Qubit carriers with internal degrees of freedom in a non-factorable state

We have studied how the quality of transfer of a qubit state depends on distinguishability of internal states of the particles carrying qubits. The transfer is implemented without any direct interaction, just by a partial exchange of photons, measurement on one of them, and conditional feed-forward correction. It appears that the quality of the transfer is only influenced by the level of distinguishability of the states of unaccessible internal degrees of freedom not used for information encoding. We have found a directly measurable parameter quantifying this level of distinguishability and proved it usefulness experimentally.