We present Quantum Cloning Machines (QCM) that transform N identical qubits
into $M>N$ identical copies and we prove that the fidelity (quality) of these
copies is optimal. The connection between cloning and measurement is discussed
in detail. When the number of clones M tends towards infinity, the fidelity of
each clone tends towards the optimal fidelity that can be obtained by a
measurement on the input qubits. More generally, the QCM are universal devices
to translate quantum information into classical information.

本文介绍了一种利用 Quantum Cloning Machines（QCM）将 N 个相同 qubits 转换成 M 个相同副本的方法，并证明这些副本的保真度是最佳的，并且讨论了克隆与量度之间的关系。当克隆的数量 M 趋近于无穷大时，每个克隆的保真度趋近于通过输入 qubits 的测量可以获得的最佳保真度。而且，QCM 通用于将量子信息转换为经典信息。

最优量子克隆机

Optimal Quantum Cloning Machines

We establish the best possible approximation to a perfect quantum cloning
machine which produces two clones out of a single input. We analyze both
universal and state-dependent cloners. The maximal fidelity of cloning is shown
to be 5/6 for universal cloners. It can be achieved either by a special unitary
evolution or by a novel teleportation scheme. We construct the optimal
state-dependent cloners operating on any prescribed two non-orthogonal states,
discuss their fidelities and the use of auxiliary physical resources in the
process of cloning. The optimal universal cloners permit us to derive a new
upper bound on the quantum capacity of the depolarizing quantum channel.

论文介绍了关于量子克隆机的研究，通过特殊的酉演化或者新型的量子隐形传态方案达到了量子克隆的最大保真度，并且讨论了两个非正交态的克隆的最优操作以及克隆过程中所使用的外部物理资源的提取。最后，提出了一个新的上限界定来衡量去极化量子信道的量子容量。