Nonlinear optics with single photons
Almost all single photon sources realized today emit light within the red or near-infrared spectral region between ca. 600 nm and 1000 nm. Owing to strong absorption in fiber transmission these photons are not well suited for quantum communication over large distances.
For the development of long-range quantum networks it is thus very important to realize single photon sources in the wavelength range of telecommunications (1310 nm, 1550 nm). A similar reasoning holds for the investigation of quantum repeater schemes: on one hand quantum information may be stored in long lived electronic states of atoms or ions or spin states of color centers (with read/write operations on optical transitions in the VIS-NIR range) but on the other hand transfer of quantum information is facilitated using photons within the telecom windows.
For realization of these goals we investigate frequency conversion of single photons from the red (ca. 750 nm) to the infra-red (1550 nm) spectral region. The method employed here is nonlinear optical difference frequency mixing of single photons with an intense mixing wave resulting in the generation of a single (idler-) photon at telecom wavelengths.
Our recent experiments focus on the setup of optical parametric oscillators for generating the mixing wave (infrared spectral range), realization of the frequency conversion process and investigation of noise processes during the nonlinear conversion.