Harnessing the quantum interference of the pair generation processes, infrared quantum spectroscopy, based on nonlinear interferometers with visible-infrared photon-pair sources, enables the extraction of the infrared optical properties of a sample through visible photon detection without the need for an infrared optical source or detector. We develop a theoretical framework for quantum Fourier-transform infrared (QFTIR) spectroscopy. The proposed Fourier analysis method, which fully utilizes the phase information in the interferogram, allows us to determine the complex transmittance and optical constants for a sample in a simple setup without the use of any dispersive optics for spectral selection. In the experimental demonstrations, the transmittance spectrum of a bandpass filter and the refractive index of silica glass are measured in the near-infrared region using QFTIR operated in a low gain regime; these results agree well with the independently measured spectrum using a conventional spectrometer and an value estimated from references. These demonstrations prove the validity and great potential of QFTIR spectroscopy.

Through the use of experimental and numerical techniques, we study coherent networks of plasmonic phase-correlated dipole domains that can spread coherent properties along ultralong distances. The phase-correlated domains occur in regions containing closely packed metallic nanoparticles with random positions, shapes, and sizes. We demonstrate that when such regions are periodically arranged, forming two-dimensional arrays, optical diffraction can coherently excite and align the plasmonic dipoles of the nanoparticles within each region, forming delocalized collective plasmon resonances associated with the in-phase coupling of the dipoles. We study the impact of the number of metallic nanoparticles in each region, determining the limit at which it becomes a phase-correlated dipole domain and establishes coherent interdomain coupling. Our results show how such a coupling can form coherent networks of packages of delocalized states with the same phase information. $\mathrm{Au}$ nanoislands with different packings, sizes, and shapes are considered as experimental models to explore phase-correlated dipole domains and interdomain coupling. The conditions in which the coherent interdomain coupling occurs via parallel or orthogonal hybridization of the Rayleigh anomaly with the delocalized plasmon resonances supported by the domains are explored. The outcomes offer alternative techniques for coherent transport of energy and information, in which phase-correlated dipole domains serve as units with large coherent spatial extension.

In this work, we show that targeted and controlled modifications of the Josephson-junction properties of a bridge-type $\mathrm{Nb}$ nanoSQUID can be achieved by an electroannealing process allowing us to tune and tailor the response of a single device. The electroannealing consists in substantial Joule heating produced by large current densities followed by a rapid temperature quench. We report on a highly nontrivial evolution of the material properties when performing subsequent electroannealing steps. As the current density is increased, an initial stage characterized by a modest improvement of the superconducting critical temperature and normal-state conductivity of the bridges, is observed. This is followed by a rapid deterioration of the junction properties, i.e., decrease of critical temperature and conductivity. Strikingly, further electroannealing leads to a noteworthy recovery before irreversible damage is produced. Within the electroannealing regime where this remarkable resurrection of the superconducting properties are observed, the nanoSQUID can be operated in nonhysteretic mode in the whole temperature range and without compromising the critical temperature of the device. The proposed postprocessing is particularly appealing in view of its simplicity and robustness.

At present, functionalities, miniaturization, and tunability are useful directions in the development of practical device applications for topological photonics and phononics. Atomically thin two-dimensional (2D) materials not only have excellent mechanical properties, but also have outstanding electrical, optical, and magnetic properties, making them great candidates for tunable microdevices via electrostatic forces, magnetoacoustic, or photoacoustic coupling, etc. In this paper, we propose an elastic topological insulator based on atomically thin 2D materials pinned on honeycomb-patterned substrates. Thanks to acoustic impedance mismatch between the suspended 2D materials and the rigid substrates, both topological edge states and corner states of elastic waves inside the 2D materials can be achieved, accompanied by a display of functional components, e.g., arbitrary waveguides, splitters, resonators. Significantly, all these components for elastic waves are frequency tunable, due to the electromechanical sensitivity of the 2D material. For example, a two-port high-Q bandpass filter is estimated to achieve 7.26% operating frequency shifting by applying only 5 V of gate voltage. These kinds of elastic topological materials and corresponding devices can strongly promote the development of 2D-material-based nanoelectromechanical systems and might have immediate applications in modern wireless communication technology at radio or microwave frequencies.

Although studies of hydrodynamic cloaking have attracted considerable attention, most pioneering studies have so far focused on creeping flows, in which nonlinear convective terms are neglected and cloaks are fabricated with inhomogeneous or anisotropic metamaterials. Here we discover that intangible uniform-viscosity hydrodynamic cloaks, which are capable of hiding obstacles immersed in narrow-gap Navier-Stokes flows with non-negligible convection, can be possibly designed. Via the convection-diffusion-balance method and the realizable thermally controlled tactic, the diffusion effect exerted by the obstacle surface can be balanced by the convection counterpart exerted by the manipulation of temperature-dependent viscosity inside the intangible cloak. This analysis has been validated by finite-element computational simulations that tackle cloaked-cylinder flows for Reynolds numbers in the laminar regime. The proposed study may lay the foundation for attempts to further explore hydrodynamic cloaks and possibly other metamaterial-related devices.

We report the influence of morphological changes on the process of charge storage for electrode materials based on ${\mathrm{Ti}\mathrm{O}}_2$ nanotubes and provide theoretical insights into the electronic properties from density-functional theory (DFT) simulations. ${\mathrm{Ti}\mathrm{O}}_2$ nanotubes are synthesized hydrothermally from nanoparticle precursors. Structural characterization analysis reveals excellent phase purity from the XRD peaks and the formation of uniform and homogeneous nanotubes from field-emission SEM images. Interestingly, cyclic voltammetry and charge-discharge measurements demonstrate that these structurally engineered ${\mathrm{Ti}\mathrm{O}}_2$ nanotubes exhibit an enhanced specific capacitance of about $1400\mathrm{F}{\mathrm{g}}^{-1}$ compared with $400\mathrm{F}{\mathrm{g}}^{-1}$ for pristine ${\mathrm{Ti}\mathrm{O}}_2$ nanoparticles. Employing DFT simulations, we present the electronic and structural properties of the ${\mathrm{Ti}\mathrm{O}}_2$ electrode for charge-storage performance. We compute the diffusion barrier of ${\mathrm{K}}^{+}$ ions in the electrolyte ($\mathrm{KOH}$), the accumulated voltage for different ${\mathrm{K}}^{+}$ concentrations, and the quantum capacitance of the ${\mathrm{Ti}\mathrm{O}}_2$ surface. A lower diffusion barrier for ${\mathrm{K}}^{+}$ ions and higher accumulated voltage contribute towards a superior charge-storage performance, which supports experimental observations. Hence, detailed experimental and theoretical analyses predict that a larger surface area in the tubular structure, increased number of delocalized carriers, improved conductivity, and enhanced mobility of the electrolytic ions contribute towards a fast and highly reversible electron-transfer process, leading to enhanced charge-storage performance in morphologically engineered ${\mathrm{Ti}\mathrm{O}}_2$ nanotube-based electrodes.

We establish a method for the determination of the single-photon optomechanical coupling rate, which characterizes the radiation pressure interaction in an optomechanical system. The estimation of the rate with which a mechanical oscillator, initially in a thermal state, undergoes a Hopf bifurcation, and reaches a limit cycle, allows us to determine the single-photon optomechanical coupling rate in a simple and consistent way. Most importantly, and in contrast to other methods, our method does not rely on knowledge of the system’s bath temperature and on a calibration of the signal. We provide the theoretical framework and experimentally validate this method, providing a procedure for the full characterization of an optomechanical system, which could be extended outside cavity optomechanics, whenever a resonator is driven into a limit cycle by the appropriate interaction with another degree of freedom.

The orbital angular momentum (OAM) of photons presents a degree of freedom for enhancing the secure key rate of free-space quantum key distribution (QKD) through mode-division multiplexing (MDM). However, atmospheric turbulence can lead to substantial modal crosstalk, which is a long-standing challenge to MDM for free-space QKD. Here, we show that the digital generation of time-reversed wavefronts for multiple probe beams is an effective method for mitigating atmospheric turbulence. We experimentally characterize seven OAM modes after propagation through a 340-m outdoor free-space link and observe an average modal crosstalk as low as 13.2% by implementing real-time time reversal. The crosstalk can be further reduced to 3.4% when adopting a mode spacing $\mathrm{\Delta }\ell$ of 2. We implement a classical MDM system as a proof-of-principle demonstration, and the bit error rate is reduced from $3.6\times {10}^{-3}$ to be less than $1.3\times {10}^{-7}$ through the use of time reversal. We also propose a practical and scalable scheme for high-speed, mode-multiplexed QKD through a turbulent link. The modal crosstalk can be further reduced by using faster equipment. Our method can be useful to various free-space applications that require crosstalk suppression.

We propose a bianisotropic hybrid metal-dielectric structure comprising dielectric and metallic cylindrical wedges wherein the composite metacylinder enables advanced control of electric, magnetic, and magnetoelectric resonances. We establish a theoretical framework in which the electromagnetic response of this meta-atom is described through the electric and magnetic multipole moments. The complete dynamic polarizability tensor, expressed in a compact form, is derived as a function of the Mie-scattering coefficients. Further, the constitutive parameters—determined analytically—illustrate the tunability of the structure’s frequency and strength of resonances in light of its high degree of geometric freedom. Flexibility in the design makes the proposed metacylinder a viable candidate for various applications in the microscopic (single meta-atom) and macroscopic (metasurface) levels. We show that the highly versatile bianisotropic meta-atom is amenable to being designed for the desired electromagnetic response, such as electric dipole-free and zero or near-zero (backward and forward) scattering at the microscopic level. In addition, we show that the azimuthal asymmetry gives rise to normal polarizability components, which are vital elements in synthesizing asymmetric optical transfer function at the macroscopic level. We conduct a precise inspection, from the microscopic to the macroscopic level, of the metasurface synthesis for emphasizing on the role of normal polarizability components for spatial optical signal processing. It is shown that this simple two-dimensional asymmetric meta-atom can perform first-order differentiation and edge detection at normal illumination. The results reported herein contribute toward improving the physical understanding of wave interaction with artificial materials composed of asymmetric elongated metal-dielectric inclusions and open the potential of its application in spatial signal and image processing.

Based on first-principles methods, we demonstrate that it would be possible to design efficient all-full-Heusler-alloy-based spintronic devices associating ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}$ electrodes and nonmagnetic full-Heusler-alloy spacers selected to preserve the half-metallicity of ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}$ up to the interface atomic layers. We focus on the ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}/{\mathrm{Fe}}_2\mathrm{Ti}\mathrm{Si}$ magnetic tunnel junction, compare the stability range of its different interface terminations, and show that the half-metallicity of the ${\mathrm{Fe}}_2$/$\mathrm{Mn}\mathrm{Si}$-terminated interface even withstands atomic intermixing. The robustness of the half-metallicity of this interface termination is also confirmed in ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}/{\mathrm{Fe}}_2\mathrm{V}\mathrm{Al}$ spin valves. With such interfaces, spintronic devices could possess very high magnetoresistive properties.

Thermal management is a current global challenge that must be addressed exhaustively. We propose the design of a nanoscale phononic analog of the Ranque-Hilsch vortex tube in which heat flowing at a given temperature is split into two different streams going to the two ends of the device, inducing a temperature asymmetry. Our nanoscale prototype consists of two carbon nanotubes (capped and open) connected by molecular chains. The results show that the structural asymmetry in the contact regions is the key factor for producing the flux asymmetry and, hence, the induced temperature-bias effect. The effect can be controlled by tuning the thermal-equilibration temperature, the number of chains, and the chain length. Deposition on a substrate adds another variable to the manipulation of the flux asymmetry but the effect vanishes at very large substrate temperatures. Our study yields insights into the thermal management in nanoscale materials, especially the crucial issue of whether the thermal asymmetry can survive phonon scattering over relatively long distances, and thus provides a starting point for the design of a nanoscale phononic analog of the Ranque-Hilsch vortex tube.

The $\mathrm{Si}\mathrm{C}/{\mathrm{Si}\mathrm{O}}_2$ interface in the termination area is a crucial component in limiting high-temperature reverse-bias (HTRB) reliability for $\mathrm{Si}\mathrm{C}$-based high-voltage devices. However, the atomic structure and carrier-trapping behavior of the $\mathrm{Si}\mathrm{C}/{\mathrm{Si}\mathrm{O}}_2$ interface defects therein and the underlying physical mechanisms of breakdown-voltage ($V_{\mathrm{BD}}$) variation are still largely unclear. Here, the $\mathrm{Si}\mathrm{C}/{\mathrm{Si}\mathrm{O}}_2$ interface defects of 4H-$\mathrm{Si}\mathrm{C}$ gate turn-off (GTO) thyristors before and after HTRB stress are investigated by transient capacitance measurements and density-functional-theory (DFT) calculations. It is found that the bias stress at 4.4 kV enlarges the interface state density at $E_C-0.60$ eV to $E_C-1.33$ eV by electron capturing. As a result, the negative interface charge is generated. As high-resolution transmission electron microscopy reveals the presence of excess carbon near the $\mathrm{Si}\mathrm{C}$ surface, DFT calculations are focused on carbon-related interface defects to clarify the atomic and electronic structures of the $\mathrm{Si}\mathrm{C}/{\mathrm{Si}\mathrm{O}}_2$ interface trap and assign them to negatively charged excess split-interstitial carbon at the interface. Furthermore, technical computer-aided-design simulation further proves that the negatively charged $\mathrm{Si}\mathrm{C}/{\mathrm{Si}\mathrm{O}}_2$ interface defect is the main cause for the observed $V_{\mathrm{BD}}$ degradation after the HTRB test, which leads to a strong electric field crowding effect. These results not only provide deep physical insights underlying $V_{\mathrm{BD}}$ degradation in HTRB-stressed high-voltage devices, but are also of significant importance in the optimizations of device structure and oxidation technology for $\mathrm{Si}\mathrm{C}/{\mathrm{Si}\mathrm{O}}_2$ interfaces in high-voltage $\mathrm{Si}\mathrm{C}$ devices.

We present a nanoparticle size-separation device based on a nanofluidic rocking Brownian motor. It features a ratchet-shaped electrostatic particle potential with increasing barrier heights along the particle transport direction. The sharp drop of the particle current with barrier height is exploited to separate a particle suspension into multiple subpopulations. By solving the Fokker-Planck equation, we show that the physics of the separation mechanism is governed by the energy landscape under forward tilt of the ratchet. For a given device geometry and sorting duration, the applied force is thus the only tunable parameter to increase the separation resolution. For the experimental conditions of 3.5 V applied voltage and 20 s sorting, we predict a separation resolution of approximately $2$ nm, supported by experimental data for separating spherical gold particles of nominal diameters of 80 and 100 nm.

In this paper we investigate an elastically linked, nonlinear, in-plane rotator system and experimentally study its nonreciprocal impulse response. The nonlinearity of the system arises from the angled elastic linkage in rotational motion. A chain of rotators coupled with such linkages reaches an acoustic vacuum when the pretension of the elastic links vanish, leading to large nonlinearity tunable via small pretension. Using an analytical model and experimental exploration, we observe a broadband nonreciprocity in a weakly pretensioned, asymmetric, three-rotator system. In addition, we use a nonlinear normal-mode (NNM) analysis, capturing the main qualitative dynamics of the response, to explain the observed nonreciprocity mechanism. The analysis shows that equal applied impulses, combined with energy-dependent frequency and mode shapes, result in robust nonreciprocity features, contrary to the reciprocal response present in the linear counterpart of this system.

Interfacing quantum mechanics and gravity is one of the great open questions in natural science. Micromechanical oscillators have been suggested as a plausible platform to carry out these experiments. We present an experimental design aiming at these goals, inspired by Schmöle et al. [Class. Quantum Grav. 33, 125031 (2016)]. Gold spheres weighing of the order of a milligram will be positioned on large silicon nitride membranes, which are spaced at submillimeter distances from each other. These mass-loaded membranes are mechanical oscillators that vibrate at about $2$ kHz frequencies in a drum mode. They are operated and measured by coupling to microwave cavities. First, we show that it is possible to measure the gravitational force between the oscillators at deep cryogenic temperatures, where thermal mechanical noise is strongly suppressed. We investigate the measurement of gravity when the positions of the gravitating masses exhibit significant quantum fluctuations, including preparation of the massive oscillators in the ground state, or in a squeezed state. We also present a plausible scheme to realize an experiment where the two oscillators are prepared in a two-mode squeezed motional quantum state that exhibits nonlocal quantum correlations and gravity at the same time. Although the gravity is classical, the experiment will pave the way for testing true quantum gravity in related experimental arrangements. In a proof-of-principle experiment, we operate a 1.7 mm diameter ${\mathrm{Si}}_3{\mathrm{N}}_4$ membrane loaded by a 1.3 mg gold sphere. At a temperature of 10 mK, we observe the drum mode with a quality factor above half a million at 1.7 kHz, showing strong promise for the experiments. Following implementation of vibration isolation, cryogenic positioning, and phase noise filtering, we foresee that realizing the experiments is in reach by combining known pieces of current technology.

High-dimensional entanglement promises to increase the information capacity of photons and is now routinely generated, exploiting spatiotemporal degrees of freedom of single photons. A curious feature of these systems is the possibility of certifying entanglement despite strong noise in the data. We show that it is also possible to exploit this noisy high-dimensional entanglement for quantum key distribution by introducing a protocol that uses multiple subspaces of the high-dimensional system simultaneously. Our protocol can be used to establish a secret key even in extremely noisy experimental conditions, where qubit protocols fail. To show that, we analyze the performance of our protocol for noise models that apply to the two most commonly used sources of high-dimensional entanglement: time bins and spatial modes.

Frequency-comb-based optical two-way time and frequency transfer (O-TWTFT) can support future ultra-precise clock networks over free-space links. However, demonstrations have thus far only operated at one corner of a complex parameter space, which balances received optical power, timing performance, and update rate. Here, we analyze the performance of O-TWTFT across this parameter space, with a specific focus on extending the link distance at constant launch power and telescope aperture. We perform experiments across a three-node network spanning 28 km of turbulent air, and successfully demonstrate a more than 2000× reduction in the required received optical power corresponding to a potential 45× increase in distance. This distance increase does come with an associated reduction in timing precision, with the uncertainty increasing from 60 as to 20 fs at 10 s averaging times. However, this level of precision is still more than adequate for most applications. In addition, it comes with a reduction in sampling rate, which potentially limits this approach to static links. Interestingly, because this system optimization does not require any hardware modifications, O-TWTFT links could be dynamically tuned to support future long-distance optical clock networks over a range of conditions and applications.

We present a theoretical study of the collective quasiparticle excitations responsible for the electromagnetic response of ultrathin plane-parallel homogeneous periodic single-wall carbon-nanotube arrays and weakly inhomogeneous single-wall carbon-nanotube films. We show that in addition to varying film composition, the collective response can be controlled by varying the film thickness. For single-type nanotube arrays, the real part of the dielectric response shows a broad negative refraction band near a quantum interband transition of the constituent nanotube, whereby the system behaves as a hyperbolic metamaterial at higher frequencies than those classical plasma oscillations have to offer. By decreasing nanotube diameters it is possible to push this negative refraction into the visible region, and using weakly inhomogeneous multitype nanotube films broadens its bandwidth.

The interactions of a moving crack with self-emitted acoustic waves are studied in ion-implanted circular plates of crystalline silicon, where complex reproducible surface patterns made of local roughness variations are observed. A simple geometrical model, considering the sole propagation of $A_0$ Lamb waves inside the assembly, allows full prediction of all of these pattern shapes and their dependence on system parameters (crack velocity, elastic properties). Acoustic waves propagating along and behind the crack front are shown to play a central role in fracture-pattern formation. When the crack front is curved, surface patterns originate independently of any edge reflection or frequency preselection from acoustic waves propagating along and behind the crack. As in case of Kelvin wake patterns, fracture patterns emerge from geometrically induced coherence.