  We demonstrate the reliable generation of 1-mJ, 31-fs pulses with an average
power of 1 kW by post-compression of 200-fs pulses from a coherently combined
Yb:fiber laser system in an argon-filled Herriott-type multi-pass cell with an
overall compression efficiency of 96%. We also analyze the output beam,
revealing essentially no spatio-spectral couplings or beam quality loss.
