  In this paper we investigate the properties of self induced transparency
(SIT) solitons, propagating in a $\Lambda$-type medium. It was found that the
interaction between SIT solitons can lead to trapping with their phase
preserved in the ground state coherence of the medium. These phases can be
altered in a systematic way by the application of appropriate light fields,
such as additional SIT solitons. Furthermore, multiple independent SIT solitons
can be made to propagate as bi-solitons through their mutual interaction with a
separate light field. Finally, we demonstrate that control of the SIT soliton
phase can be used to implement an optical exclusive-or gate.
