Spin relaxometry with nitrogen-vacancy (N-$V$) centers in diamond offers a spectrally selective, atomically localized, and calibrated measurement of microwave-frequency magnetic noise, presenting a versatile probe for condensed-matter and biological systems. Typically, relaxation rates are estimated with curve-fitting techniques that do not provide optimal sensitivity, often leading to long acquisition times that are particularly detrimental in systems prone to drift or other dynamics of interest. Here we show that adaptive Bayesian estimation is well suited to this problem, producing dynamic relaxometry pulse sequences that rapidly find an optimal operating regime. In many situations (including the system we employ), this approach can speed the acquisition by an order of magnitude. We also present a four-signal measurement protocol that is robust to drifts in spin readout contrast, polarization, and microwave pulse fidelity while still achieving near-optimal sensitivity. The combined technique offers a practical, hardware-agnostic approach for a wide range of N-$V$ relaxometry applications.

We study the physics of a magnetic molecule described by a “giant” spin with multiple ($d>2$) spin states interacting with the quantized cavity field produced by a superconducting resonator. By means of the input-output formalism, we derive an expression for the output modes in the dispersive regime of operation. It includes the effect of magnetic anisotropy, which makes different spin transitions addressable. We find that the measurement of the cavity transmission allows us to uniquely determine the spin state of the qudits. We discuss, from an effective Hamiltonian perspective, the conditions under which the qudit readout is a nondemolition measurement and consider possible experimental protocols to perform it. Finally, we illustrate our results with simulations performed for realistic models of existing magnetic molecules.

The quantum Hall system possesses topologically protected edge states, which have enormous theoretical and practical implications in both fermionic and bosonic systems. Harnessing the quantum Hall effect in optical platforms with lower dimensionality is highly desirable with synthetic dimensions and has attracted broad interests in the photonics society. Here, we introduce an alternative way to realize the artificial magnetic field in a frequency dimension, which is achieved in a pump-probe configuration with cross-phase modulations in a one-dimensional four-waveguide array. The dynamics of the topological chiral edge state has been studied and the influence from the crosstalk of the pump fields has been explored. Our work shows an all-optical way to simulate the quantum Hall system in a photonic system and holds potential applications in manipulating light in waveguide systems.

We demonstrate, numerically, the possibility of manipulating the spin states of molecular nanomagnets with shaped microwave pulses designed with quantum optimal control theory techniques. The state-to-state or full gate transformations can be performed in this way in shorter times than using simple monochromatic resonant pulses. This enhancement in the operation rates can therefore mitigate the effect of decoherence. The optimization protocols and their potential for practical implementations are illustrated by simulations performed for a simple molecular cluster hosting a single ${\mathrm{Gd}}^{3+}$ ion. Its eight accessible levels (corresponding to a total spin $S=7/2$) allow encoding an eight-level qudit or a system of three coupled qubits. All necessary gates required for universal operation can be obtained with optimal pulses using the intrinsic couplings present in this system. The application of optimal control techniques can facilitate the implementation of quantum technologies based on molecular spin qudits.

Getting wireless signals around obstacles or passing them through walls is proven to be challenging for electromagnetic waves with a short wavelength and some energy-efficient buildings due to thick insulating materials. Here, we propose a scheme of through-wall wireless communication by applying purposely designed passive metalenses for an asymmetric background medium of the wall. An ultrathin metalens consisting of three layers of metallic patterns is applied to focus the incoming waves to the other side of the wall with high efficiency. The focusing effect is verified by scanning the spatial distribution of electric fields. Due to the focusing effect, the wireless signal strength is significantly enhanced while propagating through the wall. To validate its use in the 5-GHz Wi-Fi environment, we demonstrate that the metalens can reconnect a network channel that is otherwise broken due to weak signal strength, and thus, significantly increase the data transmission rate. Our work opens an unobstructive and retrofitting approach for sustaining wireless communication in future eco-friendly buildings beyond 5G and 6G.

Surface-acoustic-wave (SAW) sensors have undergone outstanding developments during the last decades. However, for biosensing applications, they struggle to compete with optical sensing technologies. In this work, we present a Love-wave-based sensing platform that leverages the use of the local resonances of micropillars fabricated on the surface of the guiding layer. We demonstrate numerically and experimentally that the Love-wave interaction with the micropillars produces sharp attenuation dips in the transmission spectrum. These dips are associated with the excitations of flexural and torsional resonance modes of the pillars. We utilize these modes to achieve higher detection sensitivity of temperature variation and mass loading using sugar and microbeads concentrations in aqueous solutions. Integrating a lattice of micropillars into SAW-based devices offers a strong potential for detecting low particles concentrations.

We show perpendicular magnetic anisotropy (PMA) in substituted rare-earth iron garnet ${\mathrm{Y}}_{3-x}{\mathrm{Tm}}_x{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ (x = 0–3) films epitaxially grown on (111) $({\mathrm{Gd}}_{0.63}{\mathrm{Y}}_{2.37})({\mathrm{Sc}}_2{\mathrm{Ga}}_3){\mathrm{O}}_{12}$ substrates by magnetron sputtering. With $\mathrm{Tm}$ concentrations varying from x = 0 to x = 3, the effective PMA field of the 10-nm-thick films increases monotonically from 0.3 to 2.7 kOe together with a similar coercivity increase from 2 to 65 Oe. The substituted garnet films with x ranging from 0.6 to 1.2 are structurally much more robust against epitaxial strain relaxation. Magnetotransport measurements in the ${\mathrm{Y}}_{3-x}{\mathrm{Tm}}_x{\mathrm{Fe}}_5{\mathrm{O}}_{12}$/$\mathrm{Pt}$ heterostructure show that the anomalous Hall effect and spin Seebeck effect are significant and change little with $\mathrm{Tm}$ concentration, indicating that the interface spin-exchange interaction and interface spin-current transmission mainly depend on the net moment of ${\mathrm{Fe}}^{3+}$ sublattices rather than the total moment of ${\mathrm{Y}}_{3-x}{\mathrm{Tm}}_x{\mathrm{Fe}}_5{\mathrm{O}}_{12}$. Importantly, the critical current density for the spin-orbit-torque-induced switching also changes very little with a minor increase from 1.76 × ${10}^{11}$ to 1.83 × ${10}^{11}$ A m${}^{-2}$. We tentatively attribute this weak PMA-dependent current density to modification of the domain-wall depinning by the current. Our results demonstrate that substituted rare-earth iron garnets can be a general approach to tune PMA within a large range with nearly constant switching current, and yet there is plenty of work left to be done for more efficient SOT-based garnet ferrimagnet spintronics devices.

Superconducting circuits present a promising platform with which to realize a quantum refrigerator. Motivated by this, we fabricate and perform spectroscopy of a gated Cooper-pair box, capacitively coupled to two superconducting coplanar-waveguide resonators with different frequencies. We experimentally demonstrate the strong coupling of a charge qubit to two superconducting resonators, with the ability to perform voltage driving of the qubit at gigahertz frequencies. We go on to discuss how the measured device could be modified to operate as a cyclic quantum refrigerator by terminating the resonators with normal-metal resistors acting as heat baths.

Developing a critical-element-free low-cost permanent magnet is an urgent necessity in view of rapidly developing technologies and the associated huge market demand for ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$-based magnets. Here, inspired by the abundant and low-cost nature of $\mathrm{Ce}$ and these high-performance ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$ permanent magnets, we explore whether it is in fact possible to attain a useful performance in alloys based on the sister material ${\mathrm{Ce}}_2{\mathrm{Fe}}_{14}\mathrm{B}$, employing both experimental and theoretical efforts. Experimentally, we study ${\mathrm{Ce}}_2{\mathrm{Fe}}_{14}\mathrm{B}$ with $\mathrm{Co}$, $\mathrm{La},$ and $\mathrm{Zr}$ substitutions. The $\mathrm{Zr}$ substitution is explored in view of $\mathrm{Zr}$’s frequent role in enhancing magnetic anisotropy in permanent magnets, while the $\mathrm{Co}$ and $\mathrm{La}$ substitutions serve to remedy the too-low Curie point of 433 K in the base alloy. While we find no $\mathrm{Zr}$-related anisotropy enhancement either experimentally or theoretically, the cosubstitution of $\mathrm{La}$ and $\mathrm{Co}$ indeed improves the Curie temperature as well as the magnetization, $M_s$, with a potential energy product as high as 38 MG Oe. These properties together suggest optimization of the alloy ${\mathrm{La}\mathrm{Ce}\mathrm{Fe}}_{12.7}{\mathrm{Co}}_{1.3}\mathrm{B}$ (with only 7 wt % cobalt) as a critical-element-free permanent magnet. While the substituted elements do not enhance magnetic anisotropy, from theory, we find a substantial increase, to a first anisotropy constant, $K_1$, as high as 4.24 $\mathrm{MJ}/{\mathrm{m}}^3$, associated with $\mathrm{Bi}$ substitution for $\mathrm{Ce}$. Our experimental and theoretical results demonstrate the great potential of $\mathrm{La}$, $\mathrm{Co},$ and $\mathrm{Bi}$ substitutions in developing low-cost and critical-element-free ${\mathrm{Ce}}_2{\mathrm{Fe}}_{14}\mathrm{B}$-based permanent magnets.

Magnetic skyrmions are chiral spin textures with attractive features, such as ultrasmall size, solitonic nature, and easy mobility with small electrical currents that make them promising as information-carrying bits in low-power high-density memory and in logic applications. However, it is essential to guarantee the positional stability of skyrmions for reliable information extraction. Using micromagnetic simulations for the minimum energy path, we compute the energy barriers associated with stabilizing notches along a racetrack. We vary the material parameters, specifically, the strength of the chiral Dzyaloshinskii-Moriya interactions, the notch geometry, and the thickness of the racetrack, to obtain the optimal barrier height. We find that the reduction of skyrmion size as it squeezes past the notch gives rise to the energy barrier. We find a range of energy barriers up to approximately $45k_{B}T$ for a racetrack of 5-nm thickness that can provide a positional lifetime of years for skyrmions for long-term memory applications, while requiring a moderate amount of current (approximately ${10}^{10}{\text{A/m}}^2$) to move the skyrmions. Furthermore, we derive quasianalytical equations to estimate the energy barrier. We also explore other pinning mechanisms, such as local variation of the material parameters in a region, and find that notched geometry provides the highest energy barrier. Our results open up possibilities to design practical skyrmion-based racetrack geometries for spintronics applications.

Continuous-variable systems realized in high-coherence microwave cavities are a promising platform for quantum information processing. While strong dynamic nonlinear interactions are desired to implement fast and high-fidelity quantum operations, static cavity nonlinearities typically limit the performance of bosonic quantum error-correcting codes. Here we study theoretical models of nonlinear oscillators describing superconducting quantum circuits with asymmetric Josephson-junction loops. Treating the nonlinearity as a perturbation, we derive effective Hamiltonians using the Schrieffer-Wolff transformation. We support our analytical results by numerical experiments and show that the effective Kerr-type couplings can be canceled by an interplay of higher-order nonlinearities. This can be better understood in a simplified model supporting only cubic and quartic nonlinearities. Our results show that a cubic interaction allows an increase in the effective rates of both linear and nonlinear operations without an increase in the undesired anharmonicity of an oscillator which is crucial for many bosonic encodings.

We apply the magnetoconductivity characteristic and angle-resolved photoemission spectroscopy (ARPES) to visualize the evolution of surface-selected electronic features of quasi-one-dimensional material ${\mathrm{Bi}}_4{\mathrm{Br}}_4$. The transport measurements indicate the quantum-interference correction to conductivity possesses a symbolic spin-rotational characteristic correlated to the value of Berry phase with the effects of weak localization and weak antilocalization for (001) and (100) surfaces, respectively. The ARPES spectra provide the experimental evidence for a quasi-one-dimensional surface state at the side (100) surface and an anisotropic massive Dirac surface state at the top (001) surface, respectively, which is highly coincided with the angle-dependent scaling behavior of magnetoconductivity. Our results reveal the facet-dependent electronic quantum diffusion in quasi-one-dimensional ${\mathrm{Bi}}_4{\mathrm{Br}}_4$, stimulating further investigations of this topological class and applications of the feasible technologies of topological spintronics.

Significant efforts have been devoted to further increasing the photoconversion efficiency of two-dimensional (2D) organic-inorganic halide perovskites, which are promising photovoltaic materials with superior stability but lower efficiency compared to their three-dimensional counterparts. One of the main factors that limit their photoconversion efficiency is the nonradiative recombination of photoexcited carriers. A widely used strategy for tuning the photoconversion efficiency of 2D perovskites is exchanging various organic cations. However, due to mixed effects contributed to by extrinsic factors, such as defect concentration and sample morphology, experiments alone are insufficient to explain the role of organic cations in phonon-assisted carrier recombination. With time-domain simulations based on first-principles nonadiabatic molecular dynamics, we investigate six prototypical 2D Ruddlesden-Popper perovskites to reveal the impact of organic cations on tuning the nonradiative recombination time through displacing inorganic ions and affecting electron-phonon coupling. The phonon-assisted band-to-band nonradiative recombination time is on the order of a few hundred nanoseconds and can be tuned up to 5 times by modifying organic cations. A distinct correlation between the pure-dephasing time and inorganic atom distortions induced by the motion of organic parts is revealed. Compared to the pure-dephasing time, nonadiabatic coupling (NAC) associated with the strength of electron-phonon coupling plays a more important role in determining the nonradiative carrier recombination time. Notably, fluorinated organic spacers increase the NAC between frontier electronic states and speed up the nonradiative recombination process.

It is widely believed that the skyrmion Hall effect, often disruptive for device applications, vanishes for overall topologically neutral structures such as (synthetic) antiferromagnetic skyrmions and skyrmioniums due to a compensation of Magnus forces. In this paper, however, we report that in contrast to the case of spin-transfer-torque-driven skyrmion motion, this notion is generally false for spin-orbit-torque-driven objects. We show that the skyrmion Hall angle is directly related to their helicity and imposes an unexpected roadblock for developing faster and lower input racetrack memories based on spin-orbit torques.

Modeling and simulations of entanglement-based quantum-networking protocols commonly assume perfect entangled states. Some investigations have been performed, which show how imperfections cause the efficiency of the protocols to rapidly deteriorate. For polarization-entangled states created by spontaneous-parametric down-conversion (SPDC), the fundamentals of phase matching lead to a trade-off problem for the optimal properties of the state. We present a SPDC method, which circumvents the trade-off problem and allows one to obtain degeneracy, indistinguishability, and heralded-single-photon spectral purity greater than $99\mathrm{\%}$ for any target SPDC wavelength in the visible and near-infrared spectrum. Therefore, our method can, in principle, generate polarization-entangled states that are optimal for polarization-entanglement-based quantum-networking protocols.

In functional magnonic devices, spin-wave couplers serve a key role in controlling the spin-wave transmission across different magnonic units. Here we study a reconfigurable spin-wave coupler based on domain-wall channels. The design relies on the dipolar interaction between spin waves propagating in neighboring domain walls. The dipolar-coupled spin-wave modes and their propagation in domain walls are analyzed using micromagnetic simulations and analytical calculations. The distance between domain walls and thus the spin-wave coupling strength can be tuned dynamically. Such a feature results in a dynamically manipulatable spin-wave coupler that enables full control over the propagation properties of coupled spin waves. The spin-wave coupler based on domain-wall channels can be achieved in a single magnetic film, avoiding complex nanofabrication, and enabling possibilities in reconfigurable magnonic functional devices.