We demonstrate a new technique for characterizing two-photon quantum states
based on joint temporal correlation measurements using time resolved single
photon detection by femtosecond upconversion. We measure for the first time the
joint temporal density of a two-photon entangled state, showing clearly the
time anti-correlation of the coincident-frequency entangled photon pair
generated by ultrafast spontaneous parametric down-conversion under extended
phase-matching conditions. The new technique enables us to manipulate the
frequency entanglement by varying the down-conversion pump bandwidth to produce
a nearly unentangled two-photon state that is expected to yield a heralded
single-photon state with a purity of 0.88. The time-domain correlation
technique complements existing frequency-domain measurement methods for a more
complete characterization of photonic entanglement in quantum information
processing.Comment: 4 pages, 5 figure