We describe an experiment in which a physical qubit represented by the polarization state of a single-photon was probabilistically encoded in the logical state of two photons. The experiment relied on linear optics, post-selection, and three-photon interference effects produced by a parametric down-conversion photon pair and a weak coherent state. An interesting consequence of the encoding operation was the ability to observe entangled three-photon Greenberger-Horne-Zeilinger states.
@article{arxiv.quant-ph/0312097,
title = {Probabilistic Quantum Encoder for Single-Photon Qubits},
author = {T. B. Pittman and B. C Jacobs and J. D. Franson},
journal= {arXiv preprint arXiv:quant-ph/0312097},
year = {2009}
}