Characterization Of Narrow-Bandwidth Single Photon State Generated From Cold Atom Ensembles

The single-photon state has become one of the most important research objects of be-cause its application prospect in quantum information processing,quantum communication and quantum computation.The single photon can be generated from a variety of media,for example,Nitrogen-vacancy center,atomic gases,quantum dots,single atoms and single molecules.No matter how the single photons are generated,it is necessary to test its quality,such as spectral linewidth,spectral brightness,the state purity,indistinguishability,density matrix in the photon-number basis and so on.The research work in thesis mainly uses two types of quantum measurement technology,i.e.,photon counting and photo-current detec-tion,to measure the photon coherence time,the indistinguishability,the purity and the density matrix of the heralded single photons,which are generated from a 85Rb cold atomic system using spontaneous four-wave mixing.This thesis can be divided into three sections:In the first section I introduce the single-photon source,and the generation of heralded single pho-tons through four-wave mixing in a⁸⁵Rb cold atomic ensemble trapped by a two-dimension magneto-optical trap.In the second section,the quantum tomography technology based on photo-current detection is applied on the characterization of the narrowband single photons.This is the first time that quantum tomography is operated on the characterization of the single-photon state which is modulated from electromagnetically-induced transparency?EIT?.From the experimental measurements,the quasi-probability distribution function,i.e.,Wigner func-tion,which describes the photon numbers of the quantum state,and its corresponding density matrix in Fock state,and the density matrix of the single-photon temporal state are obtained.The third section focuses on the purity of the photonic temporal state,which is obtained from measuring the coincidence counts and correlation function of photons.We utilize two inde-pendent single photon sources to produce their Hong-Ou-Mandel interference,from which the indistinguishability and state purity of individual single photon source are measured.For the first time,we demonstrate in this work,the purity of the narrow-band single photons can be up to 80%with the resolution of the photon detector smaller than 10 ns.The results from this thesis work have potential application in quantum information processing,quantum com-munication and photonic quantum computation.