Frontiers of quantum physics

Abstract:

Ideal dense coding protocols allow one to use prior maximal entanglement to send two bits of classical information by the physical transfer of a single encoded qubit. We investigate the case when the prior entanglement is not maximal and the initial state of the entangled pair of qubits being used for the dense coding is a mixed state. We find upper and lower bounds on the capability to do dense coding in terms of the various measures of entanglement. Our results can also be reinterpreted as giving bounds on purification procedures in terms of dense coding capacities. © 2000 Taylor & Francis Group, LLC.

Abstract:

We show that on exceeding a certain degree of mixedness (as quantified by the von Neumann entropy), entangled states become useless for teleportation. By increasing the dimension of the entangled systems, this entropy threshold can be made arbitrarily close to maximal. This entropy is found to exceed the entropy threshold sufficient to ensure the failure of dense coding.

Abstract:

We present a generalization of quantum teleportation that distributes quantum information from a sender's d-level particle to N0 particles held by remote receivers via an initially shared multiparticle entangled state. This entangled state functions as a multiparty quantum information distribution channel between the sender and the receivers. The structure of the distribution channel determines how quantum information is processed. Our generalized teleportation scheme allows multiple receivers at arbitrary locations, and can be used for applications such as optimal quantum information broadcasting, asymmetric telecloning, and quantum error correction.

Abstract:

Lower bounds for two-state teleportation fidelity were computed using a nonmaximally entangled pure state as a channel, and the exact result for the two-state fidelity with no entanglement. Only pure entangled states were considered as channels for teleportation. The results provide additional information on the respective roles of classical information and quantum entanglement in the new field of quantum information processing.