Quantum spectroscopy by frequency-correlated photon pairs

The new approach is experimentally realized for spectroscopic investigation of matter, where frequency-correlated photon pairs generated in the process of spontaneous parametric down-conversion (SPDC) are used. The doped crystal of YAG:Er$^{3+}$ under investigation was placed one of the channels of Brown–Twiss interferometer scheme, while the wavelength control was made by a monochromator in another channel. The criteria of fidelity is a comparison with absorption spectra measured with the help of classical light source.