  Annual Review of Astronomy and Astrophysics, Volume 51, page 511e studied for
coherently driven single photon sources, such as atoms and quantum dots.
Maximal squeezing is realized, if the electronic subsystem of the emitter is in
a pure quantum state. The purification is achieved by using a cavity as a
second decay channel, besides the incoherent coupling to the electromagnetic
vacuum. For realistic cavities this yields a purity of the electronic state of
more than 99%. Aside from numerical calculations, we also derive approximate
analytical results. Based on the approximations, effects are studied which
originate from the environment of the emitter, including radiationless
dephasing and incoherent pumping of the emitter and the cavity mode. The
fragility of squeezing against decoherence is substantially reduced, so that
squeezing persists even under hostile conditions. The measurement of squeezing
from such light sources is also considered.
