Nonclassical light and interferometry techniques
Quantum noise of the electro-magnetic field is one of the major noise sources in second-generation ground-based interferometric gravitational wave detectors. The well-known Standard Quantum Limit (SQL) for continuous position measurement arises because the time-evolving Heisenberg operator of the position of a free test mass does not commute with itself at different times. Intensive research during the last 20 years has revealed that the SQL can be surpassed by employing quantum correlations, which could arise naturally during the measurement process, or could be produced by injecting nonclassical modes of light. Techniques that beat the SQL are generally called "Quantum Non-Demoliton" (QND) techniques. In the last years our understanding has considerably improved and more and more practical schemes for beating the SQL have been developed.
At low frequencies opto-mechanical resonances will allow for sensitivities beyond the SQL. Also ponderomotive squeezing, formed naturally in conventional interferometer configurations, can be taken advantage of with the help of optical filters. Second-generation detectors will reach the SQL at about 100 Hz. In this regime the interferometer sensitivity can be increased by increasing the laser power and by applying non-classical techniques. It is planned to inject appropriately designed squeezed modes of the electromagnetic field into the interferometer. The most efficient nonlinear process for squeezed mode generation is optical parametric oscillation in crystals. The goal is to reduce the shot noise by factors of 2 to 3. Rigorous designs of new topologies ("speed meters") will aim at easing these approaches.