Quantum Optics with Electron-Photon Pairs

Electron microscopy is a highly developed technology that employs the wave properties of electrons to resolve structures at an atomic level. In this project we want to utilize Cherenkov radiation - which is generated by uniformly moving charged particles (electrons) with velocities exceeding the speed of light in a nearby dielectric medium - to create correlated electron-photon pairs, within a transmission electron microscope. This would enable a powerful new platform to study interesting quantum phenomena with far reaching applications, due to their different physical properties: the massive electron with picometer de Broglie wavelength, enabling atomic resolution, and the Cherenkov photon with micrometer wavelength, which is easy to guide, manipulate and detect in a phase coherent manner. We envision to bridge the very successful fields of quantum optics and electron microscopy.

Electron microscopes can do more than make extremely small objects visible: in this project, we showed that they can also create and use quantum links between single electrons and single particles of light. This is an important step toward a new kind of microscopy in which electrons and light work together to reveal information that is inaccessible to classical physics. Electron microscopes are essential tools in materials science, nanotechnology and biology. They can image structures far smaller than optical microscopes, but the high-energy electrons used for imaging can damage delicate samples. Our project explored whether concepts from photonic quantum optics can help overcome this limitation. A central result was the experimental generation and detection of paired electrons and photons within a transmission electron microscope. When an electron passed through a very thin silicon membrane, it could emit a photon. By measuring both particles at the same time, we showed that their positions and momenta were connected in a way that cannot be explained classically. This verified, for the first time, quantum entanglement between a free electron and a photon. We then used these correlations for ghost (coincidence) imaging. In this method, one particle interacts with the object, while the other provides the spatial information needed to reconstruct the image. In our experiment, the photon probed the object, while the electron carried the information used to build the image. This demonstrated that photonic quantum imaging concepts can be transferred to electron microscopy. We also developed a practical method to certify such electron-photon entanglement in realistic, noisy microscope experiments. In the long term, these methods may help obtain more information from fewer damaging electrons, especially for radiation-sensitive materials, biological samples and nanotechnology.