  Squeezed states of the optical field were theoretically described in the
early 1970s and first observed in the mid 1980s. The measured photon number of
a squeezed state is correlated with the measured photon numbers of all other
squeezed states of the same ensemble, providing sub-Poissonian statistics.
Today all gravitational-wave observatories use squeezed light as the
cost-efficient alternative to further scaling up the light power. This user
application of quantum correlations was made possible through dedicated
research and development of squeezed light between 2002 and 2010.
