Quantum limits and symphotonic states in free-mass gravitational-wave   antennae
  Quantum mechanics sets severe limits on the sensitivity and required circulating energy in traditional free-mass gravitational-wave antennas. One possible way to avoid these restrictions is the use of intracavity QND measurements. We analyze a new QND observable, which possesses a number of features that make it a promising candidate for such measurements and propose a practical scheme for the realization of this measurement. In combination with an advanced coordinate meter, this scheme makes it possible to lower substantially the requirements on the circulating power. 
Vladimir B.Braginsky, Michael L.Gorodetsky, Farid Ya.Khalili