A single photon, delocalized over two optical modes, is characterized by means of quantum homodyne tomography. The reconstructed four-dimensional density matrix extends over the entire Hilbert space and thus reveals, for the first time, complete information about the dual-rail optical quantum bit as a state of the electromagnetic field. The experimental data violate the Bell inequality albeit with a loophole similar to the detection loophole in photon counting experiments.
@article{arxiv.quant-ph/0312135,
title = {Homodyne tomography characterization and nonlocality of a dual-mode optical qubit},
author = {S. A. Babichev and J. Appel and A. I. Lvovsky},
journal= {arXiv preprint arXiv:quant-ph/0312135},
year = {2009}
}