  Charged particle bunch length detection is one of the most challenging
measurements in particle and accelerator physics, especially when the bunch
length reduces below tens of femtoseconds. Since these measurements are most
critical in high energy electron accelerators and devices with ultrashort
electron bunches - such as the plasma wakefield and related accelerators - this
discussion is limited to electron bunch detection, although the results clearly
have significance for any ultra-relativistic particle bunches.
