Does an electron wavefunction collapse when it radiates?

Research of cavity quantum electrodynamics (CQED) has enabled new capabilities in quantum optics, quantum computation, and various quantum technologies. So far, all the work in this field has included light interacting with bound-electron systems such as atoms, quantum dots, and quantum circuits. In contrast, free-electron systems enable fundamentally different physical phenomena, as their energy distribution is continuous and not discrete, and allow for tunable transitions and selection rules.

We have developed a platform for studying free-electron CQED at the nanoscale and demonstrated it by observing coherent electron interaction with a photonic cavity for the first time. Our platform includes femtosecond lasers in an ultrafast transmission electron microscope, which created what is, in many respects, the most powerful nearfield optical microscope in the world today. We resolve photonic bandstructures as a function of energy, momentum, and polarization, simultaneously with capturing the spatial distribution of the photonic modes at deep-subwavelength resolution.

These capabilities open new paths toward using free electrons as carriers of quantum information. As examples, we show how to create free-electron qubits and implement quantum gates with femtosecond lasers. We further show how to measure quantum decoherence in space and time using the free-electron quantum interactions. Such interactions also enable new avenues for tunable X-ray sources, as we demonstrate with theory and experiments.

 References:

·       N. Rivera and I. Kaminer, Light–matter interactions with photonic quasiparticles, Nature Reviews Physics 2, 538–561 (2020) (Review)

·       R. Dahan†, S. Nehemia†, M. Shentcis, O. Reinhardt, Y. Adiv, K. Wang, O. Beer, Y. Kurman, X. Shi, M. H. Lynch, I. Kaminer, Resonant phase-matching between a light wave and a free-electron wavefunction, Nature Physics 16, 1122-1130 (2020)

·       I. Kaminer, et al., Quantum Čerenkov Radiation: Spectral Cutoffs and the Role of Spin and Orbital Angular Momentum, Phys. Rev. X 6, 011006 (2016)

·       K. Wang, R. Dahan, M. Shentcis, Y. Kauffmann, A. Ben-Hayun, O. Reinhardt, S. Tsesses, I. Kaminer, Coherent Interaction between Free Electrons and Cavity Photons, Nature 582, 50 (2020)

·       M. Shentcis, et al., Tunable free-electron X-ray radiation from van der Waals materials, Nature Photonics 14, 686–692 (2020)

·       L. J. Wong†, I. Kaminer†, O. Ilic, J. D. Joannopoulos, and M. Soljačić, Toward Graphene Plasmon-Based Free-Electron IR to X-ray Sources, Nature Photonics 10, 46 (2016)