Experimental characterization of frequency-dependent squeezed light

We report on the demonstration of broadband squeezed laser beams that show a frequency-dependent orientation of the squeezing ellipse. Carrier frequency as well as quadrature angle were stably locked to a reference laser beam at 1064 nm. This frequency-dependent squeezing was characterized in terms of noise power spectra and contour plots of Wigner functions. The latter were measured by quantum state tomography. Our tomograph allowed a stable lock to a local oscillator beam for arbitrary quadrature angles with ±1° precision. Frequency-dependent orientations of the squeezing ellipse are necessary for squeezed states of light to provide a broadband sensitivity improvement in third-generation gravitational-wave interferometers. We consider the application of our system to long-baseline interferometers such as a future squeezed-light upgraded GEO 600 detector