A family of asymmetric quantum cloning machines is introduced that produce two approximate copies of a single quantum bit, each copy emerging from a Pauli channel. A no-cloning inequality is derived, describing the balance between the quality of the copies. The Pauli cloning machine is also shown to put a limit on the quantum capacity of Pauli channels.

• Received 30 March 1998

DOI:https://doi.org/10.1103/PhysRevLett.84.4497