A fiber-optic molecular dispersion spectrometer using code-division multiplexing to make simultaneous phase measurements of a hydrogen cyanide (${\mathrm{H}}^{13}{\mathrm{C}}^{14}\mathrm{N}$) $2{\nu }_3$-band P10 rovibrational transition at multiple optical frequencies is demonstrated. A second-derivative anomalous dispersion response is digitally synthesized from sets of these concurrent measurements. The synthesized measurement demonstrates 3 orders of magnitude noise suppression relative to individual measurements, has intrinsic wavelength tuning linearization, and a spectroscopic detection limit of $146\mathrm{ppb}\mathrm{m}/\sqrt{\mathrm{Hz}}$.

The coexistence of quantum and classical signals over the same optical fiber with minimal degradation of the transmitted quantum information is critical for operating large-scale quantum networks over the existing communications infrastructure. Here, we systematically characterize the quantum channel that results from simultaneously distributing approximate single-photon polarization-encoded qubits and classical light of varying intensities through fiber-optic channels of up to 15 km. Using spectrally resolved quantum process tomography with a Bayesian reconstruction method that we develop, we estimate the full quantum channel from experimental photon counting data, both with and without classical background. Furthermore, although we find the exact channel description to be a weak function of the pump polarization, we nevertheless show that the coexistent fiber-based quantum channel has high process fidelity with an ideal depolarizing channel when the noise is dominated by Raman scattering. These results provide a basis for the future development of quantum repeater designs and quantum error-correcting codes for real-world channels and inform models used in the analysis and simulation of quantum networks.

The traditional Fourier single-pixel imaging (FSI) utilizes cosinusoidal (grayscale) illumination patterns to reconstruct the image of an object. In this paper, we propose a universal mechanism in which FSI can be implemented via arbitrary periodic structured illumination not merely cosinusoidal illumination patterns. The proposed theory is verified by numerical simulations and optical experiments. In the experiment, we demonstrate the result of FSI with phase-shifting sawtooth (trapezoid, square) structured illumination modulated by a light projector. Moreover, in order to underline the high efficiency of the proposed mechanism, we perform FSI utilizing binary (square) illumination patterns generated directly by the digital micromirror devices (DMDs). Therefore, this mechanism can promote the high-performance and extensive applications of FSI in various scenarios.

Asymmetric superconducting tunnel junctions with gaps ${\mathrm{\Delta }}_1>{\mathrm{\Delta }}_2$ have been proven to show a peculiar nonlinear bipolar thermoelectric effect. This arises due to the spontaneous breaking of electron-hole symmetry in the system, and it is maximized at the matching-peak bias $|V|=V_p=({\mathrm{\Delta }}_1-{\mathrm{\Delta }}_2)/e$. In this paper, we investigate the interplay of photon-assisted tunneling (PAT) and bipolar thermoelectric generation. In particular, we show how thermoelectricity, at the matching peak, is supported by photon absorption and emission processes at the frequency-shifted sidebands $V=\pm V_p+n\hslash \omega$, $n\in \mathbb{Z}$. This represents a sort of microwave-assisted thermoelectricity. We show the existence of multiple stable solutions, being associated with different photon sidebands, when a load is connected to the junction. Finally, we discuss how the nonlinear cooling effects are modified by the PAT. The proposed device can detect millimeter-wavelength signals by converting a temperature gradient into a thermoelectric current or voltage.

Information reconciliation significantly impacts the performance of the practical continuous-variable quantum key distribution (CV QKD) system. Fixed-rate error-correction codes limit the potential applications of the CV QKD because they lead to reduced reconciliation efficiency when the signal-to-noise ratio of the quantum channel changes, further deteriorating the system’s performance. Therefore, we propose a rate-adaptive polar-coding-based reconciliation scheme for practical CV QKD systems with the time-variant quantum channel. Experimental results verify that the proposed scheme can successfully extract secret keys within the range of signal-to-noise ratios from $-0.5$ to $-4.5$ dB, and the minimum frame-error rate can be less than ${10}^{-3}$. Moreover, the proposed scheme can promote the application of the CV QKD system in a realistic environment.

Measurement-device-independent quantum key distribution (MDI QKD) is a promising method for remote key sharing that can eliminate all detector side-channel attacks. However, current security proofs often overlook the potential information leakages from the legitimate users’ devices. In a quantum version of Trojan-horse attack (THA), a malicious eavesdropper can inject bright light into the sources of a MDI QKD system and then analyze the back-reflected light to obtain their setting choices, thereby compromising the final security. Here, we derive the finite-key security bounds of decoy-state MDI QKD in the presence of THA, which significantly outperform previous analyses in terms of the secret-key rate and transmission distance. Specifically, we analyze a symmetric three-intensity decoy-state MDI QKD protocol and an efficient four-intensity decoy-state MDI QKD protocol. Our results represent a fundamental step in guaranteeing the implementation security of quantum communication systems.

The generation of electromechanical frequency combs in both air and liquid environments using a capacitive microresonator array is presented in this paper. In contrast to frequency-comb generation in purely mechanical resonators, we show that the damping-dependent threshold for comb generation can be reduced by parametrically coupling a resonant electrical circuit to the mechanical resonator. A one-dimensional lumped parameter model of the proposed system is presented and semianalytical solutions are developed to investigate the parameters influencing frequency-comb formation under various operating conditions. The results obtained with numerical simulations are experimentally validated using a commercially available microelectromechanical resonator, and frequency combs with a repetition rate sensitive to the force on the mechanical resonator are generated with a single electrical drive in air and in a liquid-filled microfluidic channel. In contrast to prior work on electromechanical frequency combs, this work represents a simple yet robust approach to generating stable combs, thereby enabling its practical use in applications, such as gas sensing and microfluidics.

Laser-driven neutrons arouse outstanding interest because of their promising uses in several fields, from basic science to materials inspection. Many experiments achieved neutron yields [${10}^8-{10}^{10}\mathrm{n}/(\text{sr s})$] suitable for applications. These results were obtained by exploiting high-energy (approximately $10$–$100\mathrm{J}$) lasers working at low repetition rates. Instead, adopting advanced target configurations like near-critical double-layer targets (DLTs) and compact, commercial lasers was slightly considered. Here, a theoretical study is performed to address neutron generation with commercial ($40$–$400\mathrm{TW}$, $1$–$15\mathrm{J}$) systems and DLTs. We investigate proton acceleration and interaction with various materials to induce ($p,n$) reactions. DLTs allow achieving $1$–$2$ orders of magnitude larger neutron yields and maximum energies 3 times higher than with single-layer targets. Then, the feasibility of two materials characterization techniques, namely fast neutron activation analysis and pulsed fast neutron resonance radiography, is assessed. The results indicate that they can be performed with commercial lasers and DLTs.

Dispersion forces (van der Waals force and Casimir force) originating from quantum fluctuations are crucial in the cohesion of microscale and nanoscale particles. The nonadditivity of these forces and the complex geometry of realistic particles make conventional additive algorithms produce unacceptable errors, and it is experimentally challenging to identify the contribution of dispersion forces from the many forces that constitute the cohesion. In this study, we use the fluctuating surface current technique, an exact scattering theory-based nonadditive algorithm, to accurately quantify the influence of geometry on dispersion forces. To characterize the complex geometries, we employ a data-adaptive spatial filtering method and three-level descriptors. Our results determine the influence of multiscale surface fluctuations on dispersion forces. Additionally, we establish a convenient formula for predicting the dispersion forces between realistic particles with complex shapes from the exact Lifshitz solution via multistage corrections.

We report depth-resolved measurements of the Meissner screening profile in several surface-treated $\mathrm{Nb}$ samples using low-energy muon spin rotation. In these experiments, implanted positive muons, whose stopping depths below $\mathrm{Nb}$’s surface are adjusted between approximately $10\mathrm{nm}$ and $150\mathrm{nm}$, reveal the field distribution inside the superconducting element via their spin precession (communicated through their radioactive decay products). We compare how the field screening is modified by different surface treatments commonly used to prepare superconducting radio-frequency cavities used in accelerator beamlines. In contrast to an earlier report [A. Romanenko et al., Appl. Phys. Lett. 104, 072601 (2014)], we find no evidence for any “anomalous” modifications to the Meissner profiles, with all data being well described by a London model. Differences in screening properties between surface treatments can be explained by changes to the carrier mean free paths resulting from dopant profiles near the material’s surface.

We investigate the thermoelectric properties of a hybrid nanodevice composed of a two-dimensional carbon-based material and a superconductor. This system presents nonlinear bipolar thermoelectricity, as induced by the spontaneous breaking of the particle-hole (PH) symmetry in a tunnel junction between bilayer graphene (BLG) and a Bardeen-Cooper-Schrieffer superconductor. In this scheme, the nonlinear thermoelectric effect, predicted and observed in superconductor-insulator-${\mathrm{superconductor}}^{\prime }$ junctions, is not affected by the competitive effect of the Josephson coupling. From a fundamental perspective, the most intriguing feature of this effect is its bipolarity. The capability to open and control the BLG gap guarantees improved thermoelectric performances that reach up to 1 mV/K, regarding the Seebeck coefficient, and a power density of $1\mathrm{nW}/\text{μ}{\mathrm{m}}^2$ for temperature gradients of tens of kelvin. Furthermore, the externally controlled gating can also dope the BLG, which is otherwise intrinsically PH symmetric, giving us the opportunity to investigate the bipolar thermoelectricity, even in the presence of the controlled suppression of the PH symmetry. The predicted robustness of this system could foster further experimental investigations and applications in the near future, thanks to the available nanofabrication techniques.

Open-path dual-comb spectroscopy has emerged as a promising technique for regional multigas monitoring with its conspicuous advantages of broadband spectral coverage, high spectral resolution, and rapid update rate. However, it is challenging to realize its full potential due to the undesirable mutual coherence of the dual-comb source and turbulence in the air path, which hinder it from field-deployed open-path applications. Here, phase-sensitive open-path dual-comb spectroscopy based on free-running combs is reported, in which dual-purpose compensation is proposed to provide immunity against both the time jitter of comb sources and turbulent noise. Broadband and high-fidelity atmospheric amplitude and phase spectra containing gas absorption and dispersion information over a 900-m turbulent air path are acquired. For the rovibrational resonances of ${\mathrm{CO}}_2$ and ${\mathrm{H}}_2\mathrm{O}$ in 6250–6660 ${\mathrm{cm}}^{-1}$, the achieved residual of the amplitude spectrum is no more than 0.01, and the average residual of the phase spectrum is 0.2 mrad, corresponding to about 0.2 as of relative timing noise or a refraction-index change of about 6 × ${10}^{-14}$ over the target path. The precision of concentration retrieval is about 3 ppm for ${\mathrm{CO}}_2$ in 30 s. A simulated gas-leakage measurement validates the dynamic monitoring capability of this system. This highly effective noise-compensation method provides the possibility of deploying the portable configuration and holds the potential to propel environmental protection and atmospheric science.

This paper presents an analytical framework for the analysis of time-varying metal-based metamaterials. Specifically, we particularize the study to time-modulated metal-air interfaces embedded between two different semi-infinite media that are illuminated by monochromatic plane waves of frequency ${\omega }_0$. The formulation is based on a Floquet-Bloch modal expansion, which takes into account the time periodicity of the structure ($T_s=2\pi /{\omega }_s)$ and integral-equation techniques. It allows us to extract the reflection and transmission coefficients as well as to derive nontrivial features about the dynamic response and dispersion curves of time-modulated metal-based screens. In addition, the proposed formulation has an associated analytical equivalent circuit that gives a physical insight into the diffraction phenomenon. Similarities and differences between space- and time-modulated metamaterials are discussed via the proposed circuit model. Finally, some analytical results are presented to validate the present framework. Good agreement is observed with numerical computations provided by a self-implemented finite-difference time-domain (FDTD) method. Interestingly, the present results suggest that time-modulated metal-based screens can be used as pulsed sources (when ${\omega }_s\ll {\omega }_0$), beam formers (${\omega }_s\sim {\omega }_0$) to redirect energy in specific regions of space, and analog samplers (${\omega }_s\gg {\omega }_0$).

The interfacial Dzyaloshinskii-Moriya interaction (iDMI) originating from a heavy-metal/ferromagnet (FM) interface plays a crucial role in stabilizing chiral spin textures in magnetic multilayers. Recently, graphene based on the low-spin-orbit-coupling (SOC) element carbon ($\mathrm{C}$) has shown a significant iDMI owing to the Rashba SOC when deposited on top of $\mathrm{Co}$. A similar enhanced SOC has been reported earlier for other $\mathrm{C}$ allotropes, namely fullerene (${\text{C}}_{60}$) and carbon nanotubes, and a sizeable DMI can be expected to emerge from them. In this context, we elucidate the magnetization-reversal mechanism, the domain-wall (DW) dynamics, and the strength of the iDMI in a $\mathrm{Pd}(4.0\mathrm{nm})/\mathrm{Co}(0.5\mathrm{nm})/{\text{C}}_{60}(t_{{\mathrm{C}}_{60}})/\text{Pd}(2.0\mathrm{nm})$ system with varying $t_{{\mathrm{C}}_{60}}$. The field-induced DW velocity measured in the creep region shows a DW velocity 2 orders of magnitude higher for a sample with 1.6 nm of ${\text{C}}_{60}$, due to a lower depinning field and spin-dependent hybridization at the $\text{Co}/{\text{C}}_{60}$ interface. Further, DMI measurements performed by use of the asymmetric DW expansion method exhibit a systematic increase in the iDMI from $-0.07$ to $-0.46\mathrm{mJ}/{\mathrm{m}}^2$ with an increase in $t_{{\mathrm{C}}_{60}}$. Such an enhanced iDMI turns an achiral Bloch wall into a chiral Néel wall. The emergence of a finite DMI ($D_{\mathrm{Co}/{\mathrm{C}}_{60}}\sim -0.11\mathrm{mJ}/{\mathrm{m}}^2$) from the $\text{Co}/{\text{C}}_{60}$ interface is particularly encouraging for the use of $\mathrm{C}$-based materials in chiral-DW-based device applications.

Although the field of two-dimensional (2D) materials has democratized materials science by making high-quality samples accessible cheaply, due to the atomically thin nature of these systems, an integration with nanostructures is almost always required to obtain a significant optical response. Traditionally, these nanostructures are fabricated via electron beam lithography or focused ion beam milling, which are expensive, and large-area fabrication can be further time-consuming. In order to overcome this problem, we report the integration of 2D semiconductors on a cost-effective and large-area fabricated nanocone platform. We show that the plasmon modes of our nanocone structures lead to photoluminescence enhancement of monolayer ${\text{WSe}}_2$ by about eight to ten times compared with the nonplasmonic case, consistent with finite-difference time-domain simulations. Excitation power-dependent measurements reveal that our nanocone platform enables a versatile route to engineering the relative exciton trion contributions to the emission.

Inhomogeneous broadening is a major limitation for the application of quantum emitters in hexagonal boron nitride to integrated quantum photonics. Here we demonstrate that so-called blue emitters with an emission wavelength of 436 nm are less sensitive to electric fields than other quantum emitters in hexagonal boron nitride. Our measurements reveal a weak, predominantly quadratic Stark effect that indicates a negligible transition dipole moment of the defect. Using these results, we discuss implications for the symmetry of the defect and use density-functional-theory simulations to identify a likely atomic structure of blue emitters in hexagonal boron nitride.

Charge noise is one of the main sources of environmental decoherence for spin qubits in silicon, presenting a major obstacle in the path towards highly scalable and reproducible qubit fabrication. Here we demonstrate in-depth characterization of the charge noise environment experienced by a quantum dot in a CMOS-fabricated silicon nanowire. We probe the charge noise for different quantum dot configurations, finding that it is possible to tune the charge noise over two orders of magnitude, ranging from $1\text{μ}{\mathrm{eV}}^2/\mathrm{Hz}$ to $100\text{μ}{\mathrm{eV}}^2/\mathrm{Hz}$. In particular, we show that the top interface and the reservoirs are the main sources of charge noise, and their effect can be mitigated by controlling the quantum dot extension. Additionally, we demonstrate a method for the measurement of the charge noise experienced by a quantum dot in the few-electron regime. We measure a comparatively high charge noise value of $40\text{μ}{\mathrm{eV}}^2/\mathrm{Hz}$ at the first electron, and demonstrate that the charge noise is highly dependent on the electron occupancy of the quantum dot.

Manipulating charge-to-spin conversion in a single magnetic layer holds great promise for optimizing efficiency of spintronic devices. Here we present experimental evidence uncovering a giant conversion anisotropy of up to 400% in fourfold-symmetric anisotropy $\mathrm{Fe}$ films on $\mathrm{Mg}\mathrm{O}(100)$ substrate by changing the direction of applied radio frequency microwave currents from the magnetic hard axis to the easy one. Together with the anisotropy of Gilbert damping and anisotropic magnetoresistance, the fourfold-symmetric anisotropy is found to stem from large emergent spin-orbit coupling in $\mathrm{Fe}$ films with local lattice distortions, which is further confirmed by selected area electron diffraction patterns in different regions. Given that $\mathrm{Fe}$ films are deposited by magnetron sputtering at room temperature, our results suggest a simple route for designing future energy-efficient magnetic devices for mass production.