Integrated quantum photonics is a promising strand of research that involves guiding light in submicron-scale structures and utilizing the quantum properties of the fundamental particle of light—the photon. One of these quantum properties, the indistinguishability of the photons, is used to achieve high-visibility quantum interference, which is one of the key resources in photonic quantum information processing. Here, we demonstrate that the faster and simpler heralded and unheralded second-order correlation functions ($g^{(2)}$) estimate the visibility (indistinguishability) well without the need to perform laborious and time-consuming quantum-interference measurements.

We present a protocol for directly detecting time-dependent magnetic field waveforms with a quantum two-level system. Our method is based on a differential refocusing of segments of the waveform using spin echoes. The sequence can be repeated to increase the sensitivity to small signals. The frequency bandwidth is intrinsically limited by the duration of the refocusing pulses. We demonstrate detection of arbitrary waveforms with approximately $20\mathrm{ns}$ time resolution and approximately $4\mu \mathrm{T}/\sqrt{\mathrm{Hz}}$ field sensitivity using the electronic spin of a single nitrogen-vacancy center in diamond.

We report on terahertz (THz) emission from a single-layer ferromagnet, which involves the generation of backflow nonthermal charge current from the ferromagnet/dielectric interface by femtosecond laser excitation and subsequent conversion of the charge current to a transverse transient charge current via the anomalous Hall effect, thereby generating THz radiation. The THz emission can be either enhanced or suppressed or the polarity can be reversed by introducing a magnetization gradient in the thickness direction of the ferromagnet. Unlike spintronic THz emitters reported previously, it does not require an additional nonmagnetic layer or Rashba interface.

By using supervised learning, we train a recurrent neural network to recognize and classify ultrafast magnetization processes that are realized in two-dimensional nanosystems with Dzyaloshinskii-Moriya interactions. Our focus is on different types of skyrmion dynamics driven by ultrafast magnetic pulses. Each process is represented as a sequence of sorted magnetization vectors that are inputted into the network. The trained network can perform an accurate classification of the skyrmionic processes at zero temperature over a wide range of magnetic pulse widths and damping factors. The network performance is also demonstrated on different types of unseen data, including finite-temperature processes. Our approach can be easily adapted for creating an autonomous control system on skyrmion dynamics for experiments or data-storage devices.

Quantum sensing exploits fundamental features of quantum mechanics and quantum control to realise sensing devices with potential applications in a broad range of scientific fields ranging from basic science to applied technology. The ultimate goal are devices that combine unprecedented sensitivity with excellent spatial resolution. Here, we propose an new platform for all-electric nanoscale quantum sensing based on a carbon nanotube double quantum dot. Our analysis demonstrates that the platform can achieve sensitivities that allow for the implementation of single-molecule magnetic resonance spectroscopy and therefore opens a promising route towards integrated on-chip quantum-sensing devices.

Twisted-light beams with orbital angular momentum provide an additional degree of freedom in controlling light-matter interactions, which are interesting for fundamental and applied research. Although there are various methods that can produce twisted laser beams at submicrometer or shorter wavelengths, it is still challenging to extend such beams to midinfrared (mid-IR) wavelengths with relativistic intensities. Here, we present a promising scheme to generate such pulses, which are converted through frequency downshift of intense driver optical pulses via a plasma-based photon decelerator. The resulting near-single-cycle vortex pulses cover a broad mid-IR spectral range up to 18 µm with an energy-conversion efficiency of 4.8% (energy approximately 150 mJ) in the wavelength range above 7 µm. These long-wavelength infrared pulses at the terawatt level can be focused to relativistically high intensities, which may offer significant opportunities for high-field physics and ultrafast applications.

Parasitic crosstalk in superconducting quantum devices is a leading limitation for quantum gates. We demonstrate the suppression of static ZZ crosstalk in a two-qubit, two-coupler superconducting circuit, where the frequency of a tunable coupler can be adjusted such that the ZZ interactions from each coupler destructively interfere. We verify the crosstalk elimination with simultaneous randomized benchmarking, and use a parametrically activated iswap interaction to achieve a Bell-state preparation fidelity of 98.5% and a $\sqrt{\mathrm{i}\mathrm{swap}}$-gate fidelity of 94.8% obtained via quantum-process tomography.

Being nonlinear dynamic systems, magnetic read sensors should respond to an excitation signal of a frequency considerably different from their natural ferromagnetic resonance (FMR) frequencies. Because of the magnetization dynamics’ inherent nonlinear nature, the sensors’ response should be measured at the dc, excitation frequency, and its multiples (harmonics). In this paper, we present results of such measurements, accomplished using a one-port nonlinear vector network analyzer (NVNA), which show distinct resonances at fractional frequencies of the free layer (FL) FMR mode. Identification of these resonances, resulting from the nonlinear nature of the spin-torque-(ST-)induced magnetization dynamics, is performed using micromagnetic modeling. In particular, we show that the measured dc response at the above-mentioned fractional frequencies can be explained by a low-order nonlinearity and strong magnetodipolar feedback between magnetic layers adjacent to a $\mathrm{Mg}\mathrm{O}$ barrier. Additionally, we determine that the simulated harmonic response is strongly enhanced by the mutual ST effect between these layers. Finally, we demonstrate that the read sensors’ nonlinear magnetization dynamics and, by extension, their harmonic response are highly sensitive to various magnetic and ST parameters. Thus, this study shows that using NVNA measurements in conjunction with micromagnetic modeling can clarify the uncertainty in the definition of these parameters.

An on-chip light source is one of the essential components for integrated photonic circuits and quantum-information processing chips. To date, it has been a great challenge to construct an on-chip light source with high directionality, high collection efficiency, and ultrasmall feature size simultaneously in the optical communication range. Here, an on-chip light source, consisting of a circular paraboloid etched in an aluminum-doped zinc-oxide film, which is an epsilon-near-zero material, is experimentally realized at 1550 nm. Photons emitted from the a $\mathrm{PbS}$ quantum-dot cluster placed near the focal point of this paraboloid are reflected into a directional parallel beam due to the large impedance mismatch between aluminum-doped zinc oxide and ambient medium. An ultrasmall emission divergence angle of only $\pm 3^{\circ }$, and an ultrahigh directional emission contrast ratio of 44 dB are achieved from the ultracompact device with a feature size of only $1.5\mu \mathrm{m}$. Also, an ultrahigh collection efficiency of up to 92% is predicted by simulation. This work not only opens an alternative way for the realization of integrated photonic devices based on epsilon-near-zero materials, but also provides another method for the precise assembling of composite functional nanostructures.

Bioelectronic devices residing within the human body often rely on wireless signals to interact with the external world. The transmission of wireless signals from the body, however, is generally inefficient and energy demanding, in large part due to reflection incurred at the interface between biological tissue and surrounding space. Here, we show that tailored diffractive patterns placed on the surface of the body can enhance wireless transmission by nearly an order of magnitude via the conversion of evanescent waves generated by total internal reflection into propagating waves that transmit into the far field. We develop a theoretical model of this effect at planar interfaces and verify the transmission enhancement numerically and experimentally for nonplanar body surfaces. We then demonstrate clinical utility by increasing the strength of Bluetooth signals received from a wireless device implanted in a large animal model. This enhanced wireless access paves the way for more functional and longer-lasting bioelectronic devices.

Generation and control of quantum states of light on an integrated platform have become essential tools for scalable quantum technologies. Chip-scale sources such as nonlinear optical microcavities have been demonstrated to efficiently generate entangled biphoton states. However, these systems have little control over the continuous-variable time-energy entanglement of the photons. We demonstrate such control by preparing biphoton states with asymmetric temporal wave functions. This is done by selectively modifying the density of states of the cavity modes participating in the interaction using Rayleigh-scattering-induced strong coupling of optical modes of a resonator. These states reveal exotic coherence properties and show a path forward for continuous-variable quantum-state engineering on a chip.

A method for the effective excitation of Highly Confined Surface Plasmon Polaritons (HC SPPs) that can generate x-rays is presented based on the electron beam excitation of hot carriers and the graphene hot carrier quantum Cherenkov effect. Compared with the widely used methods of HC SPPs’ excitation, in this method, no periodic structure is needed, so it is possible to achieve a miniaturized system; it can also excite monochromatic HC SPPs with a guide wavelength as small as 1/300 of the vacuum wavelength. The working point of HC SPPs is not sensitive to the velocity of the electron beam and can be dynamically tuned through the Fermi energy of graphene. At the same time, the working efficiency can be significantly improved (to about 37.28%). Based on the above-mentioned characteristics, when the HC SPPs and the electron beam in this method are used in the electron-plasmon scattering system, x-ray emission can be achieved with rather low electron beam energy (35 KeV). Since the electron beam can simultaneously act as the SPPs’ source and the scatterer, it is possible to integrate an x-ray source and the HC SPPs’ emitter that drives the x-ray source in the same cavity, realizing a unified system.

Preconcentration of microparticles or biological entities to a measurable level is required in many disease detection applications. Motivated by this utilitarian scope, we demonstrate a controlled maneuvering of polystyrene particles on a simple “paper-and-pencil”-based device by exploiting the underlying electrokinetic phenomena with primary contribution from dielectrophoretic (DEP) forces. In sharp contrast to other reported DEP devices, the present configuration does not demand any sophisticated laboratory module for creating a nonuniform electric field, which is an essential requirement in DEP settings. We explore negative dielectrophoresis to trap a 1-µm size polystyrene particle in a low-conductivity suspending medium, at an applied field strength of approximately 10 V/cm. The flexibility of the device design offers controlled and yet localized and selective trapping as per requirements. Furthermore, switching of the trapping direction (negative to positive dielectrophoresis) can be simply achieved by manipulating the conductivity of the medium. Our study brings out an optimum range of the medium's pH for efficient trapping. Considering the potential utilization of this technique, we demonstrate bacterial aggregation, which will be helpful in designing alternative point-of-care diagnostic devices (particularly for sample preconcentration) within resource-limited settings.

Tuning magnetic anisotropy (MA) by electric field is crucially important for the development of low-power-consumption magnetoelectric devices. In general, the MA is determined by magnetocrystalline anisotropy, magnetoelastic coupling, or exchange bias. We present an alternative approach that tunes the MA of the (110)-orientated ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{Mn}\mathrm{O}}_3$ (LSMO) film by modulating the phase structure of its oxide cap layer. The LSMO layer exhibits a $[1\overline{1}0]$ easy direction when it is capped by a ${\mathrm{Sr}\mathrm{Co}\mathrm{O}}_{2.5}$ layer. Driving the cap layer to transit between the ${\mathrm{Sr}\mathrm{Co}\mathrm{O}}_{2.5}$ and ${\mathrm{Sr}\mathrm{Co}\mathrm{O}}_{3-\delta }$ phases via ionic-liquid gating, a bidirectional tuning between the [001] and $[1\overline{1}0]$ directions is achieved for the MA of the LSMO layer. More importantly, the electric tuning of the MA is very reversible. Based on x-ray linear dichroism analysis, we show that the phase transition of ${\mathrm{Sr}\mathrm{Co}\mathrm{O}}_x$ causes a variation of the electron population of the $\mathrm{Mn}$ 3d orbitals of LSMO, resulting in the MA switching of the MA of LSMO. The preferentially occupied orbital is $d_{x^2-y^2}$ in ${\mathrm{Sr}\mathrm{Co}\mathrm{O}}_{2.5}/\mathrm{LSMO}$ and $d_{3z^2-r^2}$ in ${\mathrm{Sr}\mathrm{Co}\mathrm{O}}_{3-\delta }$/LSMO. This work demonstrates a unique approach to reversibly tune the MA of LSMO, paving the way toward electrically tunable magnetoelectric devices based on correlated oxides.

The ubiquitous presence of $1/f$ flux noise was a significant barrier to long coherence times in superconducting qubits until the development of qubits that could operate in static, flux-noise-insensitive configurations commonly referred to as “sweet spots.” Several proposals for entangling gates in superconducting qubits tune the flux bias away from these spots, thus reintroducing the dephasing problem to various degrees. Here we revisit one such proposal, where interactions are parametrically activated by rapidly modulating the flux bias of the qubits around these sweet spots, and study the effect of modulation on the sensitivity to flux noise. We explicitly calculate how dephasing rates depend on different components of the flux-noise spectrum, and show that, although the qubits are parked at flux-insensitive points, the modulation results in increased dephasing rate due to the multiplicative $1/f$ noise and white noise components. Remarkably, we find a sweet spot under flux modulation, which we dub the “ac sweet spot,” that is insensitive to $1/f$ flux noise. We show that simple filtering of the flux control signal additionally protects parametric entangling gates from white noise in the control electronics at this ac sweet spot, allowing interactions of quality that is limited only by higher-order effects and other sources of noise.

We explore the implementation of a spoof–fluid-spoof (SFS) acoustic waveguide, which is the acoustic analogue of a metal-insulator–metal waveguide in plasmonics. The proposed structure consists of two corrugated rigid surfaces separated by a fluid layer. The properties of the guided acoustic modes between corrugated rigid parallel plates are analytically and numerically studied by deriving their dispersion relation and through a careful analysis of symmetric and antisymmetric modes supported by this structure. Similar to plasmonic waveguides, it is shown that the dispersion relation of the proposed SFS acoustic waveguide, and the supported group and phase velocities, can be largely controlled by changes in the geometrical parameters of the waveguide, opening the possibility for application of this technology for acoustic delay lines, modulators, and sensors. The analytical and numerical results are validated through experiments in a controlled environment.

Information reconciliation is crucial for continuous-variable quantum key distribution (CV QKD) because its performance affects the secret key rate and maximal secure transmission distance. Fixed-rate error-correction codes limit the potential applications of the CV QKD because of the difficulty of optimizing such codes for different low SNRs. In this Paper, we propose a rateless reconciliation protocol combined multidimensional scheme with Raptor codes that not only maintains the rateless property but also achieves high efficiency in different SNRs using just one degree distribution. It significantly decreases the complexity of optimization and increases the robustness of the system. Using this protocol, the CV QKD system can operate with the optimal modulation variance, which maximizes the secret key rate. Simulation results show that the proposed protocol can achieve reconciliation efficiency of more than 95% within the range of SNR from $-20$ to 0 dB. It also shows that we can obtain a high secret key rate at arbitrary distances in a certain range and achieve a secret key rate of about $5\times {10}^{-4}$ bits/pulse at a maximum distance of 132 km (corresponding SNR is $-20$ dB) that is higher than previous works. The proposed protocol can maintain highly efficient key extraction under the wide range of SNRs and paves the way toward the practical application of CV QKD systems in flexible scenarios.

We report on the design, fabrication, and experimental characterization of high-$Q$ acoustic metasurfaces, which enable emission enhancement of sound plane-wave sources and the creation of collimated acoustic laser beams. A rigorous transfer-matrix theory is proposed to describe such phenomena. It is found that for optimal structural parameters, the emission-enhancement factor can reach a maximum of 43. Such emission enhancement almost does not increase the electrical impedance and thus input electrical power for the sound source. Experimental results, theoretical analysis, and numerical simulations agree well with each other. Our work suggests an alternative method to create high-intensity collimated sound beams, and may find applications in sound manipulation and nonlinear acoustics.

We report a comprehensive study of the maximization of the spin Hall ratio (${\theta }_{\mathrm{SH}}$) in $\mathrm{Pt}$ thin films by the insertion of submonolayer layers of $\mathrm{Ti}$ to decrease carrier lifetime while minimizing the concurrent reduction in the spin Hall conductivity (${\sigma }_{\mathrm{SH}}$). We establish that the intrinsic ${\sigma }_{\mathrm{SH}}$ and carrier lifetime sets a practical upper bound of ${\theta }_{\mathrm{SH}}$ of $\mathrm{Pt}$, while robust against the strain and the moderate interruption of crystal order caused by these insertions, begins to decrease rapidly at high resistivity level because of the shortening carrier lifetime. The unavoidable trade-off between the intrinsic ${\sigma }_{\mathrm{SH}}$ and carrier lifetime sets a practical upper bound of ${\theta }_{\mathrm{SH}}$ ≥ 0.8 for heterogeneous materials where the crystalline $\mathrm{Pt}$ component is the source of the spin Hall effect and the resistivity is increased by shortening the carrier lifetime. This work also establishes a very promising spin Hall metal of [$\mathrm{Pt}$ 0.75 nm/$\mathrm{Ti}$ 0.2 nm]${}_7$/$\mathrm{Pt}$ 0.75 nm for energy-efficient, high-endurance spin-orbit-torque technologies (e.g., memories, oscillators, and logic) due to its combination of a giant ${\theta }_{\mathrm{SH}}$ ≈ 0.8, or equivalently, a dampinglike spin-torque efficiency per unit current density ${\xi }_{\mathrm{DL}}^j\approx 0.35$, with a relatively low resistivity (90 µΩ cm) and high suitability for practical technology integration.