A quantum telecloning process combining quantum teleportation and optimal quantum cloning from one input to M outputs is presented. The scheme relies on the establishment of particular multiparticle entangled states, which function as multiuser quantum information channels. The entanglement structure of these states is analyzed and shown to be crucial for this type of information processing.

We demonstrate that local transformations on a composite quantum system can be enhanced in the presence of certain entangled states. These extra states act much like catalysts in a chemical reaction: they allow otherwise impossible local transformations to be realized, without being consumed in any way. In particular, we show that this effect can considerably improve the efficiency of entanglement concentration procedures for finite states.

We find a minimal set of necessary and sufficient conditions for the existence of a local procedure that converts a finite pure state into one of a set of possible final states. This result provides a powerful method for obtaining optimal local entanglement manipulation protocols for pure initial states. As an example, we determine analytically the optimal distillable entanglement for arbitrary finite pure states. We also construct an explicit protocol achieving this bound.PACS numbers: 03.67.HkThe existence of nonlocal correlations, or entanglement, between parts of a composite quantum system is at the heart of quantum information theory and its applications [1]. In recent years, much effort has been expended on the problem of how to define and quantify the entanglement of a given state in physically meaningful ways. One very fruitful approach, first pursued by Bennett and coworkers [2][3][4], is to regard entanglement in terms of the limitations that exist to the manipulation of a composite system when each subsystem is operated on locally. A paradigmatic situation is as follows: suppose Alice and Bob each possess part of a quantum system, which is prepared in a state r. Qualitatively, the existence of entanglement implies that some transformations of r which are in principle possible cannot be realized if Alice and Bob are allowed to perform only local operations on their respective subsystems, and to exchange classical communication. In this paper, transformations of this type will be referred to as "local transformations," or "LQCC" for short.A quantitative way of expressing this fact is in terms of so-called entanglement monotones (EMs) [5]. These are functions´͑r͒ of the quantum state that can, on average, never increase under LQCC [6]. There are many known EMs, for example the entanglements of distillation [2][3][4] and formation [4,7], and the relative entropy of entanglement [8] (in fact, any reasonable measure of entanglement must by definition be an entanglement monotone, and vice versa). Despite their different physical interpretations, they all share a common feature: a transformation which, on average, increases any single EM cannot be realized locally. In other words, they provide necessary conditions any local transformation T must satisfy.A natural question that presents itself is then: what are sufficient conditions for T to be local? In other words, we would like to have a set ͕´i͖ of entanglement monotones such that, if the average ͗´i͑T ͓r͔͒͘ #´i͑r͒ for all i, then T is local. Ideally, this set should also be minimal, in the sense that these conditions should not be redundant [9]. An important result in this direction was recently presented by Nielsen [10], who found sufficient conditions for the locality of transformations that take one given pure state to another with 100% probability. In the present Letter, we extend Nielsen's theorem to the case where the transformation need not be deterministic, that is, when T may lead to several possible final states. We demonstrate that, for this ca...

We present an alternative scheme for the generation of a two-qubit quantum gate interaction between laser-cooled trapped ions. The scheme is based on the ac Stark shift ͑light shift͒ induced by laser light resonant with the ionic transition frequency. At specific laser intensities, the shift of the ionic levels allows the resonant excitation of transitions involving the exchange of motional quanta. We compare the performance of this scheme with respect to that of related ion-trap proposals and find that, for an experimental realization using traveling-wave radiation and working in the Lamb-Dicke regime, an improvement of over an order of magnitude in the gate switching rate is possible.

We analyze approximate transformations of pure entangled quantum states by local operations and classical communication, finding explicit conversion strategies which optimize the fidelity of transformation. These results allow us to determine the most faithful teleportation strategy via an initially shared partially entangled pure state. They also show that procedures for entanglement manipulation such as entanglement catalysis ͓Jonathan and Plenio, Phys. Rev. Lett. 83, 3566 ͑1999͔͒ are robust against perturbation of the states involved, and motivate the notion of nonlocal fidelity, which quantifies the difference in the entangled properties of two quantum states.