Abstract

Quantum communication is hampered by the difficulty to generate maximally entangled states between distant nodes. Due to loss and decoherence, in reality we can only generate partially entangled states. Entanglement purification techniques are needed to concentrate maximally entangled states from partially entangled states. For qubit systems, efficient entanglement purification protocols have been found. Recently, quantum communication has been extended from qubit systems to continuous variable systems, and continuous variable teleportation has been demonstrated experimentally by using two-mode squeezed light [A. Furusawa, et al, Science 282, 706 (1998)]. For continuous variable systems, there is also inevitable noise resulting from the finite squeezing and from the light transmission loss. The latter will bring a two-mode squeezed state to a mixed Gaussian state. It is thus desirable to have some continuous variable entanglement purification protocol which can generate a useful maximally entangled state from realistic Gaussian continuous entangled states. Here we present an entanglement purification scheme with the following properties: (i) For pure two-mode squeezed states it reaches the maximal allowed efficiency in the asymptotic limit (when the number of pairs of modes goes to infinity); (ii) It can be readily extended to distill maximally entangled states from a relevant class of mixed Gaussian states which result from losses in the light transmission. Furthermore, we propose and analyze a scheme to implement this protocol experimentally using high finesse cavities and cross-Kerr nonlinearities.

© 2000 IEEE