  Optical losses degrade the sensitivity of laser interferometric instruments.
They reduce the number of signal photons and introduce technical noise
associated with diffuse light. In quantum-enhanced metrology, they break the
entanglement between correlated photons. Such decoherence is one of the primary
obstacles in achieving high levels of quantum noise reduction in precision
metrology. In this work, we compare direct measurements of cavity and mirror
losses in the Caltech 40m gravitational-wave detector prototype interferometer
with numerical estimates obtained from semi-analytic intra-cavity wavefront
simulations using mirror surface profile maps. We show a unified approach to
estimating the total loss in optical cavities (such as the LIGO gravitational
detectors) that will lead towards the engineering of systems with minimum
decoherence for quantum-enhanced precision metrology.
