A soft ferromagnetic plate uniformizes tesla-level fields generated by attached permanent magnets, producing a smooth and electronically tunable surface field on the opposite side. By arranging $n$ precisely fabricated rectangular plates, a nearly ideal magnetic quadrupole field with a substantial gradient can be created at the center. This robust and rapidly tunable field configuration is well suited for two-dimensional magneto-optical trapping (2D MOT) and magnetic guiding of cold atoms. By aligning an optical nanofiber (ONF) along the zero-field line of a planar 2D MOT in a two-plate assembly, we demonstrate quasicontinuous, field-free operation of the quantum optical interface without switching off the magnetic field. Transient transmission spectroscopy with nanosecond laser pulses is performed on the ${}^{87}\mathrm{Rb}D2$ line at a measurement repetition rate as high as 250 kHz. The observed line broadening, while not yet fully understood, is partially attributed to residual magnetic fields in the $n=2$ assembly. Through additional measurements and simulations, we verify that these residual fields can be fully eliminated in an $n=4$ assembly, resulting in an ultrastraight 2D trap that supports uniform light-atom interaction over exceptionally long, field-free distances. We extend our discussion to $n=6$, $n=8$ designs with similar uniformity but multiple zero-field lines. With its strong gradient for magnetic trapping, the ferromagnetic devices also enable quantum optical scenarios featuring interactions between coguided atoms and photons at ONF interfaces.

Strongly correlated insulators, such as Mott or charge-transfer insulators, exhibit a strong temperature dependence in their resistivity. Consequently, self-heating effects can lead to electrothermal instabilities in planar thin-film devices of these materials. When the electrical bias current exceeds a device-specific threshold, the device can switch from a high- to a low-resistance state through the formation of metallic filaments. However, since the filaments are sustained by local Joule heating after they form, reducing the bias current below a second threshold causes them to disappear, returning the device to the high-resistance state. Hence, electrothermal resistive switching is usually volatile. Here, on the contrary, we report on nonvolatile resistive switching in a planar ${\mathrm{Nd}\mathrm{Ni}\mathrm{O}}_3$ thin-film device. By combining electrical transport measurements with optical wide-field microscopy, we provide evidence for a metallic filament that persists even after returning the bias current to zero. We attribute this effect to the pronounced hysteresis between the cooling and heating branches in the resistance versus temperature dependence of the device. At least $100$ intermediate resistance states can be prepared, which are persistent as long as the base temperature is kept constant. Further, the switching process is nondestructive, and thermal cycling can reset the device to its pristine state.

Coulomb crystals—ordered structures of cold ions confined in ion traps—have applications in a variety of research fields. The number and temperature of the ions forming the Coulomb crystals are two key attributes of interest in many trapped-ion experiments. Here, we present a fast and accurate approach to determining these attributes from fluorescence images of the ions based on convolutional neural networks (CNNs). In this approach, we first generate a large number of images of Coulomb crystals with different numbers of ions at different temperatures using molecular-dynamics simulations and then train CNN models on these images to classify the desired attributes. The classification performance of several common pretrained CNN models is compared in example tasks. We find that for crystals with numbers of ions in the range 100–299 and secular temperatures of 5–15 mK, the best-performing model can discern variations of a single ion with an accuracy of 93% and temperature changes of 1 mK with an accuracy of 92%. Since the trained models can be directly integrated into experiments, in situ determination of these attributes can be realized in a noninvasive fashion. This has the potential to greatly enhance the analysis and control of trapped ions in real time.

Hyperentangled states are fascinating resources in quantum information processing as they can significantly increase the channel capacity and enhance noise resistance. We explore a hyperfusion mechanism to fuse one $n$-photon hyper-$W$ state and one $m$-photon hyper-$W$ state into a large-scale $(n+m-2)$-photon hyper-$W$ state. Another mechanism to fuse one $n$-photon hyper-$W$ state, one $m$-photon hyper-$W$ state, and one $t$-photon hyper-$W$ state into an $(n+m+t-3)$-photon hyper-$W$ state is also proposed. These two hyperfusion mechanisms are constructed employing only polarizing beam splitters, balanced beam splitters, half-wave plates, single-photon detectors, and cross-Kerr nonlinearities. Conditional quantum gates, path couplers, and ancillary photons are not required in our constructions. Moreover, our fused $W$ states are hyperentangled in the polarization and spatial degrees of freedom of single-photon systems. The presence of only one garbage output state demonstrates that high efficiency can be achieved in our schemes.

Advanced applications based on semiconductor nanostructures require accurate modeling tools for the description of coherent and dissipative quantum processes, but, crucially, also microscopic fluctuations and their propagation to the contacts. Since conduction and fluctuations are intimately related by the fluctuation-dissipation theorem, the analysis of carrier transport and noise properties should be addressed at the same level of approximation to obtain a complete and unambiguous understanding of the quantum system. Starting from a quantum transport model based on the nonequilibrium Green’s function formalism, we identify a set of microscopic noise sources in terms of fundamental current and charge fluctuations in the presence of dissipative processes. The noise sources are added to the relevant conservation equations, and the power spectra of the short-circuit current fluctuations are evaluated through a linear perturbation theory equivalent to the classical impedance-field method for noise analysis. Numerical examples with textbook structures are shown to establish a connection with semiclassical noise theories. Application to a resonant tunneling diode demonstrates the potential of the proposed model as a tool to probe the electron kinetics in highly nanostructured devices. The present work concerns fluctuations associated with intraband scattering processes. The extension to number fluctuations due to interband scattering could provide information on the technological quality and physical origin of degradation mechanisms in state-of-the-art optoelectronic devices such as type-II superlattice photodetectors.

The one-dimensional side gate based on graphene edges shows a significant capability of reducing the channel length of field-effect transistors, further increasing the integration density of semiconductor devices. The nanoscale electric field distribution near the edge provides the physical limit of the effective channel length; however, its imaging under ambient conditions is still lacking, which is a critical aspect for the practical deployment of semiconductor devices. Here, we used scanning nitrogen-vacancy (N-$V$) microscopy to investigate the electric field distribution near edges of single-layer graphene. Real-space scanning maps of photocharged floating graphene flakes were acquired with a spatial resolution of approximately 10 nm, and the electric edge effect was quantitatively studied by analyzing the N-$V$ spin energy-level shifts due to the electric Stark effect. Since the graphene flakes are isolated from external electric sources, we brought out a theory based on the photothermionic effect to explain the charge transfer from graphene to the oxygen-terminated diamond probe with a disordered distribution of charge traps. Real-time tracing of electric fields detected the photothermionic emission process and the recombination process of the emitted electrons. This study provides a perspective for graphene-based one-dimensional gates and optoelectronics with nanoscale real-space imaging and, moreover, offers a method to tune the chemical environment of diamond surfaces based on optical charge transfer.

We study van der Waals heterostructures consisting of monolayer ${\mathrm{Mo}\mathrm{Se}}_2$ and few-layer ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ fully encapsulated in hexagonal boron nitride using low-temperature photoluminescence and polar magneto-optical Kerr-effect measurements. Photoluminescence characterization reveals a partial quenching and a change of the exciton-trion emission ratio in the heterostructure as compared to the isolated ${\mathrm{Mo}\mathrm{Se}}_2$ monolayer. Under circularly polarized excitation, we find that the exciton-trion emission ratio depends on the relative orientation of excitation helicity and ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ magnetization, even though the photoluminescence emission itself is unpolarized. This observation hints at an ultrafast, spin-dependent interlayer charge transfer that competes with exciton and trion formation and recombination.

Ultracold atoms trapped in optical lattices have emerged as a scalable and promising platform for quantum simulation and computation; however, gate speeds remain a significant limitation for practical applications. In this work, we employ quantum optimal control to design fast, collision-based two-qubit gates within a superlattice based on a Fermi-Hubbard description, reaching errors in the range ${10}^{-3}$ for realistic parameters. Numerically optimizing the lattice depths and the scattering length, we effectively manipulate hopping and interaction strengths intrinsic to the Fermi-Hubbard model. Our results provide five times shorter gate durations by allowing for higher energy bands in the optimization, suggesting that standard modeling with a two-band Fermi-Hubbard model is insufficient for describing the dynamics of fast gates, and we find that four to six bands are required. Additionally, we achieve nonadiabatic gates by employing time-dependent lattice depths rather than using only fixed depths. The optimized control pulses not only maintain high efficacy in the presence of laser-intensity and phase noise but also result in negligible interwell couplings.

The possibility of driving vortex lines in type II superconductors with an oscillating magnetic field could be useful in many applications. For example, it can be used for the removal of undesired trapped flux from contactless elements of superconducting devices. We investigate the dynamics of vortex lines in a superconducting film with a ratchet thickness profile driven by an oscillating magnetic field applied parallel to the film. We numerically simulate the dynamics of a single flux line modeled as an elastic string with a variable length. Exploring the behavior for different frequencies and amplitudes of the oscillating magnetic field, we find several dynamic regimes. For moderate frequencies, the average velocity is finite only within specific amplitude ranges. A notable feature is the presence of extended velocity plateaus, which correspond to regimes when the line moves by integer multiples of the spatial period $w$ during integer multiples of the time period $T$. The transitions to these plateau states are rather steep, especially at low frequencies. The plateau at velocity $w/T$ dominates at intermediate frequencies but vanishes at high frequencies. The onset field amplitude of finite velocity nonmonotonically depends on the frequency and passes through a minimum at a certain frequency. At low frequencies, the velocity exceeds $w/T$ and progressively increases with the amplitude. These findings provide valuable insights into the dynamic behavior of vortex lines driven by an oscillating magnetic field in patterned superconducting films, offering potential pathways for controlling the magnetic flux in superconducting devices.

We present an extensive study of 4H-$\mathrm{Si}\mathrm{C}$ nanomechanical resonators electrochemically etched out of a monocrystalline wafer. Combining piezo-driven interferometric determination of the mechanical spectra with scanning laser Doppler vibrometry, an unambiguous assignment of resonance peaks to flexural and torsional modes is achieved. The investigation of multiple harmonic eigenmodes of singly and doubly clamped resonators with varying geometry allows for a comprehensive characterization. Excellent intrinsic mechanical quality factors up to $2\times {10}^5$ are found at room temperature, approaching the thermoelastic limit at eigenfrequencies exceeding 10 MHz. Mechanical stress is essentially absent. Young’s modulus is in agreement with the literature. These findings are robust under postprocessing treatments, in particular atomic layer etching and high-temperature thermal annealing. The resulting on-chip high-quality mechanical resonators represent a valuable technological element for a broad range of applications. In particular, the monolithic architecture meets the requirements of spin-based photonic quantum technologies on the upcoming $\mathrm{Si}\mathrm{C}$ platform.

The introduction of time as a new degree of freedom to control wave-matter interactions has opened alternative avenues to fully control wave propagation in four dimensions (x,y,z,t). Time interfaces (rapid changes of the constitutive relationships of the medium where a wave propagates) have recently become popular, as they are the temporal analogue of spatial interfaces. While recent groundbreaking experimental work has demonstrated time interfaces from water waves, microwaves, and the optical regime, rapidly changing, for instance, the permittivity of the medium requires carefully engineered structures. Here, we study the possibility of implementing smooth temporal transitions of the permittivity and permeability of the medium (while considering time-independent impedance) to mimic the response of the time-refracted waves produced by photonic time interfaces. It is shown that indeed, as long as the signal is inside the structure during the modulation time, there are some values of rising or falling time of the smooth modulation that enable a full approximate emulation of time refraction. These results may open further avenues to explore by relaxing the speed at which the time modulation should be introduced when designing four-dimensional media.

In the pursuit of scalable superconducting quantum computing, tunable couplers have emerged as a pivotal component, offering the flexibility required for complex quantum operations of high performance. In most current architectures of superconducting quantum chips, such couplers are not equipped with dedicated readout circuits to reduce complexity in both design and operation. However, this strategy poses challenges in precise characterization, calibration, and control of the couplers. In this work, we develop a hardware-efficient and robust technique based on adiabatic control to address this issue. The critical ingredient of this technique is adiabatic swap operation between a tunable coupler and nearby qubits. Using this technique, we have characterized and calibrated tunable couplers in our chips and achieved straightforward and precise control over these couplers. For example, we have demonstrated the calibration and correction of the flux distortion of couplers. In addition, we have also expanded this technique to tune the dispersive shift between a frequency-fixed qubit and its readout resonator over a wide range.

Temporal fluctuations in the superconducting qubit lifetime, $T_1$, present additional challenges in the pursuit of fault-tolerant quantum computing. Although the exact mechanisms remain unclear, $T_1$ fluctuations are generally attributed to strong coupling between the qubit and a few near-resonant two-level systems (TLSs), which can exchange energy with an ensemble of thermally fluctuating two-level fluctuators (TLFs) at low frequencies. Here, we report $T_1$ measurements of qubits with varying geometrical footprints and surface dielectrics as a function of temperature. By analyzing the noise spectrum of the qubit depolarization rate, ${\mathrm{\Gamma }}_1=1/T_1$, we disentangle the contributions of TLSs, nonequilibrium quasiparticles (QPs), and equilibrium (thermally excited) QPs to the variance in ${\mathrm{\Gamma }}_1$. We find that the ${\mathrm{\Gamma }}_1$ variance in qubits with smaller footprints is more susceptible to QP and TLS fluctuations than that in larger-footprint qubits. Furthermore, the QP-induced variances in all qubits align with the theoretical framework of QP diffusion and fluctuation. These findings offer valuable insights for future qubit design and engineering optimization.

We study the steady-flow-induced deformation between an incompressible non-Newtonian fluid and a three-dimensional deformable channel. Specifically, we provide a comprehensive experimental-theoretical framework for such flows of constant-viscosity viscoelastic (i.e., Boger) fluids, which allows us to quantify the influence of the fluid’s viscoelasticity on the flow rate–pressure drop ($q$–$\mathrm{\Delta }p$) relation. For a flow-rate-controlled regime and weakly viscoelastic flow, we find excellent agreement between the theoretical prediction and experimental results, specifically providing an experimental demonstration of how both wall compliance and fluid viscoelasticity decrease the pressure drop. To definitively demonstrate the coupled effects of wall compliance and fluid viscoelasticity (change of cross-sectional area and curving of streamlines), we perform experiments in an equivalent rigid channel, showing that both the Boger fluid and a viscosity-matched Newtonian fluid have the same pressure drop. Thus, the experimentally verified theory for the $q$–$\mathrm{\Delta }p$ relation of weakly viscoelastic flows of Boger fluids in compliant channels can now be used for the design of microfluidics and soft hydraulic systems operating with complex fluids.

Training-free quantum architecture search (QAS) has shown substantial promise in automating quantum circuit design for variational quantum algorithms (VQAs) with minimal computational expense. However, various VQA tasks and search spaces often favor specific training-free proxies. Furthermore, circuit performance is influenced by multiple factors, such as expressibility, trainability, and circuit topology, making it challenging for a single proxy to assess circuit performance accurately. To address these limitations, we propose an adaptive QAS framework that dynamically integrates multiple training-free proxies, combining their strengths and mitigating individual biases. This framework uses a mixture-of-experts (MoE) model that adapts to different VQA tasks and search spaces. Specifically, we develop an expert model for each training-free proxy, where each expert focuses on a distinct feature, such as expressibility, trainability, or topology. A gating network then assigns weights to each expert based on the specific VQA task and search space. Our proposed QAS framework consistently outperforms state-of-the-art QAS algorithms across diverse VQAs, including variational quantum eigensolvers (VQEs) and quantum neural networks (QNNs), as well as multiple search spaces, such as gatewise, layerwise, and blockwise constructions. Moreover, our approach demonstrates transferability across different VQA tasks and search spaces.

We derive a method for extracting the trap density of states (DOS) in electrolyte-gated thin-film transistors (EG-TFTs) using only transfer measurements. We follow the approach of a well-established scheme for extracting the DOS in thin-film transistors [M. Grünewald et al., Phys. Stat. Sol. B 100, K139 (1980)], combined with the Gouy-Chapman model for electrical double layers. We apply the method to the transfer characteristics of a poly(3-hexylthiophene)-based EG-TFT with water as the electrolyte. The extracted DOS is consistent with results from prior studies on low-voltage organic thin-film transistors, demonstrating the method’s validity and potential for broader application.

Quantum key distribution (QKD) offers a theoretically secure method to share secret keys, yet practical implementations face challenges due to noise and loss over long-distance channels. Traditional QKD protocols require extensive noise compensation, hindering their industrial scalability and lowering the achievable key rates. Alternative protocols encode logical qubits in noise-resilient states but at the cost of using many physical qubits, increasing susceptibility to loss and limiting transmission distance. In this work, we introduce a logical-qubit encoding that uses antisymmetric Bell states in the continuous photonic degrees of freedom, frequency and time. By leveraging the continuous space, we overcome this noise-loss robustness trade-off by minimizing the number of photons per logical qubit while optimizing the encoding resilience over noise fluctuations. We analyze the security of our encoding and demonstrate its robustness compared to existing state-of-the-art protocols. This approach provides a path toward scalable, efficient QKD implementations under realistic noise conditions.

Since Benjamin Franklin proposed the first bifocal lenses in 1784 and Bernard Maitenaz invented the first varifocal lenses in 1959, researchers worldwide have been actively working on developing electronically switchable lenses for spectacles with the goal of addressing vision issues faced by individuals with both presbyopia and myopia. Liquid-crystal (LC) refractive or diffractive Fresnel lenses have emerged as a promising solution. However, the degradation of incoherent imaging, caused by microstructures, diffraction, and chromatic aberration, makes LC refractive or diffractive Fresnel lenses not suitable for spectacle applications. LC gradient index (GRIN) lenses have been suggested and successfully demonstrated in laboratories as an alternative to LC Fresnel lenses. However, how to bridge the gaps between engineering and physics as well as from laboratory to industry is still a question. To address these gaps, we present here electrically switchable spectacles based on LC GRIN lenses, which have the potential for mass production. Additionally, we discuss the optical principles underlying polarization-independent LC GRIN lenses, based on a wave-plate model. We present a breakthrough that is a significant advancement in LC-based tunable spectacles and that may provide vision correction for individuals worldwide. The impacts of this study are not only ophthalmic applications but also machine vision for artificial intelligence as well as augmented reality.