We use the thickness of Cu layers to control all-optical switching of magnetization in adjacent ${\mathrm{Gd}}_{24}$(${\mathrm{Fe}}_{90}{\mathrm{Co}}_{10}{)}_{76}$ films. While increasing the Cu thickness from $5$ to $900$ nm has no effect on the switching threshold, it significantly enlarges the fluence and pulse duration at which multiple domains emerge. Having shown that thermally activated multidomain formation limits the maximum fluence and pulse duration for controlled switching, we demonstrate that continuous magnetization reversal precedes multidomain formation in ${\mathrm{Gd}}_{18}{\mathrm{Dy}}_4{\mathrm{Co}}_{78}$ films excited with fluences slightly larger than the multidomain threshold.

The manifold of the fundamental domain of the Brillouin zone is always considered to be a torus. However, under the synthetic gauge field, the Brillouin manifold can be modified by the projective symmetries, resulting in unprecedented topological properties. Here, we realize a real-projective-plane hybrid-order topological insulator in a phononic crystal by introducing the $Z_2$ gauge field. Such an insulator hosts two momentum-space nonsymmorphic reflection symmetries, which change the Brillouin manifold from a torus to a real projective plane. These symmetries can simultaneously lead to a Klein-bottle insulator and quadrupole insulator phases in different bulk gaps. The nonsymmorphic reflection symmetries on Brillouin real-projective-plane, edge states of the Klein-bottle insulator, and corner states of the quadrupole insulator are observed. These results evidence the hybrid-order topology on the Brillouin manifold beyond the torus, and enrich the topological physics.

We present the results of Fourier-transform measurements for determination of refractive indices of four semiconductor alloys in the midinfrared spectral range from 10 to 300 K. We investigate double-sided-polished, undoped $\mathrm{In}\mathrm{P}$ and $\mathrm{Ga}\mathrm{As}$ substrates as well as ${\mathrm{In}}_{0.53}{\mathrm{Ga}}_{0.47}\mathrm{As}/\mathrm{In}\mathrm{P}$ and $\mathrm{Al}\mathrm{As}/\mathrm{Ga}\mathrm{As}$ distributed Bragg reflector (DBR) mirrors with their operational wavelength designed for 4 µm. Through analysis of measured transmittance spectra supported by ellipsometry it is possible to determine the temperature-dependent refractive indices of $\mathrm{In}\mathrm{P}$ and $\mathrm{Ga}\mathrm{As}$ from 1.3 to 11 µm. Furthermore, by using reflectivity measurements supported by numerical modeling of the two types of designed DBR structures the refractive indices of ${\mathrm{In}}_{0.53}{\mathrm{Ga}}_{0.47}\mathrm{As}$ and $\mathrm{Al}\mathrm{As}$ for around 4 µm are determined with their temperature dependencies. Data acquired through this method could lead to improvement of already existing optical semiconductor devices expanding their application for longer wavelengths.

For most comb-based applications, the signal-to-noise ratio (SNR) of the beat note between the comb and the continuous-wave laser is a critical parameter that affects the stability of the system and the accuracy of the measurement. We analyze the SNR model of the beat note and discuss some key aspects of implementing an optimized gated-detection scheme. High-speed balanced detection is introduced to enable detection of more interfering photons, while suppressing optical-intensity-noise interference. The gated detection is achieved by our combining a self-made low-noise, delay-adjustable pulse-generator circuit, which can suppress phase-random white noise. Compared with traditional direct-detection schemes, we demonstrate a 20-dB improvement in performance and achieve a beat-note SNR of more than 60 dB within a 300-kHz resolution bandwidth. Further experiments on frequency-comb locking show that this scheme has high reliability and robustness.

A key challenge for the realization of future skyrmion devices comprises the controlled creation, annihilation, and detection of these topologically nontrivial magnetic textures. In this study, we report an all-optical approach for writing, deleting, and reading skyrmions in the chiral magnet ${\mathrm{Fe}}_{0.75}{\mathrm{Co}}_{0.25}\mathrm{Si}$ based on thermal quenching. Using focused femtosecond laser pulses, patches of a thermally metastable skyrmion lattice state are created and annihilated locally, demonstrating unprecedented control of skyrmions in chiral magnets. The skyrmion state is read out by analyzing the microwave spin excitations in time-resolved magneto-optical Kerr effect measurements. Extracting the magnetic field and laser-fluence dependence, we find well-separated magnetic field regimes and different laser-fluence thresholds for the laser-induced creation and annihilation of skyrmions. The all-optical skyrmion control, as established in this study for a model system, represents a promising and energy-efficient approach for the realization of skyrmions as magnetic bits in future storage devices, reminiscent of magneto-optical storage devices in the past.

This paper proposes a light-wave polarization response model based on a kind of subwavelength metallic grating (SMG) by controlling the rearrangement of surface free electrons and net positive charges. The action of charging interdigital SMGs to form a capacitor-shaped architecture can be used to modulate the surface charge distribution, leading to a decrease in the transmission of the incident light waves with initial polarization orientations parallel to the long axis of the grating (parallel waves or P waves) and an increase in the transmission of the incident light waves with initial polarization orientations perpendicular to the long axis of the grating (vertical waves or S waves), thus contributing to the modulation of the polarization state of the SMGs. By using serrated-shaped sidewalls for each nanostrip in the SMGs, the surface net charges can be more efficiently pumped onto the nearest apex in a linear nanotip array to clearly enhance or suppress the penetration of P waves or S waves in a wide wavelength range of approximately 200 to 2500 nm. The developed graphene-covered SMG metasurface further demonstrates a typical feature: the transmittivity of the P wave is maximally increased by 10.24% and that of the S waves is maximally decreased by 21.84% when a low dc voltage is applied because of the controlled conductive action of the graphene film formed by sequentially stacking 30 layers of single-crystal graphene over the surface of the serrated SMGs. The above behavioral attributes provide an alternative way of realizing the adjustability and controllability of the polarization response characteristics of the incident light wave and performing the fast and fine acquisition of polarization information in polarization imaging.

We propose a scheme to generate nonreciprocal entanglement between an yttrium iron garnet (YIG) sphere and two nitrogen-vacancy-center ensembles by magnon Kerr effects in a transmission-line resonator. By driving the YIG sphere appropriately, strong Kerr nonlinearities emerge and then induce magnon parametric amplification and magnon frequency shift, resulting in the appearance of optimal entanglements. Depending on the direction of the bias magnetic field, macroscopic nonreciprocal entanglement is realized in this all-solid-state quantum system, which may find potential applications in chiral quantum information processing.

Variational quantum eigensolvers (VQEs) are one of the most important and effective applications of quantum computing, especially in the current noisy intermediate-scale quantum (NISQ) era. There are two main approaches for VQEs: problem-agnostic and problem-specific. For problem-agnostic methods, they often suffer from trainability issues. For problem-specific methods, their performance usually relies upon the choice of initial reference states, which are often hard to determine. In this paper, we propose an entanglement-variational hardware-efficient ansatz (EHA), and numerically compare it with some widely used ansatzes by solving benchmark problems in quantum many-body systems and quantum chemistry. Our EHA is problem-agnostic and hardware-efficient, is especially suitable for NISQ devices, and has potential for wide applications. Our EHA can achieve a higher level of accuracy in finding ground states and their energies in most cases, even compared with problem-specific methods. The performance of the EHA is robust to the choice of initial states and to parameter initialization, and it has the ability to quickly adjust the entanglement to the required amount, which is also the fundamental reason for its superiority.

Quantum technologies, holding the promise of exponentially superior performance in comparison to their classical counterparts for certain tasks, have consistently encountered challenges, including instability in quantum light sources, quantum decoherence, and vulnerability to losses that topological photonics happens to adeptly address. Here, we theoretically put forth a quantum Su-Schrieffer-Heeger-type chain designed to greatly enhance single-photon blockade (single PB) effect with topological protection. By designing the deliberate coupling strengths, the quantum level lattices take the form of a one-dimensional array with a topological edge state in single-excitation space and a two-dimensional square breathing lattice with topological corner states in two-excitation space, resulting in enhanced single-photon excitation and the suppression of two-photon transitions. Therefore, the second-order correlation function is diminished by up to 2 orders of magnitude at the cavity resonance frequency, accompanied by stronger brightness. Furthermore, the PB effect is robust to local perturbations in cavity-qubit coupling and qubit frequency, benefitting from topological protection.

In thermally activated delayed-fluorescence (TADF) materials, the fluorescence efficiency is enhanced through reverse intersystem crossing (RISC) from triplet to singlet excitons. The performance of organic light-emitting devices (OLEDs) based on TADF materials highly depends on the thermal conversion efficiency of the lowest triplet exciton $T_1$ into the lowest singlet exciton $S_1$, which relates to the energy difference $\mathrm{\Delta }{E}_{S_1-T_1}$. Here we investigate the RISC process in two TADF compounds with vastly different $\mathrm{\Delta }{E}_{S_1-T_1}$ using magneto-optical spectroscopies that include magnetophotoinduced absorption (MPA) in films and magnetoelectroluminescence (MEL) in OLEDs. The photoinduced absorption spectrum of the fast RISC material clearly shows that both singlet and triplet excitons coexist under steady-state conditions. Since the MPA response of the singlet and triplet excitons are similar and have the same polarity, we conclude that the magnetic field does not influence the RISC process in the studied compounds. We also find that the MEL response in OLEDs based on these compounds originates from the injected polaron pair species before they decay into $S_1$ and $T_1$ excitonic states in the emissive TADF layer.

This work proposes a graphical impedance-matching method for superconducting parametric amplifiers with quantum-limited noise performance using a modified Smith chart. It enables one to quickly apply the impedance-matching methods in microwave engineering to the design of parametric amplifiers. When the matching networks are composed of resonators and their equivalents, the matching of a parametric amplifier can be reduced to that of a negative-resistance amplifier, which is generally invalid due to the connection between the signal-frequency and idler-frequency networks introduced by the parametrically modulated component. Several examples are presented to demonstrate the effective and efficient matching of parametric amplifiers using this method, circumventing the need for complex calculations. This method also offers intuitive insight into how variations in pump strength and bias affect the amplification behavior of the circuit, which is useful in characterizing the parametric amplifier and finding the optimal operating state.

We introduce and demonstrate a hybrid optically pumped magnetometer (HOPM) that simultaneously measures one dc field component and one rf field component quadrature with a single atomic spin ensemble. The HOPM achieves sub-$\mathrm{pT}/\sqrt{\mathrm{Hz}}$ sensitivity for both dc and rf fields, and is limited in sensitivity by spin projection noise at low frequencies and by photon shot noise at high frequencies. We demonstrate with the HOPM an alternative application of multiparameter quantum sensing: background-canceling spread spectrum magnetic communication. We encode a digital message as rf amplitude, spread among 16 channels from 29 to 33 kHz in a noisy magnetic environment, and observe quantum-noise-limited rf magnetic signal recovery enabled by quantum-noise-limited dc noise cancelation, reaching noise rejection of 15 dB at 100 Hz and more than 20 dB at 60 Hz and below. We measure signal fidelity versus signal strength and extrinsic noise in communication of a short text message. The combination of high sensitivity, quantum-noise-limited performance, and real-world application potential makes the HOPM ideally suited for the study of high-performance multiparameter quantum sensing.

We show that weakly entangled states can improve communication over a qubit channel using only separate, interference-free, measurements of individual photons. We introduce a communication task corresponding to the cryptographic primitive known as secret sharing and show that all steerable two-qubit isotropic states provide a quantum advantage in the success rate using only product measurements. Furthermore, we show that such measurements can even reveal communication advantages from noisy partially entangled states that admit no quantum steering. We then go further and consider a stochastic variant of secret sharing based on more-sophisticated, yet standard, partial Bell-state analyzers, and show that this reveals advantages also for a range of unsteerable isotropic states. By preparing polarization qubits in unsteerable states, we experimentally demonstrate increased success rates of both secret-sharing tasks beyond the best entanglement-unassisted qubit protocol. Our results reveal the capability of simple and scalable measurements in entanglement-assisted quantum communication to overcome large amounts of noise.

We present a successful realization of four-dimensional semimetal bands featuring tensor monopoles, achieved using superconducting quantum circuits. Our experiment involves the creation of a highly tunable diamond energy diagram with four coupled transmons, and the parametric modulation of their tunable couplers, effectively mapping momentum space to parameter space. This approach enables us to establish a four-dimensional Dirac-like Hamiltonian with fourfold degenerate points. Moreover, we manipulate the energy of tensor monopoles by introducing an additional pump microwave field, generating effective magnetic and pseudoelectric fields and simulating topological parity magnetic effects emerging from the parity anomaly. Utilizing nonadiabatic response methods, we measure the fractional second Chern number for a Dirac valley with a varying mass term, signifying a nontrivial topological phase transition connected to a five-dimensional Yang monopole. Our work lays the foundation for further investigations into higher-dimensional topological states of matter and enriches our comprehension of topological phenomena.

Thin-film solar cells demonstrate high power conversion efficiencies, but are complex in their structure. To fully understand their functionality and to devise further efficiency improvements, a physical cell model or digital twin is required. As the thin film’s bulk and interface material parameters might change during subsequent device processing, this model needs to be inferred from characterization of the finished solar cell only. In this work, we derive one-dimensional optoelectronic models based on a variety of optoelectronic experimental results. As an example for thin-film devices, we take three ${\mathrm{Cu}(\mathrm{In},\mathrm{Ga})\mathrm{Se}}_2$ solar cells, two with high efficiency (19% and 20%) and one with moderate efficiency at around 15%. The development of the models uses the simulation tool Synopsys TCAD as the subprogram of a routine that fits the manifold optoelectronic experiments simultaneously. By the newly developed iterative one-dimensional simulation, we are able to fit the measured time-resolved photoluminescence at varied excitation and forward bias, the current-voltage curves in dark and under illumination, the (biased) external quantum efficiency, the voltage-dependent capacitance, and the drive-level capacitance of the solar cell. As an outcome, we obtain a digital model comprising the effective values for the doping densities in all layers, the charge carriers’ lifetimes and mobilities in the absorber, and the band alignment between the absorber and the buffer. Among these material parameters, the electron minority carrier lifetime appears to be of paramount importance. It exhibits values of 57 and 74 ns in the highly efficient cells and 2.5 ns in the moderately efficient cell. A hole mobility being substantially smaller compared to that of electrons is irremissible to explain the voltage dependence of the time-resolved photoluminescence of all investigated cells. Furthermore, a small positive conduction-band offset between 100 and 200 meV at the ${\mathrm{Cd}\mathrm{S}/\mathrm{Cu}(\mathrm{In},\mathrm{Ga})\mathrm{Se}}_2$ interface is the result of the model for all investigated cells. Finally, the digital twin of a highly efficient cell is used to assess the dominant recombination processes and to calculate a hypothetical device performance if these dominant processes could be inhibited by material optimization.

We demonstrate a closed-loop light-pulse atom-interferometer inertial sensor that can realize continuous decoupled measurements of acceleration and rotation rate. The sensor operates with double-loop atom interferometers, which share the same Raman light pulses in a spatially separated Mach-Zehnder configuration and use continuous cold atomic beams propagating in opposite directions from two ${\text{2D}}^{+}$ magneto-optical trappings. Acceleration and the rotation rate are decoupled and simultaneously measured by the sum and difference of dual-atom-interferometer signals, respectively. The sensitivities of inertial measurements are also increased to be approximately 1.86 times higher than that of a single atom interferometer. The acceleration phase shift is compensated in real time by phase locking these interferometers via the Raman laser phases from the sum-interferometer signal, and the gyroscope performance is improved. We achieve long-term stabilities of $6.1\text{μ}\text{g}$ and 840 nrad/s for the acceleration and the rotation rate, respectively, using a short interrogation time of 0.87 ms (interference area $A=0.097{\text{mm}}^2$). This work provides a building block for an atomic interferometer-based inertial measurement unit for use in field applications that require a high data rate and high stability.