  Ultrashort electron bunches are crucial for an increasing number of
applications, however, diagnosing their longitudinal phase space remains a
challenge. We propose a new method that harnesses the strong electric fields
present in a laser driven plasma wakefield. By transversely displacing driver
laser and witness bunch, a streaking field is applied to the bunch. This field
maps the time information to a transverse momentum change and, consequently, to
a change of transverse position. We illustrate our method with simulations
where we achieve a time resolution in the attosecond range.
