  Contemporary gravitational-wave detectors are fundamentally limited by
thermal noise -- due to dissipation in the mechanical elements of the test mass
-- and quantum noise -- from the vacuum fluctuations of the optical field used
to probe the test mass position. Two other fundamental noises can in principle
also limit sensitivity: test-mass quantization noise due to the zero-point
fluctuation of its mechanical modes, and thermal excitation of the optical
field. We use the quantum fluctuation-dissipation theorem to unify all four
noises. This unified picture shows precisely when test-mass quantization noise
and optical thermal noise can be ignored.
