Cross-correlation Experiments Research Articles

Insights into Ligand-Mediated Activation of an Oligomeric Ring-Shaped Gene-Regulatory Protein from Solution- and Solid-State NMR

The 91 kDa oligomeric ring-shaped ligand binding protein TRAP (trp RNA binding attenuation protein) regulates the expression of a series of genes involved in tryptophan (Trp) biosynthesis in bacilli. When cellular Trp levels rise, the free amino acid binds to sites buried in the interfaces between each of the 11 (or 12, depending on the species) protomers in the ring. Crystal structures of Trp-bound TRAP show the Trp ligands are sequestered from solvent by a pair of loops from adjacent protomers that bury the bound ligand via polar contacts to several threonine residues. Binding of the Trp ligands occurs cooperatively, such that successive binding events occur with higher apparent affinity but the structural basis for this cooperativity is poorly understood. We used solution methyl-TROSY NMR relaxation experiments focused on threonine and isoleucine sidechains, as well as magic angle spinning solid-state NMR 13C–13C and 15N-13C chemical shift correlation spectra on uniformly labeled samples recorded at 800 and 1200 MHz, to characterize the structure and dynamics of the protein. Methyl 13C relaxation dispersion experiments on ligand-free apo TRAP revealed concerted exchange dynamics on the µs-ms time scale, consistent with transient sampling of conformations that could allow ligand binding. Cross-correlated relaxation experiments revealed widespread disorder on fast timescales. Chemical shifts for methyl-bearing side chains in apo- and Trp-bound TRAP revealed subtle changes in the distribution of sampled sidechain rotameric states. These observations reveal a pathway and mechanism for induced conformational changes to generate homotropic Trp-Trp binding cooperativity.

A set of cross-correlated relaxation experiments to probe the correlation time of two different and complementary spin pairs

Intrinsically disordered proteins (IDPs) defy the conventional structure-function paradigm by lacking a well-defined tertiary structure and exhibiting inherent flexibility. This flexibility leads to distinctive spin relaxation modes, reflecting isolated and specific motions within individual peptide planes. In this work, we propose a new pulse sequence to measure the longitudinal 13C′ CSA-13C′-13Cα DD CCR rate ΓzC′/C′Cα and present a novel 3D version of the transverse ΓxyC′/C′Cα CCR rate, adopting the symmetrical reconversion approach. We combined these rates with the analogous ΓxyN/NH and ΓzN/NH CCR rates to derive residue-specific correlation times for both spin-pairs within the same peptide plane. The presented approach offers a straightforward and intuitive way to compare the correlation times of two different and complementary spin vectors, anticipated to be a valuable aid to determine IDPs backbone dihedral angles distributions. We performed the proposed experiments on two systems: a folded protein ubiquitin and Coturnix japonica osteopontin, a prototypical IDP. Comparative analyses of the results show that the correlation times of different residues vary more for IDPs than globular proteins, indicating that the dynamics of IDPs is largely heterogeneous and dominated by local fluctuations.

Spin cross-correlation experiments in an electron entangler.

Correlations are fundamental in describing many-body systems. However, in experiments, correlations are notoriously difficult to assess on a microscopic scale, especially for electron spins. Even though it is firmly established theoretically that the electrons in a Cooper pair1 of a superconductor form maximally spin-entangled singlet states with opposite spin projections2-4, no spin correlation experiments have been demonstrated so far. Here we report the direct measurement of the spin cross-correlations between the currents of a Cooper pair splitter5-13, an electronic device that emits electrons originating from Cooper pairs. We use ferromagnetic split-gates14,15, compatible with nearby superconducting structures, to individually spin polarize the transmissions of the quantum dots in the two electronic paths, which act as tunable spin filters. The signals are detected in standard transport and in highly sensitive transconductance experiments. We find that the spin cross-correlation is negative, consistent with spin singlet emission, and deviates from the ideal value mostly due to the overlap of the Zeeman split quantum dot states. Our results demonstrate a new route to perform spin correlation experiments in nano-electronic devices, especially suitable for those relying on magnetic field sensitive superconducting elements, like triplet or topologically non-trivial superconductors16-18, or to perform Bell tests with massive particles19,20.

Dynamics of Ultrafast Phase Transitions in (001) Si on the Shock-Wave Front.

We demonstrate an ultrafast (<0.1 ps) reversible phase transition in silicon (Si) under ultrafast pressure loading using molecular dynamics. Si changes its structure from cubic diamond to β-Sn on the shock-wave front. The phase transition occurs when the shock-wave pressure exceeds 11 GPa. Atomic volume, centrosymmetry, and the X-ray-diffraction spectrum were revealed as effective indicators of phase-transition dynamics. The latter, being registered in actual experimental conditions, constitutes a breakthrough in the path towards simple X-ray optical cross-correlation and pump-probe experiments.

TRACT revisited: an algebraic solution for determining overall rotational correlation times from cross-correlated relaxation rates.

Accurate rotational correlation times (τc) are critical for quantitative analysis of fast timescale NMR dynamics. As molecular weights increase, the classic derivation of τc using transverse and longitudinal relaxation rates becomes increasingly unsuitable due to the non-trivial contribution of remote dipole-dipole interactions to longitudinal relaxation. Derivations using cross-correlated relaxation experiments, such as TRACT, overcome these limitations but are erroneously calculated in 65% of the citing literature. Herein, we developed an algebraic solutions to the Goldman relationship that facilitate rapid, point-by-point calculations for straightforward identification of appropriate spectral regions where global tumbling is likely to be dominant. The rigid-body approximation of the Goldman relationship has been previously shown to underestimate TRACT-based rotational correlation time estimates. This motivated us to develop a second algebraic solution that employs a simplified model-free spectral density function including an order parameter term that could, in principle, be set to an average backbone S2 ≈ 0.9 to further improve the accuracy of τc estimation. These solutions enabled us to explore the boundaries of the Goldman relationship as a function of the H-N internuclear distance ( r ), difference of the two principal components of the axially-symmetric 15N CSA tensor (ΔδN), and angle of the CSA tensor relative to the N-H bond vector (θ). We hope our algebraic solutions and analytical strategies will increase the accuracy and application of the TRACT experiment.

Ultralong-distance quantum correlations in three-terminal Josephson junctions

The production of entangled pairs of electrons in ferromagnet-superconductor-ferromagnet or normal metal-superconductor-normal metal three-terminal structures has aroused considerable interest in the last twenty years. In these studies, the distance between the contacts is limited by the zero-energy superconducting coherence length. Here, we demonstrate nonlocality and quantum correlations in voltage-biased three-terminal Josephson junctions over the ultralong distance that exceeds the superconducting coherence length by orders of magnitude. The effect relies on} the interplay between the time-periodic Floquet-Josephson dynamics, Cooper pair splitting and long-range coupling similar to the two-terminal Tomasch effect. We find cross-over between the ``Floquet-Andreev quartets'' (if the spatial separation is smaller than the superconducting coherence length), and the ``ultralong-distance Floquet-Tomasch clusters of Cooper pairs'' if the separation exceeds the superconducting coherence length, possibly reaching the same $\simeq 30\,\mu$m as in the Tomasch experiments. The effect can be detected with DC-transport and zero-frequency quantum current-noise cross-correlation experiments, and it can be used for fundamental studies of superconducting quasiparticle quantum coherence in the circuits of quantum engineering.

An Easy-Implemented On-Chip Waveguide Coupled Single Photon Source Based on Self-Assembled Quantum Dots Membrane

In recent years, many groups and institutions have been committed to the research of integrated quantum photonic circuit technologies, of which the key components are waveguide coupled single photon sources. In this study, we propose an on-chip waveguide-coupled single photon source that is easily implemented as the waveguide is directly made from the quantum dot membrane. In order to scatter light out of the on-chip waveguide plane into the detection apparatus, grating output couplers are made at both ends of the waveguide. The photon statistics of the on-chip photon source were investigated by second-order correlation function g(2)(τ) measurements using a Hanbury Brown and Twiss interferometer. From the spectra and cross-correlation experiments by collecting emission at the point of quantum dot and out coupler, the emitting of single photons from the same quantum dot and propagating via the waveguide to the out couplers was confirmed. These results show that we have achieved an on-chip single photon source that is easily implemented and easily integrated into quantum photonic circuits.

Normal modes separation by use of ambient noise cross-correlation function in shallow sea

The possibility to extract the time-domain Green's function from ambient noise cross-correlation function was demonstrated by a number of authors. This approach has a weak point: the accumulation time of the noise signal. The accumulation time is significantly reduced when using mode signals and separating normal modes. Both numerical study of the spectrogram of the noise cross-correlation function and experiment in Arctic shelf demonstrated that dispersion curves observed in the spectrogram in the case of large distances between hydrophones, with a decrease in this distance are transformed into separate maxima localized both in frequency and in time delay. These maxima arise due to the presence of a stationary phase point near the minimum of the group velocities of the modes, and also due to modes interference, which is more efficient at small distances between hydrophones. At small distances, working with a signal in the frequency band where only the lowest hydroacoustic mode is excited has a number of advantages, the main of which is a decrease in the accumulation time. We demonstrate the change in the regime of modes separation with a change in the distance between hydrophones and discuss the optimal conditions for the applicability of the method.

(Invited) Coupled Quantum Defects of DNA Functionalized Carbon Nanotubes

Single wall carbon nanotubes (SWCNTs) with sp3 defects created via covalent bonding of organic functional groups are rapidly emerging as a new material system for chemistry, physics, material science, engineering and quantum technologies. They are often referred to as “organic color centers” as they can mimic quantum mechanical properties of solid-state color centers.1 Their unique properties include highly enhanced molecularly wavelength-tunable fluorescence and room temperature single photon generation with 99% single photon purity at 1.55 µm telecommunication wavelength.2 To date, diazonium chemistry has been the chemists’ tool of the choice for creating sp3 defects sparsely distributed at random positions along the sidewall of SWCNTs. While such distribution is ideally suited for single photon generation, an ability to incorporate multiple coupled quantum defects in a well-defined spatial pattern is highly desirable for more sophisticated quantum information science applications involving entanglement. Recent discovery of covalently functionalizing SWCNTs with single-stranded DNA (ssDNA)3 open a possible path toward achieving this goal. In this process, SWCNTs form covalent bonds to the guanine nucleotides when aqueous suspensions of SWCNTs coated with ssDNA are exposed to singlet oxygen. As a result, creating a regular array of covalent defect with spacing defined by the ssDNA sequence become a possibility. To further explore this exciting possibility, we investigated intrinsic PL emission properties of ssDNA functionalized SWCNTs through low temperature single tube PL spectroscopy. In contrast to individual aryl functionalized SWCNTs that usually emit a single sharp isolated PL peaks, multiple PL peaks with their emission wavelengths varying from 1050 to 1300 nm were observed from individual ssDNA functinalized SWCNTs. We further conducted 2nd order photon correlation (g(2)) and cross-correlation experiments to investigate their quantum light emission properties. We observed photon anti-bunching behavior when an individual PL emission peak is spectrally isolated indicating that ssDNA functionalization creates a series of quantum defects along the length of SWCNTs. Interestingly, when the g(2) experiment is performed on combined emission of multiple (2—4) PL peaks, an incomplete antibunching with g(2)(0) values in the range of 0.2-0.4 is observed. Because these values are significantly smaller than g(2)(0) values estimated for 2-4 independent quantum emitters (0.5-0.75), we interpret that the series of defects created via ssDNA functionalization are coupled. Our photon cross-correlation experiment performed between two emission peaks of the group also reveal evidence of coupling. 1 Srinivasan, K. & Zheng, M. Nat. Photon. 11, 535 (2017). 2 Brozena, A.H., Kim, M., Powell, L.R. & Wang, Y. Nat. Rev. Chem. 3, 375-392 (2019). 3 Zheng, Y., Bachilo, S. M., Weisman, R. B. ACS Nano, 13, 8222 (2019)

A novel high-dimensional NMR experiment for resolving protein backbone dihedral angle ambiguities

Intrinsically disordered proteins (IDPs) are challenging established structural biology perception and urge a reassessment of the conventional understanding of the subtle interplay between protein structure and dynamics. Due to their importance in eukaryotic life and central role in protein interaction networks, IDP research is a fascinating and highly relevant research area in which NMR spectroscopy is destined to be a key player. The flexible nature of IDPs, as a result of the sampling of a vast conformational space, however, poses a tremendous scientific challenge, both technically and theoretically. Pronounced signal averaging results in narrow signal dispersion and requires higher dimensionality NMR techniques. Moreover, a fundamental problem in the structural characterization of IDPs is the definition of the conformational ensemble sampled by the polypeptide chain in solution, where often the interpretation relies on the concept of ‘residual structure’ or ‘conformational preference’. An important source of structural information is information-rich NMR experiments that probe protein backbone dihedral angles in a unique manner. Cross-correlated relaxation experiments have proven to fulfil this task as they provide unique information about protein backbones, particularly in IDPs. Here we present a novel cross-correlation experiment that utilizes non-uniform sampling detection schemes to resolve protein backbone dihedral ambiguities in IDPs. The sensitivity of this novel technique is illustrated with an application to the prototypical IDP alpha-Synculein for which unexpected deviations from random-coil-like behaviour could be observed.

Advantages of the cross-correlation method for estimating nanoparticle sizes in suspensions

This article presents a cross-correlation method for estimation the size of nanoparticles in monodisperse colloidal solutions and suspensions. The original scheme of the experimental setup for carrying out cross-correlation experiments is presented. The results of measurements for a liquid sample with dispersed particles are presented for the cross-correlation method and the classical dynamic light scattering method, in which the autocorrelation function is calculated instead of the cross-correlation function of the scattering signals. The results obtained demonstrate the fact that for highly diluted samples, autocorrelation and cross-correlation produce the same result. Since the cross-correlation method is also applicable to turbid samples, this gives it a significant advantage over the dynamic light scattering method.

Accessing the dark exciton spin in deterministic quantum-dot microlenses

The dark exciton state in semiconductor quantum dots (QDs) constitutes a long-lived solid-state qubit which has the potential to play an important role in implementations of solid-state-based quantum information architectures. In this work, we exploit deterministically fabricated QD microlenses which promise enhanced photon extraction, to optically prepare and read out the dark exciton spin and observe its coherent precession. The optical access to the dark exciton is provided via spin-blockaded metastable biexciton states acting as heralding states, which are identified by deploying polarization-sensitive spectroscopy as well as time-resolved photon cross-correlation experiments. Our experiments reveal a spin-precession period of the dark exciton of (0.82 ± 0.01) ns corresponding to a fine-structure splitting of (5.0 ± 0.7) μeV between its eigenstates ↑⇑±↓⇓. By exploiting microlenses deterministically fabricated above pre-selected QDs, our work demonstrates the possibility to scale up implementations of quantum information processing schemes using the QD-confined dark exciton spin qubit, such as the generation of photonic cluster states or the realization of a solid-state-based quantum memory.

Coincidence timing of femtosecond optical pulses in an X-ray free electron laser

Femtosecond resolution pump-probe experiments are now routinely carried out at X-ray Free Electron Lasers, enabled by the development of cross-correlation “time-tools” which correct the picosecond-level jitter between the optical and X-ray pulses. These tools provide very accurate, &amp;lt;10 fs, measurement of the relative arrival time, but do not provide a measure of the absolute coincidence time in the interaction. Cross-correlation experiments using transient reflectivity in a crystal are commonly used for this purpose, and to date no quantitative analysis of the accuracy or stability of absolute coincidence time determination has been performed. We have performed a quantitative analysis of coincidence timing at the SACLA facility through a cross-correlation of 100 ± 10 fs, 400 nm optical pulses with 7 fs, 10.5 keV X-ray pulses via transient reflectivity in a cerium-doped yttrium aluminum garnet crystal. We have modelled and fit the transient reflectivity, which required a convolution with a 226 ± 12 fs uncertainty that was believed to be dominated by X-ray and laser intensity fluctuations, or assuming an extinction depth of 13.3 μm greater than the literature value of 66.7 μm. Despite this, we are able to determine the absolute coincidence time to an accuracy of 30 fs. We discuss the physical contributions to the uncertainty of coincidence time determination, which may include an uncharacterised offset delay in the development of transient reflectivity, including cascading Auger decays, secondary ionisation and cooling processes. Additionally, we present measurements of the intrinsic short-term and long-term drifts between the X-rays and the optical laser timing from time-tool analysis, which is dominated by a thermal expansion of the 25 m optical path between tool and the interaction region, seen to be ∼60 fs over a period of 5 h.

VICS82: The VISTA–CFHT Stripe 82 Near-infrared Survey

Abstract We present the VISTA–CFHT Stripe 82 (VICS82) survey: a near-infrared (J+K s ) survey covering 150 square degrees of the Sloan Digital Sky Survey (SDSS) equatorial Stripe 82 to an average depth of J = 21.9 AB mag and K s = 21.4 AB mag (80% completeness limits; 5σ point-source depths are approximately 0.5 mag brighter). VICS82 contributes to the growing legacy of multiwavelength data in the Stripe 82 footprint. The addition of near-infrared photometry to the existing SDSS Stripe 82 coadd ugriz photometry reduces the scatter in stellar mass estimates to dex for galaxies with at , and offers improvement compared to optical-only estimates out to , with stellar masses constrained within a factor of approximately 2.5. When combined with other multiwavelength imaging of the Stripe, including moderate-to-deep ultraviolet (GALEX), optical and mid-infrared (Spitzer-IRAC) coverage, as well as tens of thousands of spectroscopic redshifts, VICS82 gives access to approximately 0.5 Gpc3 of comoving volume. Some of the main science drivers of VICS82 include (a) measuring the stellar mass function of galaxies out to ; (b) detecting intermediate-redshift quasars at (c) measuring the stellar mass function and baryon census of clusters of galaxies, and (d) performing cross-correlation experiments of cosmic microwave background lensing in the optical/near-infrared that link stellar mass to large-scale dark matter structure. Here we define and describe the survey, highlight some early science results, and present the first public data release, which includes an SDSS-matched catalog as well as the calibrated pixel data themselves.

Simple Floquet-Wannier-Stark-Andreev viewpoint and emergence of low-energy scales in a voltage-biased three-terminal Josephson junction

A three-terminal Josephson junction consists of three superconductors coupled coherently to a small nonsuperconducting island, such as a diffusive metal, a single or double quantum dot. A specific resonant single quantum dot three-terminal Josephson junction $({S}_{a},{S}_{b},{S}_{c})$ biased with voltages $(V,\ensuremath{-}V,0)$ is considered, but the conclusions hold more generally for resonant semiconducting quantum wire setups. A simple physical picture of the steady state is developed, using Floquet theory. It is shown that the equilibrium Andreev bound states (for $V=0$) evolve into nonequilibrium Floquet-Wannier-Stark-Andreev (FWS-Andreev) ladders of resonances (for $V\ensuremath{\ne}0$). These resonances acquire a finite width due to multiple Andreev reflection (MAR) processes. We also consider the effect of an extrinsic linewidth broadening on the quantum dot, introduced through a Dynes phenomenological parameter. The dc-quartet current manifests a crossover between the extrinsic relaxation dominated regime at low voltage to an intrinsic relaxation due to MAR processes at higher voltage. Finally, we study the coupling between the two FWS-Andreev ladders due to Landau-Zener-St\"uckelberg transitions, and its effect on the crossover in the relaxation mechanism. Three important low-energy scales are identified, and a perspective is to relate those low-energy scales to a recent noise cross-correlation experiment (Y. Cohen et al., arXiv:1606.08436).

Cascaded emission of single photons from the biexciton in monolayered WSe2.

Monolayers of transition metal dichalcogenide materials emerged as a new material class to study excitonic effects in solid state, as they benefit from enormous Coulomb correlations between electrons and holes. Especially in WSe2, sharp emission features have been observed at cryogenic temperatures, which act as single photon sources. Tight exciton localization has been assumed to induce an anharmonic excitation spectrum; however, the evidence of the hypothesis, namely the demonstration of a localized biexciton, is elusive. Here we unambiguously demonstrate the existence of a localized biexciton in a monolayer of WSe2, which triggers an emission cascade of single photons. The biexciton is identified by its time-resolved photoluminescence, superlinearity and distinct polarization in micro-photoluminescence experiments. We evidence the cascaded nature of the emission process in a cross-correlation experiment, which yields a strong bunching behaviour. Our work paves the way to a new generation of quantum optics experiments with two-dimensional semiconductors.

Single-photon electroluminescence for on-chip quantum networks

An electrically driven single-photon source has been monolithically integrated with nano-photonic circuitry. Electroluminescent emission from a single InAs/GaAs quantum dot (QD) is channelled through a suspended nanobeam waveguide. The emission line has a linewidth of below 6 μeV, demonstrating the ability to have a high coherence, electrically driven, waveguide coupled QD source. The single-photon nature of the emission is verified by g(2)(τ) correlation measurements. Moreover, in a cross-correlation experiment, with emission collected from the two ends of the waveguide, the emission and propagation of single photons from the same QD is confirmed. This work provides the basis for the development of electrically driven on-chip single-photon sources, which can be readily coupled to waveguide filters, directional couplers, phase shifters, and other elements of quantum photonic networks.

Study on magnetohydrodynamics angular rate sensor under non-uniform magnetic field

Purpose The evaluations of the magnetohydrodynamics angular rate sensor (MHD ARS) in its applications necessitate further improvements in the sensor’s dynamic measurement ability. The magnetic field of the MHD ARS is a key factor in the sensor’s modeling and error analysis. The aim of this study is to illustrate the influence of a non-uniform magnetic field on the sensor. Design/methodology/approach Numerical simulation is made using ANSYS FLUNET with the magnetic field calculated by 3D-Magnetostatic. The comparison of the simulation results between uniform and non-uniform magnetic fields is made to reveal and explain the effects of magnetic field inhomogeneity (MFI) on the flow and electric field in detail. Two different structures with different MFIs are designed to confirm the MFI effect on the sensor’s output in simulation and experiment. A cross-correlation experiment and an adaptive filter are carried out to extract the signal to identify the error of the sensor output caused by MFI. Findings The MFI effect on the flow field in MHD ARS is found to be insignificant, while its effect on the electric potential is considerable. The comparisons between two kinds of MHD ARS in numerical simulation and experiment show that the MFI effect on the sensor error can be identified by fitting the sensor output. The deviation is mainly generated at the peaks and valleys of an angular vibration. Originality/value The study of the MHD ARS under the influence of a non-uniform magnetic field can offer an understanding of the MFI effect on the sensor and an evaluation method of the sensor error caused by the MFI effect.

Time-Resolved and Dynamic Studies of Riboswitches

Riboswitches are a class of regulatory RNA molecules widely distributed mostly across prokaryotic organisms. The discovery of the first riboswitches, by the Breaker lab and others, opened the door to a previously unimagined layer of genetic regulation. The majority of riboswitches undergo large scale conformational rearrangements upon binding target metabolites in their aptamer domain. Riboswitches are found predominantly in the 5' untranslated region of a number of genes. As a nascent strand is being transcribed by an RNA polymerase, the switch can fold to either occlude the expression platform or to expose it. The former leads to termination of the transcript, while the latter leads to gene expression. It is hypothesized that the concentration of the riboswitch's target metabolite governs gene expression. Thus, if the switch has bound its target metabolite prior to moving past the critical terminator stem, gene expression is regulated. Conversely, if the polymerase moves past the terminator stem before binding the target metabolite, the riboswitch and metabolite will not have time to reach equilibrium before an on/off decision is made. Accordingly, the mechanism of regulation may be highly dependent on the kinetic on-rate. For these reasons, understanding the dynamics of the riboswitch both on a global and local level is critical to determining the molecular mechanism by which riboswitches regulate gene expression. NMR T1, T2, NOE, cross-correlation, and dispersion experiments would be presented to delineate the site-specific dynamics involved in riboswitch function. These local dynamics measurements are combined with other biophysical techniques that we use to track global structural changes on a wide range of time-scales. These studies would likely provide a glimpse of the conformational switching necessary for riboswitch function.

Single-Nanocrystal Photoluminescence Spectroscopy Studies of Plasmon-Multiexciton Interactions at Low Temperature.

Using thick-shell or "giant" CdSe/CdS nanocrystal quantum dots (g-NQDs), characterized by strongly suppressed Auger recombination, we studied the influence of plasmonic interactions on multiexciton emission. Specifically, we assessed the separate effects of plasmonic absorption and plasmonic emission enhancement by a systematic analysis of the pump fluence dependence of low-temperature photoluminescence (low-T PL) derived from individual CdSe/CdS g-NQDs deposited on nanoroughened silver films. Our study reveals that (1) the multiexciton (MX) emissions in g-NQD coupled to silver films were enhanced not only through the creation of more excitons via enhancement of absorption but also through the direct modification of the competition between the radiative and nonradiative recombination processes of MXs; (2) strong enhancement in absorption is not necessary for strong multiexciton emission; and (3) the emission of MXs can become stronger with the increase of multiexciton order. We also exploited the strong enhancement of MX emission to perform second-order photon correlation and cross-correlation experiments using very low pump fluences and observed a strong photon bunching that decays with increasing pump fluence.