One of the big challenges of current electronics is the design and implementation of hardware neural networks that perform fast and energy-efficient machine learning. Spintronics is a promising catalyst for this field with the capabilities of nanosecond operation and compatibility with existing microelectronics.

Physical Review Applied is pleased to present a Collection on Photovoltaic Energy Conversion, in recognition of the imminent need to harness solar energy, and the key role that Applied Physics plays in that endeavor. Contributions to this collection will be published throughout 2021 and into 2022. The invited articles, plus an editorial by Guest Editors Shanhui Fan and Zetian Mi, are linked in the Collection.

Quantum information processing requires qubits that can retain their energy for a long time, and one also needs to be able to rapidly manipulate and measure the qubits. The authors utilize the spatial extent of a microwave resonator to meet both of these requirements. They protect a superconducting qubit from energy decay by simply shifting the position of the coupling port in a conventional device. This study demonstrates a simple technique that enables fast readout and reset of a superconducting qubit without compromising its lifetime. The technique could immediately be incorporated into the design of a superconducting quantum computer.

The authors introduce a setup for energy harvesting in magnetic resonance imaging (MRI) scanners that efficiently converts circularly polarized radio-frequency electromagnetic fields. Such an approach allows doubling of performance, compared to traditional harvesting coils that convert linearly polarized field components. The setup can be used as a wireless power supply for pieces of additional equipment used within MRI scanners. Importantly, a series of experiments with two commonly used MRI scanners demonstrates that the proposed coil does not degrade the quality of MRI images.

Microwave parametric amplifiers operating at the quantum noise limit have become indispensable tools for a range of cryogenic quantum technologies. These amplifiers are typically constructed from nonlinear Josephson junctions, which limit the ability to amplify high-power signals. This study reports a device based instead on the weakly nonlinear kinetic inductance intrinsic to a superconducting film of niobium titanium nitride. The amplifier offers large phase-sensitive gain and high power handling, plus a simple design and fabrication process. As it contains no junctions, it is robust to electrostatic discharge and potentially operable under high temperatures and large magnetic fields.

The cyclotron-resonance method reveals the drift mobility of carriers in semiconductors, which determines a device’s (opto)electronic functionality. However, determining the intrinsic mobility value without interference from other carriers, dislocations, impurities, etc. remains challenging. By minimizing the density of photoexcited carriers in ultrapure diamond, the authors find an extraordinarily narrow cyclotron-resonance curve for electrons in diamond at 3 K. In this manner they obtain a corrected mobility value of 10${}^8$ cm${}^2$ V${}^{-1}$ s${}^{-1}$, a 16-fold increase compared to the previous record value for diamond.

A series of $III-V$ alloy-based avalanche photodiodes are recently seen to demonstrate superior performance such as low excess noise, but the origin of such behavior is not completely understood. The authors use atomistic modeling of the material and transport properties to deconstruct the underlying physical mechanisms, which are attributed to a combination of engineered minigaps, increased effective mass, and spin-orbit coupling. These attributes selectively limit the ionization rate of one carrier type, and are simplified here into a set of inequalities that could potentially be useful for the design of future high-performance avalanche photodiodes.

Rock on! Understanding time-dependent diffusion processes in complex heterogeneous media is of great importance in physics, chemistry, biology, materials science, geophysics, and petroleum engineering. Here the authors further study the recently discussed $diffusion$ $spreadability$, by computing it for a variety of two- and three-dimensional model structures that span the nonhyperuniform and hyperuniform classes. The lessons learned are used to ascertain crucial structural characteristics of a Fontainebleau sandstone. Spreadability is a powerful, dynamics-based figure of merit for probing real microstructures across length scales and enabling materials design.

Coherent control of collective spin excitations (magnons) with acoustic phonons is a key technology for future hybrid spintronic devices, thanks to the short wavelengths and low radiation loss involved. However, a tiny coupling efficiency limits the controllability and functionality of devices. The authors develop a planar cavity-magnomechanical system and show coherent magnon-phonon transduction with a cooperativity exceeding unity. This approach paves the way for the development of alternative magnomechanical technologies for both quantum and classical applications.

Designing filter devices with precisely desired transmission spectra is of utmost importance in wave physics. For sharp spectra (such as elliptic filters with transmission zeros), brute-force methods to directly optimize the spectrum face severe numerical challenges, while circuit models and coupled-mode theory apply only in certain limits. The authors provide a set of universal analytical criteria based on quasinormal-mode theory (QNMT) for designing 2-port systems, and apply the method to a variety of microwave metasurface filters configured for polarization-preserving transmission, reflective polarization conversion, or diffractive anomalous reflection.

Understanding exciton diffusion in organic solar cells is crucial to understanding the recent rise in power-conversion efficiencies afforded by using non-fullerene acceptor molecules (NFAs). This study introduces a technique called pulsed-PLQY for measuring exciton diffusion lengths in organic semiconductors. Compared to existing techniques, pulsed-PLQY is faster, easier, requires less specialized equipment, and is less sensitive to experimental conditions. Using this method, the authors find that modern non-fullerene acceptor semiconductors exhibit longer diffusion lengths than their fullerene forerunners, and that this increase is driven by increases in diffusivity.

Complete characterization of the errors that occur in using sets of logic gates is critical to developing the technology of fault-tolerant quantum computing, but current tomography methods are either slow or include unchecked assumptions. This study presents a self-consistent method for process tomography that is both fast and flexible. The technique complements the broad suite of existing characterization tools, and may potentially allow for pulse optimization to further increase gate fidelities.

Guest Editors Shanhui Fan and Zetian Mi introduce a collection of papers in Physical Review Applied on photovoltaic energy conversion, a subject of ever-increasing importance.

APS Editor in Chief, Michael Thoennessen, discusses a new opportunity for communicating authors to include their pronouns together with their contact email in order to promote a more respectful, inclusive, and equitable environment.

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