Roadmap to ultrafast quantum degenerate free electron beams*

While degenerate Fermi gases have been extensively studied, and neutron and pion antibunching due to the Pauli force in Hanbury Brown-Twiss experiments demonstrated, quantum degenerate free electron beams have yet to be produced. Electron antibunching has been reported using a high-brightness DC field emitter [1], but the effects of Coulomb repulsion were neglected. We have shown theoretically [2] that pulsed, ultrafast photoemission from nanotips may lead to partial to complete quantum coherences, paving the way for the observation of macroscopic effects due to quantum degeneracy. We report here a coincidence experiment with an ultrafast W nanotip photoemitter that observed an antibunching signal of 1 part in 4, stronger by a factor of 300 compared with [1]. We attribute this sub-Poissonian distribution to transverse Coulomb repulsion of photoemitted electrons belonging to the same pulse. We present a roadmap, based on theory constrained by our experimental data, and with electron pulse duration, nanotip apex radius of curvature, and the coherence time and length as parameters, that can guide us to the achievement of quantum degenerate free electron beams.

[1] H. Kiesel, et al., Nature 418, 392 (2002).

[2] S. Keramati, et al., Adv. Imaging Electron Phys. 213 (2020), 3-26.