Abstract
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 $1064\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. 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 $\ifmmode\pm\else\textpm\fi{}1\ifmmode^\circ\else\textdegree\fi{}$ 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.
Highlights
Gravitational wavesGWswere long ago predicted by Einstein using the theory of general relativity, but so far have not been directly observed1͔
If some apparatus changes the phase of only one sideband by the angle ⌽ keeping the carrier as well as the other sideband unchanged, we find that the quadrature angle of constructive interference changes by half the amount of the single sideband rotation, a Ј
The colored spectra in Figs. 3͑aand 4͑aclearly show that the squeezed laser beam investigated carried frequencydependent squeezing
Summary
Gravitational wavesGWswere long ago predicted by Einstein using the theory of general relativity, but so far have not been directly observed1͔. Further improvement in sensitivity can be achieved by signal recycling8͔, an advanced technique which will be implemented in second-generation GW interferometers and is already successfully implemented in GEO 600 ͓9,10͔ It was first proposed by Caves11͔ that squeezed light injected into the dark port of a GW interferometer can reduce the high laser power requirements. Harms et al ͓21͔ have generally shown that signal-recycled interferometers will benefit from squeezed light to conventional interferometers as mentioned above This result has strongly motivated further investigations on squeezed light because second-generation detectors like Advanced LIGO will combine arm cavities and signal recyclingthe so-called resonant sideband extraction22,23͔͒ and will be quantum-noise limited over a substantial fraction of the detection spectrum. In the shot-noise-limited regime of a signal-recycled interferometer a single filter cavity can provide the optimal frequency dependence for the squeezing ellipse.
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