We report on the experimental demonstration of single-photon state generation and characterization in an electron microscope. In this aim we have used low intensity relativistic (energy between 60 and 100 keV) electrons beams focused in a ca. 1 nm probe to excite diamond nanoparticles. This triggered individual neutral nitrogen-vacancy centers to emit photons which could be gathered and sent to a Hanbury Brown-Twiss intensity interferometer. The detection of a dip in the correlation function at small time delays clearly demonstrates antibunching and thus the creation of nonclassical light states. Specifically, we have also demonstrated single-photon states detection. We unveil the mechanism behind quantum states generation in an electron microscope, and show that it clearly makes cathodoluminescence the nanometer scale analog of photoluminescence. By using an extremely small electron probe size and the ability to monitor its position with subnanometer resolution, we also show the possibility of measuring the quantum character of the emitted beam with deep subwavelength resolution.
We report on the experimental demonstration of single-photon state generation and characterization in an electron microscope. In this aim we have used low intensity relativistic (energy between 60 and 100 keV) electrons beams focused in a ca. 1 nm probe to excite diamond nanoparticles. This triggered individual neutral nitrogen-vacancy centers to emit photons which could be gathered and sent to a Hanbury Brown-Twiss intensity interferometer. The detection of a dip in the correlation function at small time delays clearly demonstrates antibunching and thus the creation of nonclassical light states. Specifically, we have also demonstrated single-photon states detection. We unveil the mechanism behind quantum states generation in an electron microscope, and show that it clearly makes cathodoluminescence the nanometer scale analog of photoluminescence. By using an extremely small electron probe size and the ability to monitor its position with subnanometer resolution, we also show the possibility of measuring the quantum character of the emitted beam with deep subwavelength resolution.
Quantum optics performed at the nanometer scale is an important challenge, especially for quantum emitters characterization. They can be point defects in material (few ang- ströms) or confined structures of a few nanometers. A way to reach this scale is by using cathodoluminescence (CL) performed in a scanning transmission electron microscope (CL- STEM), which has only recently been done [1]. However, when aiming at studying the statistical properties of the light coming out of a CL experiment, which is necessary to e. g. study the quantum nature of Single Photon Emitters (SPE) emission, dedicated expe- riments on top of regular CL ones have to be designed. Few months before my arrival in the STEM-group of the LPS, an intensity interferometry experiment (HBT) that measures the autocorrelation function g(2) of the CL signal intensity was built [2]. It is well known that the clear signature of SPE as measured in photoluminescence (PL) is antibunching in the g(2)(τ), namely that the autocorrelation function is always less than one. It was re- cently demonstrated on a famous SPE, the Nitrogen vacancy (NV) defect in diamond, that CL-STEM is similar to PL when only one SPE is involved. In this thesis we will see how CL-STEM allowed to characterize a new point defect in h-BN, showing the relevance of HBT experiments in a CL-STEM for discovering and characterizing new SPE. However, by studying the excitation of multiple SPE in CL, we discovered a new emission phenomenon, characterized by a huge bunching effect of the g(2)(τ) function (g(2)(0) > 35), in complete contradiction to PL measurements and expectations (g(2)(0)<1). In my thesis manuscript, this surprising effect will be experimentally investigated, theoretically explained and applied to lifetime measurement at the nanometer scale. Because quantum optics is often linked to quantum plasmonics, I will present, to conclude, a theoretical proposal, in collaboration with J. Garcia de Abajo, about quantum plasmonics measurement in a STEM.
Electron beams are an exceptional tool for probing nanophotonic systems [1]. Combining electron microscopy with external optical excitation in ultrafast transmission electron microscopes has enabled nanoscale [2,3] and attosecond phase-resolved mapping of optical near-fields [4] by stimulated inelastic scattering between electrons and light. Recently, the underlying interaction strength of this process has been increased significantly by harnessing resonant enhancement of the optical field and velocity phase matching [5]. Such an increased interaction strength also benefits the spontaneous scattering of electrons with light, thus promoting single-particle-based scattering experiments [6]. Here, we demonstrate the generation of nonclassical multiphoton states via coherent cathodoluminescence (CL) from an optical waveguide, approaching near-unity coupling in electron spectroscopy of nanophotonic systems [7]. To this end, we scatter swift electrons at the optical mode of a custom silicon nitride photonic chip. Specifically, inside of a transmission electron microscope, a beam of free electrons (100-keV kinetic energy) passes parallel to the waveguide surface in an aloof geometry (cf. Fig. 1a). The electrons can inelastically scatter with the evanescent tail of the guided optical mode, generating photons at a wavelength around 1400 nm in the fundamental quasi-TM mode, while the electrons lose multiples N of the corresponding photon energy of 0.88 eV (cf. Fig. 1b). The photons are then coupled via optical fibers to a balanced beam splitter and two single-photon avalanche diodes (SPADs), implementing a Hanbury Brown and Twiss (HBT) setup (cf. Fig. 1c). Probing the ensemble average using the 2nd-order correlation function g(2), we observe Poissonian photon statistics in accordance with the underlying coherent scattering mechanism (Fig. 1d, top). A parallel single-particle-resolved measurement using an event-based electron camera (Timepix3 ASIC) coupled to an EEL spectrometer provides information on both electron energy and arrival time. This allows us to further investigate the photon statistics of the generated light as a function of the electron energy. Using this electron-heralded HBT setup, we verify the generation of single photons following an energy loss of E = ℏω (Fig. 1d, bot), and photon pairs for double scattering events. Characterizing the electron-driven photon generation using established metrics for heralded single-photon sources, such as spontaneous parametric down-conversion (SPDC), we find an intrinsic single-photon heralding efficiency of 10% and a coincidence-to-accidental ratio > 30. This establishes a new source for electron-heralded quantum light, which may find its applications in quantum photonics. Practically, efficient electron-photon coupling and event-based detention, offers a new pathway for coincidence-enhanced electron spectroscopy of photonic materials [8]. Besides eliminating undesired and formerly unrecoverable averaging, access to the full scattering distribution of the involved particles can strongly suppress noise in ultrafast electron experiments [6]. Furthermore, for systems that can be directly pumped by single electrons, ultrafast dynamics can be studied using continuous electron beams, achieving sub-nanosecond resolution, only limited by the employed detector technology. Finally, this work represent an essential step toward quantum-enhanced electron imaging and enables the heralding of individual electrons for dose-sensitive applications. a) Electrons are scattered at an extended waveguide, generating light by coherent cathodoluminescence. b) Electron energy loss into discrete sidebands is observed, spaced by the photon energy ℏω. c) Event-based coincidence spectroscopy enables electron-heralded detection of photon number states. d) The discrete 2nd-order correlation function g(2)[q] shows Poissonian statistics for unheralded photon detection, while conditioning on an ℏω energy loss produces high-contrast single-photon states.
Defects in diamond have evolved as unique object for their potential application for Optical imaging and Nano-photonics. High pressure high temperature (HPHT) grown type-Ib diamond particles were milled down to the size of ~ 80 nm and purified to remove iron & graphitic carbon impurities and irradiated with 35 KeV He+ ions to create vacancies in the diamond lattice. Vacancies diffuse close to substitutional nitrogen impurities of nano-diamond particles upon thermal treatment. Characteristic sharp emission of NV centers were observed at 575.5 nm and 637.9 nm corresponding to NV° and NV− charged states in addition to a broad emission in the range 550-750 nm. Structural and photo-physical behavior of these fluorescent diamonds was found to depend on the method of purification adopted. Samples purified through acid reflux were found suitable for bio-imaging applications while, sample purified through air oxidation shows properties desirable for Nano-photonic applications. Photon-correlation measurements of emission from individual diamond nanoparticles using Hanbury Brown and Twiss interferometer (HBT) shows anti-bunching curve with g2(t)= 0 confirming their suitability as single photon source. Such sources of indistinguishable photons are highly desirable for quantum computation and quantum optics applications.
Atomically confined spins are emerging as active components in quantum optoelectronic devices such as quantum bits and sensors. However, interrogating single spins at atomic length-scales remains a sizeable challenge, limited by diffraction in conventional optics. Here we show that the highly-local excitation provided by injecting energetic charge carriers from the atomically sharp probe of a scanning tunneling microscope can trigger single-photon emission from individual atomic vacancy centers in a layered semiconductor. With an effective spatial resolution of <1 nm, we show that the captured light closely mirrors the orbital symmetry of the bound-state wavefunction of the vacancy center while photon correlation measurements confirm single-photon emission, as reflected in clear photon anti-bunching signatures. Our results constitute an important step toward the realization of an electrically addressable single-atom quantum light source and solid-state spinphoton interface, addressed at the atomic-scale.
The development of quantum-enhanced technologies requires single-photon sources, as well as methods for their characterization and verification. Here, we describe a methodology for measuring the correlation function of a single-photon source using an experimental setup that comprises a laser-excited fluorescence microscope equipped with a Hanbury Brown–Twiss intensity interferometer as one of the detection systems. Measurements of the response function of the device and the reference samples are performed. The second-order autocorrelation function of the exciton state of GaAs quantum dots in AlGaAs nanowires is obtained and reveals a single-photon emission.
Electron beams are a formidable tool for probing nanophotonic systems [1]. Combining electron microscopy with external optical excitation in ultrafast transmission electron microscopes has enabled the mapping of optical near-fields [2] and the bunching of electrons in attosecond pulses [3, 4]. These developments are based on the stimulated inelastic scattering between electrons and light [5, 6], in which the electrons gain energy from or lose energy to the optical field. Recently, the underlying interaction strength of this process has been increased significantly by harnessing resonant enhancement of the optical field and velocity phase matching [7]. Such an increased interaction strength also benefits the spontaneous scattering of electrons with light, thus promising applications like generating complex quantum states of light [8]. In this presentation, we discuss a set of experiments on free electron quantum optics that investigate single-electron-single-photon interactions and probe the resulting entanglement. Specifically, we consider scenarios in which optical waveguide structures are placed inside a transmission electron microscope (TEM) facilitating the interaction. Electrons passing by, for example, an integrated photonic structure (cf. Fig. 1a) can inelastically scatter with the optical mode, leading to the emergence of electron-photon pair states [9]. The resulting correlation between energy-loss electrons and photons, illustrated in Fig. 1b, can be probed employing time- and energy-resolved detection of both particles, revealing a pronounced coincidence peak between their arrival times, schematically shown in Fig. 1c. Extending the photon detection setup by a second detector to allow for Hanbury Brown and Twiss (HBT) measurements, the photon statistics heralded by such energy-loss electrons can be studied [10]. The observed anti-bunching behavior indicates a nonclassical optical state with single-photon properties. The inelastic electron-light scattering underlying the electron-photon pair generation is further expected to induce entanglement in the particle energies [11], however, direct observation of quantum entanglement would be exceedingly difficult given the large number of independent quantum states available in these degrees of freedom. Therefore, we propose experiments based on the quantum eraser scheme to demonstrate this free-electron-photon entanglement in different degrees of freedom [12,13]. Specifically, we consider a double-slit geometry coherently illuminated by an electron beam such that an interference pattern can be observed on a distant screen (cf. Fig. 2a). If electrons passing through the left or right (denoted |Land |R, respectively) slit generate distinct marker photons with vertical (|V)or horizontal (|H) polarization, this interference pattern would disappear due to the entanglement of electron path and photon polarisation (cf. Fig. 2b). However, if this which-path information carried by the marker photon is erased, e.g. by mixing the polarisation states, the interference pattern can be recovered when detecting electrons and photons in coincidence. We describe an extension of this idea to an entanglement test, before discussing possible realistic implementations and presenting preliminary results on several experimental key aspects. In summary, we discuss the inelastic electron-light scattering at integrated photonic structures with applications ranging from coherent electron beam modulation using continuous-wave optical fields to the electron-driven generation of quantum optical states of light. Delving deeper into single electron-single photon interactions, we outline feasible experiments to generate and verify electron-photon entanglement with a potential extension to electron pairs based on entanglement swapping. All this adds to the toolbox of free electron quantum optics, potentially enabling novel sources of quantum light as well as quantum-enhanced sensing in electron microscopy. Generation of correlated electron-photon pairs. (a) Electrons pass over a photonic integrated circuit placed in the sample plane of a TEM, generating photons in the optical waveguides via inelastic scattering. Energy-loss electrons are detected and time-tagged behind an imaging spectrometer in coincidence with photons exiting the waveguide. (b) Inelastic scattering links the electron energy loss (green, right ladder) to the generation of photons (red, left ladder), creating correlations in energy and time. (c) A strong temporal correlation between the loss electrons and detected photons, indicated by the red dot, is observed relative to the uncorrelated zero-loss background electrons (blue line). Free-electron-photon quantum eraser. (a) Coherent electron illumination of a double-slit structure results in interference between the two pathways |Land |R.(b) If the electrons generate distinguishable marker photons, e.g. of orthogonal polarizations |Vand |H, no interference pattern can be observed due to the electron-photon entanglement. (c) Collecting these marker photons and erasing the which-path information, e.g. using a half-wave plate (λ/2), the electron interference can be recovered using electron-photon coincidence detection with single-photon detectors (SPDs) placed behind a polarizing beam splitter (PBS).