We are pleased to announce the winner of the Best Presentation Award that Instruments (ISSN: 2410-390X) sponsored at the Physics and Applications of High Brightness Beams Workshop 2023, held from 19 to 23 June 2023, in San Sebastian, Spain. Congratulations to Simona Borrelli!

“Direct Observation of Temporal Sub-Poissonian Statistics of Free Electrons with Sub-Picosecond Resolution” by Simona Borrelli

Directly observing the statistical properties of free electrons has been a long-lasting scientific challenge, due to the limitations of available detectors in measuring over very short time intervals. Such a measurement involves counting the number of electrons reaching a detector simultaneously within a specific time window and determining the corresponding statistical distribution.

The Poisson distribution is commonly employed to model the counting statistics when observing electron emission from a thermal source over ‘long’ time intervals. However, at shorter time scales, the random nature of the electron emission process may be lost due to the emergence of electron–electron anti-correlations, resulting in sub-Poissonian statistics. These anit-correlations arise from the quantum-mechanical properties of electrons governed by the Pauli exclusion principle, which is the expression of their fermionic nature. Additionally, the Coulomb repulsion between electrons can also influence their dynamics and lead to anti-correlation within the time window characteristics of the interaction.

Sub-Poissonian statistics manifest as electron anti-bunching, causing a lower coincidence rate on a detector within a specific time interval than the classical Poisson behavior. Typically, the relevant time interval for observing such anti-correlation ranges from tens of picoseconds to femtoseconds. Observing such anti-bunching thus requires precise determination of particle arrival times within extremely short time windows.

We have introduced a novel method to directly observe the arrival time statistics of free electrons with sub-ps resolution. This technique has facilitated the first direct observation of sub-Poissonian statistics in a continuous electron beam on time scales ranging from a few picoseconds to a few hundred femtoseconds.

Our measurements have revealed Poissonian-like behavior on the nanosecond timescale, while increasingly pronounced sub-Poissonian statistics were observed as the time window decreased from 2 picoseconds to 340 femtoseconds.

These findings offer valuable insights into the underlying dynamics of free electron emissions. The proposed method not only enables the characterization of emissions from diverse sources, but also holds promise for developing highly coherent electron sources exhibiting sub-Poissonian behavior. Additionally, this research opens up possibilities for directly observing the fermionic nature of electrons, shedding light on their fundamental properties.