  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.
