Quantum-state tomography of single-photon entangled states

We have performed quantum-state tomography of several different single-photon entangled states, that is, states in which a single photon is shared between two possible paths. We do this by projecting the states corresponding to the two paths onto a tomographically complete set of states (within the one-photon subspace) using interference. We simultaneously verify that our states exist in a single-photon subspace by measuring the degree of second-order coherence, $g^{\left( 2 \right)}\left( 0 \right)$. We are able to create high purity, path-entangled states. The measured states are found to have fidelities of larger than 0.95 when compared to the states that we had intended to prepare.