Experimental And Theoretical Study Of Spatial Multiplexed Atom-photon Entanglement Interface

The light-matter quantum interface that can create quantum-correlation or entanglement between a photon and one atomic collective excitation is a fundamental building block for quantum repeater.There are two main methods to generate entangled photons pairs.The first main method is cavity-enhanced spontaneous parametric down-conversion(SPDC).Another is spontaneous raman scattering(SRS).We first perform quantum memory of light in room temperature vapor and cold atomic ensemble.Besides that,we generate entangled photon pairs by cavity-enhanced SPDC with a type-?periodically poled KTiOPO4(PPKTP)crystal and SRS in cold atomic ensemble.These experiments provide experimental foundation for realizing remote quantum communication,distributed quantum computation and quantum repeater.The main accomplished works in this dissertation are as follows:(1)We report an experimental and theoretical investigation to extend lifetimes of light storages by applying guiding magnetic fields in a room-temperature atomic vapor.The storages are based on dynamic electromagnetically induced transparency(EIT).Retrieval efficiencies versus storage time are experimentally measured for different strengths of the guiding magnetic fields.The measured results show that the 1/e storage times are ~ 6 ?s and ~ 59 ?s for the guiding field B0z=0 and B0z=93mG,respectively.Physical processes causing decoherence in an atomic ensemble have been discussed and analyzed.A theory model which is used to evaluate the decoherence caused by fluctuations of transverse magnetic fields is developed.Based on this evaluation,the fact that storage lifetimes can be increased by applying guiding magnetic fields is well explained.(2)We experimentally realized storing and retrieving quadrature values of coherent state in a warm rubidium vapor by dynamic electromagnetically induced transparency.Balanced homodyne detection measurements on the input and retrieved signal are made to measure the quadrature values of coherent light state.We measured the quadrature values as a function of local oscillator phase with three different input signal photon levels and the retrieved signals at different storage time.The experimental result indicated that the amount of quantum noise of quadrature value is distributed evenly in time and the amplitude of quadrature values is increase with increasing the input signal photon level.Besides that,the lifetime of quantum memory for quadrature values is 32?s.The method of measuring quadrature measurements of coherent state is providing experimental foundation for quantum memory for continuous variables.(3)Using the process of cavity-enhanced spontaneous parametric down-conversion(SPDC),we generate a narrow-band polarization-entangled photon pair resonant on the rubidium(Rb)D1 line(795 nm).The degenerate single-mode photon pair is selected by multiple temperature controlled etalons.The linewidth of generated polarization-entangled photon pairs is 15 MHz which matches the typical atomic memory bandwidth.The measured Bell parameter for the polarization-entangled photons S=2.73±0.04 which violates the Bell-CHSH inequality by ~18 standard deviations.The presented entangled photon pair source could be utilized in quantum communication and quantum computing based on quantum memories in atomic ensemble.(4)Long-lived and high-fidelity memory for photonic polarization qubit(PPQ)is crucial for constructing quantum networks.Here we present an EIT-based millisecond storage system in which a moderate magnetic field is applied on a cold-atom cloud to lift Zeeman degeneracy.PPQ states are stored as two magnetic-field-insensitive spin waves.Especially,the influence of magnetic-field-sensitive spin waves on the storage performances is almost totally avoided.The measured average fidelities of polarization states are 98.6% at 200 ?s and 78.4% at 4.5 ms,respectively.(5)By using 6 spin-wave-photon entanglement sources,a switching network and feed-forward controls,we build a multiplexed light-matter interface and then demonstrate a ~6-fold(~4-fold)probability increase in generating entangled atom-photon(photon-photon)pairs.The measured compositive Bell parameter for the multiplexed interface is 2.49±0.03 and memory lifetime up to ~60?s.