Preparation Of Non-classical Source Based On Quasi Phase Matched Crystals

Non-classical quantum source plays an important role in many applications. Such as denying the hidden variable theory, observation of EPR paradox, quantum key distribution and quantum teleportation. In this paper we introduce a series of work based on photon and photon-pair preparation. Based on the process of spontaneous parametric down conversion(SPDC) in periodically poled potassium titanyl phosphate(PPKTP) crystals, we prepared telecom band high quality polarization entangled photon pair source, cavity enhanced narrow linewidth high intensity photon pair at both infrared and visible wavelength. In addition, we also realized mode tunable HG mode narrow linewidth photon pair based on type I crystal in OPO cavity. To pump OPO cavity we studied low pump power high efficient UV light generation.The Main Content of This Paper1. We create CW pumped 1550 nm telecom wavelength polarization entangled photon pair source with a single type-II PPKTP crystal in a Sagnac-loop configuration. Various measurements are performed to characterize the quality of the entangled source. The two-photon HOM interference has 95.3% ± 1.6% visibility and 2.4 nm signal and idler photon bandwidth. High HOM interference visibility is reserved by tuning the central wavelength of the photon over 20 nm. The two-photon Bell-type interference fringe at 45° basis has visibility of 96.4% ± 2.0%. The Bell-type visibilities keep unchanged by varying the pump power from 15 mW to 120 mW or tuning the temperature from 15℃to 55℃. The measured Clauser-Horne-Shimony-Holt (CHSH) inequality S parameter is 2.63 ± 0.08, which violates the inequality with 8 standard deviations. We also perform state tomography of the entangled state, the experimental measured fidelity is 0.935 ± 0.021. These results clearly show the high performance of our entangled source. Our photon source is in telecom wavelength and fiber coupled output. It is suitable for all-fiber quantum communications.2. We realize a high-efficiency UV laser at 397.5 nm based on cavity-enhanced SHG with low pump power. The high beam quality and long-term stability of the laser make it suitable for pumping of an OPO operating far below threshold. A total of 49 mW of UV light can be generated for a pump power of 110 mW. This relatively low pump power is a significant advantage for our setup. Our SHG system has some significant advantages in certain area. A tunable range of only 7 GHz is required to cover the full transition spectrum of the D2 lines. So a TA is not needed in case of using our system. Our system only needs an ECDL. We can also remove the EOM and modulate the LD current directly by using ECDL. This will make the system more compact and enhance its performance. Also, fast current feedback can improve the locking performance of the SHG cavity, which leads to greater SH power stability. These advantages demonstrate the possibility of applying our system in commercial SHG lasers, particularly in atomic-level applications.3. We realized a type-II cavity enhanced SPDC to generate narrow band photon-pair. We observed comb-like structure in cross-correlation function of multi-mode photon-pair. Signal-to-noise ratio is better than 100:1. After using a filter cavity to filter single-mode photon-pair we observed the disappearance of comb-like structure. Signal-to-noise ratio is better than 20:1. This proves that we generated single-mode narrow band photon-pair. After fitting the data, we obtain the bandwidth of the photon-pairs as 11.8 MHz, fitting well with atom’s intrinsic linewidth. Photon’s wavelength is at 780 nm which is in the transition spectrum of the D2 lines of Rb. It is suitable for quantum memory and quantum repeater. In the future we will use cavity enhanced SHG to achieve higher pump power to increase photon-pair generation rate, and thus increase quantum communication and quantum memory rate.4. A bright photon source in the telecom regime based on a type II periodically poled potassium titanyl phosphate (PPKTP) nonlinear crystal is generated. By locking the cavity to the pump laser, and by finely tuning the temperature of the crystals, we produce simultaneous resonances of the pump and the two down-converted fields. The bandwidth of the photon generated is 8 MHz and the estimated spectral brightness of our OPO source is 134±25 s-1MHz-1mW-1. Our photon source is in telecom wavelength and fiber coupled output It is suitable for all-fiber quantum communications.5. We hope to generate photon-pair with different spatial modes. We made some primary attempt We generate only HG mode photon. An OPO cavity is locked to it’s high order eigenmode. Then HG mode photon output from cavity. We proved the generated photon-pair has no-classical correlations. The bandwidth of the photon pairs obtained experimentally are 11.4 MHz and 20.8 MHz for two different HG modes. The wavelength of photon-pair is 780 nm. Which can be coupled to high dimensional quantum communications based on atoms. Then we realized a LG mode non-degenerate cavity. Different LG modes resonate at different frequency. Thus different LG mode can’t couple each other that lead to become HG mode. No ideal cavity is needed to make LG mode become eigenmode of cavity. This established the foundation for narrow band OAM photon-pair generation.The Innovations of This Paper1. In the paper, the spontaneous parametric down-conversion process based on quasi phase match crystals has been systematically studied, which includes both the single-pass and the cavity enhanced regimes, both the visible and infrared wavelength ranges, both the product state and entangled states, both the base mode and the high order spatial modes. Our work is systematically and comprehensive that covers many situations of SPDC process in quasi phase matched cryataks.2. We realized telecom band polarization entangled photon pair generation. The entangled photons have high fidelity, intensity and good spectrum property. It is suitable for quantum communications.3. We realized single mode narrow band photon source preparation. We first perform a SHG system, then prepare multi-mode photon, and filter it using a cavity, realized triple-resonance single mode narrow band photon source preparation. We studied single mode narrow band photon source preparation systematically and comprehensive from the shallower to the deeper.4. We realized multi-mode narrow band HG mode photon pair preparation. Very different from those old methods that transform photon from the base mode to the high order mode, our method can achieve very high fidelity. It is very useful in certain areas.