  We present a joint theoretical and experimental characterization of
thermo-refractive noise in high quality factor ($Q$), small mode volume ($V$)
optical microcavities. Analogous to well-studied stability limits imposed by
Brownian motion in macroscopic Fabry-Perot resonators, microcavity
thermo-refractive noise gives rise to a mode volume-dependent maximum effective
quality factor. State-of-the-art fabricated microcavities are found to be
within one order of magnitude of this bound. We confirm the assumptions of our
theory by measuring the noise spectrum of high-$Q/V$ silicon photonic crystal
cavities and apply our results to estimate the optimal performance of proposed
room temperature, all-optical qubits using cavity-enhanced bulk material
nonlinearities.
