We report experimental generation of a noisy entangled four-photon state that exhibits a separation between the secure key contents and distillable entanglement, a hallmark feature of the recently established quantum theory of private states. The privacy analysis, based on the full tomographic reconstruction of the prepared state, is utilized in a proof-of-principle key generation. The inferiority of distillation-based strategies to extract the key is exposed by an implementation of an entanglement distillation protocol for the produced state.
@article{arxiv.1010.4575,
title = {Experimental Extraction of Secure Correlations from a Noisy Private State},
author = {Krzysztof Dobek and Michal Karpinski and Rafal Demkowicz-Dobrzanski and Konrad Banaszek and Pawel Horodecki},
journal= {arXiv preprint arXiv:1010.4575},
year = {2011}
}