Quantum optical technologies with free electrons

The quantum free electron holds promise as a new and exciting versatile quantum probe, offering high coherence, sub-femtosecond and sub-nanometer resolution, tunability, and precise coherent control. Recent experimental and theoretical developments have highlighted the quantum nature of free electron interactions with light and bound-electron qubits, demonstrating strong electron-photon coupling, coincidence measurements, and proposals for quantum light generation.

In this talk, I will introduce the field of free-electron quantum optics, where free electrons join photons and optical emitters as a key element in quantum optics. I will first discuss how free electrons can be integrated with cavity QED, enabling applications such as quantum sensing of strong coupling effects and imaging ultrafast vacuum Rabi oscillations. Coherent interference between electron-atom and electron-photon interactions is imprinted on the free-electron wavefunction, allowing enhanced sensitivity to emitter position, dipole orientation, and direct imaging of vacuum Rabi oscillations. When the vacuum Rabi splitting is resolvable in electron energy loss spectroscopy, it also enables quantum state readout of polaritonic excitations.

Next, I will explore the potential of free electrons as flying qubits for ultrafast quantum information processing. This approach is based on a novel free-electron–polariton blockade effect, where electrons interact with a nonlinear quantum system in cavity QED.

Finally, I will discuss how strongly coupled free electrons and quantum light can serve as a new tunable source of single-photon nonlinearity. Specifically, we propose using low-energy electrons as nonlinear emitters to implement the Jaynes-Cummings interaction. We also examine ways to enhance interaction strength and nonlinearity for higher-energy electrons guided by light in hollow-core waveguides and fiber structures. These advancements open new possibilities for macroscopic quantum light generation and multimode nonlinear quantum optics.

With rapid experimental progress complementing these theoretical developments, free-electron quantum optics presents a promising avenue for novel quantum technologies with capabilities beyond traditional approaches.