  We analytically show that probabilistic amplification of a weak coherent
state stored inside an atomic medium can be achieved via detection of
coherently scattered photons. We show that this is because of collective
excitations created among atoms in the ensemble. We describe the physics of the
amplification and identify the failure events, which occur during the
amplification process. The amplification is realized by coherently mapping a
weak coherent state in an ensemble of $\Lambda$-level atoms followed by
detection of multiple Raman scattered photons, conditionally projecting the
coherent state into an amplified state upon retrieval.
