Photomultiplier tubes convert photons to electrical signals. When a photon strikes the photocathode, it liberates a photoelectron. The electron's initial energy determines how precisely its arrival time can be measured — or so the reasoning went. Higher-energy photoelectrons should be easier to track, their trajectories more predictable.
The sub-10-picosecond RF photomultiplier (arXiv:2603.11887) achieves extreme timing resolution by exploiting the opposite. Multi-alkali photocathodes emit photoelectrons with surprisingly low initial energies — 0.1 to 0.3 electron volts. These slow electrons are easy to manipulate with radiofrequency fields because their trajectories respond strongly to small applied forces. Fast electrons resist RF steering; slow electrons submit to it.
The RF cavity acts as a temporal lens. Photoelectrons entering at different phases of the RF cycle are accelerated differently, and the timing information is encoded in the energy they gain. Sub-10-picosecond resolution follows because the slow electrons' trajectories are almost entirely determined by the RF field rather than by their initial conditions. The measurement dominates the noise.
The detector's weakness — low photoelectron energy — is precisely what enables its precision. A photocathode that emitted higher-energy electrons would be more efficient at detection but worse at timing, because the electrons' initial velocities would compete with the RF field's steering. The low-energy emission that looks like a performance limitation is the performance mechanism.
Compact design follows from the same source. Low-energy electrons require less voltage to control, smaller cavities to contain, shorter drift distances to separate. The physical footprint shrinks with the electron energy. Small, slow, and precise.