Single-photon sources play a pivotal role in quantum information technology and quantum sensing [1]. Quantum dots and color centers allow the creation of single photons on demand, are capable of generating higher Fock states and can be electrically driven [1], [2]. Spontaneous parametric down-conversion and spontaneous four-wave mixing, in turn, serve as widely tunable sources of heralded single photons and complex quantum states [3]. The generation of photons radiated from swift free electrons makes it possible to obtain a broad spectrum, amplification, and electron-photon entanglement. Using flatband resonances and microcavities may even enhance the processes [4], [5]. Modulation of free electrons enables the construction of photonic cat and GKP states, which are crucial for fault-tolerant quantum computing [6]. Recently, the effect of generating 1550 nm photons from free electrons directly into a nanophotonic Si3N4 ring resonator in a transmission electron microscope (TEM) at 120 keV has been experimentally shown [5].
Single-photon sources play a pivotal role in quantum information technology and quantum sensing [1]. Quantum dots and color centers allow the creation of single photons on demand, are capable of generating higher Fock states and can be electrically driven [1], [2]. Spontaneous parametric down-conversion and spontaneous four-wave mixing, in turn, serve as widely tunable sources of heralded single photons and complex quantum states [3]. The generation of photons radiated from swift free electrons makes it possible to obtain a broad spectrum, amplification, and electron-photon entanglement. Using flatband resonances and microcavities may even enhance the processes [4], [5]. Modulation of free electrons enables the construction of photonic cat and GKP states, which are crucial for fault-tolerant quantum computing [6]. Recently, the effect of generating 1550 nm photons from free electrons directly into a nanophotonic Si3N4 ring resonator in a transmission electron microscope (TEM) at 120 keV has been experimentally shown [5].
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.
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).
Developing reliable and efficient single-photon sources is crucial for advancing quantum technologies, relying on nonlinear frequency conversion or spontaneous emission from individual quantum emitters. While different types of single-photon sources excel in specific applications, none meet all criteria for an “ideal” source: exceptional brightness, high purity, and indistinguishability. To address this challenge, coupling single-photon emitters with designer nanostructures can significantly enhance emission performance, a pivotal area in quantum nanophotonics. This review summarizes recent advancements over the past decade in generating and manipulating single photons, emphasizing the pivotal role of nanostructure coupling. Single-photon emission systems—such as nonlinear crystals, solid-state defects, quantum dots, carbon nanotubes, and two-dimensional materials—are categorized quantitatively based on their ability to achieve high purity, indistinguishability, and brightness, presented in a three-dimensional technology map. Furthermore, nanostructure engineering is showcased for manipulating properties such as emission direction, polarization, chirality, and entanglement of single photons. By elucidating these critical aspects, this review aims to advance understanding of how advancements in nanostructured environments promise to shape the future of single-photon generation and manipulation within quantum nanophotonics.
Advancing quantum information and communication requires the control of quantum correlations in complementary degrees of freedom. In this work, we generate electron-photon pair states via inelastic scattering of free electrons at a high-Q photonic-chip-based microresonator. In analogy to spontaneous parametric down-conversion, time- and energy-resolved detection of both particles enables various heralding schemes. We experimentally characterize this new heralded source of single photons and free electrons. Ultimately, these results underpin the recent progress in free-electron quantum optics, promising electron-photon entanglement, tailored photon Fock states, and quantum-enhanced electron imaging.
Nowadays, single-photon sources can be generated experimentally through a wide variety of platforms [1]. Semiconductor quantum dots [2] have demonstrated impressive performance in terms of photon purity and indistinguishability, particularly in Hong-Ou-Mandel experiments. Another widely used method involves nonlinear crystals, which can generate entangled photon pairs via nonlinear processes such as spontaneous parametric down-conversion (SPDC) or spontaneous four-wave mixing (SFWM). These are heralded single-photon sources, rather than deterministic ones, yet they remain essential tools in quantum optics [3]. Moreover, among the most commonly used single photon sources, there are NV centres in diamonds [4]. These single-photon sources emit quantum states of light, i.e., those that have no classical counterpart.
Quantum information, communication, and sensing rely on the generation and control of quantum correlations in complementary degrees of freedom. Free electrons coupled to photonics promise novel hybrid quantum technologies, although single-particle correlations and entanglement have yet to be shown. In this work, we demonstrate the preparation of electron-photon pair states using the phase-matched interaction of free electrons with the evanescent vacuum field of a photonic chip-based optical microresonator. Spontaneous inelastic scattering produces intracavity photons coincident with energy-shifted electrons, which we employ for noise-suppressed optical mode imaging. This parametric pair-state preparation will underpin the future development of free-electron quantum optics, providing a route to quantum-enhanced imaging, electron-photon entanglement, and heralded single-electron and Fock-state photon sources.