Relativistic free electrons based quantum physics

Abstract

The light-matter interaction is a fundamental research field in physics. The electron is the elementary particle first found to make up matter. Therefore, the interaction between electron and light field has long been the research interest of physicists. Electrons are divided into two kinds, the bound electrons and free electrons. The quantum transition of bound electron system is constrained by selection rules and generally discrete energy levels, while free electron systems are not. In the last decade, the theoretical framework and experiments of photon-induced near field electron microscopy (PINEM) have been proposed and developed to describe the interaction between quantum free electrons (we use quantum electron wavepackets to describe them) and optical fields, and many new phenomena have been successfully discovered and explained. In the framework of macroscopic quantum electrodynamics, the concept of photon is extended to photonic quasi-particles. Solutions of maxwell's equations in medium that satisfy certain boundary conditions are called photonic quasiparticles, such as surface plasmon polaritons, phonon polaritons, or even magnetic field. The different dispersion relations of photonic quasi-particles produce abundant phenomena in the interaction between light and matter. The experimental set up of PINEM is the Ultrafast electron transmission microscopy (UTEM). The underlying information of the interaction can be inferred from the electron energy loss spectrum (EELS). It has been used for near field imaging in its infancy, by now it is not only capable of time-resolved dynamic imaging, reconstructing the dispersion relation of photonics crystal and its Bloch mode, but also capable of measuring the mode lifetime directly. Using PINEM for sample imaging is usually non-destructive, and it provides high spatial, temporal and energy resolution. Thus it is a necessary complement to other imaging methods. PINEM is also used to study free electron wavepacket reshaping, free electron comb, free electron attosecond pulse train, etc. Many interesting framework of coherent control has been proposed. Recently, this field has entered the era of quantum optics, and people use PINEM to study novel phenomena in quantum optics, such as entanglement between free electrons and cavity photons, entanglement between free electrons and free electrons, free electron qubits, and preparation of novel light quantum states etc. In this paper, the theoretical and experimental development of PINEM is reviewed, its application scenarios are demonstrated, the current difficulties are summarized, and the future development is prospected.