Magnetic exchange springs (ESs) are composed of exchange-coupled hard and soft magnetic layers, i.e., layers with high and low anisotropy, respectively. The moments in the soft layer can be wound up by applying an external field, which has to be smaller than the anisotropy field of the hard layer. Alternatively, an ES can be realized by biasing the soft magnetic layer by two adjacent hard magnetic layers with different magnetic anisotropy directions. We have fabricated an ES layer stack by magnetron sputter deposition. As the hard magnetic bottom layer, we used epitaxial $\mathrm{Fe}\mathrm{Pt}$ L1${}_0$, and as the top layer $\mathrm{Co}$ with both layers having different in-plane easy axes. These hard layers pin the moments of a soft permalloy (${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$) layer sandwiched between them, winding up an ES at remanence. The anisotropy of the polycrystalline top $\mathrm{Co}$ layer was engineered by glancing-angle deposition to have in-plane easy axis anisotropy perpendicular to the easy direction of the bottom layer. Using soft x-ray spectroscopy and magneto-optical measurements, we found the in-plane ES to extend from the soft layer into the top layer of our $\mathrm{Fe}\mathrm{Pt}$/permalloy/$\mathrm{Co}$ trilayer structure.

The properties of spin-wave quanta, referred to as magnons, make them a promising signal carrier for the transmission and processing of information in magnetic materials. In this field, called magnonics, dynamic magnonic crystals attract special attention for applications such as complex real-time spectral transformations, due to on-demand controllability of transmission gaps. Here, we report on the experimental realization of the dynamic control of spin-wave band structures in artificial crystals using the metal-insulator transition in the vanadium dioxide (${\mathrm{VO}}_2$) grating. The obtained results are confirmed by a theoretical approach predicting a dramatic variation of the reflection coefficient from the ${\mathrm{VO}}_2$ stripes upon the phase transition. The proposed structure combines the excellent versatility, fast switching speed, low power consumption, and external control it affords.

The realization of superconducting single-photon detectors operating above the liquid-helium temperature is currently the focus of intense research efforts. Here, we present the fabrication of ultraclean encapsulated nanowires from commercially available thin films of $\mathrm{YBa}$${}_2$$\mathrm{Cu}$${}_3$$\mathrm{O}$${}_{7-\delta }$ by high-energy oxygen-ion irradiation. The structured nanowires, ranging from 0.1 to $5\text{μ}\mathrm{m}$ in width, exhibit sharp resistive transitions to the superconducting state above 85 K. The $I$-$V$ characteristics reveal that the evolution from the superconducting to the normal state shows a large voltage jump in the volt range. This demonstrates that the entire nanowire transits—a prerequisite for the development of a hot spot upon absorption of a single photon. Our results pave the way for the fabrication of $\mathrm{YBa}$${}_2$$\mathrm{Cu}$${}_3$$\mathrm{O}$${}_{7-\delta }$-based scalable superconducting single-photon-detection devices.

The change in the arrangement of magnetic dipole moments in a magnetic metasurface, due to the influence of an external static magnetic field, is discussed. Each meta-atom of the metasurface is composed of three identical disk-shaped resonators (trimer) made of magnetically saturated ferrite. To provide physical insight, full-wave numerical simulations of the near fields and transmission characteristics of the metasurface are complemented by the theoretical description based on the multipole decomposition method. With these methods, the study of eigenmodes and scattering conditions of a single magnetic resonator, trimer, and their array forming the metasurface is performed. It is found that the magnetic dipole-based collective hybrid mode of the trimer can be gradually transformed from the radial (pseudomonopole) to azimuthal (toroidal) order and vice versa by varying the bias magnetic field strength. This is because the magnetic dipole moment of each individual disk constituting the trimer undergoes rotation as the bias magnetic field strength changes. This transition between two orders is accompanied by various patterns of localization of the electric field inside the meta-atoms. Due to the unique field configuration of these modes, the proposed metasurface can be considered for designing magnetic field sensors and nonreciprocal devices.

Recent advances in laser technology demand more-effective measurement of ultrashort optical pulses in fiber optics and nanophotonics devices. Here we propose an in-fiber strategy to characterize ultrashort pulses by incorporating a gallium selenide–assisted microfiber, as a frequency-doubling block, in an all-fiber interferometer. Pulses to be retrieved were first split into the two arms of the interferometer and then overlapped, exciting the interferometric autocorrelated second harmonic in the microfiber due to high second-order nonlinearity of gallium selenide. In the experiment, the autocorrelator shows high accuracy in measuring 7.13-ps, 1239-fs, and 683-fs pulses. In comparison with a commercial frequency-resolved optical gating, the relative errors are only 1.53%, 3.74%, and 6.86%, respectively. Moreover, by attenuation of the incident laser of one arm of the interferometer, the measured peak-to-background ratio was less than the conventional theoretical value. The proposed autocorrelator could be very compatible with current systems for future applications of in-fiber or on-chip ultrafast optical technology.

Resistive switching in memristors is being amply investigated for different applications in nonvolatile memory, neuromorphic computing, and programmable logic devices. Memristors are conducting devices in which the conductance depends on one or more slow internal state variables, and they exhibit strongly nonlinear properties and intense memory effects. Here, we address the characterization of current-controlled memristors by small-perturbation frequency-resolved impedance techniques. We show that the equivalent circuit obtained at different stationary points provides essential information about the dynamic behavior in voltage cycling and transient response to a square perturbation. The general method enables the analysis of stability and hysteresis in current-voltage curves. The current-controlled memristor very naturally produces a negative capacitance effect, and we review several devices reported in the literature, including discharge tubes and metal-oxide memristors, to expose the deep connections between the sign of the capacitance and the type of hysteresis.

Niobium nitride ($\mathrm{Nb}\mathrm{N}$) is a particularly promising material for quantum technology applications, as it shows the degree of reproducibility necessary for large-scale superconducting circuits. We demonstrate that resonators based on $\mathrm{Nb}\mathrm{N}$ thin films present a one-photon internal quality factor above ${10}^5$ maintaining a high impedance (larger than $2\mathrm{k}\mathrm{\Omega }$), with a footprint of approximately $50\times 100\text{μ}{\mathrm{m}}^2$ and a Kerr nonlinearity of few tenths of a hertz. These quality factors, mostly limited by losses induced by the coupling to two-level systems, have been maintained for kinetic inductances ranging from tenths to hundreds of picoHenry per square. We also demonstrate minimal variations in the performance of the resonators during multiple cooldowns over more than nine months. Our work proves the versatility of niobium nitride high-kinetic-inductance resonators, opening perspectives towards the fabrication of compact, high-impedance, and high-quality multimode circuits, with sizable interactions.

Quantum sensing has performance advantages that far exceed classical sensing, where sensing with photons is one of the most useful branches, and fiberization is a significant development approach to achieve a broader range of applications. However, the birefringence effect in optical fibers can cause polarization drift, which severely reduces the sensitivity of measurement on weak signals and influences the development of quantum sensing. In order to solve this problem, we propose a scheme that builds four time-division-multiplexing channels in the system by switching the polarization states of different optical pulses, and achieves the effect of suppressing polarization drift by analyzing and integrating the effective information of the four polarization channels. Moreover, the sensitivity and linear dynamic range of the system can be adjusted by inserting a reference phase. We demonstrate the feasibility of the system via theoretical analysis, and the numerical simulation results show that the scheme can demodulate the signal well and improve the signal-to-noise ratio by 30 dB, even in the case of the signal of the direct demodulation method is drowned out by noise and completely distorted.

Non-Hermiticity has emerged as a promising tool to induce topological phase transitions, allowing for the design of tunable and functional topological devices such as topological lasers and switches on on-chip integrated optical platforms. In this work, we experimentally demonstrate loss-modulated photonic topological edge states in anti-parity-time-symmetric waveguide arrays. Loss is introduced via carefully designed subwavelength grating (SWG) waveguides, allowing for on-chip control of topological responses. Our results show that loss not only induces topological phase transition but also enhances topological responses, leading to the increased localization of topological edge states and the anomaly reduced intrinsic loss of the topological edge states. Our work provides a platform based on SWG waveguides for exploring non-Hermiticity-induced photonic topological responses and corresponding applications.

We apply inverse-design methods to produce two-dimensional triangular-lattice plasma-metamaterial (PMM) devices, which are then constructed and demonstrated experimentally. Finite-difference frequency-domain simulations are used along with forward-mode automatic differentiation to optimize the plasma densities of each of the plasma elements in the PMM to perform beam steering and demultiplexing under transverse magnetic polarization. The optimal device parameters are then used to assign plasma-density values to elements that make up an experimental version of the device. Device performance is evaluated against both the simulated results and human-designed alternatives, showing the benefits and disadvantages of in silico inverse design and paving the way for future fully in situ optimization.

Remote implementation of quantum operation between two distant nodes is crucial for many quantum applications. Here we consider the implementation of an arbitrary single-photon operation upon a remote quantum system exploiting photon pairs simultaneously entangled in the polarization and temporal degrees of freedom, which is a fascinating resource in long-distance quantum communication with its outstanding merits. To improve the level of security for the above protocol, we also consider a controlled implementation of quantum operation scenario by adding a supervisor, which is necessary for distributed quantum computation in a quantum network. Both of the protocols can be fulfilled by bidirectional state teleportation with the aid of the effective cross-Kerr interaction. Furthermore, the simple construction circuits together with available existing optical elements and common single-photon detectors facilitate the realization of our protocols with current sufficient technologies.

Bound states in the continuum (BICs) in open cavities have attracted considerable attention in wave physics due to their ability to confine light and produce high-quality-factor resonances with promising applications for filtering and sensing. One of the most interesting types of BICs is Friedrich-Wintgen (FW) BICs, which result from destructive interference of two interacting modes belonging to the same radiation channel. Here, we investigate theoretically and experimentally FW BICs in a photonic and plasmonic T-shaped cavity made of two horizontal guides of lengths $d_2$ and $d_3$ coupled to a vertical stub of length $d_1$. We demonstrate that the necessary condition for obtaining BICs consists in taking the lengths of the two horizontal guides $d_2$ and $d_3$ commensurate. This BIC is a common mode of the guides of lengths $d_2$ and $d_3$, such as the electric field vanishes at their connection point with the stub of length $d_1$; this BIC is independent of $d_1$ and the infinite waveguide to which the whole cavity will be attached. We show that, depending on $d_1$, the FW BIC appears as the consequence of the interaction between two eigenmodes of the originally isolated cavity where the width of one mode vanishes giving rise to FW BIC, while the width of the second mode becomes broad. In addition, we show that by slightly deviating from the BIC condition, the latter transforms to either electromagnetically induced transparency (EIT) or reflection or Autler-Townes splitting (ATS) resonances. Both EIT and ATS effects are qualified as a transparency window between two transmission zeros, but with different physical origins. We exploit the Akaike’s information criterion test to discern EIT from ATS and distinguish the regime where the EIT or ATS effect dominates. The theoretical results, obtained by means of the Green’s function method, are validated both by experimental measurements using coaxial cables in the radiofrequency domain and numerical simulations using metal-insulator-metal plasmonic waveguides operating in the infrared domain. The sensitivity of the PIT (the plasmonic analogue of EIT) resonances to the dielectric inside the waveguides can be used to design a highly sensitive sensor, which makes it suitable for an on-chip optical sensing platform.

We show how to generate Bell states and $N$-partite $W$ states of long-distance superconducting (SC) qubits in a SC waveguide QED system, where SC qubits are coupled to an open microwave transmission line. In the two-qubit case, the Bell state of two long-distance qubits can be a dark state of the system by choosing appropriate system parameters. If one proper microwave pulse drives one of two qubits, the two qubits will evolve from their ground states to a Bell state. Further, we extend this scheme to the multiqubit case. We show that $W$ states of $N$ long-distance qubits can also be generated. Because both the Bell and $W$ states are decoupled from the waveguide (i.e., dark states of the system), they are steady and have very long lifetimes in the ideal case without decoherence of qubits. In contrast to the ideal case, the presence of decoherence of qubits limits the lifetimes of the Bell and $W$ states. Our study provides an alternative scheme for generating Bell states and $N$-partite $W$ states in SC waveguide QED, which can be used to entangle long-distance nodes in waveguide quantum networks.

Quantum key distribution (QKD) based on the fundamental laws of quantum physics can allow the distribution of secure keys between distant users. However, imperfections in realistic devices may lead to potential security risks, which must be accurately characterized and considered in practical security analysis. High-speed optical modulators, being one of the core components of practical QKD systems, can be used to prepare the required quantum states. Here, we find that optical modulators based on $\mathrm{Li}\mathrm{Nb}\mathrm{O}$${}_3$, including phase modulators and intensity modulators, are vulnerable to photorefractive effects caused by external light injection. By changing the power of external light, eavesdroppers can control the intensities of the prepared states, posing a potential threat to the security of QKD. We have experimentally demonstrated the influence of light injection on $\mathrm{Li}\mathrm{Nb}\mathrm{O}$${}_3$-based optical modulators and analyzed the security risks caused by the potential light-injection attack, along with the corresponding countermeasures.

We describe the generation of entangling gates on superconductor-semiconductor hybrid qubits by ac voltage modulation of the Josephson energy. Our numerical simulations demonstrate that the unitary error can be below ${10}^{-5}$ in a variety of 75-ns-long two-qubit gates (CZ, $i\mathrm{SWAP}$, and $\sqrt{i\mathrm{SWAP}}$) implemented using parametric resonance. We analyze the conditional $ZZ$ phase and demonstrate that the CZ gate needs no further phase-correction steps, while the $ZZ$ phase error in swap-type gates can be compensated by choosing pulse parameters. With decoherence considered, we estimate that qubit relaxation time needs to exceed $70\text{μ}\mathrm{s}$ to achieve the 99.9% fidelity threshold.

Quantum digital signatures (QDSs), generating correlated bit strings among three remote parties for signatures through quantum law, can guarantee nonrepudiation, authenticity, and integrity of messages. Recently, a one-time universal-hashing QDS framework, exploiting the quantum asymmetric encryption and universal hash functions, has been proposed to significantly improve the signature rate and ensure unconditional security by directly signing the hash value of long messages. However, similar to quantum key distribution, this framework utilizes keys with perfect secrecy by performing privacy amplification that introduces cumbersome matrix operations, thereby consuming large computational resources, causing delays, and increasing failure probability. Here, we prove that, different from private communication, imperfect quantum keys with partial information leakage can be used for digital signatures and authentication without compromising the security while having eight orders of magnitude improvement on signature rate for signing a megabit message compared with conventional single-bit schemes. This study significantly reduces the delay for data postprocessing and is compatible with any quantum key generation protocols. In our simulation, taking two-photon twin-field key generation protocol as an example, QDS can be practically implemented over a fiber distance of 650 km between the signer and receiver. For the first time, this study offers a cryptographic application of quantum keys with imperfect secrecy and paves a way for the practical and agile implementation of digital signatures in a future quantum network.

Recently, three-dimensional (3D) metamaterials have undergone a revolution driven mainly by the popularization of 3D-printing techniques, which has enabled the implementation of modern microwave and photonic devices with advanced functionalities. However, the analysis of 3D metamaterials is complex and computationally costly in comparison with the analysis of their one-dimensional and two-dimensional counterparts due to the intricate geometries involved. In this paper, we present a fully analytical framework based on Floquet-Bloch modal expansions of the electromagnetic fields and integral-equation methods for the analysis of 3D metamaterials and metagratings. Concretely, we focus on 3D configurations formed by periodic arrangements of rectangular waveguides with longitudinal slot insertions. The analytical framework is computationally efficient compared with full-wave solutions and also works under oblique-incidence conditions. Furthermore, it comes associated with an equivalent circuit that allows one to gain physical insight into the scattering and diffraction phenomena. The analytical equivalent circuit is tested against full-wave simulations in cst. Simulation results show that the proposed 3D structures provide independent polarization control of the two orthogonal polarization states. This key property is of potential interest for the production of full-metal polarizers, such as the one illustrated here.

Tuning $\mathrm{Ga}\mathrm{As}$-based quantum emitters to telecom wavelengths makes it possible to use existing mature technology for applications in, e.g., long-haul ultrasecure communication in fiber networks. A promising method redeveloped recently is to use a metamorphic $(\mathrm{In},\mathrm{Ga})\mathrm{As}$ buffer that redshifts the emission by reducing strain. However, the impact of such a buffer also causes the simultaneous modification of other quantum dot (QD) properties. Knowledge of these effects is crucial for actual implementations of QD-based nonclassical light sources for quantum communication schemes. Here, we thoroughly study single $\mathrm{Ga}\mathrm{As}$-based quantum dots grown by molecular-beam epitaxy on specially designed, digital-alloy $(\mathrm{In},\mathrm{Ga})\mathrm{As}$ metamorphic buffers. With a set of structures varying in the buffer indium content and providing quantum dot emission through the telecom spectral range up to 1.6 µm, we analyze the impact of the buffer and its composition on QD structural and optical properties. We identify the mechanisms of quantum dot emission shifts with varying buffer compositions. We also look at charge-trapping processes and compare excitonic properties for different growth conditions with single-dot emission successfully shifted to both the second and third telecom windows.

Topological insulators that exhibit localized states at the edges or interference regions have been extensively investigated. Among these, waterborne acoustic topological interface states have significant potential in applications such as particle manipulation. However, the large impedance of water can no longer be ignored and sets difficulty in the construction of the waveguide in observing the topological states. In this paper, we propose an effective strategy to circumvent this issue and achieve the direct observation of the topological interface state in a waterborne acoustic waveguide as well as the associated nonzero Zak phases. Moreover, the robustness of the interface state against disorder is validated, suggesting a viable method for studying underwater topological acoustic fields.