  We propose and demonstrate a coherent control scheme for stable phase locking
of squeezed vacuum fields. We focus on sideband fields at frequencies from 10Hz
to 10kHz which is a frequency regime of particular interest in gravitational
wave detection and for which conventional control schemes have failed so far. A
vacuum field with broadband squeezing covering this entire band was produced
using optical parametric oscillation and characterized with balanced homodyne
detection. The system was stably controlled over long periods utilizing two
coherent but frequency shifted control fields. In order to demonstrate the
performance of our setup the squeezed field was used for a nonclassical
sensitivity improvement of a Michelson interferometer at audio frequencies.
