Photonic Quantum State Tomography Using Free Electrons

The tomography of photonic quantum states is key in quantum optics, impacting quantum sensing, computing, and communication. Conventional detectors are limited in their temporal and spatial resolution, hampering high-rate quantum communication and local addressing of photonic circuits. Here, we propose to utilize free electron-photon interactions for quantum state tomography, introducing electron homodyne detection with potential for femtosecond-temporal and nanometer-spatial resolutions. The detectable quantum information level depends on the electron-photon interaction strength. Our Letter opens avenues for free-electron-based photodetection utilizing the ultrafast, subwavelength, nondestructive nature of free electrons.