  We investigate decoherence channels that are modelled as a sequence of
collisions of a quantum system (e.g., a qubit) with particles (e.g., qubits) of
the environment. We show that collisions induce decoherence when a bi-partite
interaction between the system qubit and an environment (reservoir) qubit is
described by the controlled-U unitary transformation (gate). We characterize
decoherence channels and in the case of a qubit we specify the most general
decoherence channel and derive a corresponding master equation. Finally, we
analyze entanglement that is generated during the process of decoherence
between the system and its environment.
