  We experimentally study a protocol of using the broadband high frequency
squeezed vacuum to detect the low-frequency signal. In this scheme, the lower
sideband field of the squeezed light carries the low frequency modulation
signal and the two strong coherent light fields are applied as the bichromatic
local oscillator in the homodyne detection to measure the quantum entanglement
of the upper and lower sideband for the broadband squeezed light. The power of
one of the local oscillators for detecting the upper sideband can be adjusted
to optimize the conditional squeezing in the low frequency regime by
subtracting the photocurrent of the upper sideband field of the squeezed light
from that of the low sideband field. By means of the quantum correlation of the
upper and lower sideband for the broadband squeezed light, the low frequency
signal beyond the standard quantum limit is measured. This scheme is
appropriate for enhancing sensitivity of the low frequency signal by the aid of
the broad squeezed light, such as gravitational waes detection, and does not
need directly produce the low frequency squeezing in the optical parametric
process.
