Preparation Of Photon Pairs In Periodically Poled Nonlineaer Crystal

Spontaneous parametric downconversion photons have good quantum correlation as one important type of non classical field. So far, the most mature way of obtaining the entangled photon pairs is the spontaneous parametric downconversion (SPDC) method in a nonlinear crystal. SPDC photon is widely used in quantum optics and quantum information field, especially in quantum net work used as a flying bit to carry information. In another point, quantum memory (quantum reperter) is also a key component to bulid up a quantum network. The atomic system is regard as a good candidate for quantum reperter. The typical natural linewidth of atom is on the order of MHz. But the linewidth of SPDC photon is on the order of THz. It is hard to realize the efficient coupling between the atom and the photon to transfer information. So preparing a narrow-band photon is a necessary work.The main contents and key points in this dissertation are as follows:1. Second harmonic generation with periodically poled potassium titanyl phosphate. We use CW laser and pulse laser to generation second harmonic respectively. Firstly, we experimentally demonstrate an efficient CW SHG at 780 nm wavelength with a first-order type-Ⅰphase matching PPKTP crystal in a ring cavity. At an input mode-matched power of 73mW, the power of generated UV radiation is 8.8mW with the net conversion of 12%. Secondly we report on the efficient frequency doubling of a femtosecond laser at 800 nm wavelength with type-Ⅱphase matching PPKTP crystals in a single-pass configuration. The normalized conversion efficiency of 0.36%W-1 cm-1 is obtained.2. Preparation of SPDC Photon Pairs in PPKTP. At second part we study the generation of a collinearly propagating photon pair by pumping a 2.12 mm long type-I phase matching bulk PPKTP crystal with CW laser. Using a single mode fiber as a spatial filter, we detect about 7400/s net coincidence counts with 3.58 mW pump power in a Hanbury-Brown-Twiss-type experiment. The estimated photon pair production rate is about 0.73 MHz/mW per second. At second part, we experimentally demonstrate an ultrabright two-photon source with a type-I bulk PPKTP. With the 390 nm CW laser generated from our home-made frequency doubler in a ring cavity as the pump, we detect about 340 k/s coincidence counts with a pump power of 0.11 mW in a Hanbury-Brown-Twiss-type experiment in a single-pass SPDC configuration, corresponding to the production rate of 94 MHz/mW with full bandwidth of 40 nm measured at FWHM. With a 3 nm interference filter, we detect about 11.5 k/s coincidence counts with pump power of 0.11 mW, which corresponds to the production rate of 4.3 MHz/mW/nm.3. Experimental measuring the coherence length of the single photon generated via a degenerated optical parametric oscillator far below threshold. We report on experimental measuring the coherence length of the single photon, which is generated via a degenerated OPO far below threshold. The bandwidth of the cavity measured is about 7.8 MHz and the measured coherence length of the single photon is about 90μm via a Michelson interferometer due to the multimode property of the photon generated in the experiment. We also do a raw estimation about the coherence length of the single photon. The estimated coherence length is about 110μm, which is close to the measured 90μm coherence length in the experiment.4. Preparation of multi-channel photon pairs in a periodically poled Potassium Titanyl Phosphate waveguide. We propose a possible way to generate multi-channel photon pairs by using a periodically poled Potassium Titanyl Phosphate (PPKTP) waveguide via SPDC. We discuss type-Ⅱand type-Ⅰphase matching cases with a pulsed and CW laser respectively. We could make a multi-channel photon pairs source which could provide these photon pairs to several communication couples at the same time. This is very promising in quantum information network.