Ultra-broadband continuously-tunable polarization entangled photon pair source covering the C+L telecom bands based on a single type-II PPKTP crystal

In this work, we report on the preparation of an ultra-broadband continuously-tunable, highly polarization-entangled photon-pair source covering the C+L telecom bands using a single type-II phase-matched bulk PPKTP crystal. Over 60 nm turning band of the down-converted photons is achieved experimentally. The broad tuning ability is due to operating near an extended phase match point in this crystal. The S parameter value of CHSH inequality is in the range between 2.60+/-0.04 (minimum) and 2.72+/-0.07 (maximum), which clearly demonstrates the high entanglement of the source over the whole tunable range. This is the first realization of such a broadband tunable degenerate polarization entanglement pair source in the telecom band. Combined with dense wavelength-division multiplexing technique, our source could be used to improve the transmission capacity of a single communication channel in the field of quantum communication.