Harvesting of the ambient electromagnetic field energy by converting the oscillating current into a direct one using a spin-torque diode is a very important topic in studies of possible methods for green energy production. In this paper we perform detailed numerical studies of such energy harvesting using an “in-plane” resonant spin-torque diode (STD). In order to accelerate the corresponding modeling, we first suggest two new simulation protocols which enable one to compute the frequency dependence of the rectified (dc) voltage in a single simulation run. Both methods provide a very high acceleration degree and allow one to quickly obtain the desired results in a wide range of STD parameters. In the main part of this paper we investigate the fundamental limit of STD efficiency, suggesting that the dc voltage is maximized by applying a constant external field. We demonstrate that this efficiency may be greatly enhanced when the applied field is close to the switching field of the STD free layer. Keeping in mind that the energy barrier preventing the switching of the STD magnetization to its opposite equilibrium position should still be high enough, we show that an ac current flowing through the STD affects this barrier very weakly (as opposed to the dc current). This feature allows us to predict the field values for which the output dc voltage is much larger than without an external field, still keeping the field far away from the switching point to ensure thermal stability. Our results can be employed to optimize the STD working conditions and to predict the maximal voltage that could be achieved with this relatively simple STD design (standard “in-plane” multilayer nanoelements).

We demonstrate a new approach for the metrology of microwave fields based on the trap-loss spectroscopy of cold Rydberg atoms in a magneto-optical trap. Compared to state-of-the-art sensors using room-temperature vapors, cold atoms allow longer interaction times, better isolation from the environment, and a reduced Doppler effect. Our approach is particularly simple, as the detection relies on fluorescence measurements only. Moreover, our signal is well described by a two-level model across a broad measurement range, allowing one, in principle, to reconstruct the amplitude and the frequency of the microwave field simultaneously without the need for an external reference field. We report on a scale-factor linearity at the percent level and no noticeable drifts over two hours, paving the way for new applications of cold Rydberg atoms in metrology, such as calibrating black-body shifts in state-of-the-art optical clocks, monitoring the Earth’s cryosphere from space, measuring the cosmic microwave background, or searching for dark matter.

Ideal photonic quantum memories can store arbitrary pulses of light with unit efficiency. This requires operating in the adiabatic regime, where pulses have a duration much longer than the bandwidth of the memory. In the nonadiabatic regime of short pulses, memories are therefore imperfect, and information is always lost. We theoretically investigate the bandwidth limitations for setups based on individual atoms, or ensembles thereof, confined inside optical cavities. We identify an effective strategy for optimizing the efficiencies of the storage and retrieval process regardless of the duration or the shape of the pulses. Our protocol is derived almost completely analytically and attains efficiencies that are comparable to those obtained by numerical optimization. Furthermore, our results provide an improved understanding of the performance of quantum memories in several regimes. When considering pulses defined on an infinite time interval, the shapes can be divided into two categories, depending on their asymptotic behaviors. If the intensity of the pulse increases with time slower than or as an exponential function then the storage efficiency is only limited by the pulse width. For pulses defined on a finite interval, on the other hand, the efficiency is determined by the shape at the beginning of the storage or, correspondingly, at the end of the retrieval process.

Nonvolatile spintronics-based devices that utilize electron spin both to store and transport information face a great challenge when scaled to nanosized dimensions due to loss of thermal stability and stray field-induced disturbance in closely packed magnetic bits. The potential replacement of ferromagnetic materials with antiferromagnets may overcome some of these issues owing to the superior robustness of sublattice spin orientations to magnetic field disturbance as long as they are kept well below the Néel temperature, which is hard to measure with conventional methods, especially in the ultrathin limit. In this work, we have employed spin pumping from a soft ferromagnetic $\mathrm{Ni}\mathrm{Fe}$ layer into widely used ultrathin metallic antiferromagnet ${\mathrm{Ir}}_{20}{\mathrm{Mn}}_{80}$, $\mathrm{Fe}\mathrm{Mn}$, $\mathrm{Pt}\mathrm{Mn}$, $\mathrm{Pd}\mathrm{Mn},$ or $\mathrm{Ni}\mathrm{Mn}$ with thicknesses in the 0.7–3 nm range, as a probe to detect damping enhancement during magnetic phase-transitions. Independent measurements of the blocking temperature with magnetometry reveal that temperature-dependent shifts in the resonance peaks can also be used to measure the blocking temperature, enabling the analysis of the correlation between the Néel and blocking temperatures in trilayers with the permalloy and antiferromagnetic layer separated by a 3-nm-thick spacer layer. The thickness-dependent characterization of thermal stability in antiferromagnets provides a key element for scalable and ultrafast antiferromagnetic spintronics.

Nonlinear, i.e., multiphoton microscopy is a powerful technique for imaging deep into biological tissues. Its penetration depth can be increased further using adaptive optics. In this work, we present a fast, feedback-based adaptive-optics algorithm, termed C-DASH, for multiphoton imaging through multiple-scattering media. C-DASH utilizes complex-valued light shaping (i.e., joint shaping of amplitude and phase), which offers several advantages over phase-only techniques: it converges faster, it delivers higher image quality enhancement, it shows a robust performance largely insensitive to the axial position of the correction plane, and it has a higher ability to cope with, on the one hand, scattering media that vary over time and, on the other hand, scattering media that are partially absorbing. Furthermore, our method is practically self-aligning. We also present a simple way to implement C-DASH using a single reflection off a phase-only spatial light modulator. We provide a thorough characterization of our method, presenting results of numerical simulations as well as two-photon excited fluorescence imaging experiments.

Chiral metasurfaces, driven by bound states in the continuum (BICs), provide an excellent platform for studying light-matter interactions. Currently, research on chirality is limited to enhancing the quality (Q) factor in low cyclic symmetry structures. Here we propose chiral metasurfaces consisting of a periodic arrangement of unit cells of skeleton fans with $C_6$ cyclic symmetry (SFCS) that are protected by the BIC topology. If the in-plane symmetry is broken, BIC polarization singularities (V points) with integer topological charges split into pairs of circular polarization points (C points) with half-integer topological charges. The C points simultaneously possess the high-Q and high circular dichroism (CD) responses, resulting from quasi-BICs based on toroidal dipole modes. In particular, the extrinsic and intrinsic chiralities based on BICs are achieved by breaking the out-of-plane and in-plane symmetries. The results also demonstrate good performance for high-Q chiral refractive-index sensing.

We present acoustic particle trapping in a spherical microchamber (SMC) as a generic in vitro cell testing tool for continuous perfusion experiments with temperature control. The established platform technology provides noninvasive three-dimensional positioning of beads and cells without wall contact. The spherical cavity allows efficient focusing and trapping of cells over any time range from seconds to several hours with spatiotemporal control for individual cells, few cells ($<10$), or small aggregates with more than $100$ cells. This article presents the numerical simulation of the induced acoustic pressure field for particle accumulation in a perfect sphere, a spheroidal chamber, and a spheroidal chamber with microfluidic channels. The representation results in three circular planes in the $x$, $y$, and $z$ directions. The resonance frequency splits from just one frequency in a perfect sphere into three frequencies for the spheroidal chamber and additionally with microfluidic channels. The characterization and verification of the platform technology provide experimental proof of splitting the resonance frequency in the SMC. The main resonance frequencies for trapping particles in the center of the SMC could be identified as $f_1=1.76\mathrm{MHz}$ for particle movement in $y$ and $z$ directions, as $f_2=1.80\mathrm{MHz}$ in $x$ and $z$ directions, and as $f_3=1.81\mathrm{MHz}$, where the particle movement is high in $x$ and $y$ directions. Hence, we received more resonance frequencies between them, which overlapped. Those resonance frequencies are integrated into a modulated frequency range for the trapping process in the SMC. Finally, we determined the minimum driven voltage for particle trapping starting from a particle diameter of 5 µm, and we validated our SMC by trapping human blood and lung cells to prove cell viability during the trapping as a biological example for future cell diagnostics.

The effective control of low-frequency noise remains a formidable challenge due to the inherently long wavelengths involved, which typically require large-scale physical barriers to achieve meaningful attenuation. Against this backdrop, metamaterials present a viable alternative, exploiting deep subwavelength structures that capitalize on thermal losses associated with local resonances to dampen sound. Here, we introduce a hybrid porous Helmholtz resonator (HPHR) that markedly improves acoustic absorption by amalgamating the Helmholtz resonator with specially designed porous necks to optimize acoustic impedance matching. It is promising as the basic unit of the metamaterial for low-frequency broadband acoustic absorption. Based on parallel HPHRs, we expand the effective absorption (>0.8) bandwidth using a one-dimensional convolutional neural network to engineer composite structures. The thickness of the sample is approximately 1/13th of the wavelength at the lowest target frequency. Our designs integrate resonant structures and porous materials, providing promising potential for creating broadband acoustic absorption systems and low-frequency noise control.

Quantum error correction enables the preservation of logical qubits with a lower logical error rate than the physical error rate, with performance depending on the decoding method. Traditional decoding approaches rely on the binarization (“hardening”) of readout data, thereby ignoring valuable information embedded in the analog (“soft”) readout signal. We present experimental results showcasing the advantages of incorporating soft information into the decoding process of a distance-3 ($d=3$) bit-flip surface code with flux-tunable transmons. We encode each of the 16 computational states that make up the logical state $|0_L⟩$, and protect them against bit-flip errors by performing repeated $Z$-basis stabilizer measurements. To infer the logical fidelity for the $|0_L⟩$ state, we average across the 16 computational states and employ two decoding strategies: minimum-weight perfect matching and a recurrent neural network. Our results show a reduction of up to 6.8% in the extracted logical error rate with the use of soft information. Decoding with soft information is widely applicable, independent of the physical qubit platform, and could allow for shorter readout durations, further minimizing logical error rates.

Nitrogen-vacancy ($\mathrm{N}$-$V$) centers in diamond constitute a solid-state nanosensing paradigm. Specifically for high-precision magnetometry, the so-called Ramsey interferometry is the prevalent choice where the sensing signal is extracted from time-resolved spin-state-dependent photoluminescence (PL) data. Its sensitivity is ultimately limited by the photon shot noise, which cannot be sufficiently removed by averaging or frequency filtering. Here, we propose Ramsey dc magnetometry of a single $\mathrm{N}$-$V$ center enhanced by a wavelet-denoising scheme specifically tailored to suppress photon shot noise. It simply operates as a classical postprocessing applied on a collected PL time series. Our implementation is based on a method that we named template margin thresholding, which enables not only frequency but also time-dependent denoising. Thanks to this multiresolutional approach, we can enhance the PL both in near-resonant and on-resonant frequency bands and hence surpass conventional filters in photon-shot-noise suppression. We computationally benchmark that its dc magnetic field sensing signal-to-noise-ratio improvement over the raw PL data is around an order of magnitude.

Photonic topological insulators have unique advantages regarding ensuring lossless transmission and realizing stable energy locality of electromagnetic waves. More complex requirements to manipulate discrete degrees of freedom (DOF) of photons, such as orbital DOF, show great foresight. In this paper, we design and demonstrate two kinds of topological metamaterials, A and B, with arbitrary auxiliary orbital DOF. By applying continuous generalized Kekulé modulations $\pm m_0{e}^{i\phi }$, the topological Chern numbers of the A and B configurations are inverse and the orbital DOF is controlled independently. This provides a completely new idea for the controllability and applicability of the photonic topology phase. We construct three kinds of topological platforms based on the more generic type of topological photonic insulators with arbitrary orbital DOF, including a perfect electrical conductor (PEC) boundary of a finite metamaterial structure, a domain wall interface consisting of two topological phases, and line-defect channels. The findings of our system provide a new pathway for exploring multifunctional topological electromagnetic metamaterials. It is expected that the present results can become an alternative means to manipulate topological phase transition and provide an underlying platform for the exploration of additional DOF.

The noise reduction in squeezed states makes them a powerful nonclassical resource in quantum technologies, especially in quantum sensing, and an essential step for the ongoing development of non-Gaussian states. In particular, optical and mechanical squeezing can both be generated when light interacts with a mechanical oscillator. Here we use a linear optical system as a controller and use coherent optical feedback to enhance squeezing in the unresolved-sideband regime of optomechanics. Feedback control is generally profitable for cooling, manipulating, and stabilizing the dynamics of mechanical oscillators; coherent feedback is then notable for its ability to steer a quantum system toward the highest squeezing without measurement, avoiding backaction. We thoroughly maximize optical and conditional mechanical squeezing feasible with state-of-the-art experimental setups and derive the optimal conditions under which quantum squeezing is generated in the system.

This study presents a detailed experimental investigation of conductivity and Hall effect measurements in copper iodide ($\mathrm{Cu}\mathrm{I}$), which is complemented by density-functional theory and Boltzmann-transport calculations. We have evaluated the temperature dependence of the transport data of solution-grown bulk single crystals, textured thin films grown by pulsed laser deposition, and additional data published in the literature. The density of compensating donorlike defects is determined from fits comprising ionized-impurity scattering. This is considered for determining acceptor densities and corresponding activation energies. In the course of this investigation, meticulous attention has been paid to the intricate interplay between light-hole and heavy-hole contributions, as well as the experimental values observed in zincblende $\mathrm{Cu}\mathrm{I}$. This study has yielded two distinct ionization energies for acceptors in unintentionally doped $\mathrm{Cu}\mathrm{I}$, with values of $E_{\mathrm{I},1}=95$ meV and $E_{\mathrm{I},2}=330$ meV, respectively.

The control of the spin state of electrons and holes in semiconductors is of fundamental importance for spintronics. Double quantum well (DQW) systems constitute excellent platforms that allow the modulation of the spin-relaxation times by changing the relative thicknesses of the wells to induce structural inversion asymmetry. This asymmetry, through the Rashba effect, modifies the spin-relaxation time of electron-spin pairs photogenerated in photoluminescence (PL) experiments. Besides the structural dependence, the modulation of the spin-relaxation time in a DQW can be controlled by external parameters, such as magnetic or electric fields. In the case of an external electric field, the relaxation time is modulated by the Rashba effect. Here, we report on PL experiments on ($\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{As}$/$\mathrm{Ga}\mathrm{As}$/($\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{As}$ DQWs under an external electric field applied perpendicular to the surface. We show that the spin-relaxation time of the photogenerated electron-hole pairs can be controlled by the electric field. The DQWs constitute excellent structures to tune the spin-relaxation times, and this opens the possibility of using such structures in optospintronic applications.