  We report on the feedback stabilization of the zero-phonon emission frequency
of a single InAs quantum dot. The spectral separation of the phonon-assisted
component of the resonance fluorescence provides a probe of the detuning
between the zero-phonon transition and the resonant driving laser. Using this
probe in combination with active feedback, we stabilize the zero-phonon
transition frequency against environmental fluctuations. This protocol reduces
the zero-phonon fluorescence intensity noise by a factor of 22 by correcting
for environmental noise with a bandwidth of 191 Hz, limited by the experimental
collection efficiency. The associated sub-Hz fluctuations in the zero-phonon
central frequency are reduced by a factor of 7. This technique provides a means
of stabilizing the quantum dot emission frequency without requiring access to
the zero-phonon emission.
