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.

Purification schemes for multiparticle entangled states cannot be treated as straightforward extensions of those two-particle ones because of the lack of symmetry they possess. We propose purification protocols for a wide range of mixed entangled states of many particles. These are useful for understanding entanglement, and could be of practical significance in multiuser cryptographic schemes or distributed quantum computation and communication. We show that operating locally on multiparticle entangled states directly is more efficient than relying on two-particle purification. ͓S1050-2947͑98͒50906-7͔ PACS number͑s͒: 03.67.Hk Entanglement is of central importance for quantum computation ͓1͔, quantum teleportation ͓2͔, and certain types of quantum cryptography ͓3͔. Without entangled states, quantum computation and communication would be no more efficient than their classical counterparts. For two particles, the maximally entangled states are the Bell diagonal states ͉

We show that entanglement guarantees difficulty in the discrimination of orthogonal multipartite states locally. The number of pure states that can be discriminated by local operations and classical communication is bounded by the total dimension over the average entanglement. A similar, general condition is also shown for pure and mixed states. These results offer a rare operational interpretation for three abstractly defined distance like measures of multipartite entanglement.The problem of defining and understanding multiparty entanglement is a major open question in the field of quantum information. As entanglement theory becomes more useful in other areas of many body physics, multiparty entanglement becomes increasingly relevant to general physics, too. Hence, understanding the meaning of entanglement has become and interesting and important question.In the bipartite case, entanglement is fairly well understood [1]. There are many entanglement measures defined both operationally (in terms of the usefulness of states for quantum information tasks) and abstractly (such that they obey certain axioms and may be called entanglement monotones). One of the most celebrated results in bipartite entanglement theory is that for pure states essentially all measures coincide and have clear operational relevance. For more than two parties however, the operational approach quickly becomes very difficult. There are no clear "units of usefulness" and we have the possibility of inequivalent types of entanglement [2]. Some abstract measures do persist by their simplicity. In particular those measures that define "proximity" to the set of separable states [3,4,5] have natural multiparty analogues. However, due to their abstract definition, their operational meaning is not clear and remains an open question.In this Letter, we consider the connection between distance-like entanglement measures and the task of local operations and classical communication (LOCC) state discrimination with this question in mind. This task illustrates the restriction of only having local access to a system, fundamental to the use of entanglement in quantum information (and notions of locality). Indeed, LOCC measurement of quantum states is important for cryptographic protocols [6], channel capacities [7], and distributed quantum information processing [8].Intuitively we expect that entangled states are more difficult to discriminate locally, since inherently they possess properties that are non-local. Indeed it is known that entanglement can make LOCC discrimination more dif-perfectly under LOCC, the sum of the entanglement "distances" E(|ϕi ) must be less than the total dimension D (Theorem 1 and 2), thus N ≤ D/E(|ϕi ).ficult [9]. But the exact relation is thus far unclear, and there are no general quantitative results. The results that are known can be confusing. One of the earliest results on the subject reveals a set of non-entangled, product states that cannot be discriminated perfectly by LOCC [10]. Later it was shown that any two pure states ...