We present a theory to compute the stable gap (interparticle) distance between particle chains collected in the pressure node of an acoustic standing wave. The primary and secondary acoustic radiation forces are the two competing forces that act on the particles during the particle chain formation. The stable equilibrium distance between two chains is reached when both forces are in balance. Most interestingly, the density scattering coefficient appears to the second power in the theoretical prediction of the gap distance, indicating that the particle-chain formation occurs for both particles heavier than the surrounding medium and, notably, also for buoyant particles. Experimentally, the gap distance is evaluated for several different media and particle material combinations and the particle-chain formation is observed for both buoyant particles and particles heavier than the surrounding medium. The theory agrees well with experiments in the cases where the material properties of the medium and the particles are well known.

The determination of relative stopping power (RSP) via proton computed tomography (pCT) of a patient is dependent in part on the knowledge of the incoming proton kinetic energies; the uncertainty in these energies is in turn determined by the proton source—typically a cyclotron. Here, we show that reducing the incident proton beam energy spread may significantly improve RSP determination in pCT. We demonstrate that the reduction of beam energy spread from the typical 1.0% (at 70 MeV) down to 0.2% can be achieved at the proton currents needed for imaging at the Paul Scherrer Institut 250-MeV cyclotron. Through a simulated pCT imaging system, we find that this effect results in RSP resolutions as low as 0.2% for materials such as cortical bone, up to 1% for lung tissue. Several materials offer further improvement when the beam (residual) energy is also chosen such that the detection mechanisms used provide the optimal RSP resolution.

A Kerr nonlinear parametric oscillator (KPO) can stabilize a quantum superposition of two coherent states with opposite phases, which can be used as a qubit. In a universal gate set for quantum computation with KPOs, an $R_x$ gate, which interchanges the two coherent states, is relatively hard to perform owing to the stability of the two states. We propose a method for a high-fidelity $R_x$ gate by exciting the KPO outside the qubit space with parity-selective transitions, which can be implemented by only adding a driving field. In this method, the utilization of higher effective excited states leads to a faster $R_x$ gate, rather than states near the qubit space. The proposed method can realize a continuous $R_x$ gate and thus is expected to be useful for, e.g., recently proposed variational quantum algorithms.

Memristors hold great promise as building blocks for future computing architectures where memory and logic are combined at the hardware level. However, scaling down the dimensions of memristive devices has been limited by high leakage currents, thus inhibiting further progress. Recent studies have demonstrated memristors with monolayers of ${\mathrm{Mo}\mathrm{S}}_2$ and large high-to-low resistance ratios. Defects combined with metallic ion migrations are often seen as a possible explanation for this behavior. A detailed understanding of the switching mechanisms, in particular the role of metal ion diffusion into vacancy sites and crystal defects, remains elusive. Here we investigate how defect densities affect the performance of monolayer ${\mathrm{Mo}\mathrm{S}}_2$ memristors. We experimentally demonstrate that the resistive switching ratio becomes larger if the defect density in ${\mathrm{Mo}\mathrm{S}}_2$ is increased. Furthermore, by means of ab initio quantum transport simulations, we reveal the existence of an optimum range of defect densities and explore the theoretical limits of monolayer ${\mathrm{Mo}\mathrm{S}}_2$ memristors. Our results highlight the importance of defect engineering and control in transition metal dichalcogenides memristors.

Atmospheric aerosols, such as water droplets in fog, interfere with laser propagation through scattering and absorption. Femtosecond optical filaments have been shown to clear foggy regions, improving the transmission of subsequent pulses. However, the detailed fog-clearing mechanism had yet to be determined. Here, we directly measure and simulate the dynamics of water droplets with a radius of about $5\mu \mathrm{m}$, typical of fog, under the influence of optical and acoustic interactions that are characteristic of femtosecond filaments. We find that, for filaments generated by the collapse of collimated near-infrared femtosecond pulses, the main droplet-clearing mechanism is optical shattering by laser light. For such filaments, the single-cycle acoustic wave launched by filament-energy deposition in air leaves droplets intact and drives negligible transverse displacement, and therefore, negligible fog clearing. Only for tightly focused nonfilamentary pulses, where local energy deposition greatly exceeds that of a filament, do acoustic waves significantly displace aerosols.

Theoretical studies of coherent atom transport have as yet mainly been restricted to one-dimensional model systems with harmonic trapping potentials. Here we investigate this important phenomenon—a prerequisite for a variety of quantum-technology applications based on cold neutral atoms—under much more complex physical circumstances. More specifically yet, we study fast atomic transport in a moving double-well optical lattice, whose three-dimensional (anharmonic) potential is nonseparable in the $x$-$y$ plane. We first propose specific configurations of acousto-optic modulators that give rise to the moving-lattice effect in an arbitrary direction in this plane. We then determine moving-lattice trajectories that enable single-atom transport using two classes of quantum-control methods: shortcuts to adiabaticity (STA), here utilized in the form of inverse engineering based on a quadratic-in-momentum dynamical invariant of Lewis-Riesenfeld type, and their recently proposed modification termed enhanced STA (eSTA). Subsequently, we quantify the resulting single-atom dynamics by numerically solving the relevant time-dependent Schrödinger equations and compare the efficiency of STA- and eSTA-based transport by evaluating the respective fidelities. We show that—except for the regime of shallow lattices—the eSTA method consistently outperforms its STA counterpart. This study has direct implications for neutral-atom quantum computing based on collisional entangling two-qubit gates and quantum sensing of constant homogeneous forces via guided-atom interferometry.

The detection dead zones and heading errors induced by light shifts are two important problems in optically pumped scalar magnetometry. We introduce an atomic-orientation-based single-beam magnetometry scheme to simultaneously solve these problems, using a polarization-reversing and path-bending Herriott cavity. Here, a reflection mirror is inserted into the cavity to bend the optical paths in the middle and divide them into two separated orthogonal regions to avoid the detection dead zone. Moreover, half-wave plates are added in the center of each optical region, so that the light polarization is flipped each time it passes the wave plates and the light-shift effects are spatially averaged out. This operation is demonstrated to eliminate the unnoticed heading errors induced by ac light shifts. The methods developed in this paper are robust to use and easy to apply other atomic devices.

Ferroelectricity is promising in emulating the synaptic characteristics of human brains. Utilizing ferroelectricity for brain-inspired computing is proposed as a feasible route to address technical challenges in memory and computing. Here, we demonstrate the use of a ferroelectric van der Waals (vdW) halide perovskite for synaptic emulation. The two-terminal ferroelectric synapse based on the vdW material (R)-(−)-1-cyclohexylethylammonium)${\mathrm{PbI}}_3$ (R-CYHEA${\mathrm{PbI}}_3$) exhibits voltage-pulse-dependent weight modulation with a total on:off ratio of 50 and good endurance up to ${10}^7$ cycles. The energy consumption per synaptic operation for both short-term plasticity and long-term plasticity reaches the picojoule level. The device also shows reasonable write linearity and small cycle-to-cycle variation, as well as promising spike-timing-dependent plasticity and a paired-pulse-facilitation function. Numerical simulations with the R-CYHEA${\mathrm{PbI}}_3$-synapse-based neural network suggest the potential of R-CYHEA${\mathrm{PbI}}_3$ synapses for pattern recognition. Ferroelectric vdW halide perovskites provide opportunities for exploiting their dimensionality, superior optoelectronic properties, and mild material-processing conditions for engineering of the synaptic device performance.

Structural symmetry breaking, which could lead to exotic physical properties, plays a crucial role in determining the functions of a system, especially for two-dimensional (2D) materials. Here, we demonstrate that multiple functionalities of 2D chromium-based materials can be achieved by breaking inversion symmetry via replacing $Y$ atoms in one face of pristine $\mathrm{Cr}$Y (Y = $\mathrm{P}$, $\mathrm{As}$, $\mathrm{Sb}$) monolayers with $\mathrm{N}$ atoms, i.e., forming Janus ${\mathrm{Cr}}_2\mathrm{N}Y$ monolayers. The functionalities include gapless spin, very low work function, inducing carrier doping, and catalytic activity, which are predominately ascribed to the large intrinsic dipole of Janus ${\mathrm{Cr}}_2\mathrm{N}Y$ monolayers, giving them great potential for various applications. Specifically, ${\mathrm{Cr}}_2\mathrm{NSb}$ is found to be a spin-gapless semiconductor, ${\mathrm{Cr}}_2\mathrm{NP}$ and ${\mathrm{Cr}}_2\mathrm{NHPF}$ can simultaneously induce n- and p-type carrier doping for two graphene sheets with different concentrations (forming an intrinsic p-n vertical junction), and ${\mathrm{Cr}}_2\mathrm{N}Y$ exhibits excellent electrocatalytic hydrogen-evolution activity, even superior to that of benchmark $\mathrm{Pt}$. The results confirm that breaking symmetry is a promising approach for the rational design of multifunctional 2D materials.

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.