If the symmetry, (an operator
J satisfying
J=J∗=J−1) which defines the Krein space, is replaced by a (not necessarily self-adjoint) unitary, then we have the notion of an
S-space which was introduced by Szafraniec. In this paper, we consider
S-spaces and study the structure of completely
U-positive maps between the algebras of bounded linear operators. We first give a Stinespring-type representation for a completely
U-positive map. On the other hand, we introduce Choi
U-matrix of a linear map and establish the equivalence of the Kraus
U-decompositions and Choi
U-matrices. Then we study properties of nilpotent completely
U-positive maps. We develop the
U-PPT criterion for separability of quantum
U-states and discuss the entanglement breaking condition of quantum
U-channels and explore
U-PPT squared conjecture. Finally, we give concrete examples of completely
U-positive maps and examples of
3⊗3 quantum
U-states which are
U-entangled and
U-separable.