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Photonics for Quantum (PfQ)

Generating correlated pairs of free electrons and waveguide photons

Jan-Wilke Henke, Armin Feist, Guanhao Huang, Germaine Arend, Yujia Yang, Arslan Sajid Raja, Fee Jasmin Kappert, Jiahe Pan, Hugo Lourenço‐Martins, Zheru Qiu, Junqiu Liu, Ofer Kfir, Tobias Jan Kippenberg, Claus Ropers

Published Aug 2, 2024• 0 citations
Publisher

Abstract

In this talk, we establish chip-based integrated silicon nitride photonics as a platform for experiments on the interactions between free electrons and light. Placing the fibre-coupled microresonators in a transmission electron microscope, we observe a quantised loss of energy for electrons passing the waveguide in an aloof geometry and inelastically scattering off the initially empty cavity modes while generating photons. Coincidence measurements performed on both particles reveal the common origin of these correlated electron-photon pairs, while post-selection allows for enhanced imaging of the resonator’s optical modes and promises applications as a high-fidelity heralded photon Fock state source.

DOI
10.1117/12.3027148

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In this talk, we establish chip-based integrated silicon nitride photonics as a platform for experiments on the interactions between free electrons and light. Placing the fibre-coupled microresonators in a transmission electron microscope, we observe a quantised loss of energy for electrons passing the waveguide in an aloof geometry and inelastically scattering off the initially empty cavity modes while generating photons. Coincidence measurements performed on both particles reveal the common origin of these correlated electron-photon pairs, while post-selection allows for enhanced imaging of the resonator’s optical modes and promises applications as a high-fidelity heralded photon Fock state source.
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.
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.
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.
Photonic chips with silicon nitride (Si3N4\mathrm{Si_3N_4}Si3​N4​) microring resonators are well established as heralded single-photon sources, but their operation as frequency-degenerate twin-photon sources has not previously been demonstrated. Here, we realise a twin-photon source at telecommunication wavelengths in a Si3N4\mathrm{Si_3N_4}Si3​N4​ ring microresonator via an inverse four-wave mixing (FWM) process, in which two photons from spectrally distinct pumps are converted into a pair of identical twin photons. The measurements show a maximum coincidence-to-accidental ratio (CAR) of 5.4±0.65.4\pm0.65.4±0.6. In addition, the microresonator functions as a heralded single-photon source through pump-degenerate spontaneous four-wave mixing (SFWM), exhibiting a spectral purity of P=0.67±0.05P=0.67\pm0.05P=0.67±0.05 and a heralded anti-bunching of gh(2)(0)=0.0042±0.0015g^{(2)}_h(0)=0.0042\pm0.0015gh(2)​(0)=0.0042±0.0015. Together, these results demonstrate both photon-generation schemes on a single integrated Si3N4\mathrm{Si_3N_4}Si3​N4​ platform, highlighting its potential for scalable, tailored quantum light generation.