title: Sequential quantum cloning.
abstract: Not all unitary operations upon a set of qubits can be implemented by sequential interactions between each qubit and an ancillary system. We analyze the specific case of sequential quantum cloning, 1-->M, and prove that the minimal dimension D of the ancilla grows linearly with the number of clones M. In particular, we obtain D=2M for symmetric universal quantum cloning and D=M+1 for symmetric phase-covariant cloning. Furthermore, we provide a recipe for the required ancilla-qubit interactions in each step of the sequential procedure for both cases.
authors.affiliation: Sección Física, Departamento de Ciencias, Pontificia Universidad Católica del Perú, Apartado Postal 1761, Lima, Peru.
authors.forename: Y
authors.initials: Y
authors.lastname: Delgado
authors.forename: L
authors.initials: L
authors.lastname: Lamata
authors.forename: J
authors.initials: J
authors.lastname: León
authors.forename: D
authors.initials: D
authors.lastname: Salgado
authors.forename: E
authors.initials: E
authors.lastname: Solano