  Light-phase-sensitive techniques, such as coherent multidimensional
spectroscopy, are well-established in a broad spectral range, already spanning
from radio-frequencies in nuclear magnetic resonance spectroscopy to visible
and ultraviolet wavelengths in nonlinear optics with table-top lasers. Here,
the ability to tailor the phases of electromagnetic waves with high precision
is essential. In the present contribution we achieve phase control of
extreme-ultraviolet pulses from a free-electron laser (FEL) on the attosecond
timescale in a Michelson-type all-reflective interferometric autocorrelator. By
varying the relative phase of the generated pulse replicas with sub-cycle
precision we observe the field interference, i.e. the light-wave oscillation
with a period of 129 as. The successful transfer of a powerful optical method
towards short-wavelength FEL science and technology paves the way towards
utilization of advanced nonlinear methodologies even at partially coherent soft
X-ray FEL sources that rely on self-amplified spontaneous emission.
