  The evolution of an open system is usually associated with the interaction of
the system with an environment. A new method to study the open-type system
evolution of a qubit (two-level atom) state is established. This evolution is
determined by a unitary transformation applied to the qutrit (three-level atom)
state, which defines the qubit subsystems. This procedure can be used to obtain
different qubit quantum channels employing unitary transformations into the
qutrit system. In particular, we study the phase damping and
spontaneous-emission quantum channels. In addition, we mention a proposal for
quasiunitary transforms of qubits, in view of the unitary transform of the
total qutrit system. The experimental realization is also addressed. The
probability representation of the evolution and its information-entropic
characteristics are considered.
