Inside a nonlinear crystal (BBO) an ultraviolet photon (wavelength 490nm) may spontaneously split into two infrared photons (780 nm). The down-conversion photons (A and B) have orthogonal polarizations (H or V). In the two beams along the intersections of the cones we observe a polarization-entangled two-photon state.
For the experimental realisation of quantum teleportation it was necessary to use pulsed down-conversion. Only if the pulse width of the UV-light, and thus the time of generating photon pairs is shorter than the coherence time of the down-converted photons, interferometric Bell-state analysis can be performed. In the experiment the pulses from a mode-locked Ti:Saphire laser have been frequency doubled to give pulses of 200fs duration. The interfering light was observed after passage through IR-filters of 4nm bandwidth giving a coherence time of about 520fs. (photographs of down-conversion set-up and down-conversion light).
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Text: HW
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