Guest Editors Shanhui Fan and Zetian Mi reflect on the Collection upon its closing.

Injected or photogenerated charge carriers in disordered organic semiconductors relax in energy to form an occupied density of states (ODOS) that is inherently difficult to probe. Thus little is known about nonequilibrium ODOS properties, although they are important in e.g. solar cells and LEDs. This work presents an optical technique for monitoring the ODOS distribution of relaxed charge carriers at low temperatures. The distribution that forms under nonequilibrium conditions is always narrower than that of the full DOS, in a universal ratio of about 2/3 that is reproduced by kinetic Monte Carlo simulations assuming spatially correlated disorder.

Sensing with undetected photons is a fascinating technique that allows imaging at wavelengths outside a particular detector’s range—enabling, for example, biological imaging without perturbing live cells. In this interference technique, performance can be very sensitive to both sample transmission and losses. This study shows that by balancing the nonlinear interferometer’s arms, one can completely recover fringe visibility caused by signal loss. Moreover, intentional unbalancing offers a reduced-interaction regime where the detected power can be 200 times higher than the power reaching the sample, for ultrasensitive midinfrared sensing well below the conventional noise floor.

Measurement and coherent manipulation of Andreev bound states (ABSs) in planar superconductor-semiconductor heterostructures could enable gate-controllable, small-footprint superconducting spin qubits with long coherence times. So far, readout capabilities and device coherence have been limited by lossy $III-V$ substrates, but here researchers overcome this limitation with a flip-chip approach to microwave experiments on ABSs. Superconductor-semiconductor devices and microwave resonators are placed on different chips and inductively coupled via a vacuum gap, enabling high-quality readout without sacrificing device properties.

Long-range couplings beyond the nearest neighbors can trigger exotic topological phases in metamaterials but have been overlooked in many topological models. The authors study the effect of the long-range couplings on the topological properties, and demonstrate that a Su-Schrieffer-Heeger model including the third nearest coupling, implemented by acoustic metamaterials, can lead to topological phases of large winding numbers. The experimental results show additional topological end states and rotonlike dispersions for sound waves, highlighting the potential of these long-range metamaterials in creating rich topological features.

Reversible computing exploits thermodynamic principles to replace irreversible Boolean logic with higher-efficiency reversible logic, and could change the future of computers. How best to implement reversible gates and architectures remains an open problem. Superconducting hardware features negligible intrinsic damping, which allows for exploration of many digital circuit types with varying degrees of reversibility. This study simulates Josephson junctions, where single flux quanta serve as bits in this logic type, with the goal of replacing an irreversible shift-register memory. The authors find that asynchronous access, and therefore simpler architecture, is possible.

This paper provides a key solution for integrating spintronic memories with photonic integrated circuits, for all-optical switching and reading of nanoscale magnetic bits. Problems associated with nonlinear absorption and the intrinsically weak magneto-optical effect are solved by a hybrid scheme, exploiting plasmonic and photonic resonators. Thus the authors have developed a device concept that paves the way toward ultrafast, energy-efficient advanced on-chip applications.

The authors present a platform for quantum experiments on levitated particles with masses ranging from nanograms to micrograms. They exploit the Meissner effect to magnetically levitate superconducting microparticles on a chip, in vacuum at ultralow temperatures. Integrated magnetic readout enables coupling to superconducting quantum circuits. This platform allows for precise measurements of force and acceleration, with potential applications in sensing, including the search for dark matter.

Lithium niobate (LN) devices have been widely adopted in systems for quantum key distribution (QKD), but the photorefractive effect in LN devices can be exploited as a potential loophole for malicious attacks by eavesdroppers. This work points out a vulnerability of LN devices of which manufacturers and users must become aware, so that appropriate countermeasures may be implemented. Moreover, the physical mechanism and techniques introduced in this work open an important avenue to the security analysis of QKD systems.

Understanding nonradiative recombination is important for improving semiconductor devices. For Cu(In,Ga)Se${}_2$ (CIGS) solar cells, antisite defects have long been considered the main recombination centers, yet the underlying mechanism has remained elusive. Here first-principles calculations show that these “killer centers” themselves cannot capture holes efficiently for effective recombination. However, internal conversion to the distorted neutral DX center does open an efficient hole-capture pathway, and DX’s stability in CIGS increases with Ga concentration, which resolves the longstanding issue of why the efficiency of CIGS solar cells decreases at high Ga concentration.

Ionic migration in solid phases is actively explored as a mechanism for neuromorphic computing elements, for example, but direct imaging of the ionic migration and its correlation with macroscopic electronic transport are tricky. Through multiple state-of-the-art microscopy techniques the authors visualize Ag${}^{+}$ migration in AgBiP${}_2$Se${}_6$, and how it modulates the interfacial barrier to induce switchable diodelike transport in a device. Illuminating the device can be used to switch off its ionic tunability, expanding its functionality. These results provide direct insight into the mechanism of ion-mediated transport in ionically active semiconductors.

“Ugh, not again! Something’s wrong with this printer…” In inkjet printing the nozzles in the printhead have intermittent idle periods, during which the ink can evaporate from the nozzle exit. Inks are usually multicomponent, and each component has its own characteristic evaporation rate, resulting in concentration gradients within the ink that can alter the jetting process. Through experiments, analytical modeling, and numerical simulations, the authors unravel the complex physicochemical hydrodynamics associated with the drying of ink at a printhead nozzle.

Single electrons trapped on semiconductor-defined quantum dots (QDs) are a promising platform for large-scale quantum computing. The authors demonstrate a reliable, automated method to identify the capacitive coupling between QDs. The approach combines machine learning with traditional fitting, to take advantage of the desirable properties of each. Also, analysis of cross capacitance may be used for automatic identification of the spurious QDs that occasionally form during device tuning. These techniques can autonomously flag devices with spurious dots near the operating regime, which is crucial information for reliable tuning for qubit operations.

Point-defect spin qubits with an optical interface may become the building blocks of future quantum networks. In this study, ion implantation at low fluences is used to create neutral In donors in ZnO that demonstrate desirable qubit properties. Notably, the authors also observe the signature of the indium electron-nuclear hyperfine interaction, which could enable access to a long-lived nuclear-spin memory. This result is an important step toward scalable formation of donor qubits in ZnO with optical access to a nuclear-spin memory.

Magnetometry with diamond nitrogen-vacancy (NV) ensembles has enabled devices with picotesla sensitivity at static and low-frequency fields, but their performance at higher frequencies lags far behind. The authors solve the technical challenges and demonstrate a microwave-frequency NV magnetometer with picotesla sensitivity, by implementing a pulse scheme for noise cancellation and employing a custom-grown diamond. This sensitivity enhancement could be extended into a far broader range of frequencies using spin-locking and quantum frequency mixing. These results could lead to applications such as near-field antenna characterization and microwave circuitry imaging.

Multimode lasing has a variety of applications in the fields of sensing, spectroscopy, signal processing, and communication. However, controllability of this phenomenon is commonly quite limited. This Letter demonstrates spectrum tunability of multimode lasing in a cooled optical-fiber ring. An intriguing connection between the observed optical spectrum and the sequence of prime numbers is found. This tunability of multimode lasing can be exploited for sensing, communication, and quantum data storage.

The ability to control thermal emission is critical for the development of infrared sources and on-demand energy production, but good emitters usually entail complex architectures. This study presents an analytical framework to design tunable narrowband emitters based on a simple, lithography-free Salisbury screen configuration that includes a phase-change material. Simulated devices show near-unity on-off switching and arbitrarily large spectral shifting between two emission wavelengths, which opens opportunities for tunable infrared sources, and more generally for midinfrared photonics.

Injected or photogenerated charge carriers in disordered organic semiconductors relax in energy to form an occupied density of states (ODOS) that is inherently difficult to probe. Thus little is known about nonequilibrium ODOS properties, although they are important in e.g. solar cells and LEDs. This work presents an optical technique for monitoring the ODOS distribution of relaxed charge carriers at low temperatures. The distribution that forms under nonequilibrium conditions is always narrower than that of the full DOS, in a universal ratio of about 2/3 that is reproduced by kinetic Monte Carlo simulations assuming spatially correlated disorder.

Sensing with undetected photons is a fascinating technique that allows imaging at wavelengths outside a particular detector’s range—enabling, for example, biological imaging without perturbing live cells. In this interference technique, performance can be very sensitive to both sample transmission and losses. This study shows that by balancing the nonlinear interferometer’s arms, one can completely recover fringe visibility caused by signal loss. Moreover, intentional unbalancing offers a reduced-interaction regime where the detected power can be 200 times higher than the power reaching the sample, for ultrasensitive midinfrared sensing well below the conventional noise floor.

Measurement and coherent manipulation of Andreev bound states (ABSs) in planar superconductor-semiconductor heterostructures could enable gate-controllable, small-footprint superconducting spin qubits with long coherence times. So far, readout capabilities and device coherence have been limited by lossy $III-V$ substrates, but here researchers overcome this limitation with a flip-chip approach to microwave experiments on ABSs. Superconductor-semiconductor devices and microwave resonators are placed on different chips and inductively coupled via a vacuum gap, enabling high-quality readout without sacrificing device properties.

Long-range couplings beyond the nearest neighbors can trigger exotic topological phases in metamaterials but have been overlooked in many topological models. The authors study the effect of the long-range couplings on the topological properties, and demonstrate that a Su-Schrieffer-Heeger model including the third nearest coupling, implemented by acoustic metamaterials, can lead to topological phases of large winding numbers. The experimental results show additional topological end states and rotonlike dispersions for sound waves, highlighting the potential of these long-range metamaterials in creating rich topological features.

Reversible computing exploits thermodynamic principles to replace irreversible Boolean logic with higher-efficiency reversible logic, and could change the future of computers. How best to implement reversible gates and architectures remains an open problem. Superconducting hardware features negligible intrinsic damping, which allows for exploration of many digital circuit types with varying degrees of reversibility. This study simulates Josephson junctions, where single flux quanta serve as bits in this logic type, with the goal of replacing an irreversible shift-register memory. The authors find that asynchronous access, and therefore simpler architecture, is possible.

This paper provides a key solution for integrating spintronic memories with photonic integrated circuits, for all-optical switching and reading of nanoscale magnetic bits. Problems associated with nonlinear absorption and the intrinsically weak magneto-optical effect are solved by a hybrid scheme, exploiting plasmonic and photonic resonators. Thus the authors have developed a device concept that paves the way toward ultrafast, energy-efficient advanced on-chip applications.

The authors present a platform for quantum experiments on levitated particles with masses ranging from nanograms to micrograms. They exploit the Meissner effect to magnetically levitate superconducting microparticles on a chip, in vacuum at ultralow temperatures. Integrated magnetic readout enables coupling to superconducting quantum circuits. This platform allows for precise measurements of force and acceleration, with potential applications in sensing, including the search for dark matter.

Lithium niobate (LN) devices have been widely adopted in systems for quantum key distribution (QKD), but the photorefractive effect in LN devices can be exploited as a potential loophole for malicious attacks by eavesdroppers. This work points out a vulnerability of LN devices of which manufacturers and users must become aware, so that appropriate countermeasures may be implemented. Moreover, the physical mechanism and techniques introduced in this work open an important avenue to the security analysis of QKD systems.

Understanding nonradiative recombination is important for improving semiconductor devices. For Cu(In,Ga)Se${}_2$ (CIGS) solar cells, antisite defects have long been considered the main recombination centers, yet the underlying mechanism has remained elusive. Here first-principles calculations show that these “killer centers” themselves cannot capture holes efficiently for effective recombination. However, internal conversion to the distorted neutral DX center does open an efficient hole-capture pathway, and DX’s stability in CIGS increases with Ga concentration, which resolves the longstanding issue of why the efficiency of CIGS solar cells decreases at high Ga concentration.

Ionic migration in solid phases is actively explored as a mechanism for neuromorphic computing elements, for example, but direct imaging of the ionic migration and its correlation with macroscopic electronic transport are tricky. Through multiple state-of-the-art microscopy techniques the authors visualize Ag${}^{+}$ migration in AgBiP${}_2$Se${}_6$, and how it modulates the interfacial barrier to induce switchable diodelike transport in a device. Illuminating the device can be used to switch off its ionic tunability, expanding its functionality. These results provide direct insight into the mechanism of ion-mediated transport in ionically active semiconductors.

“Ugh, not again! Something’s wrong with this printer…” In inkjet printing the nozzles in the printhead have intermittent idle periods, during which the ink can evaporate from the nozzle exit. Inks are usually multicomponent, and each component has its own characteristic evaporation rate, resulting in concentration gradients within the ink that can alter the jetting process. Through experiments, analytical modeling, and numerical simulations, the authors unravel the complex physicochemical hydrodynamics associated with the drying of ink at a printhead nozzle.

Single electrons trapped on semiconductor-defined quantum dots (QDs) are a promising platform for large-scale quantum computing. The authors demonstrate a reliable, automated method to identify the capacitive coupling between QDs. The approach combines machine learning with traditional fitting, to take advantage of the desirable properties of each. Also, analysis of cross capacitance may be used for automatic identification of the spurious QDs that occasionally form during device tuning. These techniques can autonomously flag devices with spurious dots near the operating regime, which is crucial information for reliable tuning for qubit operations.

Point-defect spin qubits with an optical interface may become the building blocks of future quantum networks. In this study, ion implantation at low fluences is used to create neutral In donors in ZnO that demonstrate desirable qubit properties. Notably, the authors also observe the signature of the indium electron-nuclear hyperfine interaction, which could enable access to a long-lived nuclear-spin memory. This result is an important step toward scalable formation of donor qubits in ZnO with optical access to a nuclear-spin memory.

Magnetometry with diamond nitrogen-vacancy (NV) ensembles has enabled devices with picotesla sensitivity at static and low-frequency fields, but their performance at higher frequencies lags far behind. The authors solve the technical challenges and demonstrate a microwave-frequency NV magnetometer with picotesla sensitivity, by implementing a pulse scheme for noise cancellation and employing a custom-grown diamond. This sensitivity enhancement could be extended into a far broader range of frequencies using spin-locking and quantum frequency mixing. These results could lead to applications such as near-field antenna characterization and microwave circuitry imaging.