Photon Correlation Measurements Research Articles

Room-temperature quantum emission from interface excitons in mixed-dimensional heterostructures

The development of van der Waals heterostructures has introduced unconventional phenomena that emerge at atomically precise interfaces. For example, interlayer excitons in two-dimensional transition metal dichalcogenides show intriguing optical properties at low temperatures. Here we report on room-temperature observation of interface excitons in mixed-dimensional heterostructures consisting of two-dimensional tungsten diselenide and one-dimensional carbon nanotubes. Bright emission peaks originating from the interface are identified, spanning a broad energy range within the telecommunication wavelengths. The effect of band alignment is investigated by systematically varying the nanotube bandgap, and we assign the new peaks to interface excitons as they only appear in type-II heterostructures. Room-temperature localization of low-energy interface excitons is indicated by extended lifetimes as well as small excitation saturation powers, and photon correlation measurements confirm antibunching. With mixed-dimensional van der Waals heterostructures where band alignment can be engineered, new opportunities for quantum photonics are envisioned.

Carbon-Related Quantum Emitter in Hexagonal Boron Nitride with Homogeneous Energy and 3-Fold Polarization.

Most hexagonal boron nitride (hBN) single-photon emitters (SPEs) studied to date suffer from variable emission energy and unpredictable polarization, two crucial obstacles to their application in quantum technologies. Here, we report an SPE in hBN with an energy of 2.2444 ± 0.0013 eV created via carbon implantation that exhibits a small inhomogeneity of the emission energy. Polarization-resolved measurements reveal aligned absorption and emission dipole orientations with a 3-fold distribution, which follows the crystal symmetry. Photoluminescence excitation (PLE) spectroscopy results show the predictability of polarization is associated with a reproducible PLE band, in contrast with the non-reproducible bands found in previous hBN SPE species. Photon correlation measurements are consistent with a three-level model with weak coupling to a shelving state. Our ab initio excited-state calculations shed light on the atomic origin of this SPE defect, which consists of a pair of substitutional carbon atoms located at boron and nitrogen sites separated by a hexagonal unit cell.

A Hanbury Brown and Twiss renascence: measurement of photon correlations yields spatio-temporal coherence

Investigations of photon correlations similar to the Hanbury Brown and Twiss experiment are since the advent of quantum optics in the center of both fundamental and application-oriented research. We derive a factorial moment-generating function of the probability distribution of electromagnetic radiation emitted by a source and absorbed by a photodetector from which we calculate an analytical expression for the second-order correlation coefficient g^{(2)} which reflects spatial coherence properties of the emitted light. The measurement of g^{(2)} thus allows to retrieve the spatial coherence parameter of the light source in terms of the combination of source area, source-detector distance, wavelength and detector area. The validity of the concept is proven by investigating g^{(2)} of true thermal light from a Xenon arc lamp and from the sun for various detector areas, in excellent agreement with the theory. Finally, we suggest a novel scheme for determining light source diameters by exploiting the spatially dependent statistics and confirm its validity by exemplary calculations. These results based on radiation thermodynamics and photon statistics give fresh insight into quantum optical properties of classical light sources, photon correlations and photodetection with promising applications perspectives, more than 68 years after the original Hanbury Brown and Twiss experiment.

Magnetically Tunable Spontaneous Superradiance from Mesoscopic Perovskite Emitter Clusters

Perovskite emitters are promising materials as next-generation optical sources due to their low fabrication cost and high quantum yield. In particular, the superradiant emission from a few coherently coupled perovskite emitters can be used to produce a bright entangled photon source. Here, we report the observation of superradiance from mesoscopic (<55) CsPbBr3 perovskite emitters, which have a much smaller ensemble size than the previously reported results (>106 emitters). The superradiance is spontaneously generated by off-resonance excitation and detected by time-resolved photoluminescence and second-order photon correlation measurements. We observed a remarkable magnetic tunability of the superradiant photon bunching, indicating a magnetic field-induced decoherence process. The experimental results can be well explained using a theoretical framework based on the microscopic master equation. Our findings shed light on the superradiance mechanism in perovskite emitters and enable low-cost quantum light sources based on perovskite.

Photon-Emission-Correlation Spectroscopy as an Analytical Tool for Solid-State Quantum Defects

Photon-emission-correlation spectroscopy is an indispensable tool for the study of atoms, molecules, and, more recently, solid-state quantum defects. In solid-state systems, its most common use is as an indicator of single-photon emission, a key property for quantum technology. Beyond the single-photon purity of an emitter, however, photon-correlation measurements can provide a wealth of information that can reveal details about its electronic structure and optical dynamics that are hidden by other spectroscopy techniques. This tutorial presents a standardized framework for using photon-emission-correlation spectroscopy to study quantum emitters, including discussion of theoretical background, considerations for data acquisition and statistical analysis, and interpretation. We highlight important nuances and best practices regarding the commonly used g(2)(τ=0)<0.5 test for single-photon emission. Finally, we illustrate how this experimental technique can be paired with optical-dynamics simulations to formulate an electronic model for unknown quantum emitters, enabling the design of quantum control protocols and assessment of their suitability for quantum information science applications.Received 1 August 2022Revised 13 December 2022DOI:https://doi.org/10.1103/PRXQuantum.4.010202Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasColor centersQuantum engineeringQuantum information architectures & platformsPhysical SystemsMoleculesQuantum dotsTechniquesFluorescence spectroscopyPhoton correlation spectroscopyPhoton countingQuantum InformationCondensed Matter, Materials & Applied PhysicsAtomic, Molecular & Optical

Probing the dipole–dipole interaction in single 0D/2D heterostructures by tightly focused cylindrical vector beams

Energy transfer from zero-dimensional (0D) quantum dots (QDs) to two-dimensional (2D) materials has attracted much attention for both the manipulation of fundamental material properties and their potential device applications. An understanding of the effect of dipole interactions on energy transfer rate in the hybrid dimensional system is essential for improving optoelectronic device performance. Here, we report the dipole-orientation-dependent energy transfer from individual core–shell CdSe/ZnS QDs to bilayer molybdenum disulfide (MoS2) by utilizing tightly focused azimuthally and radially polarized cylindrical vector beams. With second-order photon correlation measurements [g2(τ)], we show the single-photon emission behavior from QDs in 0D/2D heterostructures, indicating that the investigated heterostructure is constructed from single QDs. By polarization resolved photoluminescence (PL) imaging and PL lifetime measurements, we observe a fast energy transfer rate of the system excited with azimuthally polarized beams and interpret it based on dipole–dipole interactions with Förster energy transfer theory. Our work provides an in-depth understanding of the dipole-orientation-dependent energy transfer mechanism in 0D/2D systems, which could offer guidance for designing the related optoelectronic device applications.

Low-Divergence hBN Single-Photon Source with a 3D-Printed Low-Fluorescence Elliptical Polymer Microlens.

Efficiently collecting light from single-photon emitters is crucial for photonic quantum technologies. Here, we develop and use an ultralow fluorescence photopolymer to three-dimensionally print micrometer-sized elliptical lenses on individual precharacterized single-photon emitters in hexagonal boron nitride (hBN) nanocrystals, operating in the visible regime. The elliptical lens design beams the light highly efficiently into the far field, rendering bulky objective lenses obsolete. Using back focal plane imaging, we confirm that the emission is collimated to a narrow low-divergence beam with a half width at half-maximum of 2.2°. Using photon correlation measurements, we demonstrate that the single-photon character remains undisturbed by the polymer lens. The strongly directed emission and increased collection efficiency is highly beneficial for quantum optical experiments. Furthermore, our approach paves the way for a highly parallel fiber-based detection of single photons from hBN nanocrystals.

Subradiant states for two imperfect quantum emitters coupled by a nanophotonic waveguide

Coherent interactions between quantum emitters in tailored photonic structures is a fundamental building block for future quantum technologies, but remains challenging to observe in complex solid-state environments, where the role of decoherence must be considered. Here, we investigate the optical interaction between two quantum emitters mediated by one-dimensional waveguides in a realistic solid-state environment, focusing on the creation, population and detection of a sub-radiant state, in the presence of dephasing. We show that as dephasing increases, the signatures of sub-radiance quickly vanish in intensity measurements yet remain pronounced in photon correlation measurements, particularly when the two emitters are pumped separately so as to populate the sub-radiant state efficiently. The applied Green's tensor approach is used to model a photonic crystal waveguide, including the dependence on the spatial position of the integrated emitter. The work lays out a route to the experimental realization of sub-radiant states in nanophotonic waveguides containing solid-state emitters.

Adversarial autoencoder ensemble for fast and probabilistic reconstructions of few-shot photon correlation functions for solid-state quantum emitters

Second-order photon correlation measurements [${g}^{(2)}(\ensuremath{\tau})$ functions] are widely used to classify single-photon emission purity in quantum emitters or to measure the multiexciton quantum yield of emitters that can simultaneously host multiple excitations -- such as quantum dots -- by evaluating the value of ${g}^{(2)}(\ensuremath{\tau}=0)$. Accumulating enough photons to accurately calculate this value is time consuming and could be accelerated by fitting of few-shot photon correlations. Here, we develop an uncertainty-aware, deep adversarial autoencoder ensemble (AAE) that reconstructs noise-free ${g}^{(2)}(\ensuremath{\tau})$ functions from noise-dominated, few-shot inputs. The model is trained with simulated ${g}^{(2)}(\ensuremath{\tau})$ functions that are facilely generated by Poisson sampling time bins. The AAE reconstructions are performed orders-of-magnitude faster, with reconstruction errors and estimates of ${g}^{(2)}(\ensuremath{\tau}=0)$ that are lower in variance and similar in accuracy compared to Maximum likelihood estimation and Levenberg-Marquardt least-squares fitting approaches, for simulated and experimentally measured few-shot ${g}^{(2)}(\ensuremath{\tau})$ functions (\ensuremath{\sim}100 two-photon events) of InP/ZnS/ZnSe and CdS/CdSe/CdS quantum dots. The deep-ensemble model comprises eight individual autoencoders, allowing for probabilistic reconstructions of noise-free ${g}^{(2)}(\ensuremath{\tau})$ functions, and we show that the predicted variance scales inversely with number of shots, with comparable uncertainties to computationally intensive Markov chain Monte Carlo sampling. This work demonstrates the advantage of machine learning models to perform uncertainty-aware, fast, and accurate reconstructions of simple Poisson-distributed photon correlation functions, allowing for on-the-fly reconstructions and accelerated materials characterization of solid-state quantum emitters.

Probing the Ginzburg–Landau Potential for Lasers Using Higher-order Photon Correlations

Lasing transition is known to be analogous to the second-order phase transition. Furthermore, for some cases, it is possible to define the Ginzburg-Landau (GL) potential, and the GL theory predicts the photon statistical properties of lasers. However, the GL potential for lasers is surprising, because lasers are operating far from equilibrium. In this paper, we theoretically examine the validity of the GL theory for lasers in terms of various parameters, particularly, the ratio between photon and carrier lifetimes. For this purpose, we use stochastic rate equations and higher-order photon correlation functions. With higher-order photon correlation measurements, we can check whether or not laser dynamics are described by the GL theory. We demonstrate that, for low-$\beta$ lasers, the GL theory is applicable even when the photon lifetime is comparable to the carrier lifetime and that photon-carrier relaxation oscillation is the fundamental origin of the breakdown of the GL theory, which can be understood in the framework of center manifold reduction.

(Invited) Quantum Emission Assisted By Energy Landscape Modification in Pentacene-Decorated Carbon Nanotubes

Photoluminescent carbon nanotubes are expected to become versatile room-temperature single-photon sources that have applications in quantum information processing. Quantum emission from carbon nanotubes is often induced by localization of excitons or exciton-exciton annihilation. Here, we modify the local energy landscape of excitons by decorating nanoscale pentacene particles onto air-suspended single-walled carbon nanotubes [1]. Directional exciton diffusion from the undecorated region to the decorated site is demonstrated, suggesting exciton trapping induced by local dielectric screening from pentacene particles. Photoluminescence and photon correlation measurements on a representative carbon nanotube reveal enhanced exciton-exciton annihilation and single-photon emission at room temperature. Pentacene particles are shown to promote strong photon antibunching at the decorated site, indicating that noncovalent functionalization using molecules can be an effective approach for energy landscape modification and quantum emission in carbon nanotubes.Work supported in part by MIC (SCOPE 191503001), JSPS (KAKENHI JP20H02558, JP19J00894, JP20J00817, JP20K15121, JP20K15137, JP20K15112 and JP20K15199), and MEXT (Nanotechnology Platform JPMXP09F19UT0079). K.O. and D.Y. are supported by the JSPS Research Fellowship. D.K. is supported by the RIKEN Special Postdoctoral Researcher Program. We acknowledge RIKEN Materials Characterization Team for access to the transmission electron microscope, and RIKEN Advanced Manufacturing Support Team for technical assistance.[1] Z. Li, K. Otsuka, D. Yamashita, D. Kozawa, Y. K. Kato, ACS Photonics 8, 2367 (2021).

Tracking Exciton Diffusion and Exciton Annihilation in Single Nanoparticles of Conjugated Polymers by Photon Correlation Spectroscopy

AbstractA fundamental question relating to the nature of light emission and absorption in organic semiconductors is the dimension of the domain within a bulk material responsible for the interaction of light and matter. How large can a nanoparticle become to retain the quantized nature of light emission? Excitons are only a few nanometers in size, but because they diffuse in space, they probe a much larger volume than the single molecule. When excitons meet, they may decay non‐radiatively by singlet–singlet or singlet–triplet annihilation (SSA or STA). Fluorescence photon statistics reveal whether single photons are emitted (photon antibunching) or arrive in randomly spaced packets (photon bunching), offering direct insight into excitonic mobility. Single multichain nanoparticles of ladder‐type poly(para‐phenylene) (LPPP) are examined. The effect of SSA and STA is seen in the photon antibunching and bunching, respectively, which both decrease in fidelity as the size of the nanoparticle increases. Time resolving the photon correlation measurement yields microscopic annihilation rates for SSA and STA in agreement with values obtained from bulk LPPP films. Even though triplets in LPPP are known to be highly mobile, the results show that, on the timescale of the singlet exciton lifetime, triplet diffusion is not of significance in the STA process.

Room Temperature Fluorescence Blinking in MoS2 Atomic Layers by Single Photon Energy Transfer

AbstractThe quantum optical phenomena, such as single‐photon emission, in transition metal dichalcogenides (TMDCs) have triggered extensive research on 2D material‐based quantum optics and devices. By far, most reported quantum optical emissions in TMDCs are based on atomic defects or the local confinement of excitons. In contrast, energy transfer between two materials could also manipulate the photon emission behaviors in materials, even at the single‐photon level. Here, fluorescence blinking from MoS2 atomic layers in quantum dot (QD)/MoS2 hybrid heterostructure at room temperature is reported. The single‐photon nature of the QDs in heterostructures by second‐order photon correlation measurements is demonstrated. Based on the transient PL spectroscopy and PL time trajectories, the fluorescence blinking behavior in MoS2 is attributed to the single‐photon energy transfer from QD to MoS2. This work could open the possibility to achieve correlated quantum emitters in TMDCs at room temperature.

Higher-Order Photon Statistics as a New Tool to Reveal Hidden Excited States in a Plasmonic Cavity

Among the best known quantities obtainable from photon correlation measurements are the g(m) correlation functions. Here, we introduce a new procedure to evaluate these correlation functions based on higher-order factorial cumulants CF,m that integrate over the time dependence of the correlation functions, that is, summarize the available information at different time spans. In a systematic manner, the information content of higher-order correlation functions as well as the distribution of photon waiting times is taken into account. Our procedure greatly enhances the sensitivity for probing correlations and, moreover, is robust against a limited counting efficiency and time resolution in experiment. It can be applied even in case g(m) is not accessible at short time spans. We use the new evaluation scheme to analyze the photon emission of a plasmonic cavity coupled to a single quantum dot. We derive criteria that must hold if the system can be described by a generic Jaynes–Cummings model. A violation of the criteria can be explained by the presence of an additional excited quantum dot state.

Low-Temperature Bonding of Nanolayered InGaP/SiO2 Waveguides for Spontaneous-Parametric Down Conversion

The integration of III–V materials on photonic integrated circuits has enjoyed a lot of attention because of the necessity of merging photon sources with silicon electronics. Nevertheless, III–V source integration technologies are not sufficiently mature, inhibiting the employment of such platforms to a broader range of applications. Here, we present a novel approach that enables the transfer of III–V nanolayers (≈250 nm) to silicates via oxide bonding. Through use of a thick photoresist scaffold (≈30 μm), complete removal of the substrate can be performed while preserving the relative structure morphology. The use of the III–V native oxide without depositing interfacial oxide layers greatly reduces the processing time and cost. The transfer of an array composed of 1 mm long InGaP waveguides with 250 nm thickness and widths spanning from 0.7 to 11.2 μm to a SiO2 substrate has been experimentally demonstrated, evidencing the feasibility of the technique for wafer-scale processing. The application of the nanolayered waveguides in the spontaneous-parametric down-conversion process has been tested by photon-correlation measurement, showing good agreement with the theoretical model.

Quantum Emission Assisted by Energy Landscape Modification in Pentacene-Decorated Carbon Nanotubes

Photoluminescent carbon nanotubes are expected to become versatile single-photon sources that have applications in quantum information processing. Quantum emission from carbon nanotubes is often induced by localization of excitons or exciton–exciton annihilation. Here, we modify the local energy landscape of excitons by decorating nanoscale pentacene particles onto air-suspended single-walled carbon nanotubes. Directional exciton transfer from the undecorated region to the decorated site is demonstrated, suggesting exciton trapping induced by local dielectric screening from pentacene particles. Photoluminescence and photon correlation measurements on a representative carbon nanotube reveal enhanced exciton–exciton annihilation and single-photon emission at room temperature. Pentacene particles are shown to promote strong photon antibunching at the decorated site, indicating that noncovalent functionalization using molecules can be an effective approach for energy landscape modification and quantum emission in carbon nanotubes.

Quantum Light Emission from Coupled Defect States in DNA-Functionalized Carbon Nanotubes.

Solid-state single-photon sources are essential building blocks for quantum photonics and quantum information technologies. This study demonstrates promising single-photon emission from quantum defects generated in single-wall carbon nanotubes (SWCNTs) by covalent reaction with guanine nucleotides in their single-stranded DNA coatings. Low-temperature photoluminescence spectroscopy and photon-correlation measurements on individual guanine-functionalized SWCNTs (GF-SWCNTs) indicate that multiple, closely spaced guanine defect sites within a single ssDNA strand collectively form an exciton trapping potential that supports a localized quantum state capable of room-temperature single-photon emission. In addition, exciton traps from adjacent ssDNA strands are weakly coupled to give cross-correlations between their separate photon emissions. Theoretical modeling identifies coupling mechanism as a capture of band-edge excitons. Because the spatial pattern of nanotube functionalization sites can be readily controlled by selecting ssDNA base sequences, GF-SWCNTs should become a versatile family of quantum light emitters with engineered properties.

A Bayesian approach to luminescent down-conversion.

Photon conversion embodies a range of promising possibilities in pushing the theoretical Shockley-Queisser efficiency model of classical solar cells. Luminescent down-conversion, despite its potential, is held back in practical applications due to the difficulty of proper characterization in no small part because of concurrent luminescent downshifting events. Recent advances have demonstrated the opportunity provided by photon correlation measurement for down-conversion characterization. In this methodological work, we present a general method based on Bayesian probabilities for deriving auto-correlation functions analytically. This method is then applied to the five down-conversion mechanisms reported in the literature and successfully tested against numerical simulations. We show that the zero delay auto-correlation function can be the most direct way to demonstrate down-conversion and assess its efficiency. Our analysis offers additional useful tools for the design of characterization experiments and emphasizes some universal behavior valid for all reported conversion mechanisms.

Temperature Dependence of Excitonic and Biexcitonic Decay Rates in Colloidal Nanoplatelets by Time-Gated Photon Correlation.

Excitons in colloidal semiconductor nanoplatelets (NPLs) are weakly confined in the lateral dimensions. This results in significantly smaller Auger rates and, consequently, larger biexciton quantum yields, when compared to spherical quantum dots (QDs). Here we report a study of the temperature dependence of the biexciton Auger rate in individual CdSe/CdS core–shell NPLs, through the measurement of time-gated second-order photon correlations in the photoluminescence. We also utilize this method to directly estimate the single-exciton radiative rate. We find that whereas the radiative lifetime of NPLs increases with temperature, the Auger lifetime is almost temperature-independent. Our findings suggest that Auger recombination in NPLs is qualitatively similar to that of semiconductor quantum wells. Time-gated photon correlation measurements offer the unique ability to study multiphoton emission events, while excluding effects of competing fast processes, and can provide significant insight into the photophysics of a variety of nanocrystal multiphoton emitters.

Induced Photon Correlations Through the Overlap of Two Four‐Wave Mixing Processes in Integrated Cavities

AbstractInduced photon correlations are directly demonstrated by exploring two coupled nonlinear processes in an integrated device. Using orthogonally polarized modes within an integrated microring cavity, phase matching of two different nonlinear four‐wave mixing processes is achieved simultaneously, wherein both processes share one target frequency mode, while their other frequency modes differ. The overlap of these modes leads to the coupling of both nonlinear processes, producing photon correlations. The nature of this process is confirmed by means of time‐ and power‐dependent photon correlation measurements. These findings are relevant to the fundamental understanding of spontaneous parametric effects as well as single‐photon‐induced processes, and their effect on optical quantum state generation and control.