  Decoherence-free states protect quantum information from collective noise,
the predominant cause of decoherence in current implementations of quantum
communication and computation. Here we demonstrate that spontaneous parametric
down-conversion can be used to generate four-photon states which enable the
encoding of one qubit in a decoherence-free subspace. The immunity against
noise is verified by quantum state tomography of the encoded qubit. We show
that particular states of the encoded qubit can be distinguished by local
measurements on the four photons only.
