  Accelerating particles to relativistic energies over very short distances
using lasers has been a long standing goal in physics. Among the various
schemes proposed for electrons, vacuum laser acceleration has attracted
considerable interest and has been extensively studied theoretically because of
its appealing simplicity: electrons interact with an intense laser field in
vacuum and can be continuously accelerated, provided they remain at a given
phase of the field until they escape the laser beam. But demonstrating this
effect experimentally has proved extremely challenging, as it imposes stringent
requirements on the conditions of injection of electrons in the laser field.
Here, we solve this long-standing experimental problem for the first time by
using a plasma mirror to inject electrons in an ultraintense laser field, and
obtain clear evidence of vacuum laser acceleration. With the advent of PetaWatt
class lasers, this scheme could provide a competitive source of very high
charge (nC) and ultrashort relativistic electron beams.
