  Earth-based gravitational-wave detectors will be limited by quantum noise in
a large part of their spectrum. The most promising technique to achieve a
broadband reduction of such noise is the injection of a frequency dependent
squeezed vacuum state from the output port of the detector, whit the squeeze
angle rotated by the reflection off a Fabry-Perot filter cavity. One of the
most important parameters limiting the squeezing performance is represented by
the optical losses of the filter cavity. We report here the operation of a 300
m filter cavity prototype installed at the National Astronomical Observatory of
Japan (NAOJ). The cavity is designed to obtain a rotation of the squeeze angle
below 100 Hz. After achieving the resonance of the cavity with a
multi-wavelength technique, the round trip losses have been measured to be
between 50 ppm and 90 ppm. This result demonstrates that with realistic
assumption on the input squeeze factor and on the other optical losses, a
quantum noise reduction of at least 4 dB in the frequency region dominated by
radiation pressure can be achieved.
