  Radio-frequency particle accelerators are engines of discovery, powering
high-energy physics and photon science, but are also large and expensive due to
their limited accelerating fields. Plasma-wakefield accelerators (PWFAs)
provide orders-of-magnitude stronger fields in the charge-density wave behind a
particle bunch travelling in a plasma, promising particle accelerators of
greatly reduced size and cost. However, PWFAs can easily degrade the beam
quality of the bunches they accelerate. Emittance, which determines how tightly
beams can be focused, is a critical beam quality in for instance colliders and
free-electron lasers, but is particularly prone to degradation. We demonstrate,
for the first time, emittance preservation in a high-gradient and
high-efficiency PWFA while simultaneously preserving charge and energy spread.
This establishes that PWFAs can accelerate without
degradation$\unicode{x2014}$essential for energy boosters in photon science and
multistage facilities for compact high-energy particle colliders.
