Quantum Light Source For Quantum Repeaters

The distribution of long-distance quantum entanglement is one of the main challenges in quantum communication,and its realization is crucial for building a global quantum network.Photons are the most commonly used carriers for encoding quantum information,but the inevitable photon loss in optical fibers makes it difficult to distribute long-range entanglement over hundreds of kilometers.Fortunately,the quantum repeater scheme provides a solution to create entanglement between distant nodes and construct quantum networks.The basic idea is to divide the long-distance quantum communication link into several short-distance elementary links.First,the entanglement of the nodes in the elementary link is established,and then the entanglement is transferred between adjacent links through the entanglement swapping operation until the entanglement is distributed to two target nodes.The quantum entangled light source and quantum memories are the building blocks of quantum repeaters.Towards quantum repeater applications,the goal of this paper is to develop the entangled photon pair source for absorptive solid-state quantum memories.The main achievements include the heralded entanglement through entanglement swapping,the elementary link of a quantum repeater based on absorptive memories and the developing of deterministic entanglement for absorptive quantum memories.The study of quantum entangled photon sources in the quantum repeater is an important step towards the establishment of ultra-long-distance entanglement and large-scale quantum networks.The main research contents I completed during my postgraduate period are as follows:1.Entanglement swapping of narrowband entangled photon sourcesEntanglement swapping is an essential technology for the quantum repeater.Based on the spontaneous parametric down-conversion process,we first generate highbrightness photon pairs in the PPKTP waveguide and use the post-selection method to generate polarization entanglement.Then,the linewidth of the light source is narrowed down to 1 GHz by direct filtering in order to interface with the Nd ion-doped crystal.Our experiment completes the entanglement swapping of two entangled photon pairs,and the fidelity of heralded entanglement reaches 82.6%.In the meantime,we measure the CHSH-type Bell’s inequality and obtain a violation of more than 3 standard deviations.The final rate of heralded entanglement is two orders of magnitude higher than previous work,making it possible to demonstrate a quantum repeater scheme with solid-state quantum memories.2.Demonstration of quantum repeater based on absorptive quantum memoryThe current demonstrations of elementary links for quantum repeater are all based on emissive quantum memories,which is not compatible with multi-mode storage and deterministic entanglement generation at the same time.Quantum repeater based on absorptive quantum memories can overcome this issue.Here,based on absorptive quantum memories and external entangled photon source,we realize the heralded entanglement of two quantum nodes separated by 3.5 meters.The solid-state quantum memory based on Nd ions can realize the storage of four time modes simultaneously,and its comprehensive performance reaches the optimal level of the current broadband solidstate quantum memories.This work confirms the feasibility of constructing a quantum repeater based on absorptive quantum memories,demonstrates the acceleration effect of multiplexing in quantum repeater for the first time.This new kind of quantum repeaters can support deterministic quantum light source and multiplexed memories simultaneously,paving the way for the construction of practical high-speed quantum networks.3.Generation and regulation of entangled photon sources based on quantum dotsIn principle,deterministic photon emission can be achieved based on quantum dots.The emission has no multi-photon noise,thus has great advantages for quantum repeater application.In this paper,two-photon resonance excitation is utilized to obtain polarization-entangled photon pairs in InAs quantum dots,and a six-legs piezoelectric device is utilized to tune the fine-structure splitting and wavelength.The wavelength of the entangled photon source is around 980 nm,which can efficiently interface with broadband multi-mode quantum memories based on Yb doped crystals.At the same time,we study the spectral properties and storage properties of Yb doped crystals preliminary,laying the foundation for the realization of multi-mode storage of deterministic entangled photons.The innovations of this dissertation are as follows:1.The experiment realizes the entanglement swapping of the high-brightness narrow-band entangled photon sources,the brightness is two orders of magnitude higher than the previous work,the wavelength and linewidth are suitable for the solid-state quantum memories;2.The experiment combines external entangled light sources and solid-state quantum memories,demonstrates a multi-mode quantum repeater elementary link based on absorptive solid-state quantum memories for the first time,which promotes the practical development of quantum repeater network;3.We investigate the quantum dot entangled photon source to interface with solidstate quantum memories,utilize two-photon excitation to generate entangled photon pairs,then tune the fine-structure splitting and the radiation wavelength of quantum dots via external stress.