Towards observation of quantum degeneracy in electron beams via variation of pulse width.

An observation of the Pauli exclusion principle in free electron beams that is isolated from the effects of Coulomb forces remains one of the challenges in fundamental experimental physics [1,2]. Coulomb expansion models for pulsed photoemitted electrons from surfaces using ultrashort lasers suggest an asymptotic saturation in coincidence detection of electrons with decreasing pulse width [3]. This reveals a parameter regime in which the effects of quantum degeneracy can be studied in isolation. In situ laser pulse width measurement at the position of the electron source is thus needed. Using third-order intensity autocorrelations of the electron current allows the bypass of pre-hoc chirp corrections for the pulse duration. This is compared with measurements using a two-photon absorption fluorescence technique [4,5]. The variation of zero delay coincidences as a function of the pulse width can be compared to theory. This may reveal an experimentally feasible operating range that has a robust degeneracy signature distinguishable from the saturated Coulomb background. In addition, we are training machine learning models to detect the difference between simulated Coulomb pressure and quantum degeneracy pressure for multi-electron pulses. This illustrates that the traditional parameter range for previous experiments could even be extended to meet the challenges of degeneracy measurements.

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[2] Meier, S., Heimerl, J. & Hommelhoff, P. Few-electron correlations after ultrafast photoemission from nanometric needle tips. Nat. Phys. 19, 1402–1409 (2023). 

[3] Reed, B., Femtosecond electron pulse propagation for ultrafast electron diffraction. J. Appl. Phys. 100 (3), 034916 (2006). 

[4] Syed R., Uiterwaal, C. Ultrafast pulse duration measurement method of near-infrared pulses for a broad range of wavelengths using two-photon absorption in a liquid and fluorescent dye solution. Rev. Sci. Instrum. 94 (10), 103006 (2023). 

[5] Mahmood, A., Puente, Raul., Batelaan, H., Syed, R., Uiterwaal, C. arXiv:2411.01708 (2024).