Decoherence free subspaces for two--access quantum channels

In this paper we consider construction of decoherence free subspaces for two--access random unitary channels. First, we concentrate on hermitian unitary noise model $U$ for a bi--unitary channel and show that in this case a code exists if the space of Schmidt matrices of an eigensubspace of $U$ exhibits certain properties of decomposability. Then, we show that our technique is also applicable in a generic case and consider its application two tri- (quatro-, ...) unitary channels.