On pure quasi quantum quadratic operators of M_2(C)
In the present paper we study quasi quantum quadratic operators (q.q.o) acting on the algebra of $2\times 2$ matrices $M_2(C)$. It is known that a channel is called pure if it sends pure states to pure ones. In this papers, we introduce a weaker condition, called $q$-purity, than purity of the channel. To study $q$-pure channels, we concentrate ourselves to quasi q.q.o. acting on $M_2(C)$. We describe all trace-preserving quasi q.q.o. on $M_2(C)$, which allowed us to prove that if a trace-preserving symmetric quasi q.q.o. such that the corresponding quadratic operator is linear, then its $q$-purity implies its positivity. If a symmetric quasi q.q.o. has a Haar state $τ$, then its corresponding quadratic operator is nonlinear, and it is proved that such $q$-pure symmetric quasi q.q.o. cannot be positive. We think that such a result will allow to check whether a given mapping from $M_2(C)$ to $M_2(C)øM_2(C)$ is pure or not. On the other hand, our study is related to construction of pure quantum nonlinear channels. Moreover, it is also considered that nonlinear dynamics associated with quasi pure q.q.o. may have differen kind of dynamics, i.e. it may behave chaotically or trivially, respectively.