Second-order Autocorrelation Research Articles

Remote characterization of nonlinear distortions in ultrashort pulses transmitted through dynamic fiber optic links

Ultrashort pulse sources are complex and resource-intensive. To reduce overhead and simplify operations, we had previously developed a method to deliver ultra-short pulses through fiber-optic links to multiple locations and to characterize them remotely using a compact detector module. We created a pulse pair with varying delays at the central location using a pulse shaper before launching them into the fiber links and measured the first and second-order autocorrelations at the satellite location. However, this method proved inadequate for detecting the effects of optical nonlinearities as the spectral broadening seen by a pulse pair with varying degrees of overlap differs from that of a pair of pulses undergoing nonlinear broadening separately. To overcome this drawback, we propose to launch a variable-delay pulse pair with no temporal overlap to avoid combined nonlinear distortions in the fiber link and measure the autocorrelations at the output by adding a fixed-delay interferometer to our detector module. The in-house fabricated fixed-delay element consisted of a quartz plate with its surfaces coated by partially reflecting Bragg mirrors. Using this modified setup, we have been able to detect the nonlinear distortions encountered by sub∼400fs pulses in the delivery links.

Full-field amplitude speckle decorrelation angiography.

We propose a new simple and cost-effective optical imaging technique, full-field amplitude speckle decorrelation angiography (FASDA), capable of visualizing skin microvasculature with high resolution, and sensitive to small, superficial vessels with slow blood flow and larger, deeper vessels with faster blood flow. FASDA makes use of a laser source with limited temporal coherence, can be implemented with cameras with conventional frame rates, and does not require raster scanning. The proposed imaging technique is based on the simultaneous evaluation of two metrics: the blood flow index, a contrast-based metric used in laser speckle contrast imaging, and the adaptive speckle decorrelation index (ASDI), a new metric that we defined based on the second-order autocorrelation function that considers the limited speckle modulation that occurs in partially-coherent imaging. We demonstrate excellent delineation of small, superficial vessels with slow blood flow in skin nevi using ASDI and larger, deeper vessels with faster blood flow using BFI, providing a powerful new tool for the imaging of microvasculature with significantly lower hardware complexity and cost than other optical imaging techniques.

Reducing Multiphoton Noise in Multiplexed Single-Photon Sources

Multiplexed single-photon sources can produce indistinguishable single photons with high probability in near-perfect spatial modes. Such systems, realized with optical elements having losses, can be optimized—that is, both the optimal number of multiplexed units in the sources and the optimal mean number of photon pairs generated in a multiplexed unit, for which the output single-photon probability is maximal, can be determined. The accompanying multiphoton noise of the sources, arising from the probabilistic nature of the underlying physical processes in these systems, can be detrimental in certain applications. Inspired by this fact, we develop a procedure aimed at decreasing the multiphoton noise of multiplexed single-photon sources. The procedure is based on the reoptimization of the system for the chosen value of the normalized second-order autocorrelation function characterizing the multiphoton noise. The results of this reoptimization are shown for two types of spatially multiplexed single-photon sources. We find that by applying the proposed procedure, the multiphoton noise can be considerably decreased along with a relatively low decrease in the single-photon probability. Although the method presented here is for two spatially multiplexed single-photon sources, it can be applied straightforwardly for any type of multiplexed single-photon source.

DLS homemade setup: reviewing first and second-order coherence and autocorrelation concepts of a light source in the context of nanoparticle sizes synthesized by PLAL

The size of the nanoparticles (NP) is one of the most important and essential characteristics to know the properties of the synthesized nanostructures. The most common characterization procedures are related to Scanning Electronic Microscopy (SEM), Transmission Electronic Microscopy (TEM), and Atomic Force Microscopy (AFM). Unfortunately, from a practical point of view, they represent a time-consuming procedure and require expensive equipment, which limits its application to specialized research groups. Significant attention has been paid to Dynamic Light Scattering (DLS) as a simple, fast, and reproducible method for sizing nanoparticles. However, inadequate representation of the fundamental principles of DLS and data interpretation represents two of the most important challenges related to this technique. In this work we try to provide the fundamental principles of the DLS technique, the fundamental mathematical treatment of data obtained during the optical scattering studies and provide the MATLAB code to configure non-commercial DLS equipment. Additionally, analyzes of nanoparticles obtained by pulsed laser ablation of Ag, Au, Si and W and commercial Au nanoparticles were carried out. The particle size results are compared with SEM images to calculate the percentage error of the DLS measurements. The results show an error of 5%, 3.8%, 2.1% for the Ag, Au and Si nanoparticles respectively, which proves to be an excellent approximation to the real values of nanoparticle diameter. Meanwhile, the error in size for W nanoparticles by the same technique and commercial Au nanoparticle is 29% and 12%, which shows the effect of the hydrodynamic diameter of the nanoparticles. This work ends with the analysis of the concentration of nanoparticles and its importance in reliable results of DLS measurements.

Statistical analysis of bistable feature in a 1.55-[formula omitted] vertical cavity emitting laser

Optical bistability in telecom-band lasers has attracted significant attention because of its immense potential for applications in the fields of optical communications and optical computing. In this paper, we report on an experimental investigation of the statistical properties of a 1.55-μm bistable VCSEL. The VCSEL exhibits mode switching at a current of 5.41 mA and 4.15 mA when the pump continuously increases and decreases, respectively. Both of these switching exhibit a bistable region, indicating a polarization bistability induced by the gain saturation. By calculation of second-order autocorrelation functions with injection current, we identify different regions associated with the dynamics of different dominant modes. In particular, the statistical analysis reveals more properties of the bistable laser, such as the intensity fluctuations induced by different kinds of noise.

Quantum Light Generation Based on GaN Microring toward Fully On-Chip Source.

An integrated quantum light source is increasingly desirable in large-scale quantum information processing. Despite recent remarkable advances, a new material platform is constantly being explored for the fully on-chip integration of quantum light generation, active and passive manipulation, and detection. Here, for the first time, we demonstrate a gallium nitride (GaN) microring based quantum light generation in the telecom C-band, which has potential toward the monolithic integration of quantum light source. In our demonstration, the GaN microring has a free spectral range of 330GHz and a near-zero anomalous dispersion region of over 100nm. The generation of energy-time entangled photon pair is demonstrated with a typical raw two-photon interference visibility of 95.5±6.5%, which is further configured to generate a heralded single photon with a typical heralded second-order autocorrelation g_{H}^{(2)}(0) of 0.045±0.001. Our results pave the way for developing a chip-scale quantum photonic circuit.

Spatial and temporal patterns of land surface temperature in Greenland from 2000-2019

Temperature dynamics on the island of Greenland are an important factor in shaping ecological events. Investigating the land surface temperature (LST) patterns is critical for understanding ecological dynamics across different regions. Further melting of the Greenland ice sheet could deva state marine and terrestrial ecosystems. This study used data from Moderate Resolution Imaging Spectroradiometer satellites to understand the seasonal patterns and patterns of LST over the entire island. Focusing on the period between 2000 and 2019, this study used a natural cubic spline model to identify seasonal patterns for all sub-regions. The data were seasonally adjusted and filtered with a second-order autocorrelation component. The spline was fitted again to identify the LST pattern, and a multivariate regression model was then used to adjust for spatial correlation. We illustrate that most of the land surface of Greenland hasstable temperature trends. These observed patterns in LST in Greenland during the study period suggest that the observed ice-sheet melting in Greenland within the last two decades could be due to other factors, not necessarily LST patterns.

Single photon emitter deterministically coupled to a topological corner state.

Incorporating topological physics into the realm of quantum photonics holds the promise of developing quantum light emitters with inherent topological robustness and immunity to backscattering. Nonetheless, the deterministic interaction of quantum emitters with topologically nontrivial resonances remains largely unexplored. Here we present a single photon emitter that utilizes a single semiconductor quantum dot, deterministically coupled to a second-order topological corner state in a photonic crystal cavity. By investigating the Purcell enhancement of both single photon count and emission rate within this topological cavity, we achieve an experimental Purcell factor of Fp = 3.7. Furthermore, we demonstrate the on-demand emission of polarized single photons, with a second-order autocorrelation function g(2)(0) as low as 0.024 ± 0.103. Our approach facilitates the customization of light-matter interactions in topologically nontrivial environments, thereby offering promising applications in the field of quantum photonics.

Magnon statistical properties in multiphoton-catalyzed optomagnonics

Generating magnon states with the nonclassical behavior is a key step of implementing quantum magnonic technologies for applications. We proposed a strategy of magnonic state production by using the conditional measurement, derived from the jointed action of a cavity optomagnonics device and a beam splitter. At the case of the quantum catalysis applying to the optical field of optomagnonic system, the characters of yielding catalyzed state rely not only upon the parameters of optomechanic system but also upon the catalysis photon number and the beam splitter angle. We theoretically create a kind of the catalyzed photon–magnon entangled state and examine the statistical properties of the magnon mode of two mode optomagnonic states in the fields of the mean magnon number, the magnon number distribution, the Mandel Q parameter, the second-order autocorrelation function and the Wigner function. We demonstrate that the sub-Poisson distribution, the antibunching effect and the Wigner function negativity in the magnons can be realized via multiphoton catalysis. We also find that the magnon nonclassical statistical feature may be regulated by adjusting parameters of photon catalysis. Our work will provide a good reference for producing magnonic nonclassical state.

Autocorrelation analysis for cryo-EM with sparsity constraints: Improved sample complexity and projection-based algorithms

The number of noisy images required for molecular reconstruction in single-particle cryoelectron microscopy (cryo-EM) is governed by the autocorrelations of the observed, randomly oriented, noisy projection images. In this work, we consider the effect of imposing sparsity priors on the molecule. We use techniques from signal processing, optimization, and applied algebraic geometry to obtain theoretical and computational contributions for this challenging nonlinear inverse problem with sparsity constraints. We prove that molecular structures modeled as sums of Gaussians are uniquely determined by the second-order autocorrelation of their projection images, implying that the sample complexity is proportional to the square of the variance of the noise. This theory improves upon the nonsparse case, where the third-order autocorrelation is required for uniformly oriented particle images and the sample complexity scales with the cube of the noise variance. Furthermore, we build a computational framework to reconstruct molecular structures which are sparse in the wavelet basis. This method combines the sparse representation for the molecule with projection-based techniques used for phase retrieval in X-ray crystallography.

Controlling single rare earth ion emission in an electro-optical nanocavity

Rare earth emitters enable critical quantum resources including spin qubits, single photon sources, and quantum memories. Yet, probing of single ions remains challenging due to low emission rate of their intra-4f optical transitions. One feasible approach is through Purcell-enhanced emission in optical cavities. The ability to modulate cavity-ion coupling in real-time will further elevate the capacity of such systems. Here, we demonstrate direct control of single ion emission by embedding erbium dopants in an electro-optically active photonic crystal cavity patterned from thin-film lithium niobate. Purcell factor over 170 enables single ion detection, which is verified by second-order autocorrelation measurement. Dynamic control of emission rate is realized by leveraging electro-optic tuning of resonance frequency. Using this feature, storage, and retrieval of single ion excitation is further demonstrated, without perturbing the emission characteristics. These results promise new opportunities for controllable single-photon sources and efficient spin-photon interfaces.

Temperature-dependent photoluminescence properties of single defects in AlGaN micropillars

Single-photon emitters (SPEs) are attractive as integrated platforms for quantum applications in technologically mature wide-bandgap semiconductors since their stable operation at room temperature or even at high temperatures. In this study, we systematically studied the temperature dependence of the SPE in AlGaN micropillar by experiment. The photoluminescence (PL) spectrum, PL intensity, radiative lifetime and second-order autocorrelation function measurements are investigated over the temperature range from 303 to 373 K. The point defects of AlGaN show strong zero phonon line in the wavelength range of 800–900 nm and highly antibunched photon emission even up to 373 K. Our study reveals a possible mechanism for linewidth broadening in AlGaN SPE at high temperatures. This indicates a possible key for on-chip integration applications based on this material operating at high temperatures.

Photon superbunching in cathodoluminescence of excitons in WS2 monolayer

Cathodoluminescence spectroscopy in conjunction with second-order auto-correlation measurements of allows to extensively study the synchronization of photon emitters in low-dimensional structures. Co-existing excitons in two-dimensional transition metal dichalcogenide monolayers provide a great source of identical photon emitters which can be simultaneously excited by an electron. Here, we demonstrate large photon bunching with up to of a tungsten disulfide monolayer (WS2), exhibiting a strong dependence on the electron-beam current. To further improve the excitation synchronization and the electron-emitter interaction, we show exemplary that the careful selection of a simple and compact geometry—a thin, monocrystalline gold nanodisk—can be used to realize a record-high bunching of up to . This approach to control the electron excitation of excitons in a WS2 monolayer allows for the synchronization of photon emitters in an ensemble, which is important to further advance light information and computing technologies.

Multipair-free source of entangled photons in the solid state

We investigate the effect of multiphoton emission on polarization-entangled photon pairs from a coherently driven quantum dot by comparing quantum state tomography and second-order autocorrelation measurements as a function of the excitation power. We observe that the relative (absolute) multiphoton emission probability is as low as ${p}_{m}\phantom{\rule{4pt}{0ex}}=(5.6\ifmmode\pm\else\textpm\fi{}0.6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}[{p}_{2}=(1.5\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}]$ at the maximum source brightness, with a negligible effect on the degree of entanglement. In contrast with probabilistic sources of entangled photons, the multiphoton emission probability and the degree of entanglement remain practically unchanged against the excitation power over multiple Rabi cycles, while observing oscillations in the second-order autocorrelation function by more than one order of magnitude. Our results, explained by a model which links the second-order autocorrelation function to the actual multiphoton contribution in the two-photon density matrix, highlight that quantum dots can be regarded as a multipair-free source of entangled photons in the solid state.

Silicon Nitride Waveguides with Intrinsic Single-Photon Emitters for Integrated Quantum Photonics

The recent discovery of room temperature intrinsic single-photon emitters in silicon nitride (SiN) provides the unique opportunity for seamless monolithic integration of quantum light sources with the well-established SiN photonic platform. In this work, we develop a novel approach to realize planar waveguides made of low-autofluorescing SiN with intrinsic quantum emitters and demonstrate the single-photon emission coupling into the waveguide mode. The observed emission coupling from these emitters is found to be in line with numerical simulations. The coupling of the single-photon emission to a waveguide mode is confirmed by second-order autocorrelation measurements of light outcoupled off the photonic chip by grating couplers. Fitting the second-order autocorrelation histogram yields $g^{(2)}(0)=0.35\pm0.12$ without spectral filtering or background correction with an outcoupled photon rate of $10^4$ counts per second. This demonstrates the first successful coupling of photons from intrinsic single-photon emitters in SiN to monolithically integrated waveguides made of the same material. The results of our work pave the way toward the realization of scalable, technology-ready quantum photonic integrated circuitry efficiently interfaced with solid-state quantum emitters.

ECONOMETRIC ASSESSMENT OF THE IMPACT OF ECONOMIC INDICATORS ON THE NON-OBSERVED ECONOMY OF MOLDOVA

The non-observed economy is an integral part of the modern economic system. It is a threat to economic security in conditions of slow stagnation. The relevance of this study lies in the fact that the identification of factors influencing the non-observed economy makes it possible to develop proposals to combat this phenomenon. Also, this paper describes the problems that researchers in the Republic of Moldova face when econometrically modeling the dependence of the non-observed economy on socio-economic indicators. The novelty and purpose of the study is the construction of models with one equation, where the endogenous variable is the level of the non-observed economy in value terms and the share of the non-observed economy in Gross Domestic Product. The main research methods are regression analysis and economic and mathematical modeling. At the first step of the study, the identified main factors of influence that determine the size of the shadow economy in twenty-eight countries were systematized. In the next step, the authors established the available series of statistical data published by the National Bureau of Statistics and determined exogenous variables. The authors used the program for econometric analysis EViews 9 in constructing the models. The single equation models were tested inclusively for first and second-order autocorrelation and heteroscedasticity. The constructed models showed that the non-observed economy is positively affected by the growth of the main sectors of the national economy and by an increase in the unemployment rate. While the rise in foreign trade turnover, namely imports, negatively affects the endogenous variable. There is an identical (negative) relationship between the freight turnover and the level of the non-observed economy.

Theory of two-photon absorption with broadband squeezed vacuum

We present an analytical quantum theoretic model for nonresonant molecular two-photon absorption (TPA) of broadband, spectrally multimode squeezed vacuum, including low-gain (isolated entangled photon pairs) and high-gain (bright squeezed vacuum or BSV) regimes. The results are relevant to the potential use of entangled-light TPA as a spectroscopic and imaging method. We treat the scenario that the exciting light is spatially single mode and is nonresonant with all intermediate molecular states. In the case of high gain, we find that in the case that the linewidth of the final molecular state is much narrower than the bandwidth of the exciting light, bright squeezed vacuum is found to be equally (but no more) effective in driving TPA as is a quasimonochromatic coherent-state (classical) pulse of the same temporal shape, duration, and mean photon number. Therefore, in this case the sought-for advantage of observing TPA at extremely low optical flux is not provided by broadband bright squeezed vacuum. In the opposite case that the final-state linewidth is much broader than the bandwidth of the BSV exciting light, we show that the TPA rate is proportional to the second-order intensity autocorrelation function at zero time delay ${g}^{(2)}(0)$, as expected. We derive and evaluate formulas describing the transition between these two limiting cases, that is, including the regime where the molecular linewidth and optical bandwidth are comparable, as is often the case in experimental studies. We also show that for ${g}^{(2)}(0)$ to reach the idealized form ${g}^{(2)}(0)=3+1/\overline{n}$, with $\overline{n}$ being the mean number of photons per temporal mode, it is required to compensate the dispersion inherent in the nonlinear-optical crystal used to generate the BSV.

Heralded single-photon source based on an ensemble of Raman-active molecules

Light with high mutual correlations at different frequencies can be used to create heralded single-photon sources, which may serve as the basic elements of existing quantum cryptography and quantum teleportation schemes. One of the important examples in natural systems of light with high mutual correlations is the light produced by spontaneous Raman scattering on an ensemble of molecules. In this paper, we investigate the possibility of using Raman light to create a heralded single-photon source. We show that when using Stokes scattered light for postselection of anti-Stokes scattered light, the latter may possess single-photon properties. We analyze the influence of various negative factors on the characteristics of such a heralded single-photon source, which include a time delay between Stokes and anti-Stokes photons, the finiteness of the correlation radius of an external source, and background radiation. We show that the low value of the second-order autocorrelation function of the single-photon source is preserved even when the flow of uncorrelated photons exceeds the flow of correlated photons in scattered Raman light by an order of magnitude.

In Situ Flow Cytometer Calibration and Single-Molecule Resolution via Quantum Measurement

Fluorescent biomarkers are used to detect target molecules within inhomogeneous populations of cells. When these biomarkers are found in trace amounts it becomes extremely challenging to detect their presence in a flow cytometer. Here, we present a framework to draw a detection baseline for single emitters and enable absolute calibration of a flow cytometer based on quantum measurements. We used single-photon detection and found the second-order autocorrelation function of fluorescent light. We computed the success of rare-event detection for different signal-to-noise ratios (SNR). We showed high-accuracy identification of the events with occurrence rates below even at modest SNR levels, enabling early disease diagnostics and post-disease monitoring.

A BDS/LTE-R Enhanced Data Fusion Positioning Algorithm Based on Deep Learning

ABSTRACT Integrated positioning methods in the high-velocity train control system require auxiliary equipment leading to more construction and maintenance costs. This paper proposes a BDS/LTE-R (Beidou Navigation Satellite System/Long Term Evolution-Railway) integrated positioning system based on deep learning and establishes a 7L-CNNSeven-layer convolutional neural network) enhanced model of BDS/LTE-R data fusion positioning. Firstly, the positioning principles of the BDS and LTE-R system are analyzed to construct a data space secondorder autocorrelation matrix of the results from each single positioning system, which serves as the input of the 7L-CNN model. The positioning data are output after the depth feature extraction and feature fusion. In a test based on field data, the 7L-CNN model obtains fusion results with the second-order autocorrelation matrix of positioning data space as the input. Compared with the results obtained from the earlyfusion algorithm and CNN (convolutional neural network) fusion positioning model with the input of the original positioning data, the 7L-CNN enhanced algorithm can bring better convergence accuracy for both solving velocity and positioning results according to the velocity and position errors in the east and north directions. When a satellite is out of the lock, the 7L-CNN algorithm also has a good correction effect on the single LTE-R positioning, which can meet the requirements for high-precision and continuous real-time positioning of a train.