Research On Optical Quantum Devices Based On PPLN Waveguide

In recent decades,with the hard researches of the scientists,the quantum information has developed rapidly.Taking advantage of some quantum theories such as quantum entanglement and no-cloning theorem,people have made significant progress on quantum communication and quantum computing.Nowadays,more and more researches are being done to study scalable quantum network and its applications.Quantum network is predicted to be the platform for the application of both quantum communication and quantum computing.Quantum network is composed of many nodes,which are connected to the quantum channel.The high-speed transmission of large volumes of quantum communication and efficient distributed quantum computation can be realized by quantum network.Among them,the fiber quantum network with photons as the quantum information transfer particles is the most feasible one.Any node in the network can process the quantum information such as quantum key distribution,quantum entanglement swapping,quantum memory,logical operation and so on.Therefore,improving the quantum information processing ability of nodes is one of the important tasks to construct scalable optical fiber quantum networks.Periodically polarized lithium niobate(PPLN)crystals' second-order nonlinear coefficients are higher than those of other crystals.Its absorption loss is relatively small and the refractive index changes very gently near 1550 nm.Moreover,the integration and processing technology of the PPLN is quite mature.In a word,the PPLN is ideal for quantum optics experiments.It is used extensively in quantum entanglement source and heralded single-photon source.Comparing with bulk structure,waveguide structure has much higher conversion efficiency.Based on these properties of waveguides,we have accomplished the following experiments.?.Single-photon frequency conversion in the telecom wavelength.In optical fiber networks,to realize the information transmission between multiple users,singlephoton need to be converted by frequency conversion to switch among different channels.For quantum entanglement swapping,photons from different entanglement sources need to be converted the frequency into the same as well.Hence,single-photon frequency conversion interface has crucial significance in quantum networks.We utilize the cascaded quadric nonlinearity process [sum frequency generation(SFG)and difference frequency generation(DFG)] to realize low noise tunable single-photon frequency conversion in the telecom wavelength for the first times.what's more,our conversion interface's work range is covered all ITU channels.Our frequency conversion interface is the key component in quantum networks.?.Multiple-DWDM-channel heralded single-photon source(HSPS).As we know,the photons of different wavelengths transmit in certain channel corresponding to their wavelengths in the optical fiber networks.This method has greatly improved the transmission volume of the information.Previous researches are mainly focused on single wavelength single-photon sources.In order to match the demond of the networks,multiple HSPS have to be equipped simultaneously.However,this scheme is not conductive to the integration and practical application of networks.Using PPLN waveguides,the broadband phase matching conditions of spontaneous parametric down-conversion can be satisfied.We successfully achieve the broadband HSPS at the telecom wavelength covering over 35 ITU channels.Meanwhile,all channel sources contain almost the same brightness and quality.?.The spectral compression based on PPLN waveguide.In optical fiber networks,the nodes should have the ability to realize quantum memory.However,the property of transmitting photons are quite different form the one can be saved.We use PPLN waveguides to achieve this quantum interface.In our experiment,the spectrum of single-photon-level laser pulse is compressed by a factor of 58 through SFG,where a positively chirped single photons laser pulse and a negatively chirped classical laser pulse by fiber Bragg gratings are used to achieve a pulse with new frequency.