  We use a single trapped 40Ca+ ion as a resonant, polarization-sensitive
absorber to detect and characterize the entanglement of tunable narrowband
photon pairs from a spontaneous parametric down-conversion source.
Single-photon absorption is marked by a quantum jump in the ion and heralded by
coincident detection of the partner photon. For three polarization basis
settings of absorption and detection of the herald, we find maximum
coincidences always for orthogonal polarizations. The polarization entanglement
is further evidenced by tomographic reconstruction of the biphoton quantum
state.
