  Frequency non-degenerate entangled photon pairs have been employed in quantum
communication, imaging, and sensing. To characterize quantum entangled state
with long-wavelength (infrared, IR or even terahertz, THz) photon, one needs to
either develop the single-photon detectors at the corresponding wavelengths or
use novel tomography technique, which does not rely on single-photon
detections, such as stimulated emission tomography (SET). We use standard
quantum state tomography and SET to measure the density matrix of entangled
photon pairs, with one photon at 1550 nm and the other one at 810 nm, and
obtain highly consistent results, showing the reliability of SET. Our work
paves the way for efficient measurement of entangled photons with highly
dissimilar frequencies, even to the frequencies where single-photon detections
are not available.
