  The quantum nature of the electromagnetic field imposes a fundamental limit
on the sensitivity of optical precision measurements such as spectroscopy,
microscopy, and interferometry. The so-called quantum limit is set by the
zero-point fluctuations of the electromagnetic field, which constrain the
precision with which optical signals can be measured. In the world of precision
measurement, laser-interferometric gravitational wave (GW) detectors are the
most sensitive position meters ever operated, capable of measuring distance
changes on the order of 10^-18 m RMS over kilometer separations caused by GWs
from astronomical sources. The sensitivity of currently operational and future
GW detectors is limited by quantum optical noise. Here we demonstrate a 44%
improvement in displacement sensitivity of a prototype GW detector with
suspended quasi-free mirrors at frequencies where the sensitivity is
shot-noise-limited, by injection of a squeezed state of light. This
demonstration is a critical step toward implementation of squeezing-enhancement
in large-scale GW detectors.
