Lack of nonreciprocity—in particular, nonreciprocity of phase accumulation—is one of the major drawbacks of microwave solid-state acoustic devices, which has prevented the development of acoustic isolators and circulators. Here we report the observation of the phase nonreciprocity of hybridized surface acoustic waves (SAWs) and spin waves in a magnetoelastic heterostructure. Our system consists of a $\text{Fe-Ga-B}/{\mathrm{Al}}_2{\mathrm{O}}_3/\text{Fe-Ga-B}$ multilayer on top of a ${\mathrm{LiNbO}}_3$ crystal. Maximum values of the observed nonreciprocal phase accumulation easily exceed π radians over a broad range of field conditions, which is necessary for the development of an effective circulator. In addition, under the application of bias magnetic field, the structure demonstrates tunable giant nonreciprocity of propagation losses with isolation as high as 48 dB, necessary for the development of isolators. Theoretical calculations provide an insight into the observed phenomena and demonstrate a pathway for further improvement of nonreciprocal SAW devices based on magnetoelastic coupling.

Microwave technology plays a central role in current wireless communications, including mobile communication and local area networks. The microwave range shows relevant advantages with respect to other frequencies in open-air transmission, such as low absorption losses and low-energy consumption, and in addition, it is the natural working frequency in superconducting quantum technologies. Entanglement distribution between separate parties is at the core of secure quantum communications. Therefore, understanding its limitations in realistic open-air settings, especially in the rather unexplored microwave regime, is crucial for transforming microwave quantum communications into a mainstream technology. Here, we investigate the feasibility of an open-air entanglement distribution scheme with microwave two-mode squeezed states. First, we study the reach of direct entanglement transmission in open air, obtaining a maximum distance of approximately 500 m with parameters feasible for state-of-the-art experiments. Subsequently, we adapt entanglement distillation and entanglement swapping protocols to microwave technology in order to reduce the environment-induced entanglement degradation. The employed entanglement distillation helps to increase quantum correlations in the short-distance low-squeezing regime by up to $46\mathrm{\%}$, and the reach of entanglement increases by $14\mathrm{\%}$ with entanglement swapping. Importantly, we compute the fidelity of a continuous-variable quantum teleportation protocol using open-air-distributed entanglement as a resource. Finally, we adapt this machinery to explore the limitations of quantum communication between satellites, where the impact of thermal noise is substantially reduced and diffraction losses are dominant.

Manipulating spin waves is highly required for the development of innovative data transport and processing technologies. Recently, the possibility of triggering high-frequency standing spin waves in magnetic insulators using femtosecond laser pulses was discovered, raising the question about how one can manipulate their dynamics. Here we explore this question by investigating the ultrafast magnetization and spin-wave dynamics induced by double-pulse laser excitation. We demonstrate a suppression or enhancement of the amplitudes of the standing spin waves by precisely tuning the time delay between the two pulses. The results can be understood as the constructive or destructive interference of the spin waves induced by the first and second laser pulses. Our findings open exciting perspectives towards generating single-mode standing spin waves that combine high frequency with large amplitude and low magnetic damping.

The convolution operation is widely used in signal and image processing and represents the most computationally intensive step in convolutional neural networks. We introduce a scheme to achieve arbitrary convolution kernels in the synthetic frequency dimension with a simple setup consisting of a ring resonator incorporating a phase and an amplitude modulator. This scheme can be used to perform multidimensional convolutions. We provide an analytic approach that determines the required modulation profile for any convolution kernel. Our work points to a direction of using optical computing to remove the computational bottleneck in traditional electronic circuits and may be useful in improving machine-learning hardware in artificial-intelligence applications.

The development and performance of quantum technologies heavily relies on the properties of the quantum states, which often require careful optimization of the driving conditions of all underlying components. In quantum key distribution (QKD), optical injection locking (OIL) of pulsed lasers has recently been shown as a promising technique to realize high-speed quantum transmitters with efficient system design. However, due to the complex underlying laser dynamics, tuning such laser system is both a challenging and time-consuming task. Here, we experimentally demonstrate an OIL-based QKD transmitter that can be automatically tuned to its optimum operating state by employing a genetic algorithm. Starting with minimal knowledge of the laser operating parameters, the phase coherence and the quantum bit error rate of the system are optimized autonomously to a level matching the state of the art.

A convenient and versatile scheme to generate continuously adjustable cylindrical vector (CV) and vortex beams by multiplexing and cascading passive vortex half-wave plates (VHPs) is proposed. (−24)–24 order CV or vortex beams are generated only based on VHPs with m = 1, 3, and 8. The coaxial and small-angle interference are introduced to detect orders. For coaxial interference of $l_1$-order and $l_2$-order vortex beams, the intensity distribution is spirally fan shaped. The partition number is N = |$l_2$− $l_1$|, and the spiral direction can distinguish the sign of topological charge with higher value. Fork-shaped fringes appear in the center for small-angle interference between vortex beams with incident angle ${\beta }_1$ and ${\beta }_2$. The forking number between two fringes meets N = |$l_2$− $l_1$|, and the forks face upward when ($l_2$− $l_1$)(${\beta }_2$− ${\beta }_1$) < 0, and face downward when ($l_2$− $l_1$)(${\beta }_2$− ${\beta }_1$) > 0. Particularly, when one of the beams is a Gaussian beam, such as $l_1$ = 0, the value and sign of the topological charge $l_2$ can be directly detected. And for the interference between CV and Gaussian plane beams, the partition number N is positively correlated with the polarization order P, with N = |P| or N = 2|P| for coaxial or small-angle interference, respectively, and corresponding sign can be identified by the polarization azimuth distributions measured by Stokes parameters. It is experimentally demonstrated that the proposed scheme can effectively generate continuously adjustable CV and vortex beams by programming limited VHPs, greatly expanding order numbers with low cost, low energy consumption, and high utilization, and the coaxial and small-angle interference method can conveniently detect the corresponding orders without borrowing redundant devices.

Inspired by transformation optics, negative permeability materials enable the transfer of static magnetic fields to a distant location far away from the current source. This is useful for situations in which it is difficult to place a current source near the region where magnetic fields are required. However, existing artificial negative permeability materials are three dimensional (3D) and bulky. In this study, we theoretically design and experimentally demonstrate a metasurface with static negative permeability that displaces volumetric 3D magnetic fields to a distant region. The design overcomes the existing limitation because the dimensions of the structure are reduced to two, and the control of the magnetic field has an increased degree of freedom. The proposed method can be further extended to tools and strategies to precisely displace and/or manipulate dc or quasistatic magnetic fields in regions of interest for various applications such as medical care and measurement precision.

The efficient generation of entangled photons at telecom wavelength is essential for the success of many quantum communication protocols and the development of fiber-based quantum networks. Entangled light can be generated by solid-state quantum emitters with naturally low fine-structure splitting, such as highly symmetric $\mathrm{In}\mathrm{As}$ quantum dots (QDs) grown on (111)-oriented surfaces. The incorporation of these QDs into optical cavities is crucial to achieve sufficient signal intensities for applications but has so far shown major complications. In this work, we present droplet epitaxy of telecom-wavelength $\mathrm{In}\mathrm{As}$ QDs within an optical cavity on a vicinal ($2^{\circ }$ miscut) $\mathrm{Ga}\mathrm{As}$(111)$A$ substrate. We show a remarkable enhancement of the photon extraction efficiency compared to previous reports together with a reduction of the density that facilitates the isolation of single spectral lines. Moreover, we characterize the exciton-fine-structure splitting and employ numerical simulations under the framework of the empirical-pseudopotential and configuration-interaction methods to study the impact of the miscut on the optical properties of the QDs. We demonstrate that the presence of the miscut steps influences the polarization of the neutral excitons and introduces a preferential orientation in the ${\mathrm{C}}_{3\mathrm{v}}$ symmetry of the surface.

The transformation theory provides a distinct method for designing parameters in spatial dimensions, facilitating intriguing functions such as cloaking, concentrating, and rotating. However, with the introduction of temporal dimension, the transformation theory becomes particularly elusive because coordinate transformations apply only to static parameters. Here, we develop the transformation thermotics for designing spatiotemporal metamaterials. Specifically, we consider the transient heat-conduction equation with dynamic thermal parameters, whose transformation principles are theoretically derived and numerically confirmed. We further uncover spatiotemporal thermal cloaking, concentrating, and rotating with transformation thermotics as model applications. In contrast to conventional static parameters, dynamic parameters may provide unique opportunities for achieving thermal functions with the additional asymmetric feature. Our spatiotemporal scheme has remarkable advantages in dynamic heat regulation and provides insights into particle or plasma diffusion and wave propagation.

Rapid injection of spin polarization into an ensemble of nuclear spins is a problem of broad interest, spanning dynamic nuclear polarization (DNP) to quantum information science. We report on a strategy to boost the spin-injection rate by exploiting electrons that can be rapidly polarized via high-power optical pumping. We demonstrate this in a model system of nitrogen-vacancy center electrons injecting polarization into a bath of ${}^{13}\mathrm{C}$ nuclei in diamond. We deliver $>20$ W of continuous, nearly isotropic, optical power to the sample, constituting a substantially higher power than in previous experiments. Through a spin-ratchet polarization transfer mechanism, we show boosts in spin-injection rates by over 2 orders of magnitude. Our experiments elucidate bottlenecks in the DNP process caused by rates of electron polarization, polarization transfer to proximal nuclei, and spin diffusion. This work demonstrates opportunities for rapid spin injection employing nonthermally generated electron polarization, and has relevance to a broad class of experimental systems, including in DNP, quantum sensing, and spin-based MASERs (microwave amplification by stimulated emission of radiation).

We use Raman spectroscopy to probe in situ the hydration reaction of a foamed mineral binder, here gypsum plaster. We show the strong effect of the mixing process used for foaming on the hydration kinetics. On the contrary, we observe no effect of the cationic surfactant tetradecyltrimethylammonium bromide (TTAB) used as foaming agent and of the foam structure. By tuning the hydration time scale with a setting retardant (citric acid) and measuring in parallel the characteristic bubble radius in the foam, we show that hydration kinetics control the arrest of the bubble-size evolution in the fresh foam and the final pore size in the set gypsum.

Surpassing the Nyquist sampling limit to avoid aliasing distortion is an important issue for the task of time-varying parameter estimation. To achieve this goal, we propose a high-precision time-varying parameter estimation scheme based on split detection and postmodulation of light instead of postprocessing of data. Theoretical analysis demonstrates its advantage of surpassing the Nyquist sampling limit and the avoidance of anti-aliasing distortion. In order to illustrate the feasibility, a numerical simulation based on the acceptance rejection sampling method is performed showing the noise robustness of the scheme in terms of correlation coefficient, root-mean-square error, and mean absolute error. Our scheme provides a method for function estimation.

Kerr parametric oscillators (KPOs), which can be implemented with superconducting parametrons possessing large Kerr nonlinearity, have been attracting much attention in terms of their applications to quantum annealing, universal quantum computation, and studies of quantum many-body systems. It is of practical importance for these studies to realize fast and accurate tunable coupling between KPOs in a simple manner. We develop a simple scheme of fast tunable coupling of KPOs with high tunability in speed and amplitude using the fast transitionless rotation of a KPO in the phase space based on the shortcuts to adiabaticity. Our scheme enables rapid switching of the effective coupling between KPOs, and can be implemented with always-on linear coupling between KPOs, by controlling the phase of the pump field and the resonance frequency of the KPO without controlling the amplitude of the pump field nor using additional drive fields and couplers. We apply the coupling scheme to a two-qubit gate, and show that our scheme realizes high gate fidelity compared with a purely adiabatic one, by mitigating undesired nonadiabatic transitions.

A multimode fiber represents the ultimate limit in miniaturization of imaging endoscopes. However, such a miniaturization usually comes as a cost of a low spatial resolution and a long acquisition time. Here we propose a fast superresolution-fiber-imaging technique employing compressive sensing through a multimode fiber with a data-driven machine-learning framework. We implement a generative adversarial network (GAN) to explore the sparsity inherent to the model and provide compressive reconstruction images that are not sparse in a representation basis. The proposed method outperforms other widespread compressive imaging algorithms in terms of both image quality and noise robustness. We experimentally demonstrate machine-learning ghost imaging below the diffraction limit at a sub-Nyquist speed through a thin multimode fiber probe. We believe that this work has great potential in applications in various fields ranging from biomedical imaging to remote sensing.

Bell nonclassicality of a single-photon superposition in two modes, often referred to as “nonlocality of a single photon,” is one of the most striking nonclassical phenomena discussed in the context of foundations of quantum physics. Here we show how to robustly violate local realism within the weak-field homodyne measurement scheme for any superposition of one photon with vacuum. Our modification of the previously proposed setups involves tunable beam splitters at the measurement stations, and the local oscillator fields significantly varying between the settings, optimally being on or off. As photon-number-resolving measurements are now feasible, we advocate for the use of the Clauser-Horne Bell inequalities for detection events using precisely defined numbers of photons. We find a condition for optimal measurement settings for the maximal violation of the Clauser-Horne inequality with weak-field homodyne detection, which states that the reflectivity of the local beam splitter must be equal to the strength of the local oscillator field. We show that this condition holds not only for the vacuum–one-photon qubit input state, but also for the superposition of a photon pair with vacuum, which suggests its generality as a property of weak-field homodyne detection with photon-number resolution. Our findings suggest a possible path to employ such scenarios in device-independent quantum protocols.