  Control fields in quantum information processing are virtually always, almost
by definition, assumed to be classical. In reality, however, when such a field
is used to manipulate the quantum state of qubits, the qubits never remain
completely unentangled with the field. For quantum information processing this
is an undesirable property, as it precludes perfect quantum computing and
quantum communication. Here we consider the interaction of atomic qubits with
laser fields and quantify atom-field entanglement in various cases of interest.
We find that the entanglement decreases with the average number of photons
$\bar{n}$ in a laser beam as $E\propto\log_2 \bar{n}/\bar{n}$ for
$\bar{n}\to\infty$.
