Exploring the spatio-temporal correlation of twin photons via frequency up-conversion

The entangled photon pairs (blphotons) produced from parametric down-conversion (PDC) in a χ⁽²⁾ crystal are the key element for several quantum information and communication schemes. Recent theoretical investigations have revealed the peculiar structure of the biphotonic correlation characterizing the entanglement of twin beams in the space-time domain. The biphotonic correlation in type I PDC is characterized in the near field by a X-shaped geometry, which turns out to be non-factorable in space and time, offering thus the relevant possibility of manipulating the temporal bandwidth of biphoton entanglement by acting on their spatial degrees of freedom. The name "X-entanglement" was coined to describe this geometry. An other key feature which emerged is the extreme spatial and temporal localization of the biphotonic correlation, on the micrometer and femtosecond range, respectively.