Time-frequency domain is a promising platform for optical quantum technologies due to large Hilbert space available for quantum information encoding. Moreover, photons appear as perfect candidates for interface between different quantum systems as they can be transmitted over large distances in a low-loss manner. Obviously mostly desired would be such link at telecommunication wavelengths, because it can be integrated with classical communication schemes.
We report on a tunable telecom-wavelength photon pair source based on bulk periodically poled potassium titanyl phosphate (PPKTP) pumped by femtosecond laser pulses. The pairs are produced via type-II spontaneous parametric down conversion (SPDC). The spectra of photons, which lies in telecommunication range, in our source can be affected via both changing the spectrum of pumping laser and changing the phase matching by using crystals of different lengths. By appropriate choice of these parameters either the pair of photons occupy single-mode in frequency and is in a separable state or is multimode and entangled .
Here we report on experimental active modification the photon pairs’ spectral properties. We employ fast electro-optic temporal phase modulation to induce a deterministic change of photons’ joint spectral intensity (JSI). Central wavelength of one photon is shifted by up to 0.2 nm. The measurement of the spectrum of single photons is based upon frequency to time mapping, implemented by large group delay dispersion (GDD) in chirped fiber Bragg grating (CFBG) . An unprecedented sub-10 pm resolution of correlated single-photon spectral measurements has been achieved.
Further work will be done to modify not only the position, but also the shape of JSI by using techniques based on the time-lens principle .
 P. G. Evans, R. S. Bennink, W. P. Grice, T. S. Humble, Phys. Rev. Lett. 105, 253601 (2010)
 A. O. C. Davis, P. M. Saulnier, M. Karpinski, B. J. Smith, arXiv:1610.03040 (2016)
 M. Karpiński, M. Jachura, L. J. Wright, B. J. Smith, Nature Photonics 11, 53–57 (2017)