In electron beam lithography, we usually attempt to squeeze as much current into the beam as possible. What about the other extreme? Can we arrange to get one electron at a time in the beam, and what use might that have?
The Baum group at the Max Planck Institute of Quantum Optics has done just this. Reporting in PNAS [doi: 10.1073/pnas.1010165107], they demonstrate single-electron pulses generated by photoemission. The photoemission in turn is driven by tuned, femtosecond UV pulses. Tuning of the UV wavelength turns out to be important for the bandwidth, coherence and duration of the resulting electron pulse. The experiment results in a transverse (electron) coherence of 2.5 nm, quite adequate for diffraction studies.
Now, with a coherence length of 2.5 nm and a duration of 100 fs or less, one can study atomic-scale dynamics in condensed matter and molecules by so-called four-dimensional imaging. Many interesting phenomena become visible; see the first twelve or so references in the paper.