  In this paper we investigate the efficiency of quantum cloning of $N$
identical mixed qubits. We employ a recently introduced measure of
distinguishability of quantum states called quantum Chernoff bound. We evaluate
the quantum Chernoff bound between the output clones generated by the cloning
machine and the initial mixed qubit state. Our analysis is illustrated by
performing numerical calculation of the quantum Chernoff bound for different
scenarios that involves the number of initial qubits $N$ and the number of
output imperfect copies $M$.
