Enhanced sensitivity of low-frequency signal by using broad squeezed light and bichromatic local oscillator

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.