Abstract
No cloning distinguishes the quantum cryptography. Buzek and Hillery have developed a universal quantum cloning machine that allows providing two copies of an arbitrary qubit state with the same accuracy independently of the input-state. The fidelity has been used as a criterion to characterize the cloning. It was found that this parameter can achieve 0.85 for special subsets of quantum states, i.e, equatorial qubits. In the present paper, we investigate the effects of a magnetic field environment as a perturbation of the cloning process. The quantum copying machines studied consist of UQCM-BH and UQCM-PC. Results have been discussed using both the fidelity and the relative entropy. Much attention has been paid to the magnetic field-related decoherence of ancillary qubits before preparation. An attempt to explain the impact of this decoherence on the performance of copying machines will be presented.