  We discuss a new method for realizing number-resolving and non-demolition
photo detectors by strong coupling of light to individual single photon
emitters, which act as strong optical non-linearities. As a specific
application we show how these elements can be integrated into an error-proof
Bell state analyzer, whose efficiency exceeds the best possible performance
with linear optics even for a modest atom-field coupling. The methods are
error-proof in the sense that every detection event unambiguously projects the
photon state onto a Fock or Bell state.
