Entanglement Distillation

Quantum communication schemes, from quantum key distribution to teleportation and dense coding, offer better ways to exchange information in a network. Experiments into these areas have yielded lots of successes over small distances. However, over larger distances, optical losses, phase diffusion, and mixing with thermal states cause the signals to decohere over some finite transmission length. The most obvious way to overcome this would be to create a series of signal amplifiers to be spaced out between the source and destination where the signal could be stored onto a quantum memory device and read out again. Such quantum repeater protocols have been devised with slight variances between schemes. Due to the long lifetimes of atomic states, the most promising storage protocol so far uses the coherent spin states of cesium atoms to serve as a memory device. But along with the memory cell, the crucial ingredient for quantum repeater is represented by entanglement concentration, i.e. restoration of the deteriorated entanglement link. This project deals with a theoretical protocol for increasing the entanglement between two atomic ensembles.