Photon entanglement detection by a single atom

Abstract

We use a single trapped 40Ca+ ion as a resonant, polarization-sensitive\nabsorber to detect and characterize the entanglement of tunable narrowband\nphoton pairs from a spontaneous parametric down-conversion source.\nSingle-photon absorption is marked by a quantum jump in the ion and heralded by\ncoincident detection of the partner photon. For three polarization basis\nsettings of absorption and detection of the herald, we find maximum\ncoincidences always for orthogonal polarizations. The polarization entanglement\nis further evidenced by tomographic reconstruction of the biphoton quantum\nstate.\n