  XUV and X-ray Free Electron Lasers (FELs) produce short wavelength pulses
with high intensity, ultrashort duration, well-defined polarization and
transverse coherence, and have been utilised for many experiments previously
possible at long wavelengths only: multiphoton ionization, pumping an atomic
laser, and four-wave mixing spectroscopy. However one important optical
technique, coherent control, has not yet been demonstrated, because Self-
Amplified Spontaneous Emission FELs have limited longitudinal coherence.
Single-colour pulses from the FERMI seeded FEL are longitudinally coherent, and
two-colour emission is predicted to be coherent. Here we demonstrate the phase
correlation of two colours, and manipulate it to control an experiment. Light
of wavelengths 63.0 and 31.5 nm ionized neon, and the asymmetry of the
photoelectron angular distribution was controlled by adjusting the phase, with
temporal resolution 3 attoseconds. This opens the door to new shortwavelength
coherent control experiments with ultrahigh time resolution and chemical
sensitivity.
