The coherence of light from independent ensembles of elementary atomic emitters plays a paramount role in diverse areas of modern optics. We demonstrate the interference of photons scattered from independent ensembles of warm atoms in atomic vapor. It relies on the feasibility of the preservation of coherence of light scattered elastically in the forward and backward directions from Doppler-broadened atomic ensembles, such that photons with chaotic photon statistics from two opposite atomic velocity groups contribute to the same detection mode. While the random phase fluctuations of the scattered light caused by a large thermal motion prevent direct observability of the interference in the detected photon rate, the stable frequency difference between photons collected from scattering off counterpropagating laser beams provides strong periodic modulation of the photon coincidence rate with the period given by the detuning of the excitation laser from the atomic resonance. The presented interferometry represents a sensitive and robust methodology for Doppler-free optical atomic and molecular spectroscopy based on photon correlation measurements on scattered light.

Vapor-deposited amorphous ice, so-called amorphous solid water, exhibits complex structural and morphological transformations upon heating. A network of micropores, present at the deposition temperature (80K), collapses at 100-145K, and a glass transition takes place simultaneously above 120K. Here, we separate the two processes by allowing the micropores to collapse upon heating, which is monitored by small-angle x-ray scattering experiments. The combined micropore collapse and glass transition dynamics are studied using x-ray photon correlation spectroscopy. After cooling back down and heating a second time, we see remaining pores collapsing only near Tg. Our analysis reveals both diffusive and ballistic processes attributed to pore collapse dynamics. Fast processes (∼100Å2/s) occur only when both micropore collapse and glass transition are simultaneously at play. In other words, both processes impact on each other and lead to a speed-up. The glass transition dynamics mainly features a slow diffusive process with a diffusion coefficient of around 1Å2/s and lower. This value is in nice agreement with other work on thin and on bulk samples.

Studying photoluminescence (PL) is essential for understanding and engineering light-emitting nanomaterials. Heterogeneous samples, such as thin films and microstructures, require characterization of the spatial distribution of PL intensity, spectra, lifetimes, and photon correlations at the microscale under variable temperatures. Here we describe a cryo-optical setup for micro-PL imaging, spectroscopy, lifetime, and photon bunching measurements that addresses this need. The system combines a home-built wide-field microscope with a commercial closed-loop helium cryostat, a spectrograph equipped with an sCMOS camera, and a Hanbury-Brown and Twiss interferometer (HBT-interferometer). Optical layout and components of the setup are described, and its performance is tested on samples of CsPbBr3 nanocrystal superlattices. This work aims to demonstrate that it is feasible to develop an advanced microscopy and spectroscopy setup suitable for material scientists focused on developing novel materials, even if they initially lack extensive experience in advanced optical methods.

Single‐crystal diamond needles (SCDNs) have emerged as promising candidates for optical and quantum sensing applications due to their unique shape, high‐quality crystalline structure, and availability for relatively simple mass production. In this study, morphology modification and luminescence features of SCDNs subjected to high‐temperature oxidation in air at 650 °C and 700 °C are investigated. Significant morphological changes, including sharpening, length reduction, and surface feature formation, are revealed as a result of oxidation with scanning electron microscopy observations. The morphology modifications are dependent on oxidation temperature. Gradual sharpening and formation of surface protrusions with (100) and (111) surfaces during oxidation at 650 °C significantly accelerate at oxidation temperature increase to 700 °C. Observed formation of the surface protrusions is explained by local variation in resistance to oxidation at diamond needle surface. Furthermore, photon correlation measurements reveal that oxidation duration may be optimized allowing obtaining of SCDNs with single nitrogen‐vacancy centers situated in their tips, and confirming the viability of these diamonds for quantum sensing. This findings highlight the superior optical properties and structural integrity of SCDNs, making them highly suitable for single‐photon emission and other quantum technological applications.

Understanding protein motion within the cell is crucial for predicting reaction rates and macromolecular transport in the cytoplasm. A key question is how crowded environments affect protein dynamics through hydrodynamic and direct interactions at molecular length scales. Using megahertz X-ray Photon Correlation Spectroscopy (MHz-XPCS) at the European X-ray Free Electron Laser (EuXFEL), we investigate ferritin diffusion at microsecond time scales. Our results reveal anomalous diffusion, indicated by the non-exponential decay of the intensity autocorrelation function g2(q, t) at high concentrations. This behavior is consistent with the presence of cage-trapping between the short- and long-time protein diffusion regimes. Modeling with the δγ-theory of hydrodynamically interacting colloidal spheres successfully reproduces the experimental data by including a scaling factor linked to the protein direct interactions. These findings offer insights into the complex molecular motion in crowded protein solutions, with potential applications for optimizing ferritin-based drug delivery, where protein diffusion is the rate-limiting step.

Objectives . Gene therapy techniques based on the introduction of therapeutic nucleic acids into body cells are currently being developed for the treatment of diseases with a genetic etiology. Among modern drug delivery systems, nonviral agents based on the use of a variety of lipids to produce liposomes and micelles occupy a special place. This work sets out to synthesize and study the properties of dimeric cationic amphiphiles of irregular structure with symmetric and asymmetric hydrophobic blocks in order to determine the influence of structure on physicochemical properties and evaluate the prospects of their application as transfection agents. Methods . The formation of hydrophobic and hydrophilic blocks involves reactions of L-cystine derivatives and L-glutamic acid and diethanolamine diesters using the condensing agents: dicyclohexylcarbodiimide (DCC) + 4-(dimethylamino)pyridine (DMAP) or hexafluorophosphate benzotriazole tetramethyl uranium (HBTU) + diisopropylethylamine (DIPEA). In order to isolate the reaction products from the reaction mixture, column chromatography and/or preparative thin-layer chromatography on silica gel were used. The structure of the obtained compounds was confirmed by 1H nuclear magnetic resonance spectroscopy and mass spectrometry. Synthesized lipopeptides in aqueous medium formed liposomal dispersions whose particle size was determined by photon correlation spectroscopy. Results . Schemes for the preparation of novel dimeric cationic amphiphiles based on L-cystine derivatives were devised. The hydrophobic blocks of the obtained compounds include diesters of diethanolamine and L-glutamic acid (C10, C14, and C16). Targeted lipopeptides were used to obtain liposomal dispersed systems mixed with natural lipids. The hydrodynamic size of the particles formed in all dispersions was determined to be within the range of 50 to 200 nm. Conclusions . The physicochemical properties of aqueous dispersions based on the synthesized compounds were investigated. Dimeric amphiphiles mixed with phosphatidylcholine and cholesterol form liposomal particles. The impact of amphiphile structure on aggregate size was demonstrated. The number of L-ornithine residues (0, 1, 2) in the target products was found to be the most significant parameter affecting the particle size.

In the past couple of decades, there have been significant advances in measuring quantum properties of light, such as quadratures of squeezed light and single-photon counting. Here, we explore whether such tools can be leveraged to probe electronic correlations in the many-body quantum regime. Specifically, we show that it is possible to probe certain spin, charge, and topological orders in an electronic system by measuring the correlation functions of scattered photons. We construct a mapping from the correlators of the scattered photons to those of a correlated insulator, particularly for Mott insulators described by a single-band Fermi-Hubbard model at half filling. We show that frequency filtering before photodetection plays a crucial role in determining this mapping. We find that if the ground state of the insulator is a gapped spin liquid, a photon-pair correlation function, i.e., G ( 2 ) , can detect the presence of anyonic excitations with fractional mutual statistics. Moreover, we show that correlations between electromagnetic quadratures can be used to detect expectation values of static spin chirality operators on both the kagome and triangular lattices, thus being useful in detecting chiral spin liquids. More generally, we show that a series of hitherto unmeasured spin-spin and spin-charge correlation functions of the material can be extracted from photonic correlations. This work opens up access to probe correlated materials, beyond the linear-response paradigm, by detecting quantum properties of scattered light.

X-ray photon correlation scattering measurements are undertaken on a thermally active artificial spin ice based on the square lattice, referred to as artificial square ice, to probe the fluctuation timescales as a function of temperature as the system passes through the paramagnetic-antiferromagnetic phase transition, which belongs to the two-dimensional Ising universality class. In the paramagnetic regime, a single exponential timescale is seen, whereas at and below the critical temperature, a stretched exponential decorrelation is observed, with the stretching exponent decreasing from unity down to below one-half as the temperature reduces. This trend is confirmed by kinetic Monte Carlo simulations of a simplified point-dipolar square ice system, and is in agreement with past theoretical work on the kinetic Ising model where stretched exponential relaxation due to equilibrium domain wall dynamics below the critical temperature was found.

Experimental benchmarking of transport coefficients under extreme conditions is required for validation of differing theoretical models. To date, measurement of transport properties of dynamically compressed samples remains a challenge with only a limited number of studies able to quantify transport in high pressure and temperature matter. x-ray photon correlation spectroscopy utilizes coherent x-ray sources to measure time correlations of density fluctuations, thus providing measurements of length and timescale-dependent transport properties. Here, we present a first-of-a-kind experiment to conduct x-ray photon correlation spectroscopy in laser shock compression experiments. We report measurement of the turbulent velocity in the wake of a laser driven supersonic shock and place an upper bound on thermal diffusivity in a solid density plasma on nanosecond timescales.

Dynamics in liquids and glasses can be assessed using X-ray photon correlation spectroscopy or electron correlation microscopy, which involves measuring the temporal changes in diffraction patterns. Two methods are commonly used to evaluate these temporal changes: one-time correlation function or two-time correlation function. However, the specific characteristics of these methods have not been thoroughly studied. In this study, we investigated the differences between these methods and found that the two-time correlation function can measure dynamics for longer periods than the method relying on the one-time correlation function. Additionally, we demonstrated that the two-time correlation function exhibits a weak dependence on the amount of dose applied.

In resonance fluorescence excitation experiments, light emitted from solid-state quantum emitters is typically filtered to eliminate the laser photons, ensuring that only red-shifted Stokes photons are detected. However, theoretical analyses of the fluorescence intensity correlation often model emitters as two-level systems, focusing on light emitted exclusively from the purely electronic transition (the zero-phonon line), or they rely on statistical approaches based on conditional probabilities that neglect the quantum coherence between the emitters and the coherence between the electric fields they generate. Here, we propose a model to characterize the correlation of either zero-phonon line photons or Stokes-shifted photons. This model successfully reproduces the experimental correlation of Stokes-shifted photons emitted from two interacting molecules and predicts that this correlation is affected by quantum coherence. Besides, we analyze the role of quantum coherence in the Stokes-shifted emission from two distant emitters, showing a sharp peak at zero time delay due to the Hanbury Brown-Twiss effect.

The yielding of soft materials is critical to many natural and industrial processes, yet experimental insights into microscopic aspects of yielding are limited. This study combines angle X-ray scattering, X-ray photon correlation spectroscopy, and in situ rheology (Rheo-SAXS-XPCS) with fast lubrication dynamics simulations to examine how interparticle interactions influence yielding in charged colloidal suspensions. By tuning attraction through salt addition, we compare repulsive and attractive systems under deformation. Repulsive suspensions yield uniformly with Andrade-like creep and minimal structural change. In contrast, attractive suspensions show complex behaviors, including shear banding, delayed yielding, and resolidification, governed by transient dynamics at shear band interfaces. These results directly link microscopic particle dynamics to macroscopic flow and demonstrate how interaction potentials control rheological behavior. This work offers a framework for designing soft materials with tailored properties for applications in coatings, food processing, drug delivery, and other technologies requiring precise mechanical control.

We propose a hybrid cavity-magnon system scheme that integrates a yttrium iron garnet sphere with a microwave cavity filled with Kerr medium, aiming to achieve both conventional and unconventional photon blockade simultaneously. Our research demonstrates that by tuning the strength of the Kerr nonlinearity and adjusting the two-photon driving intensity, we can realize quantum effects associated with both types of photon blockade. Furthermore, through careful selection of system parameters, we have obtained optimal results for photon blockade under ideal conditions. This scheme fully exploits the advantages offered by both conventional and unconventional photon blockade mechanisms, effectively reducing the equal-time second-order photon correlation function while also achieving a higher mean photon number. It provides an alternative and experimentally feasible platform for preparing single-photon sources characterized by enhanced purity and brightness.

Abstract Free electrons are a universal source of electromagnetic fields, and fundamentally their quantized energy exchange may facilitate generating tunable quantum light. Because the quantum features of the emitted radiation are encoded in the joint electronic and photonic state, they can only be revealed by a measurement accessing both subsystems. Here we demonstrate the coherent parametric generation of such non-classical states of light by free electrons. Investigating electron–photon correlations, we show that the quantized electron energy loss heralds the number of photons generated in a dielectric waveguide. In Hanbury Brown–Twiss measurements, we observe an electron-heralded single-photon state using antibunching intensity correlation, whereas two-quantum energy losses of individual electrons yield pronounced two-photon coincidences. Our results will enable the tailored preparation of higher-number Fock and other optical quantum states on the basis of controlled interactions with free-electron beams.

Optimizing the quantum interference between single photons and local oscillator with photon correlations

Strongly correlated photons play a crucial role in modern quantum technologies. Here, we investigate the probability of generating strongly correlated photons in a chain of N qubits coupled to a one-dimensional waveguide. We found that disorder in the transition frequencies can induce photon antibunching and especially nearly perfect photon blockade events in the transmission and reflection outputs. As a comparison, in ordered chains, strongly correlated photons cannot be generated in the transmission output, and only weakly antibunched photons are found in the reflection output. The occurrence of nearly perfect photon blockade events stems from the disorder-induced nearly completely destructive interference of photon scattering paths. Our Letter highlights the impact of disorder on photon correlation generation and suggests that disorder can enhance the potential for achieving strongly correlated photons.

Correlation microscopy is an emerging technique for improving optical resolution. By taking advantage of the photon statistics from single-photon fluorophores, more information about the emitters (including number and location) is obtained compared with classical microscopy. Although it is known that the resolution can be improved by increasing detector numbers, as well as using photon correlations, the quantitative relationship between these two approaches is not immediately clear. Here we explore widefield photon correlation microscopy using arrays of single-photon detectors. We explicitly compare the use of N detectors used in photon counting mode vs N/2 detectors used to measure photon correlations. i.e., where there are N/2 Hanbury Brown and Twiss systems, using the same N detectors, on randomly generated two-emitter systems and triangular three-emitter systems. We find regimes where N/2 Hanbury Brown and Twiss detectors provide improved localisation compared to N photon counting detectors, as a function of emitter position and number of photons sampled.

Frequency-filtered photon correlations have been proven to be extremely useful in grasping how the detection process alters photon statistics. Harnessing the spectral correlations also permits refinement of the emission and unraveling of previously hidden strong correlations in a plethora of quantum-optical systems under continuous-wave excitation. In this work, we investigate such correlations for time-dependent excitation and develop a methodology to compute efficiently time-integrated correlations, which are at the heart of the photon-counting theory, and subsequently apply it to analyze the photon emission of pulsed systems. By combining this formalism with the —which facilitates frequency-resolved correlations—we demonstrate how spectral filtering enhances single-photon purity and suppresses multiphoton noise in time-bin-encoded quantum states. Specifically, filtering the central spectral peak of a dynamically driven two-level system boosts temporal coherence and improves the fidelity of time-bin entanglement preparation, even under conditions favoring multiphoton emission. These results establish spectral filtering as a critical tool for tailoring photon statistics in pulsed quantum light sources.

Understanding mesoscale structural dynamics of precipitation-strengthened alloys is essential for optimizing the mechanical performances of these alloys. Herein, we establish a multimodal coherent X-ray diffraction imaging framework for spatiotemporal mapping of mesoscale structural dynamics in precipitation-strengthened alloys. As a demonstrative application, we visualized the structural evolution in Mg97Zn1Gd2 during isothermal annealing at 700 K, revealing real-time dynamics of nucleation, growth, and coarsening. Ptychographic reconstruction enabled imaging of microstructural transformations across a wide field of view (~100 μm2) with temporal resolution spanning several hours. We observed decomposition of (Mg, Zn)3Gd and concurrent precipitation and coarsening of long-period stacking ordered phases. To resolve local dynamics at finer spatiotemporal scales, we combined dynamic coherent diffraction imaging with X-ray photon correlation spectroscopy, targeting selected regions (~10 μm2) with time resolution down to tens of seconds. This approach revealed the rapid formation of nanoscale precipitates within 10 s after heating, followed by coarsening over several hundred seconds. Additionally, we applied optical flow analysis-a computational method to track motion patterns-to visualize and quantify the nucleation, growth, and coarsening kinetics. The abovementioned findings demonstrate the capability of in situ coherent X-ray techniques to acquire the real-time evolutions of mesoscale structures in complex materials. Our methodology offers a robust framework for investigating dynamic phenomena in diverse material systems, including metals, polymers, and functional nanomaterials, under realistic thermal or mechanical conditions.

Colloidal semiconductor quantum dots (QDs) can generate multiple excitons (MXs) within a single QD. Owing to their large absorption cross-section, efficient utilization of MX is anticipated for the development of light-harvesting systems. However, MXs typically undergo nonradiative decay via Auger recombination (AR). In this study, we investigated the possibility of energy transfer from the MXs in a single QD to multiple cyanine dyes (Cy3) adsorbed on the QD surface. Based on the linear relationship between the AR lifetime and QD volume, formamidinium lead bromide perovskite nanocrystals (PNCs) with three distinct size distributions were synthesized. Simultaneous measurements of emission photon correlations and individual pristine PNC sizes revealed that the probability of multiphoton emission increased in PNCs larger than 12 nm. This was attributed to a slowdown in the AR rate, which was evaluated by transient absorption spectroscopy. Subsequently, multiple Cy3 dyes were adsorbed onto the PNCs as energy acceptors to evaluate the transfer of energy from the MXs to multiple Cy3 dyes. Photon correlation measurements of Cy3 emission via energy transfer─induced by excitation of a single PNC─showed an increased probability of multiphoton emission in PNC-Cy3 systems with PNC sizes exceeding 12 nm. These findings indicate that multiple Cy3 dyes were excited and emitted via the energy transfer from MXs. Thus, we clearly demonstrate that energy transfer from MXs to multiple surface dyes is feasible using large-sized PNCs.