  Next-generation plasma-based accelerators can push electron bunches to
gigaelectronvolt energies within centimetre distances. The plasma, excited by a
driver pulse, generates large electric fields that can efficiently accelerate a
trailing witness bunch making possible the realization of laboratory-scale
applications ranging from high-energy colliders to ultra-bright light sources.
So far several experiments have demonstrated a significant acceleration but the
resulting beam quality, especially the energy spread, is still far from state
of the art conventional accelerators. Here we show the results of a beam-driven
plasma acceleration experiment where we used an electron bunch as a driver
followed by an ultra-short witness. The experiment demonstrates, for the first
time, an innovative method to achieve an ultra-low energy spread of the
accelerated witness of about 0.1%. This is an order of magnitude smaller than
what has been obtained so far. The result can lead to a major breakthrough
toward the optimization of the plasma acceleration process and its
implementation in forthcoming compact machines for user-oriented applications.
