Phys. Rev. Lett. 81, 2598–2601 (1998)Optimal Universal Quantum Cloning and State EstimationReceived 1 December 1997; published in the issue dated 21 September 1998 We derive a tight upper bound for the fidelity of a universal N→M qubit cloner, valid for any M≥N, where the output of the cloner is required to be supported on the symmetric subspace. Our proof is based on the concatenation of two cloners and the connection between quantum cloning and quantum state estimation. We generalize the operation of a quantum cloner to mixed and/or entangled input qubits described by a density matrix supported on the symmetric subspace of the constituent qubits. We also extend the validity of optimal state estimation methods to inputs of this kind. © 1998 The American Physical Society URL:
http://link.aps.org/doi/10.1103/PhysRevLett.81.2598
DOI:
10.1103/PhysRevLett.81.2598
PACS:
03.67.Lx, 03.65.Bz, 89.80.+h
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