| Summary: | Gravitational Wave interferometers achieve their profound sensitivity by
combining a Michelson interferometer with optical cavities, suspended masses,
and now, squeezed quantum states of light. These states modify the measurement
process of the LIGO, VIRGO and GEO600 interferometers to reduce the quantum
noise that masks astrophysical signals; thus, improvements to squeezing are
essential to further expand our gravitational view of the universe. Further
reducing quantum noise will require both lowering decoherence from losses as
well more sophisticated manipulations to counter the quantum back-action from
radiation pressure. Both tasks require fully understanding the physical
interactions between squeezed light and the many components of km-scale
interferometers. To this end, data from both LIGO observatories in observing
run three are expressed using frequency-dependent metrics to analyze each
detector's quantum response to squeezed states. The response metrics are
derived and used to concisely describe physical mechanisms behind squeezing's
simultaneous interaction with transverse-mode selective optical cavities and
the quantum radiation pressure noise of suspended mirrors. These metrics and
related analysis are broadly applicable for cavity-enhanced optomechanics
experiments that incorporate external squeezing, and -- for the first time --
give physical descriptions of every feature so far observed in the quantum
noise of the LIGO detectors.
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