###### EDITORS' SUGGESTION

In the liquid-metal ion sources that anchor key technologies for nanoscience, above a critical electric field strength a droplet of liquid metal sprouts a conical tip that undergoes continuous sharpening and field self-enhancement, culminating in ion emission. Despite decades of research, details of this process remain mysterious. Here the authors focus on the influence of inertial effects on the self-similar process leading to divergent growth. Asymptotic analysis and numerical simulations reveal a multiplicity of tip configurations that may help to explain decades-old observations of tip pulsation, droplet emission, liquid recoil, and collapse.

Chengzhe Zhou and Sandra M. Troian

Phys. Rev. Applied **15**, 044001 (2021)

###### LETTER

Measuring electrical charge with single-electron resolution is essential for implementing spin qubits in semiconductor quantum dots. Applying quantum dot charge sensors in larger-scale quantum computing devices is being held back because these sensors are easily affected by environmental disturbances, and thus require elaborate tune-up. This study uses a feedback control circuit implemented on a field-programmable gate array to maintain sensor performance in the face of capacitive crosstalk and charge noise. The technique enables rapid tune-up of quantum dot devices and reliable single-shot electron or spin measurement, and will advance the development of large-scale spin-qubit devices.

Takashi Nakajima *et al.*

Phys. Rev. Applied **15**, L031003 (2021)

###### EDITORS' SUGGESTION

In analogy to charge in conventional circuitry, magnetic domain walls provide an alternative medium for encoding digital information. The authors exploit the lateral coupling between out-of-plane and in-plane magnetic regions induced by the interfacial Dzyaloshinskii-Moriya interaction in thin magnetic films to realize both field- and current-driven magnetic domain-wall inverters. Nonreciprocity in transport is introduced by breaking the symmetry of the inverter’s geometry, thus realizing a domain-wall diode. This innovative component enhances flexibility in the design of domain-wall circuits and extends the operation of current-driven domain-wall devices to the ac-signal regime.

Zhaochu Luo *et al.*

Phys. Rev. Applied **15**, 034077 (2021)

###### LETTER

“Molecules” of coupled quantum dots offer efficient single-photon emission and spin qubits with long coherence times, positioning them as promising platforms for quantum information processing. The authors propose a protocol for reading out a quantum dot molecule’s spin state, by means of of a microwave π-pulse and cycling of isolated optical transitions. Simulations show that the protocol can provide single-shot readout, given a realistic photon collection efficiency of 0.12%. Realizing such efficient spin readout could boost the potential of quantum dots for generating photon entanglement, storing quantum information, and serving as building blocks of quantum networks.

D. Farfurnik, R. M. Pettit, Z. Luo, and E. Waks

Phys. Rev. Applied **15**, L031002 (2021)

###### EDITORS' SUGGESTION

Superconducting qubits provide a promising architecture for scalability in quantum information processing, but their coherence times are currently limited by environmental noise, miring such processors in the noisy intermediate-scale regime. Operating at “sweet spots” (turning points in a qubit’s microwave spectrum) can substantially reduce the dephasing due to $1/f$ flux noise. The authors extend this concept to boost noise mitigation with an external drive, yielding $d\phantom{\rule{0}{0ex}}y\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}m\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}l$ sweet spots and turning static sweet “spots” into manifolds. This simple, powerful approach adds flexibility to the choice of operating points, and could enhance coherence times by more than an order of magnitude.

Ziwen Huang *et al.*

Phys. Rev. Applied **15**, 034065 (2021)

###### EDITORS' SUGGESTION

Nonlinear and quantum optics, and electromagnetic applications such as sensing, benefit from large, broadband electric field enhancements, which are usually achieved through localized resonances. That approach, however, involves a stringent compromise between field enhancement, bandwidth, and overall device size. Here the authors study the unusual phenomena arising at a $n\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}p\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}l$ $h\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}p\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}t$, a nonresonant broadband accumulation of energy in an ultrasmall volume at the end of a unidirectional waveguide. These hotspots support larger field enhancements $a\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}d$ much broader bandwidths than conventional systems, providing an attractive alternative for enhancing light-matter interaction.

Sander A. Mann, Ahmed Mekawy, and Andrea Alù

Phys. Rev. Applied **15**, 034064 (2021)

###### LETTER

Efficient generation of spin current is fundamental in spintronics. Despite a decade and more of intense research interest, whether the spin-orbit coupling (SOC) effects of magnetic interfaces can effectively generate a spin current remains an open question. Utilizing Ti/Fe-Co-B bilayers with negligible bulk spin Hall effect and strong, tunable interfacial SOC, the authors establish clean experimental evidence that magnetic interfaces $d\phantom{\rule{0}{0ex}}o$ $n\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}t$ generate any significant spin current via spin-orbit filtering or Rashba-Edelstein-like) effects.

Lijun Zhu and Robert A. Buhrman

Phys. Rev. Applied **15**, L031001 (2021)

###### EDITORS' SUGGESTION

Quantum communication will undoubtedly be deployed in telecommunication networks of the future. The current development in classical communication is motivated by the rapidly approaching limit of existing optical fibers, known as the “capacity crunch”. A promising solution is $s\phantom{\rule{0}{0ex}}p\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}e-d\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}v\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}n$ $m\phantom{\rule{0}{0ex}}u\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}p\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}x\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}g$, in which using the spatial modes supported in modern optical fibers increases their information capacity. The authors exploit related technologies to provide a source of entangled photons compatible with this latest trend in classical communication, which may serve as a standard for the next generation of entanglement-based quantum communication systems.

E.S. Gómez *et al.*

Phys. Rev. Applied **15**, 034024 (2021)

###### EDITORS' SUGGESTION

Label-free optical biosensors are an invaluable tool for drug discovery. Refractometric biosensors (the current gold standard) are sensitive and quantitative, yet due to their integrative nature are also extremely cross-sensitive to temperature and nonspecific binding. $D\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}f\phantom{\rule{0}{0ex}}f\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}m\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c$ biosensors, however, can reject these environmental noise sources via spatial lock-in of the binding signal, while remaining just as sensitive. Quantification of the biomolecular mass bound to a diffractometric sensor has remained elusive for most optical arrangements, but this study closes the gap by providing a unified theoretical framework for all two-dimensional diffractometric biosensors.

Yves Blickenstorfer *et al.*

Phys. Rev. Applied **15**, 034023 (2021)