomizing effects of Brownian motion, after being cleaved from the polymer while approaching the tip of an electrospray ion source. Their analysis confirms the feasibility of identifying a single protein molecule by delivering its amino acids in order to a mass spectrometer, with 95% likelihood of measuring the correct sequence.

Long coherence times render electron spins in quantum dots promising for scaled-up quantum computation, but large arrays of semiconductor spin qubits have yet to be realized. The authors take the next steps in scaling by demonstrating an array of $nine$ quantum dots with low electron occupancy, reproducible single-dot characteristics, and full charge-state readout. Beyond quantum information science, this also represents a major advance for the quantum-dot community, where double and triple quantum dots have been the standard for over a decade.

To reach the subnanosecond switching speeds desired for spintronic applications, antiferromagnetically coupled materials seem ideal. In this study, current-induced switching is shown to be most efficient in transition-metal—rare-earth alloys, where torques work constructively on two antiparallel sublattices of spins. There is also evidence for conservation of total angular momentum for spin-orbit-torque switching—experimental proof that the “bookkeeping picture” of the spin torque still holds in these ferrimagnetic systems.

Superconducting loops form a basis for very sensitive magnetometers. Recently, replacing the Josephson junction of a SQUID with a weak link, and probing the flux by measuring tunneling current, was proposed to realize a SQUIPT (superconducting quantum interference proximity transistor) sensor. Here the authors theorize an all-superconducting variant of this design, which would permit simplified fabrication of a device from a single material.

Manipulating the flow of heat, particularly in analogy to the flow of electrons (current), has become a major research theme. A $thermal$ $transistor$ made from a phase-change material can efficiently heat or cool to control the temperature of, say, an integrated circuit. The authors propose a far-field device exploiting the thermal hysteresis of vanadium dioxide, in which a structural phase transition is accompanied by a dramatic change in emissivity, providing jumps in temperature and heat flux at the base terminal.

In addition to heat-assisted magnetic recording (HAMR), all-optical switching (AOS) is an attractive technology for the next generation of ultrahigh-density, ultrafast, ultralow-power digital storage. The authors demonstrate cumulative magnetization switching in granular Fe-Pt-C due to multiple pulses of circularly polarized light. While this base process is statistical, adding a modest external magnetic field allows deterministic switching, which shows that this form of AOS can aid writing in a HAMR-like recording process.

To build a quantum computer, one can simplify the design of multi-qubit elements (logic gates) by treating them as series of well known one- and two-qubit gates—at the price of a slower processor. What if there were a handy means for $direct$ multi-qubit design? The authors show numerically that, under existing experimental constraints, their recently developed SuSSADE scheme can yield three-qubit gates for single-shot entangling operations with fidelity greater than 99.9%. This powerful approach brings us that much closer to a fault-tolerant solid-state quantum computer.

Reliable, simple, inexpensive methods for sequencing proteins, RNA, or DNA are of obvious interest across biology and medicine. The authors theoretically investigate the conditions under which the monomers of a biopolymer will retain their sequential order against the randomizing effects of Brownian motion, after being cleaved from the polymer while approaching the tip of an electrospray ion source. Their analysis confirms the feasibility of identifying a single protein molecule by delivering its amino acids in order to a mass spectrometer, with 95% likelihood of measuring the correct sequence.

Due to the efficient screening of charges in conventional semiconductor devices, electrons normally behave as a noninteracting liquid flowing in an external potential. The authors show that in a quantum wire under certain conditions, reduced screening allows the emergence of charge-density features with finer resolution than conventional gating can yield. This allows pinpoint engineering of electron confinement in ever-shrinking transistors.

Organic solar cells are typically fabricated from a blend of electron-donating and -accepting materials. When mixed, these materials may separate into pure components, but in some cases there may be an intermediate phase of mixed composition at the interface. Using drift-diffusion modeling, the authors reveal that there is an optimal, moderate amount of mixed-phase interfacial region for best photovoltaic device performance, depending on the sizes and morphologies of the pure-phase regions.

Compact laser frequency combs covering the midinfrared “fingerprint” spectral region are desirable light sources for high-sensitivity, high-resolution detection of target molecules. The authors leverage the nonlinear properties of chip-scale Si${}_3$N${}_4$ and periodically poled LiNbO${}_3$ waveguides to obtain a tunable, energy-efficient mid-IR frequency comb. Here the pulse energy needed for a given power per comb line is nearly two orders of magnitude lower than for conventional systems. These results are promising for applications in environmental, medical, and industrial diagnostics.

Solid-state quantum emitters such as the N-$V$ center in diamond have applications ranging from single-photon sources to quantum sensors, with some quantum properties preserved even under ambient conditions. A central challenge is to access these quantum properties efficiently, by catching as much emitted light as possible. In this study a tunable optical microcavity, with a mode volume as small as one wavelength cubed, is used to enhance spontaneous emission of single N-$V$ centers in diamond, via the Purcell effect. This allows the authors to extract large amounts of light from several individually addressable emitters, and to control their spontaneous-emission rate.

Solid-state microrefrigerators operating in the subkelvin regime are key to quantum technology. An electronic cooler based on a normal-metal/insulator/superconductor ($NIS$) tunnel junction works well in principle, but typically its $S$ electrodes are severely overheated and performance sags—especially in the powerful devices needed for applications. The authors use a second $NIS$ cooling stage to thermalize the hot superconductor at the back side of the main cooler. This cascade design could also manage heat in other cryo-electronic devices.

In both optics and nuclear magnetic resonance, two-dimensional (2D) spectroscopy has been widely used to study correlations in ensembles of molecules. The authors articulate a scheme for universal 2D quantum sensing based on dynamical decoupling, to measure nuclear spin correlations, and therefore structural features, for a sample consisting of a $single$ $molecule$—a holy grail of analytical chemistry. These results also impact NMR-based quantum information processing that employs $e.g.$ N-$V$ centers in diamond.

Long coherence times render electron spins in quantum dots promising for scaled-up quantum computation, but large arrays of semiconductor spin qubits have yet to be realized. The authors take the next steps in scaling by demonstrating an array of $nine$ quantum dots with low electron occupancy, reproducible single-dot characteristics, and full charge-state readout. Beyond quantum information science, this also represents a major advance for the quantum-dot community, where double and triple quantum dots have been the standard for over a decade.

The Second Law of Thermodynamics teaches us that energy useful for work is ultimately degraded to heat. This heat production greatly limits the performance of nanosized devices working at cryogenic temperatures. The authors show mathematically that at the nanoscale, waste heat can be converted into work and delivered without physical contacts, thanks to the Josephson effect of superconductors. This quantum thermoelectric turbine could be built with today’s nanotechnology, and seamlessly integrated into current caloritronics platforms for heat management on a chip.

Graphene-based spintronics hinges on controlling spin currents through ferromagnet/graphene interfaces. By measuring devices with different contact resistances, the authors show that the spin-transport properties of the graphene—particularly its spin resistance—can be controlled by a gate voltage. Due to reduced flow of spins from graphene into the ferromagnetic contacts, spin accumulation increases with contact resistance, scaling with the ratio of contact resistance to graphene’s spin resistance. This is an important, quantitative demonstration that the effect of this ratio on the spin signal is consistent with the theory of contact-induced spin relaxation.

Quantum computing may be the only pragmatic way to solve some problems, but when it is not absolutely necessary, is it actually worthwhile? The authors integrate the fields of heuristic optimization and optimal stopping to build a general framework for benchmarking randomized optimization algorithms. Their approach avoids bias and arbitrariness, and is particularly suited to determining the break-even point at which quantum optimization is superior to classical, when both raw performance and technology costs are taken into account.

InAs nanowires have interesting electronic properties and are widely used, and when superconducting due to the proximity effect, they are furthermore promising for research on Majorana fermions. Unfortunately, these nanowires can be tricky to handle, and imperfect, with undesirable side effects due to accidental formation of quantum dots. The authors offer a simple, clever design to measure InAs nanowires encased in Al, which provides tunnel junctions that minimize technical troubles. The resulting SETs are a platform for clean transport measurements and device physics.

Shining light on a semiconductor can create electron-hole pairs—the well known basis of all photovoltaic cells. However, the processes behind photogeneration and recombination of pairs remain unclear at the nanoscale, where Coulombic attraction becomes quite strong. The authors use FET structures to control the surface electric field and study charge separation in photoexcited ZnO nanowires Their results aid our understanding of the physics behind the responses of phototransistors and photosensors, the quantum efficiencies of solar cells, and the reactivities of photocatalytic and photochemical systems at the nanoscale.

When coupled to a tuning mechanism, a disordered medium provides a powerful means for shaping electromagnetic waveforms. The authors leverage this functionality to conduct volumetric computational imaging: A deformed cavity is outfitted with tailored, irregular surfaces, and its microwave resonant modes are projected into an imaging domain to retrieve the scene’s spatial information. This approach could be applied to biomedical imaging, security screening, or wireless power transfer or telecommunications.

With unsurpassed spectral resolution, Fabry-Pérot (FP) interferometers are essential tools in chemical and biological sensing, material analysis, and light-source characterization. Most applications, though, require a spectral $contrast$ that is naturally forbidden by a single-pass FP interferometer. The authors demonstrate a method to readily achieve a thousandfold increase in the spectral contrast of commercial FP spectrometers. This could have immediate, profound effects on many spectroscopy-based applications in biology, biotechnology, nanotechnology, and sensing.

Acoustic metamaterials have the potential to reduce environmental noise, but conventional structures are impractically large for low frequencies. Thus subwavelength systems for sound control have attracted growing attention. The authors present a simple, efficient way to absorb sound inside a waveguide by means of Helmholtz resonators, which are very small compared to the wavelength. This suggests applications such as attenuating undesirable room modes, or noise in ventilation ducts.

The performance of a field-effect transistor (FET) relies directly on the correlation between source-drain current and gate voltage. The authors offer a design principle for manipulating FET operations, using nonuniform distributions of charge (electrets) between semiconductor and dielectric layers to mimic a multigate configuration. This approach simultaneously improves a transistor’s field-effect mobility and on:off ratio, as well as tuning the threshold voltage, and shows that a high-performance FET could be made even from semiconductors with low intrinsic mobilities.