Future quantum networks will be hybrid structures, constructed from complex architectures of quantum repeaters interconnected by quantum channels that describe a variety of physical domains; predominantly optical-fiber and free-space links. In this hybrid setting, the interplay between the channel quality within network substructures must be carefully considered, and is pivotal for ensuring high-rate end-to-end quantum communication. In this work, we combine recent advances in the theory of point-to-point free-space channel capacities and end-to-end quantum network capacities in order to develop critical tools for the study of hybrid, free-space quantum networks. We present a general formalism for studying the capacities of arbitrary, hybrid quantum networks, before specifying to the regime of atmospheric and space-based quantum channels. We then introduce a class of modular quantum network architectures, which offer a realistic and readily analyzable framework for hybrid quantum networks. By considering a physically motivated, highly connected modular structure we are able to idealize network performance and derive channel conditions for which optimal performance is guaranteed. This allows us to reveal vital properties for which distance-independent rates are achieved, so that the end-to-end capacity has no dependence on the physical separation between users. Our analytical method elucidates key infrastructure demands for a future satellite-based global quantum internet, and for hybrid wired and wireless metropolitan quantum networks.

The majority of experimental realizations of single-electron sources rely on the periodic manipulation of tunnel junctions through their gate voltages, and thus require a high level of control over the system. To circumvent the necessity of external driving, we utilize the self-oscillatory behavior of an electron shuttle. By means of waiting time distributions, which have not been applied to this autonomous system before, we extensively assess the performance of the shuttle as a single-electron source. We reveal a smooth transition between three regimes, whereas previous studies at the same mean field level of description only predict a sharp bifurcation based on the time-averaged electron current. Over the parameter range of already existing experimental realizations, the electron shuttle can perform as a single-electron source, albeit with moderate precision. We propose an alternative design of the position-dependent tunneling rates, which significantly reduces the relative error of charge transmission, and thus paves the way for the realization of autonomous single-electron sources.

Anisotropic two-dimensional materials provide promising platforms for polarization-driven optoelectronic and photonic devices. In particular, layered rhenium disulfide (${\mathrm{Re}\mathrm{S}}_2$) has recently attracted intensive attention due to its unique linear dichroism. However, control over the optical properties of ${\mathrm{Re}\mathrm{S}}_2$ is mostly limited to the band-edge excitons, which significantly limits the application wavelength range. Here, we utilize ultrafast transient absorption microscopy to tune the broadband optical anisotropy of few-layer ${\mathrm{Re}\mathrm{S}}_2$. We observe a broad nonexcitonic photoinduced-absorption feature, which exhibits weak anisotropy immediately after pump excitation, but, surprisingly, shows a strong polarization dependence after a few picoseconds, leading to a broadband enhancement in the optical anisotropy. We attribute this to carrier cooling and subsequent anisotropic free-carrier absorption due to anisotropic carrier effective masses. This work provides not only a principle for the ultrafast active control of broadband polarization-sensitive photonic devices but also insights into anisotropic carrier dynamics in two-dimensional materials.

Ferrimagnetic ${\mathrm{Ni}\mathrm{Co}}_2{\mathrm{O}}_4$ (NCO) films with perpendicular magnetic anisotropy, 12.5 nm in thickness, are epitaxially deposited on (001) ${\mathrm{Mg}\mathrm{Al}}_2{\mathrm{O}}_4$ single-crystalline substrates by pulsed laser deposition. It is observed that the coercive field of the NCO films can be tuned systematically by about 20 times, while keeping the large perpendicular magnetic anisotropy, through simply increasing the deposition temperature from 250 to 350 °C. The increase of resistivity and the increase of low-temperature metal-to-insulator transition temperature suggests increased concentration of defects in NCO films deposited at elevated temperatures. The magnetic and transport characteristics are discussed based on a systematic change in cation valence and coordination in NCO films, as revealed by x-ray photoelectron emission and x-ray absorption spectroscopy measurements. The perpendicular magnetic anisotropy with tunable coercivity makes NCO films promising for multifunctional applications.

The manipulation of acoustic waves is experimentally and theoretically investigated based on the concept of coherent perfect absorption. It is shown that the absolute control of antisymmetric modes of an acoustic duct is possible using an ultrathin wire mesh. By varying the relative phase of two counterpropagating (normal or oblique) incident waves, the absorption due to wire mesh can be tuned from zero to unity. Experimental as well as numerical results demonstrate that this phenomenon is extremely broadband—allowing, for instance, the symmetrization of the acoustic wave with a short pulse in the time domain.

Superconducting nanowire single-photon detectors (SNSPDs) are receiving the interest of the scientific and industrial communities due to their unparallel high performances in the infrared. However, even though the fabrication process permits the achievement of about 98% efficiency and less than 1 cps dark-count rate, the physical mechanism inducing the detection remains unclear. It is clear however that normal core vortices play a crucial role. In this work we investigate the role of vortices in two-dimensional (2D) molybdenum silicide SNSPDs of different widths covered with an $\mathrm{Al}$ layer through the analysis of the switching current distributions from the superconducting to resistive regime, in a wide interval of temperatures from 4.5 K down to 10 mK. This analysis provides the energy scales of different mechanisms that are responsible for fluctuations and dark counts in SNSPDs. We consider two models based on vortices, the unbinding of vortex-antivortex pairs (VAPs) and vortices hopping over the edge barrier (VH) and we underline the differences among different devices made by different materials. We also estimate the energy scales of similar $\mathrm{Nb}\text{−}\mathrm{Ti}\text{−}\mathrm{N}$ and $\mathrm{Nb}\mathrm{N}$ devices and compare the results. The lower activation energies obtained for $\mathrm{Mo}\mathrm{Si}/\mathrm{Al}$ devices, explain the peculiarity of this material to work at longer wavelengths with a higher quantum detection efficiency.

Electronic voting is a very useful but challenging internet-based protocol that despite many theoretical approaches and various implementations with different degrees of success, remains a contentious topic due to issues in reliability and security. Here we present a quantum protocol that exploits an untrusted source of multipartite entanglement to carry out an election without relying on election authorities, simultaneous broadcasting, or computational assumptions, and whose result is publicly verifiable. The level of security depends directly on the fidelity of the shared multipartite entangled quantum state, and the protocol can be readily implemented for a few voters with state-of-the-art photonic technology.

Light emitters in the vicinity of a hyperbolic metamaterial (HMM) show a range of quantum optical phenomena from spontaneous decay-rate enhancement to strong coupling. In this study, we integrate a monolayer molybdenum disulfide (${\mathrm{Mo}\mathrm{S}}_2$) emitter in the near-field region of the HMM. The ${\mathrm{Mo}\mathrm{S}}_2$ monolayer has $A$ and $B$ excitons, which emit in the red region of the visible spectrum. We find that the $B$ excitons couple to the HMM differently compared to $A$ excitons. The fabricated HMM transforms to a hyperbolic dispersive medium at 2.14 eV, from an elliptical dispersive medium. The selective coupling of $B$ excitons to the HMM modes is attributed to the inbuilt field gradient of the transition. The $B$ exciton energy lies close to the transition point of the HMM, relative to the $A$ exciton. So, the HMM modes couple more to the $B$ excitons and the metamaterial functions as a selective coupler. The coupling strength calculations show that coupling is 2.5 times stronger for $B$ excitons relative to $A$ excitons. High near field of HMM, large magnitude, and the in-plane transition dipole moment of ${\mathrm{Mo}\mathrm{S}}_2$ excitons, result in strong coupling of $B$ excitons and formation of hybrid light-matter states. The measured differential reflection and photoluminescence spectra indicate the presence of hybrid light-matter states, i.e., exciton polaritons. Rabi splitting of $143.5\mathrm{meV}\pm 14.4$ meV at room temperature is observed. The low-temperature photoluminescence measurement shows mode anticrossing, which is a characteristic feature of hybrid states. Our results show that the HMM works as an energy-selective coupler for multiexcitonic systems as ${\mathrm{Mo}\mathrm{S}}_2$.

We introduce a theoretical model presenting a PT-symmetric system of two planar coupled magnonic ferromagnetic (FM) waveguides covered by thin layers of nonmagnetic metal with a strong spin-orbit interaction [heavy metal (HM)]. The eigenvalues in PT-symmetric systems change from real to complex at exceptional points. In magnonics, PT symmetry is achieved by a perfect balance of the gain and loss of spin waves. It is possible to control the voltage of exceptional-point appearance in the system by changing the distance between waveguides. The proposed system has a resonance linewidth almost 2 times narrower than that of a single FM-HM heterostructure, thus opening up opportunities for creating highly precise tunable devices, such as detectors and sensors.

Elastic metagratings that can efficiently manipulate the propagation of bulk longitudinal and transverse waves are highly desired in various applications, such as geological exploration and nondestructive detection. Here, a class of elastic metagratings consisting of a one-dimensional array of elliptical hollow cylinders carved in a steel background is proposed and studied. With the help of elastic-wave-diffraction analysis and a genetic-algorithm-based optimization approach, these intelligently designed metagratings can reflect longitudinal and transverse waves in a highly efficient and spatially separated way. Various wave-front-manipulation functionalities, such as specular reflection for longitudinal waves and anomalous reflection for transverse waves, can be simultaneously achieved with an almost perfect efficiency. Interestingly, different functionalities over both longitudinal and transverse waves can be realized by using the same cylinders with different rotation angles. Due to the nonresonant characteristics, the exhibited functionality shows robustness over both working frequencies and unit-cell configurations, which are beneficial for practical applications.

Prototypes for hot-carrier solar cells based on type-II $\mathrm{InAs}/{\mathrm{AlAs}}_{0.16}{\mathrm{Sb}}_{0.84}$ multiple quantum wells are examined with ac photoconductivity as a function of lattice temperature and photoexcitation energy to determine the photoexcited charge-carrier transport. These samples previously exhibited an excitation energy onset of a metastable regime in their short-time charge-carrier dynamics that potentially improves their applicability for hot-carrier photovoltaic cells. The transport results illustrate that the ac photoconductivity is larger in the regime corresponding to the metastability as a result of higher excitation photocarrier densities. In this excitation regime, the ac photoconductivity is accompanied by slightly lower carrier mobility, arising from the plasmalike nature of carriers scattered by Auger recombination. Outside of this regime, higher mobility is observed as a result of a lower excitation density that is more readily achievable by solar concentration. Additionally, at ambient temperatures, more scattering events are accompanied by slightly lower mobility, but the excitation dependence indicates that this is accompanied by an ambipolar diffusion length that is greater than half a micron. These transport properties are consistent with good quality in-organic elemental and III-V semiconductor solar cells and far exceed those of organic and perovskite materials. The transport results complement the dynamics observed in type-II $\mathrm{InAs}/{\mathrm{AlAs}}_{0.16}{\mathrm{Sb}}_{0.84}$ and can guide the design of hot-carrier solar cells based on these and related materials.

In this work, we propose the design of a flexoelectric sensor, which can measure sound-pressure gradient directly. It is found that the sensitivity of sound-pressure gradient of the flexoelectric sensor has a wide band for low working frequencies. Our theoretical and experimental results indicate that the position of sound sources can be predicted based on the measured sound-pressure gradients of three flexoelectric sensors, which are perpendicular to each other. Moreover, different from the previous design for a piezoelectric sensor array, only three flexoelectric sensors are needed to determine the orientation of sound sources. The flexoelectric sensor proposed here may provide an alternative way for the pressure-gradient measurement.

The equivalence between the natural minimization of energy in a dynamic system and the minimization of an objective function characterizing a combinatorial optimization problem offers a promising approach to design dynamic-system-inspired computational models and solvers for such problems. For instance, the ground-state energy of coupled electronic oscillators, under second-harmonic injection, can be directly mapped to the optimal solution of the maximum-cut problem. However, prior work has focused on a limited set of such problems. Therefore, in this work, we formulate computing models based on synchronized oscillator dynamics for a broad spectrum of combinatorial optimization problems ranging from the Max-K-Cut (the general version of the maximum-cut problem) to the traveling-salesman problem. We show that synchronized oscillator dynamics can be engineered to solve these different combinatorial optimization problems by appropriately designing the coupling function and external injection to the oscillators. Our work marks a step forward towards expanding the functionalities of oscillator-based analog accelerators and furthers the scope of dynamic system solvers for combinatorial optimization problems.

A single Bessel beam (BB) enables the scalable optical writing of waveguide devices with multiple inputs and outputs in a nonlinear medium. For this purpose, we experimentally propagate a diffracting zero-order BB in a strontium barium niobate photorefractive crystal. Such a configuration can be used for the self-writing of complex waveguiding structures. By tailoring different key parameters such as the diffraction phenomenon through the BB size, the nonlinearity strength through the applied electric field, and the photoinduced refractive-index modulation depth through input beam intensity, our optical platform enables not only the generation of optical splitters but also an all-optical control of the output intensity levels. We also analyze the stability in space and time of the different photoinduced channels. We show that adding a background illumination onto the medium permits a steady-state photoinduced refractive-index modulation with up to nine output channels. These results enrich the research on scalable optical writing techniques for the realization of complex interconnects and enlarge further possibilities for all-optical switching.

Graphene and transition-metal-dichalcogenide- (TMD) based van der Waals heterostructures in field-effect-transistor (FET) architecture exhibits extremely high sensitivity to optical radiation due to transit and physical separation of the photogenerated carriers across the heterointerface. Both the sensitivity and speed of these detectors depend on the kinetics of charge transfer, but their interdependency at room temperature (T), where these detectors would be most useful, remains largely unexplored. Here we systematically measure the T dependence of the magnitude (gain) and timescale (bandwidth) of photoresponse in graphene-TMD heterostructures well up to the room T. The gain-bandwidth product is found to be strongly dependent on the power of optical illumination and increases with decreasing power (P), becoming as large as $1\mathrm{M}\mathrm{Hz}$ in the low-P limit. We find that thermally activated back transfer of charge from graphene to the TMD determines the response time of the detector at higher temperatures under continuous illumination. Our experiment reveals the impact of charge-transfer pathways on the performance in a broad class of graphene-TMD detectors.

Acoustic manipulations of microparticles have significant implications in physics, chemistry, biology, and biomedicine. For large Mie particles, both the scattering force and the gradient force affect the trapping and manipulation behaviors, and thus, a passive acoustic tweezer (AT) for three-dimensionally manipulating a Mie particle remains challenging. Here, a passive AT based on a hybrid acoustic artificial plate is proposed to generate a hybrid acoustic focused petal beam (HAFPB) for capturing and manipulating a Mie microparticle in three dimensions. In the HAFPB, a lateral acoustic focused petal beam and an axial bifocal acoustic beam (BAB) combine to create a central zero-intensity zone for particle trapping, which can be adjusted by modulating the topological charge of the HAFPB. The acoustic radiation forces (ARFs) acting on large Mie particles with different diameters are investigated. The ARFs on the polystyrene sphere are greatly influenced by the particle size. If the particle is larger than the central zero-intensity zone, an AT with a larger topological charge and a stronger BAB is required for trapping and manipulating this larger particle. Finally, the experiments demonstrate that the AT can stably trap and manipulate a large Mie particle in three dimensions in water.