  Ultrafast processes in matter can be captured and even controlled by using
sequences of few-cycle optical pulses, which need to be well characterized,
both in amplitude and phase. The same degree of control has not yet been
achieved for few-cycle extreme ultraviolet pulses generated by high-order
harmonic generation in gases, with duration in the attosecond range. Here, we
show that by varying the spectral phase and carrier-envelope phase (CEP) of a
high-repetition rate laser, using dispersion in glass, we achieve a high degree
of control of the relative phase and CEP between consecutive attosecond pulses.
The experimental results are supported by a detailed theoretical analysis based
upon the semiclassical three-step model for high-order harmonic generation.
