Ultrahigh brightness electron beams enable novel regimes of applications, ranging from novel compact light sources for photon science to strong field
and high energy physics. For example, brightness and emittance are key performance parameters for X-ray free-electron lasers. Ultrashort, high current electron pulses
with normalized emittance of nanometer-radian level, could allow to realize hard x-ray FEL's already at few GeV electron energies. In turn, such ultralow emittance may allow
to push towards even harder photon wavelengths. Ultrahigh brightness implies ultrahigh gain, and ultrashort, ultrabright electron pulses may allow generation of single spike,
sub-femtosecond coherent X-ray pulses by brute force. This application is being explored in the related forward-looking PWFA-FEL project, funded by the UK STFC 2019-2023.